Electrodialysis as a Measure of Phosphate Availability in Soils and the Relation of Soil Reaction and Ionization of Phosphates to Phosphate Assimilation

Item Type text; Book

Authors McGeorge, W. T.

Publisher College of Agriculture, University of Arizona (Tucson, AZ)

Rights Copyright © Arizona Board of Regents. The University of Arizona.

Download date 28/09/2021 19:33:10

Link to Item http://hdl.handle.net/10150/190642

CONTENTS

PAGE: Introduction 593 Electrodialysis 595 Preliminary experiments and methods - 595 Effect of variation in amperage on rate of difference 597 Effect of variation in voltage 599 Relation of time to solution of phosphate during electrodialysis 600 Electrodialysis of phosphate from Arizona soils : 601 Electrodiffusion of phosphate from fertilized and unfertilized soils.—.. 604 Electrodiffusion of phosphate from other states 604 Interpretation of data 612 Specific solution rate 612 lonization and soil reaction : 617 Ionization of phosphates.. 617 The role of reaction (pll) in phosphate solubility and nutrition 619 Summary 624 Conclusions '.. 628 Bibliography 630

ILLUSTRATIONS

PAGE Fig. 1.—Effect of variation in amperage on the electrodiffusion of phosphate ions from soil No. 1 597 Fig. 2.—Effect of variation in amperage on the electrodiffusion of phosphate from soil No. 2 598 Fig. 3.—Rate of phosphate electrodiffusion in representative Arizona soils 601 Fig. 4.—Rate of base electrodiffusion in representative Arizona soils 602 Fig. 5.—Rate of phosphate electrodiffusion from fertilized and unfertilized

soils v 604 Fig. 6.—Rate of phosphate electrodiffusion from California (Berkeley) soils.. 606 Fig. 7.—Rate of phosphate electrodiffusion from California (Riverside) soils 607 Fig. 8.—Rate of phosphate electrodiffusion from Alabama soils _ 608 Fig. 9.—Rate of phosphate electrodiffusion from Wisconsin soils 609 Fig. 10.—Rate of phosphate electrodiffusion from Illinois soils 610 Fig. 11.—Rate of phosphate electrodiffusion from Florida soils 611 Fig. 12.—Response of tomatoes to phosphate in soil No. 1 613 Fig. 13.—Response of tomatoes to phosphate in soil No. 11 613 Fig. 14.—Comparing growth in soils Nos. 2 and 3 614 Fig. 15.—Response of millet to phosphate in soil No. 2 619 Fi#. 16.—Influence of reaction on solubility of calcium phosphates 627 ELECTRODIALYSIS AS A MEASURE OF PHOS- PHATE AVAILABILITY IN SOILS AND THE RELATION OF SOIL REACTION AND IONIZATION OF PHOSPHATES TO PHOSPHATE ASSIMILATION.* BY W. T. MCGEORGE

INTRODUCTION In some phosphate studies (4 and 5) recently conducted in this laboratory it was shown that Arizona soils are fairly well supplied with total phosphate and that it is relatively quite soluble in the dilute organic and inorganic acids often used in estimating the availability of phosphate in soils. The solubility in water, both CO2-free and CO2-saturated, varied considerably, with some relation between the degree of solubility and availability as shown in pot cultures. In the pot experiments, to which reference is made, tomato plants made practically no growth in some of the soils unless fertilized with phosphates and thus demonstrated beyond question the low degree of phosphate availability in these soils. In some of the soils which appeared from solubility studies to be amply supplied with soluble phosphate, growth of tomatoes in pot cultures was measurably increased by phosphate fertilization. Water solubility is therefore apparently of relative value only with no sharply defined distinction between soils requiring phosphate and those amply supplied with this plant food. The observations showed that factors other than solubility influence assimilation and often limit solubility interpretations. A very large percentage of the cultivated soils of Arizona are calcareous, in fact practically all those in the southern half of the State fall into this category. In the phosphate fertilization of such soils the principal problem is one of combating the fixation and reversion reactions between CaCO3 in the soil and the several commercial forms of phosphate available for fertilizer purposes, the whole of which is influenced by soil environment. We have shown(5) that the CaCO3 plays an important role in both the solubility of phosphate and its assimilation by the plant. It may be combined with calcium phosphate as a highly insoluble carbonate-phosphate of calcium and it may retard ionization of this and other calcium phosphates through a common ion effect. Aside from this influence on fixation and assimilation is the handicap

* This bulletin was received for publication in the office of the Dean and Director on February 10, 1932. P.S.B. 594 TECHNICAL BULLETIN No. 38 which CaCO3 imposes on any attempt to estimate or evaluate the phosphate requirement of the soil by chemical methods. While it can be stated definitely that the general application of chemical methods for estimating phosphate availability in soils has failed completely when applied to widespread conditions, some slight success has been met under local conditions. This is true where correlations have been developed between phosphate determinations by chemical methods and crop response in field or pot experiments. This limited success has served to keep alive investigations pertaining to laboratory methods of estimating phosphate availability. But if a soil contains CaCO3 the use of any acid solvent is valueless as the acid first attacks the carbonate rather than phosphate or other less reactive constituents. Since the CaCO3 is always present in variable amounts, similarly variable dilute acid concentrations can not easily be adjusted to compensate for this. In a selected series of Arizona soils we were able to attach only limited value to the solubility of phosphate in the soil as determined by acids as a measure of their phosphate requirements. Solubility of phosphate in water showed better correlation. But in estimating the phosphate solubility by extraction with water one is handicapped, in any soil type, by failure of adsorbed phosphate to enter solution and the fact that adsorbed phosphate is usually considered available to the plant. Then again, in any soil study involving a water extract or the soil solution, one is usually handicapped by the presence of finely divided clay particles of colloidal dimensions. While in many cases the amount may seem negligible its effect upon certain important soil properties and determinations often made on water extracts is considerable. The removal of this colloid by means of clay filters must be avoided in quantitative work because of the absorption of certain ions, notably phosphate. The only acceptable means of separating this colloid is by employing a semipermeable membrane and the process of dialysis. Ordinary dialysis with such materials as soils is too slow to be of great value. This circumstance probably prompted the application of the electric current as a means of increasing the velocity of diffusion and for attaining a more complete dissociation of the active soil compounds. Since Mattson(3) introduced electrodialysis to the study of exchangeable bases in soils the method has found rather extensive application. Previous to this, while electrodialysis had been used in many fields of investigation, other than the experiments of Cameron and Bell(l) and of a few European scientists, it had received little attention in soil investigations. At present, electrodialysis of soils, as a means of removing the soluble and exchangeable bases, is a well established PHOSPHATE AVAILABILITY AND ASSIMILATION 595 procedure, but as yet its application to the study of anions, notably phosphate, has received little or no attention. Such a procedure not only would remove the interfering colloids but also, on account of effecting an increased ionization of the phosphate compounds in the soil, should serve as a means of dissolving the active phosphate without the use of acids. That is, the ionization would not be affected by the presence of CaCO3 to a sufficient extent so that it would interfere with the method. Rost(7) determined the phosphate removed by electrodialysis from four Minnesota soils that responded and four that did not respond to phosphate treatment. He used 200 grams of soil and electrodialysed for 10 hours but found little or no difference between the two groups of soils. He did not attach any value to the method.

ELECTRODIALYSIS PRELIMINARY EXPERIMENTS AND METHODS In view of the obstacles attached to the extraction of calcareous soils with solvents the application of electrodialysis methods suggested a solution of this difficulty. Pierre and Parker (6) obtained phosphate by ordinary dialysis of soils in collodion sacks but this method is slow and troublesome. By employing an electric current to increase the rate of dialysis, transportation of phosphate ions is sufficiently rapid for many purposes as the volume of current can be varied over a wide range. Some attention has therefore been given to the solvent effect of electrodialysis or electrodiffusion * on soil phosphates. A preliminary report has already been published(4) and even without careful control of amperage the results obtained were quite promising and warranted further study. Apparatus.—The original type of apparatus as used by Mattson and many others is a three-chamber cell in which the smallest chamber containing the water and soil lies between the cathode and anode chambers. Several two-chamber cells have been devised for soil work taking advantage of the fact that both cations and water, by endosmosis, move toward the cathode. These have found application in base exchange studies where one is interested only in the contents 'of the cathode chamber. In such a set-up the anode is placed in the chamber containing the soil-water mixture. This simpler type of apparatus is not suited to the extraction of phosphate because of the reaction change which takes place in the electrode chambers. That is, if the soil were placed in the

K T. F. Buehrer of this laboratory has suggested the term electrodiffusion as being more suitable than electrodialysis for expressing ion transport during the process of electrodialysis. 596 TECHNICAL BULLETIN No. 38 anode chamber the acid reaction would increase the solubility of phosphate and the same would be true for the cathode chamber as the solubility of phosphate is also increased by excess hydroxyl ions. There seemed no other choice except the three-chamber cell for phosphate studies. Our apparatus was essentially that of Mattson(3). It was prepared from a hard rubber storage battery by cutting it into two sections which when clamped together had a capacity of three liters. The third chamber was formed by placing two layers of parchment paper between the two sections and had a capacity of about 200 cc. The cathode was of sheet copper 7 by 10 cm. and the anode of platinum gauze 5 by 12.5 cm. Amount of Soil.—After trying several different amounts of soil a weight of 10 grams was found to be most suitable for quantitative work where the Deniges colorimetric method is later used to determine phosphate. Current.—A great deal of time was spent in studying the best source as well as volume of direct current suitable for dialysis. During this time we found such a wide variation in the movement of phosphate ions that duplication of results could only be obtained by closely controlling the amperage. This was accomplished by placing an ammeter and resistance in the system and maintaining constant amperage by varying the resistance to compensate for the change in resistance of the cell as the ion concentration increased. Throughout these investigations distance between electrodes in the cell was maintained constant at 1.2 cm. After considerable experimentation the technic found to be best suited for electrodialysis was as follows: Twelve hundred fifty cc. of tap water, phosphate free, was added to each of the outer, cathode and anode, chambers of the cell. To the middle chamber was added 100 cc. of water and 10 grams of air-dried soil. The contents of this chamber was kept constantly stirred by a stream of air. The time of dialysis was usually 9 hours and 50 cc. were withdrawn at hourly periods from both the cathode and anode chambers in order that both rate and final diffusion of phosphate could be determined. The base concentration of the former was determined by titration with standard acid and the phosphate in the latter by the Deniges colorimetric method. Additional water was added after each withdrawal in order to maintain a constant volume. With the volume of current used in these experiments there was little or no increase in temperature so no allowance is made for such. The current density at .2 amperes using the electrodes of dimensions given was .0027 amperes per sq.cm. for the copper cathode and .0032 amperes per sq.cm. for the platinum anode. PHOSPHATE AVAILABILITY AND ASSIMILATION 597

In several cases the reaction (pH) of the suspended soil colloids at the end of 9 hours electrodialysis was determined. On account of the replacement of exchangeable bases by hydrogen we expected to find an acid reaction in spite of the fact that calcium carbonate was still present in the soil. The final pH of the suspended colloidal clay was pH 5.0.

EFFECT OF VARIATION IN AMPERAGE ON RATE OF DIFFUSION Having noted in our preliminary investigations that very slight variations in amperage greatly altered the rate and amount of phosphate electrodiffusion the first experiment was conducted to determine the most suitable volume of current. The best method for determination of phosphate in such a procedure is the colorimetric method. If too rapid diffusion is obtained the concentration of phosphate in the anode chamber will be so great as to seriously increase the error yet the current must be strong enough to yield readily determinate amounts of phosphate.

.2 A

./ A

^. 1.—The effect of variation in amperage on the rate of electrodiffusion of phos- phate ions in soil number 1. 598 TECHNICAL BULLETIN No. 38

J A,

<} / Z J + •*- 4, 7 ?. 9 Ho U KS Fig. 2.—The effect of variation in amperage on the rate of electrodiffusion of phos- phate ions in soil number 2.

Two soils were selected, one being very deficient in soluble phosphate, and the other well supplied with soluble phosphate. Tjhe rate of ionization in these soils was determined at .05, .1, .2 ampere with an E. M. F. of 12 volts and .5 ampere with an E. M. F. of 40 volts. Fifty cubic centimeters of solution were withdrawn from each chamber at hourly intervals for analysis. The phosphate was determined in the solutions from the anode chamber by the Deniges colorimetric method and the total bases from the cathode chamber by titration with .IN H2SO4. The data are given in Table I and shown graphically in figures 1 and 2. As shown in this experiment, the rate and amount of diffusion varies greatly with slight differences in amperage. As the amperage increases the ionization and diffusion increase. This applies to the bases as well as to the phosphate ions. From the data obtained in this experiment a current of 0.2 ampere was selected as most suitable for Arizona soils as soil No. 1 represents one of the lowest and No. 2 one of the highest in soluble phosphate which have come to our attention. PHOSPHATE AVAILABILITY AND ASSIMILATION 599

TABLE L—EFFECT OF AMPERAGE ON RATE OF ELECTRO- DIFFUSION OF PHOSPHATE IONS.

Mg. PO4 in anode chamber (1250 cc.) Soil No. 1 vSoil No. 2 Am- .05 .1 2 .5 .05 .1 .2 .5 peres Hrs.

1 .06 .12 .28 1.38 .25 .38 .63 2.15 2 .15 .26 .76 2.71 .39 .64 2.57 5.18 3 .26 .39 1.24 4.60 .53 1.29 4.67 8.23 4 .33 .53 1.61 5.40 .92 1.73 6.00 13.09 5 .35 .68 1.92 6.84 1.22 2.05 7.47 & 4.0 1 0? ?4Q 7 &7 1 M \f\ 7C 7 .56 1.23 2.97 8.90 2.08 3.78 9.79 8 .71 1.56 3.52 9.36 2.40 4.55 11.64 9 1.00 1.87 4.14 9.78 2.85 5.61

cc. .IN HL-SO+ per 50 cc. cathode solution

.7 .9 A .6 1.2 2.2 1.2 1.5 2.4 .8 1.2 2.1 3.0 1.8 22 4.6 .9 1.4 2.7 3.6 2.3 5.8 1.0 1.7 3.0 3.7 2.7 £3.65 6.6 1.3 1.9 3.7 4.0 3.1 4.1 6.9 1.4 1.9 3.7 3.5 4.4 7.1 1.5 2.2 3.9 3.7 4.6 7.3 2.3 40 4.3 4.8 7A 1.7 2^4 4.0

For all amperages up to and including 0.2 ampere a small rectifier was used delivering 12 volts, the amperage being varied and controlled by the introduction of slide wire resistance. For 0.5 ampere a motor generator was used delivering 40 volts.

EFFECT OF VARIATION IN VOLTAGE While the maintenance of equilibria conditions by which results may be duplicated and made comparable for different soils is largely a function of constant amperage the question of constant voltage was not overlooked. It was found, however, to have little significance as shown by the following experiment: Here the rate of phosphate ionization was determined in the same soil at 0.2 ampere and 32 volts from a motor generator and 0.2 ampere and 12 volts from a rectifier. The rate of movement of phosphate ions and bases in the electrodialysis cell under the influence of the different E. M. F. and constant amperage is shown by the data in Table II. 600 TECHNICAL BULLETIN No. 38

TABLE II.—EFFECT OF VARIATION IN VOLTAGE (E.M.F.) ON ELECTRODIFFUSION OF PHOSPHATE. .2 A 32 V .2 A 12 V

P.p.m. PO4 cc. .1NH,SO4 P.p.m. PO4 cc..lNH2SO4

1 Hour .3 1.2 .3 1.0 2 Hours .4 17 .4 1.7 3 Hours .8 2.1 .8 2.0 5 Hours 1.5 3.0 1.2 3.0 6 Hours 1.8 3.7 1.7 3.5 7 Hours 2.4 3.9 2.2 3.6

RELATION OF TIME TO SOLUTION OF PHOSPHATE DURING ELECTRODIALYSIS The question as to how long the process of electrodialysis should be continued is also important. That is, does the soil yield its phosphate rapidly over a short period of time or slowly for an extended period? Two soils w^ere selected to study this. As will be shown later some non-calcareous soils require a higher amperage to ionize the phosphate than do calcareous soils. In view of this a calcareous soil was electrodialysed at 0.2 ampere, 12 volts and a non-calcareous soil at 0.5 ampere, 40 volts. They were electrodialysed for extended periods and the phosphate concentration of the anode chamber determined at hourly intervals. The results are given in the following Table II-A.

TABLE II-A.—SHOWING HOURLY RATE AT WHICH PHOSPHATE IS REMOVED FROM SOILS BY ELECTRODIALYSIS. Non-calcareous soil Calcareous soil

Time Total PO4(Mg.) Hourly rate Total PO4(Mg.) Hourly rate Hrs. in anode chamber Mg. PO4 in anode chamber Mg. PO*

1.5 2.00 1.30 1.70 1.13 3 3.58 1.00 3.57 1.24 4 4.72 1.10 5.45 1.88 5 5.15 .43 6.08 .63 7 5.69 .27 6.88 .80 8 6.29 .60 7.39 .51 9 6.26 .0 7.65 .26 10 6.22 7.91 .26 11 8.17 .26 14 — — 8.68 .26 25 — — 11.93 .29

The data show that the peak of electrodiffusion is apparently passed at 4-5 hours. After this time the diffusion is practically complete at 9 hours for the non-calcareous soil but becomes a constant for the PHOSPHATE AVAILABILITY AND ASSIMILATION 601 calcareous soil and continues so for the remainder of the 24-hour period. The greater diffusion during the first 4 hours is probably due to the transport of adsorbed phosphate ions. The constant but slow rate of diffusion in the calcareous soil is due to the effect of calcium carbonate upon the ionization of the carbonate-phosphate compound. This experiment suggests that an 8- or 9-hour period of electrodiffusion should be sufficient for a quantitative comparison of phosphate ionization in soils if one is seeking comparative availability. ELECTRODIALYSIS OF PHOSPHATE FROM ARIZONA SOILS We next selected a group of Arizona soils representing variable degrees of phosphate solubility as measured by water extraction and

(availability as measured by plant growth. These were subjected to electrodialysis using a current of 0.2 amperes, 12 volts, and the rate of ionization and electrodiffusion of bases and phosphate determined at hourly intervals. These data are given in Table IV and are shown graphically in figures 3 and 4.

3 VI

i

1

/You r-^ Fig. 3.—The rate of phosphate electrodiffusion in some representative Arizona soils. Description of Soils.—The soils used in this experiment with the exception of one additional sample, No. 34, were selected from those

PHOSPHATE AVAILABILITY AND ASSIMILATION 603

TABLE III.—PHOSPHATE CONTENT OF SOILS (AIR DRY BASIS). Pounds Total Percent P04 Percent P.p.m. present (P) per Soil phosphate soluble in PO4 soluble in 1 :5 water acre Truog No. %PO4 1 % citric acid inl^KaCOs extract method

1 .147 .081 .0044 Trace 104 2 .287 .203 .0200 1.2 522 3 .246 .154 .0106 .4 522 11 .180 .046 .0024 Trace 52 29 .195 .055 .0013 Trace 52 34 .430 .250 .0060 1.3 351

The electrodiffusion data show a very close correlation with the known field performance of these soils and with the relative solubilities as determined by the various solvents. A comparison of the base curves with phosphate curves is of little or no significance and it is questionable whether any relation should be expected. TABLE IV.—RATE OF BASE AND PHOSPHATE ION TRANSPORTA- TION DURING ELECTRODIALYSIS. Mg. PO4 in anode solution (1250 cc.) Soil number 3 11 29 34 Hours 1 .37 .62 .37 .50 .12 .62 2 .76 2.52 1.26 .89 .25 1.64 3 1.15 4.62 2.56 1.42 .51 4.09 4 1.59 5.90 1.85 .90 5.75 5 1.90 7.32 2.29 1.42 8.22 6 2.43 4.66 1.73 12.38 7 2.93 9.64 5.34 1.89 15.44 3.41 11.49 3.00 2.24 4.03 6.79 2.44 17.40

cc. .IN HaSO* per 50 cc. cathode solution

1 9 1 1 1 1 1 1 2 1.1 1.1 yj.yno 2 L5 2*1 1.6 1.9 2.3 1.85 3 2.2 2.7 1.8 2.6 3.6 2.70 4 2.9 3.0 2.1 3.3 3.10 5 3.5 3.7 2.4 3.8 3.95 f. u 4T". ±1 4 3 u.o 4.4 3.9 4.5 Q7 46 7.31 f\ 4 SS O 4 n / .u 4.8 2.4 4.5 4.50 9 7.8 Original soil reaction pH 8.3 8.0 7.8 7.8 8.2 8.5 604 TECHNICAL BULLETIN No. 38

ELECTRODIFFUSION OF PHOSPHATE FROM FERTILIZED AND UNFERTILIZED SOILS We next selected soil and subsoil samples from fertilized and unfertilized plots of a phosphate field experiment in order to ascertain if a fertilizer application could be measured by this method and if the phosphate was still present in the soil in an ionizable form. The soils are designated as follows: Number 1, 6-18 inches depth from unfertilized plot, Number 2, 0-6 inches depth from unfertilized plot, Number 3, 6-18 inches depth from fertilized plot, and Number 4, 0-6 inches depth from fertilized plot. The data are given graphically in Figure 5 and show how very clearly defined are the differences between the rate of ionization in soils from fertilized and unfertilized plots.

Kz

y Hours pigf 5,—The rate of phosphate electrodiffusion in fertilized and unfertilized soils.

ELECTRODIFFUSION OF PHOSPHATE FROM SOILS OF OTHER STATES As already stated, practically all our soils are calcareous, and being desirous of applying the method of electrodialysis to other soil types we sought the aid of several other State Experiment 'Stations where soils of known phosphate response were available. California Soils.—These samples included five supplied by P. L. Hibbard of Berkeley, and four supplied by W. P. Kelley of Riverside. The following data and descriptions were kindly submitted by them along with the samples. PHOSPHATE AVAILABILITY AND ASSIMILATION 605

SOILS FROM BERKELEY PHOSPHATE EXTRACTED FROM SOILS BY VARIOUS METHODS. P.P.M. PO4 IN AIR-DRY SOTL Percolate Enuib. Deficiency Soil with with .002 N Percolate indicated by No. Total N/20 HCL H,SO* Truog with water barley plants 37 2400 494 400 49 Moderate 59 2000 20 20 Very great 64 1700 640 440 20 Much 78 450 50 Much 80 1150 1148 500 20 Much

SOILS FROM RIVERSIDE

Location from Response to Presence Soil Soil which taken and phosphate in of No. Horizon type other data pot cultures carbonatef 15913 0-12 in. Ramona Moreno Valley None None sandy loam Dry farmed soil 16157 0-12 in. Ramona Plot D-18, C.E.S. Slight* None loam Fert. Plot, No treatment Orange Grove, Riverside 16159 0-12 in. Placentia Dry farmed, noii- Slight* None loam fertilized soil, Washing- ton St., Riverside 16193 0-12 in. Sierra loam Plot B, Rubidoux fert. Slight* None plots, non-fertilized, Oranges, Riverside

These soils were subjected to electrodialysis using 0.2 ampere and 12 volts and rate of diffusion of phosphate and bases determined. The data are given in Table V and shown graphically in Figures 6 and 7. With the soils from Riverside there is very close agreement between rate of ionization or diffusion and response to phosphate. All samples are closely grouped with slightly greater ionization in 15913. The samples from Berkeley also show a very close agreement between their description and rate of phosphate ionization. No. 59 which shows the greatest response shows least ionization of phosphate. The only exception in this set of soils is No. 80, which gives much response yet shows greatest ionization. One explanation of this will be found in the data showing solubility in N/20 HCL We believe that factors other than the direct ionization of phosphate are interfering with phosphate assimilation in this soil, and this will be discussed later.

* Response to phosphate so small as to be considered negligible from a practical point of view. Tomatoes, oats, and alfalfa used in pot cultures. f Qualitative dilute acid test. 606 TECHNICAL BULLETIN No. 38

—-—" , —- •—' * ^ 3 37

^^ •—•—— — , / • / ^-^ / , 7? 1 _, — = rz i - — 12 - • _ -—— 6 A t ;Z J 7 tr 7 /-/oi/ y-s Fig. 6.—The rate of phosphate electrodiffusion in California (Berkeley) soils.

TABLE V.—RATE OF BASE AND PHOSPHATE ION TRANSPORTA- TION DURING ELECTRODIALYSIS OF CALIFORNIA SOILS. Mg. PO4 in anode solution (12oO cc.) Berkeley, California Riverside, California Soil No. Soil No. Hrs. 37 59 64 78 80 15913 16157 16159 16193

1 .25 .0 .0 .0 .75 .50 .50 .50 .30 2 .63 .25 .37 1.78 1.02 .87 1.02 .51 3 1.03 .38 .63 .25 2.35 1.31 1.12 1.31 1.15 4 1.32 .42 .91 .31 2.94 1.61 1.50 1.61 1.45 5 1.62 1.19 .39 3.45 1.87 1.88 6 1.93 — 1.37 .48 2.42 2.12 2.42 7 2.25 1.54 .55 3.67 3.01 2.39 2.47 0 2.70 3.27 9 2.92 .54 1.72 .69 4.05 3.48 2.50 3.01 2.91

cc. .IN H,SO4 per 50 cc. cathode solution 1 .9 .9 1.0 1.2 1.0 1.1 1.1 1.2 2 1.5 1.5 1.4 1.7 1.7 1.4 1.5 1.3 1.5 3 1.5 1.9 1.7 1.9 1.9 1.5 17 17 1.9 4 1.8 2.1 1.9 2.2 2.1 1.5 17 1.8 1.9 5 2.0 2.4 22 2 2 2.1 1.6 1.8 2.1 6 2.1 2.0 2.2 2.1 1.7 1.8 7 2.3 — — — — 2.2 8 2.1 9 2.1 2.5 2.3 2.2 2.1 17 1.8 1.8 2.2 Reaction of soil pH 8.1 6.7 7.1 5.4 8.2 7.4 7.7 7.6 7.6 PHOSPHATE AVAILABILITY AND ASSIMILATION 607

/J~*7/J

_ _ _ _ _ g _g _ Fig. 7.—The rate of phosphate electrodiffusion in California (Riverside) soils.

TABLE VI.—RATE OF BASE AND PHOSPHATE ION TRANSPORTA- TION DURING ELECTRODIALYSIS OF ALABAMA AND WISCONSIN SOILS.

Mg. POt in anode solution (1250 cc.) Alabama soils Wisconsin soils Current .2 A 12 V Current .2 A 12 V .5 A 40 V Soil No. 732 738 740 1 2 3 1 4 Hours 1 .0 .25 Tr. .50 .62 .2$ .0 .0 .50 .25 2 .25 .28 Tr. .89 .38 Tr. Tr. .64 .38 3 .31 .32 1.30 .77 .52 .25 .25 1.04 .64 4 .38 Tr. 1.60 1.05 .79 .38 1.29 .80 5 .39 .42 2.03 1.09 .52 6 .25 2.74 .94 1.64 z 7 .79 .54 1.85 1.08 .94 .31 5.09 1.25 1.11 1.10 .55 6.13 1.34 1.27 .56 .51 2.67 1.24 .39

cc. IN H,>SO4 per 50 cc. cathode solution

1 1.1 .9 .8 .7 1.3 1.0 1.3 1.0 2.4 1.8 2 1.9 1.45 1.3 1.5 1.4 1.5 1.6 3.7 2.4 3 2.1 2.05 1.5 2.3 2.1 1.8 2.1 1.8 3.8 3.1 4 2.30 2.9 2.1 2.0 2.1 2.1 3.5 5 2.2 2.75 1.7 3.5 2.3 2.2 2.2 6 2.70 4.1 2.3 — 3.8 3.7 7 2.6 2.0 2.3 2.4 8 2.4 2.90 2.1 4.7 — — 2.2 — 9 2.4 2.90 2.1 5.1 2.4 2.3 2.4 2.2 3.6 3.6

Reaction pH 4.1 8.0 5.6 7.7 6.6 6.4 6.5 5.8 6.6 5.8 608 TECHNICAL BULLETIN No. 38

Alabama Soils.—These soils were submitted by J. W. Tidmore of the Alabama Agricultural Experiment Station with the following descriptions: Soil i.— Responds slightly to phosphate, however it makes an average of 42 bushels of corn per acre without the addition of phosphate. Soil 732.— Responds to phosphate for all crops studied. Soil 738.— Gives great response to phosphate. Soil 740.— Gives no response. On subjecting these soils to our method of electrodialysis the data given in Table VI were obtained and these are shown graphically in Figure 8. In this set of soils there is excellent agreement between rate of dialysis of the phosphate ion and response to phosphate fertilization in the field.

79-0

i r

7*Z. 7sr ^—i 1—i r i 0 / Z 3 <+ y U 7 Fig. 8.—The rate of phosphate electrodiffusion in Alahama soils.

Wisconsin Soils.—These samples of soil were supplied by E. Truog of the Wisconsin Agricultural Experiment Station with the following comments: Soils i and 2.— Have given no response to phosphate fertilization over a period of several years when cropped to alfalfa. Soils 3 and 4.— Have given positive response to phosphate. The data obtained by the electrodialysis of these soils are given in PHOSPHATE AVAILABILITY AND ASSIMILATION 609

Table VI and shown graphically in Figure 9. This set of soils acted quite differently from the others which had been examined up to this point in our studies. While the relative rate and amount of phosphate ionization is in agreement with their response to fertilization the dissociation at 0.2 ampere is very low and scarcely measurable for soils 3 and 4. In view of this the current was increased to 0.5 ampere for soils 1 and 4 and these data are shown by the broken lines in the figure.

/ z j y *~ t> 7 Fig. 9.—The rate of phosphate electrodiffusion in Wisconsin soils.

Illinois Soils.—These soils were supplied by H. J. Snider, Illinois Agricultural Experiment Station and their properties and reaction toward phosphate fertilization are described at some length in Bulletin 337, "A Field Test for Available Phosphorus in Soils,'' published by that Station. Some of the data are reproduced here. Soil i.— Elizabethtown field, low phosphorus test, and great response to phosphate fertilization. Soil 2.— Joliet field, low phosphorus test, substantial returns from phosphate fertilization. Soil 3.— Urbana field, low phosphorus test, substantial returns from phosphate fertilization. Soil 4.— Ewing field, low phosphorus test, about the same as Urbana field, no response to rock phosphate. Soil 5.— Hartsburg field, high phosphorus test, and least response. Soil 6.— Carthage field, medium phosphorus test, and little response. When subjected to electrodialysis using a current of 0.2 ampere the Elizabethtown and Ewing samples showed only faint traces of phosphate ion just as did the Wisconsin soils. In view of this the amperage was increased to 0.5 and voltage to 40 volts for the electrodialysis of all of these soils. The data obtained are given in Table VII and shown graphically in Figure 10. 610 TECHNICAL BULLETIN No. 28

TABLE VII.—RATE OF BASE AND PHOSPHATE ION TRANSPORTA TION DURING ELECTRODIALYSIS OF ILLINOIS SOILS.

Mg. PO4 in anode solution (1250 cc.) Soil No. 1 2 3 4 5 6 Hours 1 Trace .37 Trace .87 .50 2 Trace .64 .25 Trace 1.78 1.02 3 .12 .79 .51 Trace 1.43 4 .25 .94 .25 3.22 — 5 __. .65 .37 5.22 1.61 6 1.10 1.79 7 .32 .80 .50 1.98 8 2.30 9 .37 1.39 .96 .62 5.42 2.63

cc. .IN H2SO4 per 50 cc. cathode solution

1 1.6 2.3 1.9 1.7 1.5 2 2.1 2.4 2.1 2.5 2.6 3 2.1 2.5 2.6 2.3 2.8 4 — 2.4 2.7 2.2 3.1 3.1 5 22 2.6 2.3 3.2 2.9 6 2.4 — 3.1 7 1.9 2.8 8 2.0 2.4 2.2 9 — — 2.7 22 3.1 2.7 Reaction pH 6.0 5.5 5.1 4.6 5.0 5.3

Y Hou r^ Fig. 10.—The rate of phosphate electrodiffusion in Illinois soils. PHOSPHATE AVAILABILITY AND ASSIMILATION 611

Here again there is an excellent correlation between rate and amount of phosphate electrodiffitsion and the response of the soils to phosphate fertilization in the field. The vast difference between ease of dissociation and rate of diffusion in these Wisconsin and Illinois soils as compared to our Western soils is significant and will be discussed later. Florida Soils.—These samples were supplied by R. W. Ruprecht of the Florida Agricultural Experiment Station with the following comments: Soil i.—West Florida and responds to phosphate. Soil 2.—Virgin Norfolk sand and does not respond to phosphate. The data obtained by electrodialysis of these soils, using 0.5 ampere and 40 volts are given in Table VIII and Figure 11.

0 7Z 3¥ J^ 6 79 Fig. 11.—The rate of phosphate electrodiffusion in Florida soils. TABLE VIII.—RATE OF BASE AND PHOSPHATE ION TRANSPORTA- TION DURING ELECTROD1ALYSIS OF FLORIDA SOILS. Soil No, Time 1 9 1 2 hours P.p.m. PO, cc. .IN H,SO4

1 .25 2.15 2.7 3 .32 .33 3.20 4.0 5 .51 .52 3.50 4.3 7 .58 .66 3.50 4.0 8 .68 .82 3.50 4.0 Reaction pH 67 6.0

Like the Wisconsin and some of the Illinois soils the rate of dissociation or solution of phosphate ion is very low in these soils. There is practically no difference between the two but this slight difference is in favor of the soil which does not respond to phosphate fertilization. Both these soils are very sandy types and we have observed in our studies on Arizona soils that sandy types, as compared 6 J 2 TECH NIC A L B ULLB TIN No. 28 to loams or clays, are always lower in ionizable phosphate and yet show less response to phosphate fertilization. Our investigations have shown that lower standards must be established for sandy types if comparison is to be made with loams or clay loams.

INTERPRETATION OF DATA Several interpretations stand out prominently in the consideration of these data. In using electrodialysis or electrodiffusion for the estimation of phosphate availability in soils there is much that is promising, provided certain limitations are recognized. In line with the often- observed phenomena that soluble phosphate is lower in sandy soils and phosphate availability requirements are less on account of more active root forage we found that the rate of dissociation and diffusion were less than for loams or clays. It is evident that lower standards will have to be adopted for sandy types even if the method should prove applicable to this class of soils. Then again there is a vast difference in the degree of dissociation obtained from calcareous and non-calcareous soil types in spite of the fact that the relative dissociation in the individual types agrees rather closely with field performance. That is, considering the soils from each of the states separately, the rate of dissociation is with little exception lowest in soils which respond to phosphate fertilization and highest in soils which give little or no response. The reason for this difference unquestionably lies in the nature of the phosphates which predominate in calcareous and non-calcareous types, namely the carbonate-phosphate of calcium(S) in the former, and hydrated iron and aluminum phosphates in the latter. In considering the data obtained by the electrodialysis of Arizona soils (Figure 3), all of which are calcareous, there is unquestionably a close relation between the degree of response obtained with phosphate fertilization and rate of dissociation and diffusion of soil phosphate. A number of illustrations are introduced at this point to show this relation. Figures 12 and 13 show response of tomatoes to phosphate in soils 1 and 11, and Figure 14 shows the relative growth without any fertilization in soils 2 and 3. SPECIFIC SOLUTION RATE At the suggestion of T. F. Buehrer, of this laboratory, the specific solution rate of phosphate during electrodialysis was calculated in order to determine if this solution rate is a constant and if so whether it bears any relation to the reaction of the soils toward phosphate fertilization. The rate at which the phosphate ions diffuse from the soil particles is

TABLE IX.—RATE OF ELECTRODIFFUSION OF PHOSPHATE (K) IN SOILS USING (1) PHOSPHATE SOLUBLE IN AQUA REG I

Arizona soils California soils Alabama soils Soil No. 11 29 34 37 59 64 78 15913 16157 16159 16193 732 738 Hours 1 .025 .022 .015 .028 .006 .014 .010 .067 .023 .060 .038 .013 .038 2 .026 .046 .026 .025 .006 .019 .013 .006 .011 .084 .012 .053 .040 .011 .038 .022 3 .027 .058 .036 .025 .009 .034 .015 .006 .013 .019 .094 .021 .046 .034 .017 .032 .017 4 .029 .058 .027 .012 .036 .014 .005 .014 .018 .074 .019 .048 .032 .016 .015 5 .028 .059 .027 .015 .042 .014 .012 .018 .071 .054 .017 .024 .013 .010 6 .030 .035 .015 .057 .014 .014 .019 .015 .047 .033 7 .032 .058 .035 .015 .064 .014 .014 .019 .055 .018 .046 .016 .038 .009 .033 .059 .023 .015 .015 .020 .040 .036 .036 .015 .058 .014 .012 .018 .048 .019 .038 .040 .015 .043 .010 .009- Av. K .030 .051 .030 .026 .012 .040 .014 .013 .018 .070 .018 .049 .036 .015 .036 .019 .009 Max. K .036 .059 .036 .028 .015 .064 .015 .014 .019 .094 .023 .060 .040 .017 .043 .038 .010 Min. K .025 .022 .015 .023 .006 .014 .010 . 11 .018 .048 .015 .038 .032 .011 .024 .010 .009

II

1 .019 .031 .019 .025 .006 .031 .025 .078 .051 .051 .051 .030 .025 2 .020 .067 .032 .023 .006 .043 .035 .013 .019 .098 .053 .045 .054 .026 .013 .014 3 .019 .087 .045 .026 .009 .076 .036 .013 .021 .008 089 .047 .040 .047 .037 .010 .011 4 .021 .087 .023 .011 .078 .035 .011 .024 .008 .087 .044 .041 .044 .039 .010 5 .020 .091 .024 .015 .106 .035 .025 .008 .084 .041 .042 .008 .009 .005 6 .022 .044 .015 .161 .036 .024 .008 .046 .040 .046 7 .022 .094 .044 .014 .213 .036 .024 .008 .065 .051 .039 .040 .012 .004 8 .023 .107 .020 .015 .039 .049 .012 9 .025 .046 .014 .227 .038 .006 .021 .008 .058 .047 .032 .040 .038 .013 .006 .004 Av. K .022 .038 .023 .011 .117 .035 .011 .023 .008 .058 .048 .041 .047 .036 .011 .012 .034 Max. K .025 .107 .046 .026 .015 .227 .039 .013 .025 .008 .098 .053 .051 .054 .042 .013 .025 .005 Min. K .019 .031 .019 .020 .006 .031 .025 .006 .019 .008 .058 .044 .032 .040 .026 .008 .006 .004 Phosphate availability Poor Poor Poo Poor Good Fair Very poor P< Poor Good Good Slight Slight Slight Poor Very poor ilON OF PHOSPHATE (K) IN SOILS USING (1) PHOSPHATE SOLUBLE IN AQUA REGIA AS S VALUE AND (2) A UNITARY S VALUE. I

California soils Illinois soils Florida soils 54 78 80 15913 16157 16159 16193 1 2~~

.067 .060 .123 .045 .070 311 .084 .053 .009 .135 .047 313 .019 .094 .046 .013 .045 .040 .032 314 .018 .074 .048 .005 .140 312 .018 .071 .054 .010 .006 .299 .031 .031 314 .019 .047 .033 .029 314 .019 .055 .045 .038 .009 .006 .028 .030 .029 .040 .028 .032 .032 .018 .048 .038 040 .015 .043 .008 .006 .029 .018 .070 .049 036 .015 .036 .010 .006 .035 .035 .040 314 .019 .094 .060 040 .017 .043 .013 .006 .047 .040 .070 11 .018 .048 .038 032 .011 .024 .009 .005 .028 .030 .029

II

.078 .051 .051 .030 .025 .052 .051 .025 .038 .091 .051 .098 .053 .054 .026 .013 .014 .043 .033 .019 .033 .013 .098 .051 089 .047 .047 .037 .010 .011 .046 .037 .022 .004 .027 .017 .051 .011 )24 .087 .044 .044 .039 .010 . .044 .034 .021 .006 .025 .097 )25 .084 .042 .008 .009 .005 .045 .013 .147 .035 .011 )24 .046 .046 , .053 .030 .021 .033 )24 ,065 .051 .040 .012 .004 .029 .016 .005 .012 .007 .032 .009 .010 .049 .012 .089 .033 .009 .011 121 .047 .040 .038 .013 .006 .004 .105 .034 .015 .004 .017 .011 .008 .034 )23 .048 .047 .036 .011 .012 .004 .059 .035 .020 .005 .027 .013 .007 .040 .010 .014 )2S .053 .054 .042 .013 .025 .005 ,105 .051 .025 .006 .038 .017 .003 .051 .011 .025 .044 .040 .026 .008 .006 .004 .043 .029 .015 .004 .017 .011 .006 .032 .009 .010

Poor Good Good Slight Slight Slight Poor Poor Very poor Good Good Poor Very poor Poor Poor Poor Good Good Poor Good PHOSPHATE AVAILABILITY AND ASSIMILATION 615

Converting the logarithm to base 10, this must be multiplied by 2.303

Hence 2.303 log10 S/S - c =-= Kt

From which K = 2.303/t log, 0 S/S — c The electrodiffusion data given in the preceding pages were substituted in the above formula and the K values found are given in Table IX. In Table X are shown in detail the calculations for one soil and in Table XI the effect of variation in amperage on the K value is shown for two soils. The S value was determined by digesting the soils with aqua regia and determining the phosphate dissolved. TABLE X.—SHOWING METHOD OF CALCULATING K VALUE. (ALABAMA SOIL NO. 740.)

Log 2.303 t S c S-c Log S Log S—c S/S—c Log S/S—c K

1 460 5.0 455.0 2.66275 2.55801 .00474 .0109 .0109 2 460 8.9 451.1 2.66275 2.65427 .00848 .0195 .0098 3 460 13.0 447.0 2.66275 2.65030 .01245 .0286 .0095 4 460 16.0 444.0 2.66275 2.64738 .01537 .0354 .0089 5 460 20.3 439.7 2.66275 2.64315 .01960 .0451 .0090 6 460 27.4 432.6 2.66275 2.63608 .02667 .0612 .0102 8 460 50.9 409.1 2.66275 2.61182 .05093 .1172 .0146 9 460 61.3 398.7 2.66275 2.60064 .06211 .1430 .0159

TABLE XL—RATE OF DIFFUSION (K) AT DIFFERENT AMPERES. Amps. .05 A .1 A .2 A .5 A .05 A .1 A .2 A .5 A Hours 1 .009 .013 .022 .076 .004 .008 .019 .026 2 .007 .011 .047 .099 .005 .009 .026 .102 3 .006 .015 .059 .113 .006 .009 .029 .125 4 .008 .015 .059 .152 .006 .009 .029 .114 D ,UUnn7o .Uml O^ .uuu (no 028 12=; 6 .010 .147 .006 .012 .031 .127 7 .011 • .020 .060 .006 .012 .032 .133 8 .011 .021 .060 .006 .014 .037 .126 9 .011 .024 .065 .008 .015 .033 .110

The data show that the specific solution rate is remarkably constant in practically every soil and that this constant holds during the major part of the 9-hour period of electrodialysis. In the Arizona soils there is a close correlation between this constant and the reaction of the soils toward phosphate fertilization, but this does not hold true for the other soils. This lack of correlation between K values and phosphate availability in the soils from other states may be due to their character. 616 TECHNICAL BULLETIN No. 38

All the Arizona soils are calcareous and the active forms of phosphate are similar if not the same compounds and bear the same relation to the reserve supply of phosphate in the soil. Under the circumstances they should show a definite relation between K value calculated from phosphate soluble in aqua regia and that dissolved by electrodialysis. In using similar analytical data to calculate the K values for calcareous, non-calcareous, neutral, and acid soils indiscriminately the constancy holds but it lacks correlation, in some cases, with phosphate availability. In order to make the K value universally indicative of phosphate availability it would be necessary to determine the total amount of electrodialysable phosphate to use as the S value in the equation or to adopt a unit value for S to be used for all soils. Then the K value will represent the rate of electrodiffusion per unit of electrodialysable phosphate in the soil. To determine the total amount of electrodialysable phosphate in a soil would be a ]ong and tedious process. On the other hand, since a rather definite amount of phosphate is required for crop production regardless of soil type the selection of a more or less arbitrary unit may be based on this. Since the plant roots actually come in contact with only a very small percentage of the soil particles and a 9-hour electrodialysis removes far more phosphate than a single crop it will be necessary to select a multiple of the single crop unit. We therefore recalculated the specific rate constant using 100 milligrams as the unit value for S in all the soils from states other than Arizona and 200 milligrams for the Arizona soils which are higher in electrodialysable phosphate. The K values obtained by using these values of S are given in the lower half of Table IX, and show without a single exception a definite correlation between the magnitude of K and reaction of the soil toward phosphate fertilization. Solution rate of phosphate includes both adsorbed and chemically combined phosphate and these along with difference in reaction of the soils probably explain the variations in value of K for individual soils. In some soils there is only a negligible variation in the constant throughout the entire 9-hour period, while in others it may hold only for 2 or 3 hours. The data in Table XI show that specific solution rate increases with increase in amperage but at each amperage is substantially constant. If one distinguishes between calcareous and non-calcareous soils and between sandy soils and loams or clay loams, we believe the method offers potential possibilities. The amount of dialysible phosphate is governed by the one or more solid phases which happen to predominate under the existing soil conditions. PHOSPHATE AVAILABILITY AND ASSIMILATION 617

PHOSPHATE AVAILABILITY AS A FUNCTION OF IONIZATION AND REACTION (pH) IONIZATION OF PHOSPHATES On conducting further pot experiments as illustrated by the millet plants in Figure 15, it was noted that even in one of the Arizona soils which appeared from analysis tc be very well supplied with soluble phosphate, there was some response to phosphate fertilization. While this observation of itself is very puzzling it is even more so if one compares the rate of dialysis in these same Arizona soils with that of the soils from Wisconsin and Illinois which have given no response to phosphate fertilization. As a specific case, attention is called to Figure 2 in which rate of dialysis at 0.5 ampere and 40 volts is given for Arizona soil No. 2, and to Figure 9 in which similar data are shown for Wisconsin soil No. 1. The former soil gives response to phosphate while the latter does not give response. Or we may go even further in calling attention to the fact that Arizona soil No. 2 shows as much ionizable phosphate at 0.05 ampere and 10 volts as the Wisconsin soil at 0.5 ampere and 40 volts. On first thought these observations and data appear puzzling and somewhat contradictory. But if one examines the data from the standpoint of the ionization of phosphates as well as the soil conditions affecting ionization, notably the soil reaction or the soluble salts present in the soils, the apparent contradiction is greatly clarified and some nem theories on phosphate assimilation suggest themselves.

The ionization of orthophosphoric acid, H3PO4, takes place in three stages:

(1) H3PO4 ±=> H+ + H2PO4"

(2) H2PO4 f=* H+ + HPOf "

(3) HPO4 ±5H+ + PO4"~" As pointed out in a previous bulletin (5 ) there is a common impression among students of plant nutrition that the PO4 ion is the one absorbed by plants. This, however, appears exceedingly improbable when the ionization of orthophosphate is considered, and this led us to suggest that in all probability the plant feeds upon the H2PO4 ion in most part. In studies on Nitella sap Zscheile(9) made all his calculations on the basis of the H2PO4 ion for as he points out this is the principal phosphate ion existing at pH 5.2 which is the reaction of the Nitella cell sap. While he states that nothing is known concerning which of the phosphate ions 618 TECHNICAL BULLETIN No. 38 in equilibrium enter the plant or whether the plant distinguishes between them our work has indicated that the relative ionic concentration of the three phosphate ions bears an important relation to assimilation. The sap of plants is usually within the reaction range of pH 5 to 6 and as can be shown by calculation from ionization constants, H2PO4 is the predominating ion at this reaction. Therefore it seems reasonable to assume that the plant will absorb this phosphate ion and in fact may demand it. Since an acidity is maintained in the feeding zone of the root tip, the H2PO4 ion must be the dominant ion in this zone and the other phosphate ions must change to this in establishing equilibrium as they gain entrance to the feeding zone. Outside this root-tip zone the dominant phosphate ion will be governed entirely by the soil reaction. In the following Table XII, the relative ion concentrations at reactions varying from pH 4.0 to 9.0 are given. These calculations are made for a solution containing 1 milligram per liter (1 part per million) and the figures represent milligrams of ions.

TABLE XII.—CONCENTRATION OF PHOSPHATE IONS AT VARIOUS pH VALUES OF SOLUTIONS.*

pH H3PO4 H.POr HPor- PO*

4 .0091 .99 .0019 7.0X10"13 5 .00089 .98 .0194 6.9 X10"10 6 .00076 .79 .164 5.9X10"* 7 3.1 X10"6 .33 .66 2.4X10"6 8 4.4X10~s .048 .94 3.4 X10"5 9 4.6 X10"10 .0049 .98 3.5X1CT4

* From calculations by T. F. Buehrer. It is clearly evident from the data given in this table that the concentration and proportion of PO4 ion is so very small at all reactions found in soils as to preclude its being of importance. The PO4 ion may therefore be dismissed entirely as playing any part in plant nutrition. At a point approximating < pH 6.8 soluble phosphate is present in practically equal concentrations and proportions as H2PO4 and HPO4 ions. Below this point on the pH scale H2PO4 ion increases until at pH 4.0 it reaches a maximum of 99 percent with HPO4 ion less than 1 percent. Above pH 6.8 the HPO4 ion predominates, reaching a proportion and concentration of 98 percent at pH 9.0. This is rather ^conclusive evidence that only these two ions can exist under soil conditions and that H2PO4 is the dominant ion in the immediate contact- zone of the root. The acidity of this contact-zone is well illustrated in the etching experiments often performed with plant roots on marble

620 TECHNICAL BULLETIN No. 38 with 5 parts pure water to 1 part of soil, and its phosphate content shows very active dissociation and diffusion when subjected to electrodialysis. Soil No. 34 contains easily ionizable forms of phosphate, more so in fact than soil No. 2. The following pot experiment was conducted with these two soils using 1,400 grams of soil per pot. 1. Soil No. 34 plus 0.5 gm. ammonium sulphate. 2. Soil No. 34 plus 0.5 gm. ammonium sulphate and 1 gm. double superphosphate. 3. Soil No. 2 plus 0.5 gm. ammonium sulphate. 4. Soil No. 2 plus 1.0 gm. double superphosphate. 5. Soil No. 2 plus 0.5 gm. ammonium sulphate and 1 gm. double superphosphate. Four tomato plants, 3 inches in height, were transplanted to each of these five pots. After several weeks the tops of the plants were removed, weighed, and analyzed for phosphate. The results are given in Table XIII. TABLE XIII.—WEIGHT AND PHOSPHATE CONTENT OF TOMATO PLANTS GROWN ON SOILS 2 AND 34, FERTILIZED AND NOT FERTILIZED.

Milligrams PO4 Percent Pot Weight fresh Weight dry absorbed by PO4 in number plants grams plants grams four plants dry plants

1 4.6 , .338 3.46 1.02 2 6.0 .405 9.10 2.22 3 7.5 .536 8.25 1.54 4 7.0 .760 18.50 2.44 5 14.0 1.160 24.30 2.09

Soil No. 34 has a reaction of pH 8.5 and if allowed to stand with excess of water will develop a quite strong phenolphthalein alkalinity. vSoil No. 2 has a reaction of pH 8.0. It is evident that under the conditions of the above experiment both soils have shown a response to phosphate and where fertilized have absorbed additional amounts of phosphate both as actual weight and as percentage dry matter. Then again plants grown in soil No. 34 made less growth and absorbed less phosphate than those grown in soil No. 2 in spite of the fact that the former soil (No. 34) contained more soluble phosphate than the latter. These experiments show that (1) in calcareous soils phosphate assimilation decreases with increase in pH or hydroxyl ion concentration and (2) that greater concentration of soluble phosphates are required by crops in alkaline soils (pH 7.5 to 9.0) than in neutral or slightly acid soils. It should be repeated at this point and its significance stressed PHOSPHATE AVAILABILITY AND ASSIMILATION 621 that HPO4 is the dominant ion above pH 6.8. There is considerable evidence in these experiments that crops assimilate phosphate more rapidly and more readily as H2PO± ion. If this is true it greatly clarifies the data obtained by electrodialysis of calcareous and non-calcareous, or Southwestern and Midwestern soils, for according to our observations and our theory regarding the plant preference or demand for H2PO4 ion, a greater concentration of phosphate in the soil solution would be necessary in an alkaline soil than in a slightly acid or neutral soil over and above the depression of ionization by the neutral salts and calcium carbonate present. As shown in Table IV all the Arizona soils examined fall within the reaction range of pH 7.8 to 8.5 and all give more or less response to phosphate according to the amount present in water-soluble form. The

HPO4 ion is the dominant form of soluble phosphate in these soils. By phosphate fertilization, the ratio of HPO4 to H2PO4 as shown in Table XII would remain practically unchanged but the actual concentration would be increased. On this basis then we can explain the higher concentration of phosphate which is required in the soil solution of an alkaline soil as compared with an acid soil to yield a concentration of

H2PO4 ion equivalent to that in the acid soils. In the soils from California, Berkeley soil No. 80 is a calcareous soil, is high in ionizable phosphate yet gives some response and is given a similar classification to soil No. 78 which has a reaction of pH 5.4 but much less ionizable phosphate. The Riverside soils are all quite similar in reaction, contain almost equal amounts of ionizable phosphate and give little or no response to phosphate. They all lie in a favorable reaction range. Among the Alabama series, soils Nos. 740 and 732 are calcareous and the former (pH 7.7) gives no response to phosphate while the latter (pH 8.0) gives great response. Ionization of phosphate is apparently greatly suppressed in the latter. The soils from Wisconsin all fall within a desirable reaction range from the standpoint of phosphate iom'zation and it is very significant that crops subsist on very low ionizable phosphate in these soils as compared to our Southwestern calcareous soils. The Illinois soils are all acid soils and their reaction toward phosphate fertilization is in close agreement with the relative rate of ionization under electrodialysis. Like the Wisconsin soils, as well as those from Florida, the data show that a crop can subsist on smaller amounts of ionizable phosphate in the neutral or slightly acid Illinois soils. In seeking further and more conclusive evidence on the relation of reaction to phosphate assimilation some culture experiments under more closely controlled conditions were conducted. In the first of these, wheat 622 TECHNICAL BULLETIN No. 38 plants were grown in (tap) water cultures containing 100 mg. of dipotassium phosphate and 100 mg. of sodium nitrate per liter. This nutrient solution was adjusted to the following series of reactions, pH 4.8, 6.0, 6.9, 8.0, 9.0 in 5-gallon lots. Wide-mouth bottles holding approximately 1 liter were used for growing the plants using 20 wheat plants per bottle. The culture solutions in these were changed every 24 hours. While it was realized that there would be a material reduction in pH of the alkaline cultures during this period of time, the relative trend of assimilation would be indicated in spite of this. At the end of 2 weeks time the plants were analyzed to determine the amount of phosphorus assimilated. The results are given in Table XIV along with the phosphorus content of a control set of plants grown in the same culture solution but without phosphorus. The analyses are expressed as milligrams phosphorus per 100 plants. TABLE XIV.—PHOSPHORUS ABSORBED BY WHEAT PLANTS AT REACTIONS VARYING FROM pH 4.8 TO 9.0. Reaction of culture sol. mg. P in 100 plants mg. P in 100 plants pH 1st exp. 2nd exp.

Control 153 15.3 4.8 58.8 62.6 6.0 71.4 79.0 6.9 73.9 82.9 8.0 70.5 11.3 9.0 43.1 53.2 These data clearly show that the maximum assimilation of phosphorus takes place at neutrality or at slightly acid reactions. This experiment was repeated in a slightly different manner. That is, instead of analyzing the plants the, phosphate changes in the culture solutions were determined at 8-, 24-, and 48-hour periods by the Deniges method. The culture solutions in this experiment contained originally

5 p.p.m. phosphate expressed as PO4. The results are given in the following Table XV. TABLE XV—INFLUENCE OF pH ON ABSORPTION OF PHOSPHATE BY WHEAT PLANTS. After 24 hours After 48 hours Orig. Orig. PO4 P.p.m. PO* pH p.p.m. after 8 hrs. p.p.m. POi pH p.p.m. PO4 pH

5.0 5.0 5.0 4.0 5.0 2.0 6.3 6.0 5.0 2.5 1.8 6.4 .5 6.6 7.0 5.0 1.8 1.0 7.0 .5 7.0 8.0 5.0 2.4 1.4 12 .3 7.1 9.0 5.0 5.0 4.5 8.2 2.5 7.6 PHOSPHATE AVAILABILITY AXD ASSIMILATION 623

There is additional evidence in this experiment that phosphate assimilation is largely a function of reaction. At the extreme alkaline reaction of pH 9.0 there was no measurable assimilation of phosphate until the plant, by giving off carbon dioxide, had materially reduced the alkalinity. Even after 24 hours when the reaction had been reduced to pH 8.2 there was little assimilation of phosphate, showing that even at this reaction the assimilation is seriously curtailed. The assimilation of phosphorus is also retarded by extreme soil acidities (for wheat) but there is slightly less interference than at alkaline reactions. The optimum reaction for best assimilation is on the acid side of neutrality and as shown in the columns giving reaction changes in the culture solutions the plant puts forth every effort to change the extreme acidity or alkalinity to near-neutrality. The more rapid change from alkalinity indicates this to be the less desirable reaction. In the preceding experiments tap water (which contains about 400 p.p.m. solids, 40 of which is Ca) wras used in preparing the culture solutions. The experiments were then conducted using distilled water with the proportions of potassium phosphate and sodium nitrate already given. Otherwise the experimental technic was the same. The culture solutions were analyzed after the plants had been growing in them for 7-, 24-, and 48-hour periods. The reaction (p FI) and phosphate expressed as p.p.m. PO4 are given in Table XVI. TABLE XVI.—ASSIMILATION OF PHOSPHATE FROM CULTURE SOLUTIONS AT pH 5.0, 7.0, AND 9.0.

Orig. PO., 7 hours 24 hours 48 hours

'Orig. pH p.p.m. pH p.p.m. POi pH p.p.m. PO4 pH p.p.m. PO4 5.0 5.0 5.6 3.7 5.9 3.0 6.2 0.4 7.0 5.0 6.6 4.0 6.6 3.0 6.2 1.4 9.0 5.0 8.5 5.0 7.0 3.0 7.0 2.5

These data offer still more conclusive evidence that the plant prefers the H2PO4 ion. The principal difference between tap water and distilled water in so far as phosphate assimilation is concerned lies in the 40 parts per million calcium in the former. The experiments indicate that in the absence of calcium ion the plant absorbs its phosphate much more rapidly as H2PO4 ion than if calcium is present. However, this is beside the point and will be a matter for further study. The point which we desire to emphasize in this experiment is the additional evidence that H2PO4 ion is more rapidly assimilated than the HPO4 ion, is actually preferred by the plant and that phosphate assimilation takes place, with less expenditure of the plant's energy as H2PO4 ion. 624 TECHNICAL BULLETIN No. 38

SUMMARY

The stimulus for the investigation of electrodialysis as a means of determining the more active forms of phosphate in soils arose from the need of overcoming the handicap which calcium carbonate imposes upon such solubility studies in Arizona soils. All the soils of the irrigated valleys of the State are calcareous. While the results obtained in this study are quite encouraging and show rather definite relations between the rate of dissociation of phosphate where the soil is subjected to electrodialysis and the degree of response when the soil is fertilized with phosphate, certain limitations in interpreting these data, and some significant observations on phosphate assimilation, not a part of the original objective of the investigation, were suggested by the results obtained. The nature of the phosphate compounds in calcareous soils as compared with acid and non-calcareous soils, the former being more easily dissociated from their difficultly soluble state, would be expected to show different rates of ionization, and this is found to be true. The results obtained on soils were in agreement with data obtained from the mineral phosphates of iron, aluminum, and calcium which data will appear in a later bulletin. Then again, with better aeration and therefore more active root respiration and foraging in sandy soils, crops are able to supply their phosphate needs with less expenditure of energy* in such sandy types. But after recognizing these limitations, a number of equally important factors asserted themselves. Reference is made to the influence of reaction on (1) the ionization of orthophosphates and (2) upon the assimilation of phosphate ions by the plant. On first observing the remarkable difference and the greater rate of dissociation of phosphate in our worst soils as compared to some Midwestern soils which, we had been informed, were amply supplied with phosphate, the reason for this appeared to be of paramount importance. After a thorough perusal of our data, three factors seemed to have a part in explaining this difference. 1. Neutral salts and calcium carbonate depress both phosphate solubility and assimilation in Southwestern soils. 2. Plants hold a preference for one of the three different ions formed in the ionization of orthophosphates. 3. Phosphate assimilation is a function of reaction.

1. The first of these has already been demonstrated (5) and it can be definitely stated that phosphate assimilation as well as solubility are greatly depressed by neutral salts (white alkali) and seriously depressed PHOSPHATE AVAILABILITY AND ASSIMILATION 625

by an excess of calcium carbonate. Of this difference between Southwestern and Midwestern soils we are definitely certain.

2. The fact that all plant juices are acid and that H2PO4 ion is the dominant phosphate ion which can exist at this reaction suggested that a lower expenditure of energy by the plant in the assimilation of this

ion than would be required for the assimilation of HPO4 or PO4 ions

might predispose the plant in favor of H2PO4 ion. We can always depend upon the plant to absorb its food in the easiest and simplest way. While it would be a difficult matter to definitely prove such an \dx\ preference, the evidence certainly favors this view. The soil-root contact- zone is probably just as acid as the plant sap or at most only slightly

less so. The H2PO4 ion could exist only at this reaction. When we grew plants in culture solutions varying in reaction from pH 5.0 to pH 9.0 we found no assimilation of phosphate from the p.H 9.0 culture until the plant had reduced this alkalinity. 3. Thus assimilation is a function of reaction and regardless of the phosphate concentration both extreme acidities and extreme alkalinities may militate against phosphate assimilation. At these extremes phosphate solubility ceases to be a controlling factor. So far as is known plants absorb both the bases (cations) and acids (anions) as ions only. Therefore a culture medium best suited to the plant's needs is one of near neutrality. In such a well-balanced medium, bases or acids may be absorbed at will, or as required, with a minimum expenditure of energy for one can count on the plant doing the most reasonable thing. In a medium where an excess of bases or an excess of acids (hydroxyl ions or hydrogen ions) exist, the energy consumption in attempting to absorb desired ions from an alkaline solution on the one hand and from an acid solution on the other places the plant under stress and disrupts the natural absorbing process. We feel justified in stating that all three of the preceding factors are actively concerned with phosphate availability in Arizona soils and that under our conditions larger amounts of soluble phosphate must be at the disposal of the crop than would be true for non-calcareous or non-alkaline soils. Some recent investigations at the Kentucky Experiment Station (2 and 8) yield much evidence in support of our theories and laboratory findings. Reference is made to their studies on the effect of soil reaction on the assimilation of phosphorus by lettuce, strawberries, and tomatoes which showed that an acid soil reaction is favorable to phosphate utilization by these plants and that assimilation was depressed by calcium carbonate. Where sodium carbonate was used to produce soil alkalinities, 626 TECHNICAL BULLETIN No. 38 assimilation was not so greatly depressed as with lime which is in line with our common ion studies already published(5). Aside from the reaction relations, the greater solubility of the monobasic (H2PO4) over the dibasic (HPO4) phosphates while not allowed for in Table XII and not in evidence in water cultures in which the addition of calcium is avoided, is important under soil conditions. Velocity of re-solution is reduced and therefore mobility of phosphate ions from the soil environs to the soil-contact zone where the root is actively removing the phosphate from the soil solution, is reduced. This is observed from our experiments in which ammophos and double or treble superphosphate are found to be more adaptable forms of phosphate fertilizer on calcareous soils than dicalcium phosphate (single superphosphate) or phosphate rock. Where slight alkalinities are due to basic sodium salts and the HPO4 ion is present as the sodium salt, the importance of solubility and soil-reaction relations is lessened. This has been found in our own studies and is also demonstrated by Emmert(2) who showed that plants grown on sodium-carbonate-treated soils assimilated slightly more phosphate than the checks as compared to a depression where alkalinities were obtained from calcium carbonate. The following experiment illustrates the conditions cited in the preceding paragraph and show the important influence which reaction exerts upon the solubility of calcium phosphates and therefore the phosphate solubility in calcareous soils. One-gram portions of finely ground (1) phosphate rock, (2) bone and (3) C.P. dicalcium phosphate were weighed separately into three separate one-liter graduated flasks and 1 liter of carbon-dioxide-free water added to each. They were shaken and allowed to stand 24 hours. The phosphate concentration of the solutions was determined colorimetrically as was also the reaction (pH) of the solution. Different portions of the mixture were the? treated with alkali, sodium hydroxide, and calcium hydroxide solutions, and hydrochloric acid in amounts to produce reactions ranging from pH 3.0 to 9.0. The phosphate concentration of the solutions was then determined at each change in reaction and the pH determined colorimetrically at the same time. The data are given in Table XVII and shown graphically in Figure 16. In Figure 16 the broken lines represent solubility in sodium alkalinity while the solid lines represent solubility in presence of calcium alkalinity and the point at which the two divide is the reaction and solubility of the original untreated water solution. Phosphate rock shows the least solubility, 125 p.p.m. at pH 3.0 which decreases rapidly to 8 p.p.m. at pH 7A and to practical insolubility at pH 9.0. The solubilities of bone PPM po?.

Fig. 16.—Influence of pH of solution on the solubility of phosphate in (1) phosphate rock, (2) C. P. dicalcium phosphate, (3) and bone. Broken lines repn sent sodium alkalinity and solid lines calcium alkalinity. 628 TECHNICAL BULLETIN No. and dicalcium phosphate at pH 5.0 are very similar and very high but the solubility is reduced with extreme rapidity as pH increases and both are insoluble at pH 9.0. It will be noted that in all cases phosphate ions are slightly more soluble in sodium alkalinity than calcium alkalinity. This experiment gives further evidence of the great insolubility of phosphates in calcareous and alkaline soils as compared to neutral or acid soils. TABLE XVII.—INFLUENCE OF REACTION ON SOLUBILITY OF PHOSPHATE FROM BONE, PHOSPHATE ROCK, AND DICALCTUM PHOSPHATE.

Bone Phosphate rock C. P. Dicalcium phosphate

pH P.p.m. PO. j pH P .p.m. PO4 pH P.p.m. PO4

4.6 300.0 3.0 125.0 4.8 227.0 6.2 70.0 5.8 45.0 6.2 71.0 7.6 * 9.0 6.0 40.0 6.4 42.0 9.0 (Na) 5.5 6.6 18.6 7.3* 40.0 9.0 (Ca) 0.0 7.4 8.0 8.8 (Na) 17.0 8.5 * .8 9.4 (Na) 14.0 9.4 (Na) .8 9.4 (Ca) 0.5 9.4 (Ca) Trace

* Represents reaction of original solution of phosphate.

CONCLUSIONS 1. The process of electrodialysis is an excellent means of dissolving the active or available forms of phosphate from calcareous soils with least interference from the calcium carbonate present. 2. The rate of dissociation of soil-phosphate compounds varies greatly with slight variations in amperage but is not materially modified by variations in voltage. 3. By measuring the rate of ionization and making a colorimetric determination of phosphate in the anode chamber at definite intervals, a close agreement with degree of response to phosphate fertilization was noted in a selected group of Arizona soils as well as in soils from other parts of the United States. 4. The method is sufficiently delicate to measure the increase in phosphate content of the soil following an ordinary application of phosphate fertilizer. 5. Sandy soils are lower in electrodiffusible phosphate than loams or clav loams. PHOSPHATE AVAILABILITY AND ASSIMILATION 629

6. Non-calcareous and acid soils are usually lower in electrodiffusible phosphate than calcareous soils due to the fact that acid soils have become largely depleted of their phosphate which is more soluble at this reaction. This is also due in part to the presence of a calcium carbonate- phosphate compound in calcareous soils and to hydrated iron and aluminum phosphates in most acid soils or non-calcareous types. The former are more readily ionizable than the latter. 7. Electrodialysis studies indicate that at alkaline reactions soils require more soluble phosphate to supply the needs of the crop than do neutral or slightly acid soils. 8. In acid soils available phosphate is present in largest part as the H2PO4 ion while in alkaline soils HPO4 is the important ion.

9. Since H2PO4 ion is the dominant ion in plant sap and crops subsist on lower concentrations of phosphate in the soil solution of slightly acid soils, it is suggested that the plants prefer the H2PO4 ion for nutrition purposes. 10. It is shown that wheat plants assimilate phosphate most readily at reactions close to neutrality (preferably slightly acid), less readily at acid reactions in the presence of calcium, and least readily at alkaline reactions of pH 8.0 to 9.0. 11. Phosphate assimilation is thus largely a function of reaction. 12. Electrodiffusion of phosphate can be represented by a specific rate constant which is different for different soils and at different amperages. 630 TECHNICAL BULLETIN No. 38

BIBLIOGRAPHY

1. Cameron, F. K. and Bell, J. M., 1905—The mineral constituents of the soil solution. U. S. Dept. Agric. Bur. Soils Bui. 30. 2. Emmert, E. M., 1931 —The effect of soil reaction on the growth of tomatoes and lettuce and on the nitrogen phosphorus and manganese content of the soil and plant. Ky. Agr. Exp. Sta. Research Bui. 314. 3. Mattson, Sante, 1926 — Electrodialysis of the colloidal soil material and the exchangeable bases. Jour. Agr. Res., vol. 33, p. 553. 4. McGeorge, W. T. and Breazeale, J. F., 1931 — Phosphate solubility studies in some unproductive calcareous soils. Ariz. Agr. Exp. Sta. Tech. Bui. 35. 5. McGeorge, W. T. and Breazeale, J. F., 1931—The relation of phosphate availability, soil permeability and carbon dioxide to the fertility of calcareous soils. Ariz. Agr. Exp, Sta. Tech. Bui. 36. 6. Pierre, W. H. and Parker, F. W., 1927—The use of collodion sacks in obtaining clear soil extracts for the determination of water- soluble constituents. Soil ScL, vol. 23, p. 13. 7. Rost, C. O., 1927 — Electrodialysis in studies of soil deficiencies. Proc. First Int. Cong. Soil Sci., vol. 2, Comm. 2, p. 334. 8. Waltman, C. S., 1931—Effect of hydrogen ion concentration on the growth of strawberries in sand and soil. Ky. Agr. Exp. Sta. Res. Bui. 321. 9. Zscheile, F. P., 1930—The thermodynamics of ion concentration by living plant cells. Protoplasma, Vol. 11, p. 481. INDEX TO TECHNICAL VOL. II

TECHNICAL BULLETINS 25-38

INDEXED BY KEMPTON PAYNE, M.D.

Titles of Technical Bulletins are printed in capital letters. Scientific names are in italics. Numbers of Technical Bulletins are printed in heavier type than page numbers.

of organic soil compounds. 31: 215. of raw plant material. 31: 247. Aeration of soil, conditions governing. of soil, established. 31: 216. 31: 215. studied in fresh plant material. 31: Alkali: 240. accumulation of, in soil. 26: 37. BASE-EXCHANGE PROPERTY OF results of leaching. 26: 37. ORGANIC MATTER IN SOILS, Arid soils: THE. 30: 181. base-exchange properties of. 30: 185. action of hydrogen peroxide en inor- in Southwest. 31: 215. ganic matter. 30: 195. Arizona-grown wheat: action of hydrogen peroxide on organic Early Baart strains. 27: 82. matter. 30: 192. flour yield of. 27: 83. equivalency of base exchange in organ- gluten percentages of. 27: 78. ic matter. 30: 187. varieties tested. 27: 72. nature of organic exchange compounds. "yellow berry" spotting. 27: 82. 30: 200. "yellow spotting" re soil. 27: 97. soils used for experiments. 30: 184. Artesian water: summary. 30: 211. from St. David, fluorine content of. Base replacements: 32: 277. "break-down," by organic acid solu- tooth defects, due to. 32: 270. tions. 28: 134. "building-up," by alkaline solutions. 28: 133. Base exchange: effect of basic magnesium carbonate equivalency of, in soil organic matter. upon. 26: 53. 30: 187. effect of magnesite upon. 26: 57. higher, in fermented plant material. 31: in zeolitic soils. 26: 41. 247. prevented by common ion. 26: 42. in raw plant material treated with Black alkali soil: H2O2. 31: 247. color due to lignic acid. 31: 230. valence of complex anion. 28: 133. magnesium carbonate in. 26: 55. Base-exchange capacity: phosphate solubility of. 36: 375. "break-down" of. 28: 120; 28: 121. sodium carbonate not present in most. "building-up" of. 28: 109. 31: 230. "building-up" with hydroxide solu- Breazeale, J. F. 25: 1; 26: 37; 29: 137; tions. 28: 117; 28: 119. 35: 319; 36: 361. increased by decomposition, in soils. Brown rot gummosis (Phytophthora 31: 250. [Phythiacystis] citrophthora). 37: lowest in raw organic materials. 31: 493. 248. Bryan, W. E. 27: 67. of alfalfa. 31: 250. "BUILD-UP" AND "BREAK-DOWN" of soils, increased by green manure. OF SOIL ZEOLITES, AS IN- 31: 249. FLUENCED BY REACTION, summary. 31: 249. THE SO-CALLED. 28: 101. Base-exchange complex, influence of, in "break-down." 28: 106. calcareous soil. 36: 374. "build-up." 28: 103. Base-exchange property: conclusions. 28: 133. of carbohydrates in soil. 31: 238. discussion. 28: 128. 636 ARIZONA AGRICULTURAL EXPERIMENT STATION

experiments with soils: Citrus trees : calcium-zeolite. 28: 108. damaged by "lire" ants. 37: 493. killed soil. 28: 108. transpiration of. 37: 468. Rhode Island A.E.S, 28: 108. vertical distribution of soil moisture of. "Build-up" of soil moisture 29: 152. 37: 483. Burgess, P. S. 28: 101. water consumption of. 37: 471. wilting test of, on Yuma mesa 37: 470 . Calcareous soil: Clay loam, lettuce growing on. 33: 284. alkali content re fertility. 35: 358. Crops: base replacement. 26: 37. assimilation of phosphate ion. 38: 621. conclusions. 36: 409. surface soil at'wilting percentage. 29: fertility factors of. 36: 361. 139. found in Arizona. 38: 624. D influence of base-exchan/je complex. Dates : 36: 374. Arabian imported. 34: 305. phosphate availability of. 36: 388. grown in Arizona and California. 34: phosphate solubility studies on. 35: 305. 319; 35; 336. Deglet Noor. 34: 253. physical characteristics of. 35: 322. Maktum. 34: 253. relation of phosphate availability, soil Thoory. 34: 253. permeability, and carbon dioxide to vitamin content studies. 34: 305. fertility off 36: 361. Defect of human teeth, cause of. 32: Calcium in zeolitic soils. 26: 37. 253. Carbohydrates, decay of, re humus. 31: Dolomite, base replacement property of. 238. ,26: 58; 26: 60. Carbonates: E base replacement of. 26: 48. reaction with zeolites. 26: 60; 26 62. Early Baart wheat: solubilities of. 26: 38. gluten percentage of. 27: 84. Carbon dioxide : milling and baking tests of. 27: 74. from carbohydrates. 36: 365. tests on Arizona-grown. 27: 72. yield ratio of differing strains. 27: 95. influence of, upon phosphate fixation. ELECTRODTALYSTS AS A MEAS- 36: 368. URE OF PHOSPHATE AVAIL- production of, a factor in phosphate ABILITY IN SOILS AND THE reduction. 36: 363. RELATION OF SOIL REAC- re base replacement. 26: 58. TION AND IONIZATION OF re fertility of calcareous soils. 36: 361. PHOSPHATES TO PHOS- solubility of P of phosphate rock. 36: PHATE ASSIMILATION. 38: 368." 593. solvent effect of gaseous. 36: 367. conclusions. 38: 628. solvent for phosphates. 36: 381. interpretation of data. 38: 612. Cams, A. G. 37: 413. ionization and soil reactions. 38: 617. CAUSE OF MOTTLED ENAMEL, A role of reaction (pH) in phosphate DEFECT OF HUMAN TEETH, solubility and nutrition. 38: 619. THE. 32: 253. summary. 38: 624. conclusions and summary 32: 280. Electrodialysis : endemic in St. David, Arizona. 32: time re phosphate solution during. 38: 255. 600. experiments. 32: 268. varied amperage and voltage re diffu- Cenozoic age, fossils, source of fluorine sion rate. 38: 597. in artesian water. 32: 280. Exchange properties, of organic matter, Citrus Aurcwticum L., sour orange. 37: undecomposed. 31: 240. 491. Citrus fruits : water movement of. 25: 28. "Fire" ants (Solenopsis spj. 37: 493. wilting percentage for. 29: 139. Flour : comparison of, from Early Baart Citrus maxima, "Merrill, grapefruit. 37: wheat. 27: 82. INDEX TO TBCHNJCAL VOLUME II 637

milling- and baking1, tests. 27: 73. final distribution of water used in. 37: Flour Testing Laboratory, Howard. 27: 546. 74. measurement of water used. 37: 564. Fluorine: methods for controlling soil tempera- feeding experiments with salts of. 32: ture. 37: 562. 274. soil moisture studies. 37: 495. high content of, in mastodon bones- tree response to methods used in. 37: 32: 280. 569. water examination for. 32: 276. unit head of water, determination of. Force-water-content curve, re wilting 37: 467. percentage. 25: 9. water-soil relationships. 37: 440. G K Gedroiz: Kansas City Testing' Laboratory, for theory of base exchange from salt solu- flours." 27: 74. tions. 30: 182. Kansas State Agricultural College, Mill- theory of origin of zeolitic soils. 28: ing Department. 27: 74. 104. Kinnison, A. F. 37: 413. Russian soil scientist. 28: 101. studies, on replacement. 28: 101. Grapefruit (Citrus maxima), Merrill. 37: 491. Lantz, Edith M. 32: 253. Grapefruit orchards: Lettuce: irrigation investigations of. 37: 413. clay loam for. 33: 285. maintaining desired soil-moisture con- root studies of. 33: 294. tent of. 37: 414. temperature studies in growth. 33: Green manuring. 21: 215. 287. chemical change in. 31: 215. water requirements for growth. 33: increases base-exchange- capacity c|f 285. soils. 31: 249. Lettuce seed, germination tests of. 33: 298. H Lettuce seedbed irrigation. 33: 283. Hemicelluloses: Liebig. pioneer soil chemist. 35: 327. solubility of. 31: 235. Lignin: xvlan, most abundant form, in plants. chemical composition of. 31: 246. 31: 235. colloid base-exchange agent of soil. Hobart, Charles. 33: 285. 31:216. Howard Wheat and Flour Testing Lab- humus content of. 31: 240. oratory. 27: 74. isolated from corn cobs and soil. 31: Humate salts, common-ion studies of. 217. 31: 231. re organic exchange capacity of soils. H umus: 31: 244. lignin content of. 31: 240. variation in chemical groups of. 31: studies of. 31: 216. 238. synthetic: Lignin and ligno-humates: from sucrose. 31: 239. exchange capacity of. 31: 217. preparation of. 30: 205. hydrolysis and ionization of. 31: 221. replacement capacity of. 31: 239. Lignin-like materials, compared with in- organic zeolites. 31: 230.

IRRIGATION INVESTIGATIONS M IN YOUNG GRAPEFRUIT OR- Magistad, O. C. 25: 1. CHARDS ON THE YUMA Masnesite, re base replacement. 26: 57. MESA. 37: 413. MAGNESIUM AND CALCIUM IN character of tree stock. 37: 491. ZEOLITIC SOILS. 26: 37. conclusions. 37: 586. calcium carbonate re base replacement. continuous 4-year records of. 37: 544. 26: 48. depth of irrigation. 37: 466. calcium carbonate reactions with zeo- effect of irrigation on soil temperature. lites. 26: 60; 26: 62. 27: 554/' carbon dioxide re base replacement of effect of silt deposit. 37: 488. 25: 57; 26: 58; 26: 60. 638 ARIZONA AGRICULTURAL EXPERIMENT STATION

magnesium-calcium equilibrium. 26: 51. not hereditary defect. 32: 265. See, base replacement. water supply re poor dietary. 32: 269. See, black alkali soils. solubilities of magnesium and calcium carbonates. 26: 38. summary. 26: 64. ORGANIC COMPOUNDS ASSOCI- MAINTENANCE OF M OI S T U R E ATED WITH BASE-EX. EQUILIBRIUM AND NUTRI- CHANGE REACTIONS, IN TION OF PLANTS AT AND SOILS. 31: 215. BELOW THE WILTING PER- discussion. 31: 245. CENTAGE. 29: 137. exchange properties of undecomposed absorption of potassium at wilting per- organic matter. 31: 240. centage. 29: 171. relation of lignin to organic exchange behavior of plants at wilting percent- capacity of soils. 31: 244. age. 29: 138. See, lignin. root elongation at wilting percentage. study of hemicelluloses. 31: 235. 29: 165. summary. 31: 249. soil moisture. 29: 144; 29: 163. Organic soils: summary. 29: 176. base-exchange properties of. 30: 182; wilting percentage of plants. 29: 139. 30: 192; 30: 200. wilting percentage of soil. 29: 141. base replacement of, not progressive. Manganese, experiments re tooth devel- 30: 197. opment. 32: 274. McGeorge, W. T. 30: 181; 31: 215; 35: 319; 36: 361; 38: 593. Meeker, Lucy Axline. 34: 305. Phosphate absorption, effect of soluble MILLING AND BAKING QUALI- salts on. 36: 394. TIES OF PURE LINES OF Phosphate assimilation, relation of car- ARIZONA-GROWN WHEAT bon dioxide to. 36: 388. 27: 67. Phosphate availability: baking tests. 27: 73. conclusions of electrodialytic findings grain yield per acre. 27: 92. on, 38: 628. object of investigations. 27: 70. determining factors in. 36: 363; 38: See, Early Baart wheat. 625. summary. 27: 96. function of ionization. 38: 617. Mission School, water of, re mottled its relation to fertility of calcareous teeth. 32: 279. soils. 36: 361. Mission soil, wilting coefficient of. 29: Phosphate solubility: 141; 29: 162. effect of alternate saturation and re- Molar solution, osmotic pressure of. 25: moval of carbon dioxide, on. 36: 19. 370. Mottled enamel, a defect of human teeth. effect of leaching on. 36: 372. 32: 253. specific rate of. 38: 612. co-existent with water fluorine con- PHOSPHATE SOLUBILITY STUD- tent. 32: 253. IES ON SOME UNPRODUC- endemic areas. 32: 253. TIVE CALCAREOUS SOILS. experiments on laboratory animals. 32: 35: 319. 268. conclusions of. 35: 358. found in Mission School children. 32: effect of subdivision on phosphate solu- 279. bility. 35: 340. not seen in domestic animals. 32: 268. phosphate content of good and poor produced experimentally in February, soils. 35: 332. 1931. 32: 253. progressively varying soil-water ratios. re enamel defect. 32: 255. 35: 35L studies made in St. David, Arizona. sodium salts re phosphate rock solubil- 32: 255. ity. 35: 349. theories of water contents. 32: 268. Phosphates: Mottled teeth: histological examination ionization of, re phosphate assimilation. of. 32: 255. 38: 593. incidence at Sacaton, Arizona. 32: soil reaction of, re phosphate assimila- 266. tion. 38: 593. INDEX TO TECHNICAL VOLUME II 639

Plant: soluble salts re solubility and absorp- nutrition of: tion of phosphorus. 36: 396. an electrical phenomenon. 29: 167. summary. 36: 405. dependent on ions. 29: 167; 29: 168. soil-moisture equalizer. 29: 161. s suction force of. 29: 164. Plant absorption: Sacaton, Arizona: an adaptation. 25: 23. mottled enamel among resident In- influenced by concentration of solution: dians. 32: 266. nitrogen. 25: 23. water supply of. 32: 266. phosphorus. 25: 22. Smith, G. E. P. 37: 413. potassium. 25: 22 . Smith, H. V. 32: 253. PLANT AND SOIL REACTIONS AT Smith, Margaret Cammack. 32: 253; AND BELOW THE WILTING 34: 305. PERCENTAGE. 25: 1. Soil: chemical nature of soil solution at and absorption of nutrients from. 29: 169. below wilting percentage. 25: 21. attractive forces in. 25: 6. movement of soil water. 25: 12. base absorbing property of. 31: 216. new indirect method for determination gradual loss, of fertility of. 35: 321. of wilting percentage. 25: 29. optimum moisture content. 25: 3. plant-soil equilibrium at wilting per- See, black alkali soil. centage. 25: 23. See, calcareous soil, soil film, re shrinkage and expansion of "staying power" of. 35: 330. soil. 25: 3; 25: 18. water movement: soil sheath. 25: 28. in liquid phase. 25: 12. summary. 25: 31. in vapor phase. 25: 13. water movement in soils at wilting depleted, aided by aeration. 35: 325. percentage. 25: 12. experimental, phosphate studies of. 35: Plant nutrition: 346. capillary action of roots. 29: 155; good and poor, phosphate content of. 29: 160. 35: 332. electrical phenomenon. 36: 364. non-calcareous, magnesium-calcium moisture equilibrium re wilting per- equilibrium in. 26: 51. centage. 29: 137. poor, mechanical composition and phos- re variation in soil reaction. 38: 619. phate solubility of. 36: 379. suction force of roots. 29: 141; 29: Soil colloids: 142. dye absorption by. 31: 240. toxicity of carbon dioxide toward roots. gas absorption of. 25: 5: 36: 385. Soil fertility: Plant-soil equilibrium: limiting factors of. 35: 330. at wilting percentage. 25: 1. related to water movement. 35: 32$. in desert plants. 25: 24. Soil film: maintenance of. 25: 23; 25: 29. Pressley, E. H. 27: 67. re expansion of soil. 25: 3. re shrinkage of soil. 25: 3. R Soil fineness, re soil phosphate solubility. 35: 340; 35: 342. RELATION OF PHOSPH ATE Soil infertility, carbon dioxide re phos- AVAILABILITY, SOIL PER- phorus solubility. 36: 361. MEABILITY, AND CARBON Soil moisture: DIOXIDE TO THE FERTIL- ITY OF CALCAREOUS SOILS, effect of salts upon movement of. 25: THE 36: 361. 14. carbon dioxide re root growth. 36: variations in one plant. 29: 163. 385. Soil permeability, re fertility of calcare- conclusions. 36: 409. ous soils. 36: 361. nature of soil phosphate and rock phos- Soil phosphate: phate. 36: 398. availability of, measured by electrodf- root development re carbon dioxide alysis. 38: 593. and soil dispersion. 36: 381. carbonate-phosphate compounds, least See, carbon dioxide. soluble form of. 36: 405. 640 ARIZONA AGRICULTURAL EXPERIMENT STATION

Soil phosphates: role of organic matter in base-ex- electrodialysis used to determine avail- change property of. 30: 192. ability of. 38: 612. See, black alkali soil. electrodiffusion of. 38: 604. See, calcareous soil, nature of. 36: 39.8. shrinkage of, effect of absorbed gases rate of solution of. 35: 342. upon. 25: 5. related to available organic matter. 35: study of organic compounds of. 30: 327. 192. related to soil aeration. 35: 327. swelling of. 25: 6. related to water movement. 35: 327. Solenopsis sp., "fire" ants. 37: 493. removed by dialysis. 35: 357. salts as clarifying agents. 35: 348. Sour orange (Citrus Aurantium L,.). 37: varying soil-water ratios. 35: 351. 491. Soil properties, dependent on exchange St. David, Arizona: complex. 31: 221. artesian water supply of. 32: 266. Soil reaction, relation to phosphate solu- mottled teeth, endemic in. 32: 266. bility. 36: 375. study of local dietary. 32: 268. Soil solubility, after phosphate fixation. STUDIES IN LETTUCE SEEDBED 36: 369. IRRIGATION UNDER HIGH Soil suction, action of gravity at satura- TEMPERATURE CONDI- tion point. 29: 145. TIONS. 33: 283. Soil temperature, re irrigation. 33: 291. root studies. 33: 294. Soil zeolites: seed tests. 33: 298. "break-down" of, in carbonated water. summary. 33: 302. 28: 124. temperature studies. 33: 287. "build-up" and "break-down" of: discussion. 28: 128. summary. 28: 133. common ion re base replacement. 26: Temperature studies, in lettuce growing. 64. 33: 287. found in acid Eastern soils. 28: 132. Time factor, in base replacement. 26: 63. hydrolysis of. 26: 38. loss of base-exchange capacity. 28: u 106. partial inactivation of, and subsequent United States Bureau of Soils, soil on "build-up." 28: 126. Salt River Valley Experiment Soils: Farm, Mesa, Arizona. 33: 285. base-exchange property of organic mat- ter in. 30: 181. "build-up" of, by alkali solutions. 28: 127. VITAMIN CONTENT OF THREE "build-up" of, problem of arid regions. VARIETIES OF DATES, THE. 28: 128. 34: 305. measurement of: comparative study of base-exchange vitamin A content. 34: 306. properties. 30: 185. vitamin B content. 34: 308. deflocculation of, by zeolites. 26: 37. vitamin Bi content. 34: 310. expansion of, effect of absorbed gases vitamin C content. 34: 312. upon. 25: 5. vitamin D content. 34: 314. free base fixation of. 30: 203. vitamin G content. 34: 311. H2O2 reaction on inorganic exchange summary. 34: 316. complex of. 30: 195. low gypsum content, in Arizona. 26: 37. w nitrogen content of, re base-exchange Water absorption, due to adjusted forces. capacity of. 30: 211. 29: 144. organic compounds of, and base ex- change of. 31: 215. Water content: organic exchange capacity of, re lig- change of, above wilting point. 25: 9. nin. 31: 244. change of, below wilting point. 25: 9. physical and chemical properties of Water softener, exchange capacity of. fertile. 30: 203. 30: 196. INDEX TG TECHNICAL VOLNME II 641

Water studies: from vSt. David, Arizona. 32: 270. tooth mottling re fluorine content. 32: Yuma Mesa Citrus Experiment Station: 280. climatic conditions of. 37: 435. Wharton, M. F. 33: 285. irrigation studies by Department of Wheat: Agricultural Engineering. 37: 413. climate for, southern Arizona. 27: 69. irrigation studies by Department of strains suitable for Arizona. 27: 97. Horticulture. 37: 413. strains suitable for Middle West. 27: irrigation studies on young citrus orch- 97. ards. 37: 413. Wilting percentage: location of. 37: 415. absorption of potassium from soils at. water-soil relationships of. 37: 440. 29: 171. /uma Mesa grapefruit orchards: biological phenomenon. 25: 17. method of irrigation. 37: 415. determinants of. 29: 164. slope of land of. 37: 419. error in determining. 29: 142. soil sampling of. 37: 429. for deciduous fruits. 29: 139. specific gravity of sandy. 37: 433. moisture equilibrium at and below. 29: 137. new methods for determining. 25: 29. of soils. 25: 33; 29: 141. Zeolites, treatment with hydrogen perox- plant behavior at. 29: 138. ide. 30: 195. root elongation at. 29: 165. !eolitic fraction: Wilting point: of clay, hydrolvsis of. 31: 221. definition of. 25: 2. of clay, ionization of. 31: 221. of black alkali soil. 31: 230. Zeolitic soils: base replacement in. 26: 40. calcium in. 26: 37. practical problems of. 26: 37. Xylan, preparation of. 31: 235. summary on. 26: 64.