pH, SOIL ACIDITY, AND PLANT GROWTH 67 numbers, the Danish biochemist S. P. L. Sorenson devised a system called pH for expressing the acidity or alka- pH, Soil Acidity, linity of solutions. The pH scale goes from o to 14. At pH 7, the midpoint of the scale, there and Plant Growth are equal numbers of hydrogen and hydroxyl ions, and the solution is neu- W. H. Allaway tral. pH values below 7 indicate an acid solution, where there are more hydro- When crop plants do not grow gen ions than hydroxyl ions, with the well, one of the first questions acidity (or hydrogen ion concentra- tion) increasing as the pH values get the soil scientist usually asks smaller. is, ''What is the pH of the soil?'' pH values above 7 denote alkaline solutions, with the concentration of or, 'Is the soil acid, neutral, hydroxyl ions increasing as the pH or alkaline?'' values get larger. The pH scale is based on logarithms The reason for these questions lies in of the concentration of the hydrogen the fact that the pH, or degree of and hydroxyl ions. This means that a acidity of the soil, often is a symptom solution of pH 5 has 10 times the hy- of some disorder in the chemical con- drogen ion concentration of a solution dition of the soil as it relates to plant of pH 6. A solution of pH 4 has 10 nutrition. times more hydrogen ions than one of A measurement of soil acidity or pH 5 and 10 times 10, or 100 times, alkalinity is like a doctor's measure- the hydrogen ion concentration of a ment of a patient's temperature. It re- solution of pH 6. veals that something naay be wrong but A measurement of the pH of a solu- it does not tell the exact nature of the tion of a strong, or highly ionized, acid trouble. measures essentially the total strength The acidity or alkalinity of every of the acid. But a pH determination water solution or mixture of soil and of a solution of weak, or slightly ion- water is determined by its content of ized, acid measures only a part of the hydrogen ions and hydroxyl ions. Wa- total strength of the acid, because pH ter molecules break up, or in chemical is a measure of hydrogen ions only and language, ionize, into two parts—hy- does not measure all the acid molecules drogen ions and hydroxyl ions. When that can potentially ionize to form hy- there are more hydrogen ions than drogen ions. hydroxyl ions, the solution is said to be In a soil, hydrogen ions exist in a acid. If there are more hydroxyl ions number of different chemical combi- than hydrogen ions, the solution is al- nations and states of adsorption on the kaline (or basic). Solutions with equal surfaces of solid particles. The number numbers of hydrogen and hydroxyl of hydrogen ions in the soil solution at ions are called neutral. any one time is small in relation to the Only a very small percentage of the number held in a less active form in water molecules present are broken up various nonionized molecules and on into hydrogen and hydroxyl ions at the surfaces of the solid particles. any one time. If one attempts to ex- When the soil is limed in order to press the concentration of these ions in bring it to neutrality, enough lime conventional chemical ways, some must be added to react not only with cumbersome decimal fractions result. the so-called free hydrogen ions of the In order to avoid these cumbersome soil solution but also with those held 68 YEARBOOK OF AGRICULTURE 1957 in the less active forms. This is so be- tion are neutralized or removed from cause as the neutralization of the soil the soil solution in other ways, hydro- progresses, ionization of the less active gen ions from the less active (or poten- forms of hydrogen likewise progresses tial acidity) source enter the solution. and new free hydrogen ions are formed Most of the hydrogen ions in the po- as long as the supply of less active tential acidity forms are held on sur- forms holds out. faces of solid particles of clay or soil Thus it is possible to think of the total organic matter. These clay and organic acidity of a soil as being composed of particles are very small, and conse- two parts. quently have a large surface area per One part, often called the active acid- unit weight. They make up what is ity, is made up of the hydrogen ions in called the colloidal fraction of the soil. the soil solution. These are the hydro- Since most of the potential acidity of gen ions measured when the pH of the soils is due to hydrogen ions held on soil is determined. the clay and organic particles, it fol- The second, and much larger, part lows that fine-textured soils, which are of the total soil acidity is often called high in clay and organic matter, can potential acidity. The potential acidity have a higher total acidity than sandy is due to hydrogen ions held in various soils of low clay and organic content. chemical combinations and adsorbed There are different kinds of clay in on the surfaces of solid particles. These different soils, and these different kinds hydrogen ions arc in chemical equilib- of clay can hold different amounts of rium with the free hydrogen ions of the hydrogen ions in the potential acidity active part of the soil acidity, and as form. Generally speaking, the clay the free hydrogen ions of the soil solu- found in soils of cool-temperate and

Approximate Amounts of Finely Ground Limestone Needed to Raise tfie pH of a 7-inch Layer of Soil as Indicated ^ ^ Limestone requirements From pH j.5 From pH 4..^ From pH ^.^ Soil regions and texlural classes to pH ^.5 to pll j.j to pH 6.^ Tons per acre Tons per acre Tons per acre Soils of warm-temperate and tropical re- gions: 2 Sand and loamy sand o. 3 o. 3 o. 4 Sandy loam .... .5 . ^ Loam .... .8 i. o Silt loam .... 1.2 1.4 Clay loam .... 1.5 2.0 Muck ^2.5 3.3 3.8 Soils of cool-temperate and temperate re- gions: ^ Sand and loamy sand .4 .5 .6 Sandy loam .... .8 1.3 Loam .... 1.2 1.7 Silt loam .... i.tj 2.0 Clay loam .... 1.9 2.3 Muck ^2.9 3.8 4.3 1 All limestone goes through a 2-mm. mesh screen and at least Y2 through a 0.15-mm. mesh screen. With coarser materials, applications need to be greater. For burned lime about J^ the amounts given are used; for hydratcd lime about ^^. 2 Red-Yellow Podzol, Red Latosol, etc. 3 The^ suggestions for muck soils are for those essentially free of sand and clay. For those containing much sand or clay the amounts should be reduced to values midway between those given for muck and the corresponding class of mineral soil. If the mineral soils are unusually low in organic matter, the recommendations should be reduced about 25 percent; if unusually high, increased by about 25 percent, or even more. ^ Podzol, Gray-Brown Podzol, Brown Forest, Brown Podzol, etc. 3 From USDA Handbook No. 18, p. 237. pH, SOIL ACIDITY, AND PLANT GROWTH 69 Cation Exchange Reactions When an Acid Soil Is Limed © © H^ H^ H+ H + \ I / / w // Clay Organic — „ . 2 Colloid :— —

iyig+2 Ca Ca+2Ca+2Ca+2

NEUTRAL SOIL

Replaceable Magnesium ion Replaceable Aluminum ion Replaceable Potassium ion Water

Aluminum Hydroxide arid regions can hold a greater quan- exchangeable or replaceable cations. tity of potential-acidity hydrogen ions The basic process involved when lime than can the kind of clay found in is added to an acid soil is the replace- soils of warm-temperate and tropical ment of hydrogen ions held by the clay- regions. General relationships between organic colloidal material with calcium kind of soil, pH, and the amounts of ions from the lime. lime required are shown in the table. The diagram shows this process. The The hydrogen ions of the potential small numbers by the symbols for the acidity form are held to the colloidal different cations indicate the number material because of their electrical of positive charges carried by that cat- charges. The colloidal material is pre- ion. Where no number appears, one dominantly negatively charged. The positive charge is present. The num- hydrogen ions are positively charged, bers of different kinds of ions involved and the attraction between the unlike in an exchange reaction depend on the charges accounts for most of the bind- number of charges carried by each ion. ing of hydrogen ions to the colloidal Thus one calcium ion with its two posi- surfaces. When hydrogen ions change tive charges will replace two singly from the potential to the active acidity charged hydrogen ions. Three doubly forms, their places on the clay-organic charged calcium ions are required to colloidal material are taken by other replace two triple charged (or, in chem- ions carrying positive charges. Ions of ical terms, trivalent) aluminum ions. calcium, magnesium, potassium, and Aluminum ions have a unique role sodium are positively charged and may in soil acidity. In the diagram it can take the place of hydrogen on the col- be seen that the replaceable aluminum loidal complex. ions of the acid soil have been replaced Ions with positive charges are called by calcium in the neutral soil. The cations. The process whereby positively aluminum ions thus replaced react charged ions of one kind are replaced with the soil water to form insoluble on the surfaces of the clay-organic col- hydroxides and oxides. loidal material is called cation ex- Aluminum is one of the ions of posi- change. The total amount of all kinds tive charge that react with basic so- of cations held by the clay-organic com- lution to form insoluble oxides and plex at any one time is called the cat- hydroxides. Aluminum occurs as a re- ion-exchange capacity. The ions held placeable cation in acid soils in far to the clay-organic surfaces are called greater quantity than any of the other 70 YEARBOOK OF AGRICULTURE 1957 cations that show this property. So sodium ions bring about very unde- long as the soil remains neutral, these sirable physical properties and very aluminum oxides and hydroxides re- slow movement of soil water. In these main insoluble and inert in the soil. alkali soils, hydroxyl ions in the soil so- Because calcium ions replace both lution greatly exceed the hydrogen ions aluminum and hydrogen ions during and pH values of 8 to lo are common. the neutralization of an acid soil, the One of the most important experi- aluminum ions can be said to con- ments dealing with the effect of pH on tribute to the total acidity of the soil. plant growth was conducted by D. I. In most methods for determining total Arnon and his associates in California. acidity of soils, the replaceable alu- They grew plants in water solutions in minum and hydrogen ions are lumped which the pH varied from very acid together. to very alkaline; the solutions con- Soils become acid through a process tained a liberal supply of all the im- that is almost the direct reverse of the portant plant nutrients at all pH liming process shown in the diagram. values. The plants grew well except at The soil parent materials usually con- extremely acid or extremely alkaline tain colloidal material, which is nearly pH values—values that are only very saturated with basic cations like cal- rarely encountered in field soils. cium and magnesium. But through the Throughout the range of pH values centuries during which soil develop- common in soils—that is, from about ment takes place, hydrogen ions car- 4 to 9—plant growth was not greatly ried by downward percolating waters affected by the pH of the solution. gradually replace these calcium and This experiment of Dr. Arnon and magnesium ions. The calcium and the his associates might well cause one to magnesium are carried away by the question why soil scientists so fre- drainage waters. quently measure the pH of the soil in The replacement of bases by hy- order to diagnose troubles in crop pro- drogen ions from the water moving duction. One might also question the through the soil profile is a slow proc- value of tables of the pH values at ess, but soil formation usually takes which various plants grow best. place over many centuries. The more The answer to these questions lies in water moving down through the soil, the fact that the effects of pH on plants the faster the process. Therefore the growing in soil are indirect, while Dr. soils of humid regions are generally Arnon's solution culture experiments more acid than the soils of subhumid were designed to measure the direct regions, and acid soils are rarely found effects of pH. The solution cultures in arid regions. Also, since sandy soils used contained neither deficits nor ex- can hold lesser amounts of replaceable cesses of the essential plant nutrients. bases, these sandy soils usually be- In soil, however, the solubility and come acid more rapidly than do fine- availability to plants of many impor- textured soils. tant nutrients is closely related to the Many important chemical proper- pH of the soil. It is this indirect effect ties of the soil are dependent on the of pH on the availability of plant nu- kind of replaceable cations held by the trients that justifies the frequent use of clay-organic colloidal fraction of the pH measurements for diagnosis of soil soil. Calcium, potassium, and mag- problems, and makes tables of so- nesium held as replaceable ions con- called pH preferences of plants useful stitute the major source of these im- under many conditions. portant plant nutrients in most soils. Changes in the acidity of soils may The physical properties of the soil change the availability to plants of are often afí'ected also by the replace- different nutrients in different ways. able cations. In some of the alkali soils As the pH of an acid soil is increased of the West, excesses of replaceable by the addition of lime, ions such as pH, SOIL ACIDITY, AND PLANT GROWTH aluminum, iron, manganese, copper, molybdenum is most often deficient in and zinc become less soluble. In acid acid soils, and becomes more available soils these ions may be found in dis- as the soil is limed. solved form in quantities sufficient to In any discussion of the relation- become toxic to plants. As the soil is ships between pH and nutrient avail- neutralized, these ions form inert ability in soils it should be emphasized oxides and hydroxides, and the toxic- that these relationships difier in difier- ity is corrected. As the pH of the soil ent soils. In organic soils (peats and is increased still further, the solubility mucks), the relationships between pH of these ions becomes so low that defi- and nutrient availability are not the ciencies of those (iron, copper, manga- same as for mineral soils. Copper, for nese, zinc) needed by plants may occur. example, may be deficient in acid Bacteria and other micro-organisms organic soils but is rarely so in acid living in the soil convert nitrogen, sul- mineral soils. fur, and phosphorus from organic Many soils are naturally alkaline compounds, in which these nutrients and contain an excess of lime. The are unavailable to plants, to simpler availability of plant nutrients in these inorganic forms that plants can take naturally alkaline soils quite often up. Neutralizing an acid soil usually differs from the availability of nutri- makes the soil condition more favor- ents that results when a naturally acid able to the growth of bacteria and may soil is treated with an excess of lime. thus indirectly speed up processes by For example, many naturally alkaline which important nutrients become soils have an adequate supply of available to plants. available boron, whereas overliming The bacteria that live in association of a naturally acid soil usually brings with the roots of legumes are less about boron deficiency. effective in their important role in Since the relationships between pH nitrogen fixation in acid soils than in and the availability of plant nutrients neutral or alkaline soils. are complex, pH measurements of In farm practice, compounds of soils are not easy to interpret in the calcium and magnesium are the basic solution of problems of soil fertility. materials used to treat acid soils. While the measurement of pH may Along with the decrease in acidity, give some valuable clues concerning the addition of these materials in- the reasons for poor plant growth, it is creases the supply of calcium and generally necessary to follow up these magnesium for use by plants growing clues with additional tests before an on the soil. In some cases, a lack of accurate diagnosis of the trouble can available calcium or magnesium may be made. be the most important defect of an Among the many recent publications acid soil. about chemical reactions in soils the The availability to plants of the following may be cited: Chemistry of the phosphorus in soils is changed in a Soil, by F. E. Bear (Reinhold Publish- rather complex manner when the ing Corp., New York, 1955); ^^^y ^i^- acidity of the soil changes. Phosphate eralogy, by R. E. Grim (McGraw-Hill availability in many soils is highest Book Co., Inc., New York, 1953); Cat- when the soil is neutral or slightly acid, ion Exchange in Soils, by W. P. Kelley and it declines as the soil becomes (Reinhold Publishing Corp., New York, either strongly acid or alkaline. 1948); Soil Conditions and Plant Growth, Boron and molybdenum are other by E. J. Russell and E. W. Russell plant nutrients that show changes in (Longmans, Green and Company, New availability with changes in the pH of York, 1950); "Formation Constants for the soil. Boron deficiencies frequently CU (II)—Peat Complexes," by N. T. occur when too much lime is added to Coleman, A. C. McClung, and D. P. an acid soil. On the other hand, Moore (Science, February 24, 1956).