72 YEARBOOK OF AGRICULTURE 1957 Kw=[H+] [OH-]. (i) [H+] and [OH~] are the concentra- tions of hydrogen ions and hydroxyl ions, expressed as equivalents per liter. The Chemistry of One equivalent of a singly charged ionic species is the weight in grams of Soil pH that species, which contains 6.023 ^ 10^^ particles. The value of Kw is lo"^* at 22° C, N. T. Coleman and A. Mehlich and in any aqueous system at this tem- perature the product of the concentra- Every substance dissolved in tions of hydrogen and hydroxyl ions is 10-14. water or mixed with water is Equation i can be written in a more acid, neutral, or alkaline. Vine- useful form by dividing both sides into gar, containing acetic acid, is a I and taking logarithms. That is: substance that is acid. Water ^°^¿=^°^m- ■^°Sioïp-]='4-(2) itself is neutral, as are solu- The values of log i/[H+] and log 1/ tions of salt and other such [OH"] generally are referred to as pH and pOH, respectively. These values compounds. Lime, baking soda, are indices of the acidity or alkalinity and lye give alkaline solutions. of a system. Any system in which pH and pOH The reactions of these and all other are equal is neutral. At 22^ C, when substances are typical of the substances Kw=io-i*, neutrality corresponds to themselves and of the ways in which pH=pOH = 7. When pH is less than they react with water. 7, the system is acid. When pH is W'ater itself is composed of one atom above 7, it is alkaline. of oxygen and two atoms of hydrogen. Soils vary in pH from about 4, for Its chemical formula is HgO. Liquid strongly acid soils, to about 10, for water, however, is really a mixture of alkaline soils that contain free sodium H2O, H+ and OH-. H+, called the carbonate. hydrogen ion, is a positively charged The pH range for most agricultural hydrogen atom, and OH~, the hy- soils is 5 to 8.5. droxyl ion, is a negatively charged unit consisting of one hydrogen and IN SOLUTIONS, pH is related to hydro- one oxygen atom. gen-ion concentration in a straight- The hydrogen ion and hydroxyl ions forward manner. That is not the case come from the breakdown, or ioniza- for soils, which consist of a solution tion, of the water molecule according phase, the soil water, and a solid phase, to the scheme: the mineral and organic particles of H20;=>H++0H-. the soil. The pH of a soil-water system is an approximate reflection of the This reaction does not produce very hydrogen-ion concentration of the soil many H+ or OH" ions because the solution, but it does not reflect the water molecule is very stable. In fact, total acidity of the system. This is be- only about one molecule in 10 million cause of the cation-exchange proper- is ionized at any one time. ties of soils. The extent to which water ionizes The tiny mineral and organic par- can be expressed more exactly in ticles of soils have cation-exchange terms of an ionization constant, Kw. capacities—the particle surfaces are This is defined as: negatively charged, and the positively THE CHEMISTRY OF SOIL pH 73 charged ions, or cations, sit on or near Clay minerals also have pH-depend- the particle surfaces. These positive ent charges, which result from the ions are called exchangeable cations. ionization of hydrogen ions from SiOH The cation-exchange capacity of a groups around the edges of the crys- soil is the quantity of positive ions tals and perhaps for other reasons. necessary to neutralize the negative Such charges do not develop below a charge of a unit quantity of soil, under pH of about 6, and do not contribute a given set of conditions. Cation-ex- to the efí'ective cation-exchange ca- change capacity usually is expressed as pacity of acid soils. As pH is increased milliequivalents (6.023 ^ io^^ particles) above 6, the pH-dependent charge in- of cations required to neutralize the creases progressively, reaching a maxi- negative charge of 100 grams of soil at mum at a pH of around 10. The pH-dependent charge which The cation-exchange capacity of a can be developed by clays depends on soil depends on the amounts and kinds the exposed edge area of the crystals of finely divided mineral and organicr and on the nature of the clay. It prob- particles present. Sandy soils generally ably is around 20 milliequivalents per have low cation-exchange capacities 100 grams for montmorillonoid clays, because of their small proportions of and is much smaller for micas and negatively charged material. Soils high kaolins. Cation-exchange capacity at in organic matter have substantial pH 7 includes all of the permanent cation-exchange capacities because of charge and a part of the pH-depend- the large negative charge developed by ent charge. humus. As far as clays are concerned Because of the chemical nature of soil the cation-exchange capacities of the organic matter, it probably has only montmorillonoid and vermiculite-like pH-dependent charge, resulting from minerals, found in Midwestern soils the ionization of hydrogen from car- and soils of the dry areas, are large. boxyl and phenol groups attached to Those of kaolin minerals, which pre- organic matter particles. Of these two dominate in Southeastern soils, are kinds of groups, the carboxyls ionize small. The finely divided micas are largely between pH 4 and pH 7, while intermediate. the phenol groups ionize only at alka- The fine-grained mineral and or- line reactions. The cation-exchange ganic particles of soil differ as to the capacity of soil organic matter, meas- source and characteristics of their ured at pH 7, averages near 200 milli- negative charge as well as to its magni- equivalents per 100 grams. Thus i per- tude. There are two general sources of cent of organic matter contributes cation-exchange capacity, a perma- about 2 milliequivalents per 100 grams nent charge and a pH-dependent toward the cation-exchange capacity charge. of a soil. All clay minerals appear to possess The relationships between cation- permanent charges resulting from their exchange capacity and pH for clay crystal structures. As the name implies, minerals and organic matter can be the permanent charge of soil miner- illustrated schematically, as in figure i. als persists under all conditions. The For the mineralogical clay specimens montmorillonoid minerals, which oc- the efíective cation-exchange capacity cur in many soils of the Midwest, have does not vary with pH below about 5. permanent charges of 80 to 120 milli- This may not be the case with soil equivalents per 100 grams. The kaolin clays. The eñ'ective cation-exchange minerals, which are predominant in capacity of organic matter is near zero soils of the Southeastern States, have at a pH below about 4, and increases much smaller permanent charges, continuously as the pH is raised. varying from i to about 8 milliequiva- Though clay minerals and soil or- lents per 100 grams. ganic matter generally are considered 74 YEARBOOK OF AGRICULTURE 1957 kaline soils contain large proportions of exchangeable sodium. The strength with which ions are bound to soil par- ticles depends on the nature of the ions and of the particle charges. Hydrogen is bound very strongly to pH-depend- ent charges but weakly by permanent charge.

IN MOST SOILS the amounts of ex- changeable cations are much larger than the amounts of cations in the soil water. With acid soils it is convenient to speak of "active acidity" and "total acidity." Active acidity is measured as the pH of a soil-water mixture. The total acidity of a soil is equivalent to the amount of a base (such as calcium hydroxide) necessary to neutralize it. 20 W 60 80 Negative Charge, MiHiequivalents per IGO Grams Total acidity always is greater than active acidity. It would take only /. The negative charge, or effective cation-ex- about 3 pounds of ground limestone to change capacity, of clays and organic matter varies neutralize the active acidity of the top with pH. Those of kaolinite {curve i) and mont- morillonite {curve 2) are constant below pH 6 hut 6 inches of an acre of a soil with a pH increase at more alkaline reactions. The negative of 4. Many times that amount of lime charge of humus (curve 3) increases linearly with pH. would be needed to neutralize the total acidity. to be responsible for the cation-ex- It is possible to have acid soils of change capacity of soils, other sub- identical pH but with very different stances may contribute as well. Iron, total acidities. Many of the sandy soils aluminum, and titanium oxides, as of the Atlantic Coastal Plain have a well as noncrystalline iron and alumi- pH of about 5. Often i thousand to 2 num silicates and phosphates, may add thousand pounds of calcium carbonate greatly to the cation-exchange capaci- per acre is required to neutralize soil ties of some soils. acidity. Other Coastal Plain soils, high Under any given set of soil condi- in organic matter, also have a pH near tions, the permanent charge and that 5. As much as 40 thousand pounds of part of the pH-dependent charge that calcium carbonate is required to neu- is ionized are balanced by the presence tralize the acidity of the plow layer of of cations in the vicinity of the particle many such soils. surfaces. The kinds of balancing ions, or exchangeable cations, depend on NEUTRALIZATION CURVES OR BUFFER the previous history of the soil. curves are useful for studying and illus- Hydrogen, aluminum, calcium, mag- trating the acid characteristics of soils. nesium, potassium, and sodium arc the They are prepared by adding to a soil most abundant exchangeable cations. sample small increments of an alkaline Their proportions vary from soil to substance, such as sodium or calcium soil, depending on inherited charac- hydroxide, and measuring the pH after teristics and past management prac- each addition. Curves relating soil pH tices. Hydrogen and aluminum arc the to percentage base saturation are ob- predominant exchangeable cations in tained in that way. The percentage most acid soils. Calcium and magnesi- base saturation is the proportion of the um balance the negative charge in cation-exchange capacity that is bal- nearly neutral soils, while strongly al- anced by basic metal cations such as THE CHEMISTRY OF SOIL pH 75

PH

>

^ / U / A ^ / ^^ A y^

0 4 8 12 16 20 40 60 Millicquivalents Barium Hydroxide per 100 Grams Percentage Base Saturation 2, Neutralization curves of acid soils. Curve i, 5. ThepH-percentage base saturation relationships kaolin clay; curve 2, montmorillonoid clay; curve j, jor soils having différent sources oj cation-exchange organic soil. capacity. Curve i, kaolin clay; curve 2, organic mat- ter; curve j, montmorillonoid clay. calcium, magnesium, sodium, and po- 7-8, often is taken to reflect the cation- tassium. Neutral soils are base saturat- exchange capacity. It comes at differ- ed; acid soils, containing exchange- ent additions of base for different soils. able hydrogen and aluminum, are base The third region is a zone of buffering unsaturated. at pH above about 8. This is due in If a partially base-saturated soil is part to the neutralization of weakly progressively neutralized in this way, acidic groups and in part to the de- the quantity of base necessary to reach composition of minerals which occurs pH 7 is a measure of the total acidity at strongly alkaline reactions. of the sample. When an acid-washed or The shape of the low-pH end (region otherwise base-depleted soil is treated I ) of the neutralization curve depends in the same way, cation-exchange on the base used in the titration and capacity is measured. on the nature of the soil. Generally, The second diagram illustrates the buffer curves obtained with sodium hy- kind of neutralization curves obtained droxide lie above those obtained with with several soils. The curves can be calcium hydroxide. That is, more cal- divided into three general areas. For cium than sodium hydroxide is required each soil there is a region of buffering, to raise the pH of an acid soil to a pre- or resistance to change in reaction with determined point. addition of base, in the low pH range. Differences due to the nature of the This reflects the neutralization of acidity cation-exchange material in the soil associated with the permanent charge can best be brought out by examining of the clay or the carboxyl-group charge curves in which pH is plotted against of soil organic matter. This portion of percentage base saturation. This is the buffer curve is followed by an in- illustrated by the third figure, which flection, or endpoint, evidenced by an shows such graphs for soils containing upswing in the curve between pH 6 different kinds of clay and different and 8. The midpoint of this upswing, amounts of organic matter. which commonly occurs around pH Calcium hydroxide was the base used, 76 YEARBOOK OF AGRICULTURE 1957 and neutralization was considered to be complete at pH 8.2. Observe that the curve for the soil containing mont- morillonoids and micas (the Iredell soil) lies below that for the soil con- taining kaolin minerals or organic mat- ter. This means that the former would require a larger calcium saturation to have a selected pH. To reach pH 6, for example, would require 80 percent base saturation for the Iredell soil, but only 40 percent for the Cecil soil. The course of curves would have been some- what different if cation-exchange ca- pacity had been measured at a different pH.

ANOTHER FACTOR INFLUENCES the shape of fhe low pH region of the neu-

tralization curves of acid clays. This is 0 20 40 60 80 100 120 140 the mechanism involved in the neu- Calcium Hyilroxido Addüil, IViilliequivalciits per 100 Grams tralization reaction. Acid clays con- 4. The neutralization of acid montmorillonite as tain both hydrogen ions and alumi- related to the kind oj neutralizing base and the nature num ions as exchangeable cations. The of the acid clay. Curve /, aluminum-saturated clay proportions of the two vary with the plus sodium hydroxide; curve 2, aluminum-saturated clay plus calcium hydroxide; curve j, hydrogen-satu- characteristics of the clay and the rated clay plus calcium hydroxide. previous history of the sample. Gen- erally, the permanent charge on clays is countered by metal cations such as Aluminum ions in water solution hy- calcium, magnesium, and aluminum. drolyze, or react with water, to give Appreciable quantities of hydrogen ions hydrogen ions and aluminum hydrox- do not neutralize permanent charge ide. For this reason, aluminum salt because hydrogen-clays are unstable solutions are acid. The hydrolysis reac- and spontaneously decompose to yield tion occurs between pH 4 and 5. In the aluminum ions (also magnesium, iron, neutralization of aluminum-saturated and others) and silicic acid. The former soils, however, the combination of ion migrate to exchange spots, and the clay exchange and hydrolysis reactions is becomes aluminum saturated. not complete until a higher pH is Apparently exchangeable calcium, reached. magnesium, and sodium largely neu- The fourth graph shows the shape tralize the portion of the pH-dcpendcnt and position of neutralization curves charge of clays that exists in a particu- obtained with samples of the same clay lar situation. Hydrogen counters the (montmorillonite) saturated initially part that is not ionized. with either hydrogen or aluminum ions. When hydrogen-saturated soil is neu- The curve for the hydrogen clay lies tralized with calcium hydroxide, the far below that for the aluminum clay. reaction is: Aluminum saturation of the part of the permanent charge not neutralized Soil/ + Ga(OH)2->Soil-Ga + 2H20. by calcium, magnesium, and other basic When aluminum-saturated soil is metal cations is the rule in acid soils. neutralized : About the only exchangeable hydrogen /Al /Ga in most mineral soils is due to the por- Soil— 4- 3 Ga(OH)2->Soil—Ga + 2Al(OH)3. tion of the pH-dependent charge that \A1 \Ga is neutralized between pH 6 and the THE CHEMISTRY OF SOIL pH pH chosen to make an "exchangeable hydrogen" determination. Because of this, it is more appropriate to use the term "exchange acidity" rather than "exchangeable hydrogen."

; REGION 3, THE high pH portion of the neutralization curve, is observed when the base used is sodium hydrox- ide or when calcium hydroxide is used and the soil system is protected from the carbon dioxide of the air. When an acid soil is titrated with calcium hy- droxide, equilibrium being continu- ously established with air containing a 0 40 80 120 160 200 given amount of carbon dioxide, a Milliequivalent of Calcium Hydroxide Added to 100 Grams of Clay curve similar to that in the fifth graph results. 5. The distribution oj calcium in clay-water-car- bon dioxide systems. The slanting line with a slope Under those conditions, region 3 has of ^5° shows the total amount oj calcium added to a turned into a straight line parallel to clay-water system in equilibrium with the carbon the calcium hydroxide axis. In this dioxide oj the air. For calcium hydroxide additions case, with the carbon dioxide content oj less than jo milliequivalents per 100 grams, all oj the calcium is bound to the clay. For larger addi- of the air being 0.03 percent, the pH tions oj calcium hydroxide, constant amounts oj stays constant at pH 8.3. The straight calcium bicarbonate and calcium clay are in equi- line expresses equilibrium between ex- librium with excess calcium carbonate. changeable calcium, excess calcium carbonate, water, and carbon dioxide. forward determination. That is not the The equilibrium pH, as well as the cal- case. In fact, pH, particularly in soil cium and bicarbonate concentrations systems, is a most uncertain quantity of the soil water, depends on the car- both with regard to measurement and bon dioxide level. The quantity of ex- to interpretation. cess calcium carbonate present is im- We originally defined pH as —log material as far as reaction is concerned. hydrogen-ion concentration, but have The pH of strongly alkaline soils is seen that this is not the case in soil sys- high because of the hydrolysis of ex- tems. In soil-water systems that are not changeable sodium, with the formation too concentrated, pH approximates of sodium carbonate. Free calcium and the hydrogen-ion concentration of the magnesium carbonates may be pres- soU solution. This, in turn, depends on ent, but the calcium carbonate-carbon the cation-exchange capacity of the soil dioxide equilibrium does not control and percentage base saturation, soil- soil reaction. water ratio, and electrolyte content. Just as pH is not an index to total soil The device commonly used for meas- acidity, neither is it a guide to total uring soil pH is a pH meter equipped alkalinity, or the amount of acid re- with two electrodes, which are inserted quired to bring the soil to neutrality. into a soil-water mixture. One of these, In alkaline soils the soluble and in- the glass electrode, reflects the con- soluble metal carbonates are largely centration of hydrogen ions. The essen- responsible for total alkalinity. The tial part of this consists of a thin bulb hydrolysis of exchangeable cations can of glass, which separates the soil sus- account for high pH's but not for large pension or paste from an acid solution total alkalinities. of known concentration. An electrical potential, proportional to the hydro- THE MEASUREMENT of soil pH often gen ion concentration of the solution is regarded as a simple and straight- into which the electrode dips, develops 78 YEARBOOK OF AGRICULTURE 1957 across the glass membrane. This po- The pH of a soil varies considerably tential is registered by the meter, with its water content. For acid and which has a scale that reads directly in neutral soils, pH generally is lower for pH units from o to 14. large soil-water ratios than for small. The second electrode, which is A I : I soil-water paste may have a pH placed in the soil-water mixture, is a one unit or more lower (a tenfold dif- calomel electrode. It is used as a refer- ference in "active acidity") than a i :5 ence point for measuring the potential suspension. This is another manifesta- of the glass electrode. Unfortunately, tion of the suspension effect, and gen- the potential of the calomel electrode erally has been attributed to differ- is not the same in all soil-water sys- ences in hydrogen ion concentration tems, and pH measurements made with in contact with the glass electrode. The this assembly are difficult to interpret. correct explanation appears to be much When soil pH is measured with glass more complicated and must await fur- and calomel electrodes, the value ob- ther work. tained depends on whether the salt With soils that have a high pH and bridge that connects the calomel elec- contain free sodium carbonate and trode to the soil-water system is in con- other soluble salts, pH also increases tact with soil particles or with soil so- with the water-soil ratio. That is be- lution. With acid soils, lower pH's are cause dilution increases the hydrolysis obtained when the salt bridge contacts of sodium-clay, leading to a larger soil particles. This is called the suspen- hydroxyl ion concentration in the soil sion effect. The factors responsible for solution. Hydrolysis of exchangeable the suspension effect are not completely sodium occurs very readily because understood, but its existence makes the hydrogen ions from water have a interpretation of soil pH uncertain. strong affmity for weakly acidic groups There are a number of organic com- on clay and organic matter. pounds called indicators, which change Because of the effects of soil-water color depending on the pH of their ratio on measured values of soil pH, it surroundings. One such substance is is advisable to standardize this. A ratio phenolphthalein, which is colorless be- of one part of soil to one part of water low pH 8.3 and is red above this pH. often is used. In studying the salted The indicators are widely used for soils of dry regions, the pH of a soil measuring the pH of solutions. They saturated with water often is measured. are little used for estimating soil pH Some research workers have sug- because of numerous practical diffi- gested measuring the pH of soil sam- culties. Foremost among these is the ples at field moisture contents. That is difficulty of. seeing indicator color not desirable, since the high external changes in the presence of soils. Indi- resistance that is encountered under cators have the more fundamental dis- these conditions leads to errors. advantage that they are either posi- The concentration and kind of sol- tively or negatively adsorbed by soil uble salts in the water phase pro- particles, and that they may change foundly affect soil pH. With acid soils color at different pH's in contact with this is largely an ion exchange phe- soil than in true solution. nomenon, with cations of the salt re- placing exchangeable hydrogen and THE GENERAL PH-BASE saturation re- aluminum ions from the soil particles. lationships for different kinds of soils The acidity of the solution increases already have been indicated. In addi- because of increased hydrogen ion con- tion, there arc a number of factors that centration or the partial hydrolysis of can be varied. They have large effects aluminum ions. As salt concentration on measured soil pH. These include is increased, soil pH falls rapidly at the soil-water ratio, the electrolyte first and then becomes insensitive to content, and the carbon dioxide level. further changes. Seasonal fluctuations THE CHEMISTRY OF SOIL pH .79

Yield of Soybeans, Grams per Pot Since the value of pH—^ pCa does not 20 depend at all on the soil-water ratio or the salt content, it is a more definite index to soil reaction than is pH. Another advantage is that both pH and pCa are determined on a clear soil FfFra solution, and difficulties due to the suspension effect are avoided. Considerable attention should be paid to the effect of the addition of soluble calcium salts to acid soils. When gypsum, or calcium sulfate, is added to acid soils, soil pH falls and the aluminum and manganese concen- 20 30 40 50 60 70 80 Percentage Calcium Saturation of Soil tration of the soil solution increase, sometimes to levels toxic to plants. 6. The yield of soybeans as related to the percent- The effect of salt concentration on age oj calcium saturation of two North Carolina soils. Curve /, organic soil; curve 2, White Store the pH of neutral soils is due largely soil containing montmorillonoid clay. to increases in the apparent strength of acidic groups. With alkaline soils, in- in salt content, due to fertilization or creasing salt concentration lowers pH the mineralization of organic matter, by reducing the hydrolysis of the ex- can change soil pH by 0.5 unit. changeable cations, largely sodium. Since soil reaction varies with salt The pH of soils containing free car- content, pH often is measured in salt bonates of calcium and magnesium is solutions of definite concentration. One greatly influenced by the bicarbonate normal potassium chloride and 0.01 concentration in the soil solution. The molar calcium chloride have been bicarbonate concentration is propor- used. Such procedures iron out experi- tional to the carbon dioxide content of mental fluctuations to some extent, the air in contact with the soil. The and for that reason may be desirable. carbon dioxide content of the atmos- Measurement of soil pH in dilute phere is about 0.03 percent. A soil con- calcium chloride solutions appears to taining free calcium carbonate has a ofíer several advantages. Calcium is pH of 8.3 when in equilibrium with the most abundant basic metal cation this concentration of carbon dioxide. in most soils, and addition of calcium The carbon dioxide content of the soil chloride solutions does not usually air may be much larger than that of change the proportions of the ex- the atmosphere. Plant roots and micro- changeable cations very much. organisms usually liberate carbon di- Furthermore, there is a general re- oxide faster than it can escape from lationship between the concentrations the soil to the atmosphere. Soil air may of any two ions in the water phase of a contain as much as 10 percent carbon soil-water mixture which permits a de- dioxide, although contents of about 2 scription of soil reaction in a way that percent are more common. This car- is independent of the soil-water ratio bon dioxide content would result in a and of salt content. This relation, pH of about 7.1. which has been called the "ratio law," When soil samples are taken from is that the concentration of hydrogen the field and prepared for pH measure- ions in a soil solution divided by the ment, they usually are allowed to equi- square root of the concentration of cal- librate with the atmosphere. Because cium ions is a constant that is charac- of the dependence of pH on carbon di- teristic of the soil. The ratio law may oxide level, the pH measured in the be written as: [H+]V[Ca++]= con- laboratory may be different from that stant, or as pH—^ pGa=constant. in the field.