chemistry as applied to soil management and crop production

CHEMISTRY AS APPLIED TO SOIL MANAGEMENTAND CROP PRODUCTION*

ELDROW REEVECampbell Soup Company, West Chicago, Illinois

Research work as applied to the problems of soil management andcrop production generally follows through four steps, as follows:1�research in the basic fundamental exact sciences including chemis-try, physics, geology, and botany; 2�research in the applied sciencesincluding agronomy and horticulture. (Experimental field test-plottechniques and statistical methods of analysis are essential tools foreffective research work in the applied sciences); 3�field-plot researchand demonstrational fields; 4�the culmination is reached when thenew findings are applied by farmers in a competitive agriculture.The study of soil has become so specialized that today there are

separate branches of specialization, namely; soil physics, soil chemis-try, soil microbiology, and pedology. Also, special attention is nowbeing given to the study of soil colloids. The equipment and suppliesused in a soil research laboratory are similar to those used in quantita-tive, colloidal, and physical chemistry.

THE SOIL Is A CHANGING SYSTEMSoil may be described as a dynamic natural body on the surface of

the earth in which plants grow. It is differentiated into horizons,which differ from the parent material below in morphological, physi-cal, chemical, and biological properties.The soil is a dynamic physio-physico-chemical system from which

plant roots absorb water and nutrients which, in the presence of lightand chlorophyll of green leaves, are combined with carbon dioxide ofthe air to produce sugars, starches, proteins, cellulose, hemi-cellulose,and lignins. These elaborated organic materials are the source of allour energy, and ultimately return to the soil to be decomposed by themicro-organisms that live therein. In the process of decompositionthe organically bound nutrients are released for use again by theplants. In addition, many organic acids are produced which assist inthe weathering and release of additional plant nutrients from theprimary soil minerals.There are present in the soil certain bacteria that have the capacity

to fix atmospheric nitrogen, which under optimum conditions in thelaboratory, have increased the soil nitrogen content at the rate ofapproximately thirty (30) pounds per acre per week. There are still

* Read before the Chemistry Section at the C.A.S.M.T. at its "Golden Anniversary" Convention in Chicago,November 24, 1950.

199

200 SCHOOL SCIENCE AND MATHEMATICS

other bacteria in the soil, commonly known as denitrifying bacteria,that have the power to reduce nitrates to molecular nitrogen. Themany macro- and micro-organisms that inhabit the soil are vital tothe nutrient cycle and in maintenance of soil structure. The soil istruly a changing system in which oxidations, reductions, and otherphysico-chemical reactions are proceeding continually and simul-taneously.

SOIL COLLOIDS ARE IMPORTANTFor convenience of study and classification, soil particles are ar-

ranged into groups called separates or fractions. These fractions areseparated on the basis of particle size. The size classification adoptedby the International Society of Soil Scientists is given in Table 1.

TABLE 1. SOIL SEPARATE CLASSIFICATION

Soil Separate Diameter LimitsMillimeters

Coarse sandFine sandSiltClay

2.0-0.20.2-0.020.02-0.002less than 0.002

The real significance of the different particle sizes results from theincrease in internal surface area as the particle size decreases. Thisrelationship is shown in Table 2. It will be noted that when a particleis reduced to one tenth the previous size, the number of particles isincreased one thousand times and the internal surface area ten times.

TABLE 2. NUMBERS AND INTERNAL SURFACE AREA OF PARTICLES IN ANACRE (2,000,000 POUNDS) OF SOIL

ParticleMillime

10.10.010.0010.00010.00001

Sizeiters

Number ofParticles

65376 X10765376 X101065376 X101365376 X101665376X101965376 X1022

Internal Surface AreaSquare Acres

507507X10507 X101507 X102507 X103507 X104

The clay fraction of soil is composed almost entirely of weatheredprimary rock minerals. Most of the clay particles fall within the col-loidal range (1-500 millicrons), and exhibit characteristic colloidalproperties, such as:

1. High internal surface area per unit volume in comparison withparticles of larger size.

SOIL AND CROP MANAGEMENT 201

2. High water retention within the structure of the particle, in con-trast to sand which retains water only on the surface.

3. High expansion and contraction coefficients, as manifested bycracking of clay soil upon drying.

4. Highly sensitive to electrolytes. Colloidal clay micelles carry anegative charge when in water.

5. Low osmotic pressure.

The colloidal clay micelles exist largely as films around the largersoil particles. In the presence of calcium ions the individual colloidalmicelles are flocculated, resulting in the formation of larger aggre-gates or crumbs. In practice, the formation of these aggregates is im-portant, as they favor high moisture holding capacity and good aera-tion of the soil, both of which are extremely important if high cropyields are to be obtained. On the other hand, colloidal micelles, in thepresence of monovalent cations such as sodium, are dispersed into theindividual particles, and the soil is said to have a single grain struc-ture. Under such a condition the soil is in poor physical condition forplant growth. If hydrogen ions predominate on the soil colloid, it issaid to be acid, and the soil aggregates are less stable than those de-veloped in a calcium soil. Thus, a factual basis is provided for the useof liming materials on acid soils. Aside from its nutritional value, limeimproves the physical structure of such soils, and in so doing favorsbetter plant growth.

SOIL COLLOIDS�THE SEAT OF BASE EXCHANGEAs mentioned earlier, colloidal clay micelles are negatively charged,

and the cations that are associated with them are analogous to theouter layer of an electrical double layer. The cations on the surfacemay be exchanged for other ions in the system by the process com-monly know as base exchange. This interchange of cations takes placerapidly and is reversible. The cations found associated with the claymicelles in nature are calcium, magnesium, potassium, sodium, andhydrogen. Calcium ions predominate in neutral to slightly alkalinesoils, while hydrogen is the primary replaceable ion in acid soils, andsodium in alkali soil.The principle of base exchange was applied in the reclamation of

the low lands of Holland, that were flooded with sea water duringWorld War II. When flooded with sea water the exchange complexbecame saturated with sodium ions, which destroyed the physicalstructure and thereby rendering the soil non-productive. By leachingthe sodium-soils with gypsum (a neutral calcium salt), the sodiumwas replaced with calcium ions, favorable soil structure developed,and the productive capacity was restored.

202SCHOOL SCIENCE AND MATHEMATICS

With the development of our knowledge of cation exchange, hascome a revision in our old concept that the soil is an inert mediumcontaining a solution from which plant roots absorb all their mineralnutrients. Plant roots absorb not only ions from the soil solution, butalso liberate cations from the clay micelles which in turn are absorbedby the plant. This process is commonly spoken of as contact baseexchange. It has been fairly well demonstrated that the cations whichenter the plant roots come largely from the outer layer of the claymicelles, figure 1. A plant root growing in contact with the colloidal

HAIR

FIG. 1. An over-simplified sketch of a plant root in contact with a colloidal clayparticle (micelle). The strength of the bond holding the respective ions on themicelle is represented by the lines of varying widths�the wider the line, thestronger the bond.

clay particles releases carbon dioxide (a product of respiration) to thesoil solution. In the presence of water carbonic acid (H^COa) isformed, which in turn ionizes to form H4- and HCOs" ions. The hydro-gen ion is thus free to replace calcium, magnesium, potassium, etc.on the micelle.

It has been demonstrated that for optimum growth of crops suchas alfalfa and clover, approximately 65 per cent of the total exchangecapacity should be supplied by calcium, 10 per cent by magnesium5 per cent by potassium, and 20 per cent by hydrogen. If the compo-sition of the exchange complex changes appreciably, the crop willgrow poorly, until such time as the nutrients are brought back intobalance by the addition of lime or potassium fertilizer or both. The


composition and quantity of materials required to bring about theproper balance are determined by quantitative chemical tests of thesoil in question.

THE ESSENTIAL FIFTEENThere are fifteen elements known today to be essential for plant

growth, they are: carbon, hydrogen, oxygen, phosphorus, potassium,nitrogen, sulfur, calcium, iron, magnesium, boron, manganese, copper,zinc, and molybdenum. Until about 1920 it was considered that ten,and only ten, elements were essential for plant growth. The originalten elements can be easily remembered by a mnemonic suggested bythe late Professor Cyril Hopkins of the University of Illinois, whotold his students to think of "C H 0 P K N S Ca Fe Mg," and to readit as "C. Hopk’ns’ Cafe mighty good." Since, then, boron and man-ganese have been added to the list of essential elements. Accordingly,someone suggested the mnemonic be lengthened to read, "C. HopkWCafe mighty good, but mnemonic." Now copper, zinc and molybde-num have been added to the list of essential elements, but as yet noone has suggested a way to include them in Professor Hopkins’mnemonic. As more highly purified salts become available, still otherelements will probably be found to be essential for normal plantgrowth.Carbon and oxygen are absorbed as carbon dioxide from the at-

mosphere by the leaves. The other elements are obtained from thesoil by the roots. On the basis of quantity required, the mineralnutrients are customarily spoken of as major, secondary, or trace ele-ments as shown in Table 3.

TABLE 3

Major Nutrients ^^T^enS^ Trace Elements

NitrogenPhosphorusPotassium

CalciumMagnesiumSulfur

IronManganeseCopperZincBoronMolybdenum

Of the essential plant nutrients obtained from the soil, nitrogen,phosphorus, and potassium were for many years considered the onlyones likely to become deficient in the soil. In recent years, however,soils in certain areas have become deficient in one or more of theother elements listed in Table 3. An enormous commercial fertilizerindustry has developed, as a result of the need for supplemental sup-plies of the essential mineral elements in the soil.


MIXED FERTILIZER TERMINOLOGYThe principal plant nutrients in a complete fertilizer mixture are

nitrogen (N), phosphoric acid (P206), and potash (K^O). A commonfertilizer formula like 4-12-8, contains 4 per cent total nitrogen, 12per cent available P206, and 8 per cent water-soluble K^O, and is saidto carry 24 plant nutrient units. The balance of the mixture (76 percent) may consist of inert material or conditioners to prevent themixture from caking and to maintain good drilling qualities. Such afertilizer would have an N�P206�K20 ratio of 1-3-2. If a 4-12-8fertilizer is suitable for a particular crop on a certain soil, then anyother fertilizer mixture with the same 1-3-2 ratio should be equally asgood, provided adjustment is made for difference in analysis. For ex-ample, 500 pounds of 8-24-16 would be equivalent in N�P20&�KiO,to 1000 pounds of 4-12-8.A fertilizer that carries only two of the three fertilizer elements is

sometimes referred to as an incomplete fertilizer, and if only one ofthe elements is contained it is known as a single fertilizer. For exam-ple, an 0-9-18 would be an incomplete fertilizer, and an 0-20-0 wouldbe a single fertilizer.

SOIL ACIDITY MUST BE CORRECTED FIRST

The beneficial effects of lime on soil was discovered accidentally inFrance centuries ago. However, the function of lime and its influenceon the availability of the various plant nutrient elements has becomemore fully understood only as a result of developments in modernchemistry.The ills of acid soils are due either to a deficiency in available forms

of one or more of the essential nutrient elements required by plants,or to the presence of toxic concentrations of certain non-essential ele-ments, particularly aluminum. Consequently, the influence of limeon plant growth may be either direct or indirect. It is a well knownfact that plants differ in their susceptibility to soil acidity. Certainplants are rather tolerant to high soil acidity, while others are ex-

tremely sensitive. Such a situation is explained only on the basis ofthe inherent differences in the physiology of plants. It has been pos-sible over the years to classify many of these crop plants on the basisof their susceptibility to acid soil conditions. Such a classification ispresented in Table 4.

Soil acidity is determined in modern soil laboratories by means of a

potentiometer, using glass electrodes, and is expressed in terms of pHunits (the common logarithm of the reciprocal of the hydrogen ionconcentration expressed in terms of normality). The pH range com-

monly found in agronomic soils is from about pH 4.5 to 8.5. Any pH


below 5.0 is considered to be strongly acid, between 5.0 and 6.0 asmedium, 6.5 is considered optimum for most crops, and any pHabove 7.0 indicates alkalinity.pH exhibits a differential influence upon the availability of the dif-

ferent plant nutrient elements. The optimum range for nitrogenavailability is from pH 6.0 to 8.0, for phosphorus 6.2 to 7.5, and forpotassium anything above 6.0. Calcium and magnesium are at anoptimum above 7.0, and boron, copper, and zinc below 7.0. Iron and

TABLE 4. CROP ADAPTABILITY TO SOIL ACIDITY

Strong AciditypH below 5.5

BlueberryCranberryPeanutsWatermelonAzaleaRaspberryHollyPotatoSweet PotatoLaurel

Medium Acidity

BarleyBeansBluegrassBuckwheatCauliflowerCabbageCarrotsColiardsCornCowpeasCucumbersEgg PlantKaleLespedeza

pH 5.5-6.5

Millet*

MustardOatsParsleyParsnipsPumpkinsRadishesSalsifySquashStrawberryTomatoTurnipVetchWheat

Slightly Acid toSlightly AlkalinepH 6.5 and above

AsparagusBeetsCeleryLeeksLettuceMuskmelonAlfalfaSweet CloverRed CloverOnionsSpinachPeasChard

manganese are more readilyavailable at pH levels below 6.5.11 is inter-esting to note, however, that pH 6.5 is quite satisfactory for all of theelements. This naturally suggests that for optimum crop productionin humid regions, the soil should be limed to bring the pH up to 6.5.Unless these soils are limed, maximum benefit can not be obtainedfrom any fertilizer that may be applied.

EXCHANGEABLE HYDROGEN PROVIDES A GOOD MEASUREOF THE LIME REQUIREMENT OF A SOIL

The quantity of lime required to raise the pH from 5.0 to 6.5 is notthe same for all soils. For example, 550 pounds of limestone are re-quired per acre to raise the pH of a Sassafras sand from pH 4.5 to 6.5while 7400 pounds are required to effect the same change in case ofPenn silt loam. Such a situation is due to the fact that a Sassafrassand has a very low base exchange capacity as compared to a highexchange capacity in a Penn silt loam soil. Although pH gives anindication as to the percentage saturation of the soil with bases,Table 5, it does not give a measure of exchange capacity. However,


the quantity of exchangeable hydrogen in the soil complex providesa good measure of the amount of lime required to properly adjustthe pH. Procedures have been developed whereby both pH and ex-changeable hydrogen can be measured accurately with a glass elec-trode pH meter.

Consider, for example, a sandy soil with an exchange capacity of 6milliequivalents per 100 grams of soil, and a loam soil with an ex-change capacity of 12 milliequivalents per 100 grams of soil, bothsoils at pH 5.0. For soils that follow the relationship shown in Table5, pH 5.0 indicates approximately 50 per cent saturation with bases.An exchangeable hydrogen determination would show 3 milliequiva-

TABLE 5. RELATIONSHIP BETWEEN BASE SATURATION AND pH

pH % Base Saturation

4.05.06.06.57.0

0-2040-6060-8080-90100

lents per 100 grams in the sandy soil, and 6 milliequivalents per 100.grams in the loam soil. One thousand pounds of limestone per acre ofsoil will replace one milliequivalent of hydrogen per 100 grams of soil.Accordingly, the lime requirement of the sandy soil would be 3000pounds of limestone per acre, and 6000 pounds for the loam soil.

CHEMISTS AND CROP PRODUCTION

Some fifty years ago soil chemists were making complete chemicalanalyses of soils in an attempt to find answers to soil fertility prob-lems. They reasoned that by comparing the percentages of the vari-ous plant nutrient elements in an unproductive soil with those in anormal productive soil it would be a fairly simple problem to ascer-tain the limiting factors in the unproductive soil. Unfortunately theproblem was not that simple, as crops did not always respond to ap-plications of nutrients as indicated by the complete chemical analysis.The chemists then decided to study the composition of the plants

themselves. The thought being that the elements limiting plantgrowth would be found in smaller amounts than in normal plants.Again, their efforts were not too fruitful, as they assumed that plantsabsorb elements from the soil in the exact proportion to which theyare required for normal growth. Such is not the case. If the soil per-mits, plants may absorb a much larger amount of a given elementthan is actually needed. Furthermore, a deficiency or excess of one


element may substantially alter the quantity of another elementabsorbed. Then too, plants do not always show a low percentage of acertain nutrient in limited supply, but instead make less growth,thereby keeping the chemical composition of the tissues approxi-mately constant.

Perhaps the oldest and most reliable method for determining theproductive capacity of soils and their need for supplemental plantnutrients are the field-plot experiments. However, such experimentsare slow and costly, and as a result do not have widespread applica-tion. It became imperative that agriculturists have some relativelyrapid method for determining the productive capacity and nutrientdeficiencies on individual farms. Within the past 15 years, such anapproach has come through the development of the so-called rapidchemical soil and tissue tests. These tests are not designed to measurethe total quantities of nutrients in the soils or plants, but rather thosequantities that are available for plant growth. Although there arecertain inherent shortcomings in these rapid tests, when properlyapplied and interpreted they provide a fairly reliable method for de-termining the nutrient requirements of soils and crops.

STATUS or THE MAJOR PLANT NUTRIENTS�NITROGEN, PHOSPHORUS, AND POTASSIUM

Nitrogen, is an integral component of protoplasm and is, therefore,essential for the normal growth and metabolism of plants. It is some-times spoken of as a regulatory element, since by controlling thesupply available, plant growth can be fairly well controlled. Highlevels of nitrogen in the soil favor rapid foliage growth at the expenseof root growth, retards flowering and maturation of plants, and ingeneral results in a soft succulent growth. As the nitrogen supplydeclines, plants become stunted, the leaves turn light green andgradually yellow.

Nitrogen exists in the soil largely as a constituent of organic mat-ter. This organic matter is decomposed by microorganisms, and thenitrogen converted to nitrates (NOs), the form most readily utilizedby green plants. Since nitrate production is dependent-upon the de-composition of the organic matter by microorganisms, whose activityis greatly influenced by aeration, moisture, and temperature, thenitrogen supply may be extremely variable during the course of asingle growing season. Therefore, when attempting to determinewhat supplemental nitrogen should be added to produce a good crop,one must consider the organic matter content of the soil, the possi-bilities for nitrate production, and finally the particular crop to begrown. There is considerable variation in the nitrogen requirementof different crops.


Phosphorus, enters into the formation of phospholipids and nucleicacids. A deficiency of phosphorus may interrupt the synthesis of thesecompounds and thereby retard cell division, in the meristematictissues. Phosphorus favors root growth, and the development offruits and seeds in the mature plant.

In the parent soil material phosphorus exists as a component of themineral apatite. As the parent material weathers, the phosphorus isfreed from combination with apatite, and combines with other ele-ments to form secondary phosphorus bearing minerals. Since thequantity of apatite is low in most soil forming rocks, it naturallyfollows that most soils are low in phosphorus. As phosphorus is re-leased in the soil, either from the primary and secondary minerals,organic matter, or from supplemental fertilizer application of phos-phorus, a portion of it is bound by the clay particles. As a result ofthis immobility very little phosphorus is lost from the soil by leach-ing. If the pH of the soil is below 6.0, insoluble iron and aluminumphosphates are formed, which are very slightly, if at all, availablefor plant growth. At pH 6.5 to 7.0 the phosphorus occurs in the soilas water-soluble calcium and magnesium phosphates, which arereadily available for plant utilization. If the pH exceeds 7.5 then tri-calcium phosphates are formed, which are not readily available forplant growth. It is thus apparent that the soil pH is a critical factorin regards phosphorus availability.With the present soil testing techniques it is a relatively simple

matter to test the soil for available phosphorus, and to make theapplications of supplemental phosphate fertilizers on the basis ofthese tests, so that phosphorus need not become a limiting factor incrop production. Of all the plant nutrients obtained from the soil,phosphorus is the most universally deficient.

Potassium, unlike the other ash constituents required in appreci-able amounts for plant growth, apparently is not built into organiccompounds in the plant. It is required for the formation and trans-location of carbohydrates, and appears to function in respiration,but its exact role is still obscure. In general, necrotic breakdownoccurs in the lower leaves on plants growing on a soil deficient inpotassium.

In the soil, potassium occurs largely in mineral form, and uponweathering the minerals are reduced to clay with a release of potas-sium. As weathering proceeds, some potassium is always in solutionand subject to loss from the soil by leaching, as a matter of fact, thereis probably as much potassium lost by leaching as is removed bycrops. The potassium absorbed by plants comes largely from thecolloidal clay particles in contact with the roots. Potassium deficiency


is more acute on sandy soils because they contain relatively smallamounts of the potassium-bearing minerals.As with phosphorus, the supply of readily available potassium in

the soil can be determined by chemical tests, and if found to be de-ficient, the supply can be supplemented by the proper use of fertilizer.

STATUS OF THE SECONDARY PLANT NUTRIENTS�CALCIUM, MAGNESIUM, AND SULFUR

If acid soils are properly limed and fertilized, as indicated by soiltests, calcium, magnesium, or sulfur seldom become limiting factorsin crop production. If magnesium is low, it can be supplemented bythe use of a high magnesium content limestone.

STATUS OF THE TRACE ELEMENTS

Boron, deficiencies have been found in a number of crops in theCoastal Plain and Piedmont soils of the Southeast, in New England,the Middle West, and the Pacific Coast. Plants have a narrow rangeof tolerance for boron, the amount required is small, and often timesslightly larger amounts are toxic. Borax is the common carrier of bor-on, and 20 to 30 pounds per acre represents a fairly heavy applica-tion, enough to cause injury to some crops on certain soils. Applica-tions as low as 5 to 10 pounds per acre have corrected boron defi-ciencies on apples and certain other crops. The exact role of boron inplant metabolism is not clearly understood. There seems to be a re-lationship between boron, calcium, potassium, phosphate, and ni-trate availability and utilization.

Copper, is seldom found to be deficient on the mineral soils, but isfrequently found to limit yields on muck soils, especially in onionproduction. Considerable quantities of copper are now being appliedin the form of fungicidal sprays for control of plant diseases on manycrops, so that it is unlikely that copper will become a serious limitingfactor on the mineral soils in the near future.

Iron, is one of the most important of the minor elements, althoughit is seldom necessary to supplement the supply in the soil. However,iron is applied as an iron sulfate spray to pineapples in Hawaii, andto several crops on the high lime soils in the arid West. Iron occurs asan impurity in most fertilizers.

Manganese, as manganese sulfate, is used quite extensively oncitrus and vegetable crops in Florida and the Eastern Seaboard States.

Zinc, as zinc sulfate, has been used in recent years to correct zincdeficiency diseases in corn, pecans, grapes, peaches, and other stonefruits. Its use has been largely confined to the Southern and PacificCoast States.


RAPID SOIL TESTS APPLIEDWith the aid of a glass electrode pH meter, the amount of lime

required to properly adjust the pH of an acid soil can be readily de-termined. This is a comparatively simple and rapid procedure, sothat widespread coverage of individual farms is possible.

It has been determined by correlation studies between tomatoyields grown in experimental test plots and a given rapid soil testfor phosphorus and potassium that a soil, in order to produce maxi-mum tomato yields, must contain 180 pounds available phosphorusand 300 pounds readily replaceable potassium per acre. Any soil thatgives a lower test for either of these elements, is not capable of pro-ducing a maximum yield unless the proper fertilizer is added tocorrect the deficiency.As an example of what can be accomplished by a judicious applica-

tion of rapid soil tests, a case of a tomato grower may be cited. Thefarmer’s soil was analyzed in the laboratory and found to contain110 pounds available phosphorus and 243 pounds replaceable po-tassium per acre. This test indicated that in order to meet the nutrientrequirements for a tomato crop, a fertilizer application of 900 pounds3-12-12 per acre would be required. Carefully replicated and con-trolled experiments on this particular soil gave the following results:

Fertilizer TreatmentCheck�no treatment900^ 3-12-12 per acre

Tomato Yield(Tons Per Acre)

11.6420.17

Certainly such tests should be an important consideration in theplanning of a sound agricultural program.

SELECTED REFERENCES FOR FURTHER READINGHester, J. B., and Shelton, F. A. Know Your Plant and Soil Requirements, Re-

search Monograph No. 3, Department of Agricultural Research, CampbellSoup Company, Riverton, New Jersey. January 1, 1949.

U. S. Department of Agriculture, Science in Farming, Yearbook of Agriculture,1943-1947, pp. 485-601.

The true test of civilization is not the census, nor the size of cities, not thecrops�no, but the kind of man the country turns out.�RALPH WALDO EMER-SON in his essay on "Civilization^

Voice amplifier, a portable 12-pound public address system easily carried allday with a shoulder strap, is designed for a guide or instructor escorting a partyon indoor or outdoor trips. Complete with battery, the amplifier is attached bycable to the microphone.

chemistry as applied to soil management and crop production

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