Soil Fertility and Plant Nutrition

In our research in the Department of Soils of the Missouri Experiment Station, we have labored to get soil fertility and plant nutrition linked together in a language which we all understand. Those of us who are dealing with research projects are thinking about problems on which we ought to spend moneys in the future. We are trying to make sure that we give just rewards for the money invested in the materials and for the time in the work. In opening this broader subject of soil fertility and plant nutrition, we may well be reminded that it is much broader than what is commonly included under that title. It will eventually encompass more than any of the rest of us might now include.

Much that is said about our natural scientific progress–in which we are about to believe that we have reached the pinnacle–deserves some rather critical examination. It seems that we are allowing ourselves to be so easily deceived by our success. There is a terrific danger in overconfidence. Even though we can fertilize the soil, we are not yet controlling Nature at that point of her activities. The burden of the thought is in these few words, namely, we do not yet create crops, Nature creates them.

We are delighted in the technical progress which we have built up and which raises our standard of living when you consider technologies. Technologies apply only, however, when we consider our control of dead matter. I am not quite ready to put agriculture in the class of its science under complete control, like a technology is, for example, when you look at the assembly line of an airplane or an automobile. It is more nearly correct to talk about agriculture as an art plus some science. It is not yet a case in which agriculture is under complete management. Agriculture is still an art, which we study by deduction, that is, we look at it as a natural behavior. We take a fragment out of it and put a little science into that portion. But before we can take complete control, we must have the science so well organized that we can put all the parts together and run the whole process from creation to death. Nobody as yet has been able to do that with agriculture.

Agriculture is biology first and foremost. It is technology and management second. We need only to remind ourselves of the last two seasons to be reminded how readily we use the weather as a scapegoat, when the crops didn’t behave as we thought we would like to control them. It is fitting, therefore, to provoke your thinking about the sciences applied to agriculture in a technological viewpoint only as a possible or even serious danger. Some of the agricultural troubles for which we are apologizing came about because we used technologies to upset the biology of agriculture. Much that is apt to be called agricultural science has upset the biology and we are coming now to reap the bad harvest. We are beginning to realize that the matter of agricultural production has been largely Nature’s performance. Very often we have not had very much to do with it. We have been copying and memorizing agricultural practices, but have not been comprehending the basic principles that operate under Nature.

Some Biological Processes We Have Upset

As illustrations, we may well list several cases in which we have upset the biology rather than helped it. We have been taught to believe that crop rotations build up the soil in fertility. Yet, Nature uses continuous cropping and doesn’t rotate the crops when she builds up the soils in fertility. We have upset the biology in that case very decidedly. We have had to use 65 years of trials on Sanborn Field to discover what Nature’s truth about crop rotations in relation to soil fertility really is.

We are now trying to put a grass agriculture over much of the country where Nature had tree culture. Then we are going to let the cow make up the difference between our ignorance of quality grass and the cow’s knowledge of it. We put the plow ahead of the cow. The cow is about to be extinguished if, as a matter of legal procedure, we follow the philosophy of killing cows to get rid of diseases like brucellosis, hoof and mouth disease, and others. How we ever got such a belief to prevail is strange, namely, that we can have cows when we keep killing them because we make them sick.

Primitive man lived on the dry lands. More recently in human history we began to farm those lands which have high rain falls. Then when rainfall goes back on us we run to the Federal Treasury, as though that were the place where one gets any biological help.

As another upset of biology, instead of leaving plant residues on the top of the soil, as Nature does, we bury them as deeply as we can.

Nature used different soils to make grasses on the prairies than She used in making trees in the forest. Yet there are some folks who believe that the prairie grasses make prairie soils and the forest trees make forest soils. We interchange causes and effects.

Nature uses plant roots to make soils acid in order to get the soil’s fertility into the plants. We want to put a carbonate in the soil by liming so that the root cannot put its acid out any farther than barely off of itself. Yet plants nourish themselves by making soils acid. We fight the soil acidity by means of the carbonate instead of feeding the plant with the calcium and the magnesium in the limestone.

Nature washes the soluble fertility out of the soil into the sea. We make the fertilizers soluble before we put them on the soil. If they remained there in that form they would soon be in the sea too. Or, we mine the sea salts as soluble fertilizers, like potash salts, and put them through another round of going out of the soil and into the sea.

We put cattle and grass into the Plains areas of ever-threatening drought, and succeed because that is where Nature had successfully put a similar beast in the bison. But we put them on the grass amongst the forest tree soils under high rainfall and then wonder why their reproduction is failing.

Nature doesn’t have animals live to get fat. Experiments point out that our animals are searching for anything else but fattening feeds. They are searching for those which help them protect themselves against diseases and encourage them in their reproduction. But when we feed animals we cut the amount of protein in their ration down to the limit, because we want to make cheap gains rather than to give the animals the help they need to be healthy and to multiply. Instead of letting the animals live long, we cut their life spans down to the most early maturity we possibly can and then call it “cheap gains,” and “baby beef” and “ton-litters.”

Nature placed the animal’s matings in the spring of the year. We try to have them mating all the year around.

We use artificial insemination to manage animal reproduction. In that we limit the supply of sperm per animal. We omit thereby much of the hormone treatment which the bull uses when he deposits the sperm, along with much else absorbed by the cow. We have given little thought to the fact that in the case of the so-called “hard-breeder” cows, the bull doesn’t give up. He succeeds eventually where the high costs of the repeated, failing artificial inseminations would condemn the cow to slaughter. Here Nature’s biology succeeds and the species multiplies, but our technologies fail and the species become extinct. We upset the biology, but cling to our technology and economics.

Some significant economic aspects might well be considered critically when the manipulated economics have manipulated machinery, money and technologies into agriculture, but have almost manipulated biology out. Bankers are about to believe that we can substitute capital for land. Certainly all the capital in all the banks cannot substitute for the soil of the land. We know of no bank with all its money that could by means of that wealth have a litter of pigs, lay an egg, or give birth to a calf. And yet we have folks believing that one can manipulate biology by means of controlled markets and economics. You can’t do that any more with money alone than you can with machinery and technology.

We have not yet understood, nor appreciated, agriculture as a collection of complex, but well-integrated, biological processes. We have comprehended some fragments of this natural art which science has studied deductively, but we have not yet constructed the whole of a scientific agriculture inductively. We have not seen the soil as plant nutrition and thereby as animal and human nutrition or the soil as the very foundation of all agriculture.

Confusion Will Prevail Until the Soil is Considered

Because we turned away from much of the art of agriculture in the absence of a complete science of it, we have a serious confusion. That is all the more serious now and at the moment we discover that we have rapidly mounting numbers of people and are soon running out of ample food for them. We are confused about the natural performances or about the biology in agriculture. We have permitted ourselves to be led astray and are asking the science of agriculture now to bring us back where we can understand the basic principles rather than merely mimic any practice. We dare not be mere followers of traditions. We must face the problems and solve them. All of that calls for rather clear diagnoses. Let us try and comprehend the fact, then, that soil fertility properly coupled with plant nutrition is a form of creation, a form of outdoor biology, and not a matter merely of scientific technology. In that combination, wisely used, there may be some solution for our food problem.

Now what are some of these confusions about the basic facts of soil fertility and plant nutrition? First of all, we seem to have lost sight of the fact that the creative business of agriculture has always started in the soil. That great truth was told us about six, or more thousand years ago, but we didn’t take that remark very seriously. We are beginning to appreciate it now. We shall face it more seriously when we have the least of creative capacity left in the soil, and when we need to know most about it.

In terms of wise fertilizer use the most shocking confusion prevails when we talk about soluble fertilizers, considering water as the agency for solution, and then we make laws requiring that fertilizer must be water-soluble and thereby so-called “available.” In fact, and in Nature, these soluble fertilizers are never taken out of the soil because the plant takes them into itself along with water it takes from the soil. The use of the major amount of water by the plant is that of keeping the respiring leaf tissues moist for the exchange of the gases, namely, carbon dioxide and oxygen, between the plant and the atmosphere. That escape of water from the leaf is what we call ”transpiration.” It is in that service where most of the absorbed water goes from the soil into the atmosphere. That use of soil water is controlled by the meteorological situation inviting water to evaporate from the leaves of the plants against the forces holding the water in the soil. The plant is an innocent connection between those two opposing forces acting on the water. Does the moisture in your breath move nutrients from your bloodstream into the tissues, or from your stomach into your bloodstream? But yet we take to the concept that the transpiration of the plant has something to do with the movement of nutrients from the clay of the soil into the roots. The transpiration stream of water from the soil, through the plant, and into the atmosphere is independent of the nutrient stream from the soil into the roots. That may not be true for nutrients moving within the plant’s conducting tissue. The water uptake by the roots is the result of atmospheric conditions favoring evaporation from the leaves with a set of dynamics which are more than a match against the forces holding the water on the surfaces within the soil.

Nutrient intake by crops is a function of three colloids, or possibly four, all in contact. First of all, there are the nutrients on the clay colloid, or on the organic colloid of the soil. The soil colloid is in contact with the root membrane which is another colloid. That root membrane is in contact with the contents of a cell on the inside, namely the protoplasm, or the cytoplasm, also a colloid. Then, in turn, that cell is in contact with another cell. In that you have the combination of the three or four colloids in contact. The movement of the nutrient ions from the clay into the root membrane and into the cells follows the chemical laws controlling their traverse there because of the differences in activities, adsorption capacities, interfering ions, and other factors along that line.

That movement of nutrients into the root is independent of the transpiration of water. We have demonstrated transpiration going forward regularly, or water moving from the soil through the plant to the atmosphere, when the nutrient ions were moving in the reverse direction, namely, going from the plant back to the soil. We have demonstrated the ions going into the plants regularly when there was no transpiration. You can demonstrate this when you put a bell jar with atmosphere saturated with CO2 and with water over that plant in a pot. In that case you can stop the transpiration but you don’t stop the ionic nutrient movement into the plant. Some recent work at the California Technological Institute has shown that the desert plants put water back into the soil while they are growing, therefore the water can be going back into the soil while the nutrients are going into the opposite direction. We must rid our minds of this water soluble fertilizer “bugaboo” in considering soil fertility and plant nutrition, because transpiration runs independently of our control and we need to concentrate our efforts on keeping the stream of fertility flowing more regularly into the plants.

Let us not cover either our ignorance or our responsibility toward maintaining the soil fertility by trying to blame the water situation in the soil and the rain fall. The idea persists that the “drought” is responsible for the failure of plant nutrition. But what is commonly called “drought” isn’t trouble in terms of water only. It is apt to be due to the fact that the upper layer of the soil, where the fertility is, has dried and the roots must go down through a tight clay layer which has almost no fertility. Then because of the crop failure in the absence of plant nutrition in that soil layer of stored water, we try to blame the drought or the bad weather. Drought may be merely that soil situation in which we have no soil fertility deep enough to feed the plants when they are compelled to have their roots go deeper to get stored water. We have emphasized the water so much that the situation suggests itself as a relic of the old “saloon” days, when men thought they had to stay in a saloon and drink, but forgot to take some groceries home for the family. Plants will scarcely emphasize drink to that much neglect of food. Our confused thinking about drink for plants emphasizes the water facts as an alibi for our ignorance of plant nutrition and the soil fertility factor where the emphasis properly belongs. During the drought we don’t use the water to the best of our ability. We neglect to remind ourselves that the plant is about 95% air, water, and sunshine, and only about 5% fertility. We are too indifferent to that fact to consider carefully how we can use that 5% as the requirement to produce the other 95% of plant growth–a performance which offers chances as a gamble better than one would scarcely anticipate.

We blame the water. We blame the weather. The water of transpiration from the plants is like the water going over the millwheel, only a part of that coming down the millstream. The amount of grist that one grinds in the mill is determined not so much by the amount of water going over the millwheel, the amount of which is fixed or limited, as by the diligence with which grain is kept going into the mill stones for 24 hours a day at full capacity. We haven’t been keeping the soil fertility well and properly supplied to the crop plant and are, therefore, in error when for disturbed yields we blame the drought.

The Problem Analyzed

The problem of relating soil fertility to the plant’s nutrition as well as to the plant’s drink was approached and put under study at the Missouri Experiment Station many years ago. It seems fitting to review the history of the mental procedures by which we attacked this problem of adequate soil fertility and its services in the growth of plants. We made a kind of problem analysis. Our first division of the problem into its parts divided it according to the soil texture, namely, the sand, the silt, and the clay. We decided on the clay as the part of the soil we should study first. That choice was expectable because in Missouri we have almost four million acres of claypan soils. About those it was commonly said, “Oh, they will make only about 20-30 bushels to the acre. Your crops drown out in the spring and you dry out in the summer. The Putnam claypan soil is a nice silt loam. It has a level topography and it would be fine, but it just doesn’t deliver by production. Farmers on it have some silly ideas and ways about handling it. They ‘bar off’ corn in the spring, as they say, and then they ‘hill it up’ in the summer. It is a terribly acid or sour soil. You can’t grow legumes on it. You can’t build up its nitrogen.” Yet in spite of all the kind and unkind things said about this silt loam as a claypan soil it is still a good one in contrast to the 10 ½ million acres of stony land we have in Missouri. As an outcome, we picked first on this Putnam silt loam and worked on its clay until now that clay of this claypan soil is known all over the world by its technical name, “Putnam clay.” This basic research in the laboratory, in the green house, and later in the field has moved this soil into the corn growers’ contests for the winning high yields per acre.

Let us follow with the next soil separate and inquire, “What is in the sand as fertility for plant nutrition?” You may well reply “It is largely quartz, it contains only silicon and oxygen. Those mineral grains do not weather down.” That is the reason the quartz crystals are still big grains. It never has weathered. It is a kind of soil skeleton. “Consequently for the time being,” we said, “let’s throw that soil separation out of our mind and concentrate on the silt.” It also can be quartz. But then too, it can be other-than-quartz. Its composition depends on the place where it is. The farther east one goes in the United States, and to the higher rainfalls, the more quartz there is in the silt of the soil. Silt doesn’t have much capacity to hold fertilizer, neither does the sand. Because of their large particle size, these two soil separates have little capacity to absorb and to exchange nutrients to plants. But yet there is much silt blown in here from the floodplains of the Missouri River and from the West. It piles up along the river bluffs to give what is fairly good soil. Thus the soil separates were catalogued for their order of importance for research attention. The silt fraction was set aside for later study when the initial study took to the clay separate.

Clay Research Laid the Foundation

We began with the claypan soil and its high content of clay since almost everybody wanted to enlist himself in what might be a fight with that tight clay. The early researchers bought and used much dynamite on it. They dug ditches of various kinds in it. They pushed it around with powerful machines, but about the time they would have the treatment complete the soil was behaving just about the way it was before. Physical properties of the soil are to be considered the result of the chemical ones, not vice versa.

Either, fortunately or unfortunately, this clay has little or nothing inside of its crystal form of significant fertility contribution. We tried it in some of the early work we did with it in the laboratory. We tried bubbling carbon dioxide through it only to discover that if you really treated it long and hard with carbonic acid we could break out of it no more than about the iron that would be required to grow a crop. One might get a little magnesium out of it, but as a contributor of fertility we might credit it with iron, and one would be generous in doing that. From those early studies and the light they shed on the importance of the clay separate of the soil, there developed the research studies at Missouri Station leading to our better understanding of soil fertility and plant nutrition.

“But what about the organic matter as a colloid similar to the clay?”, you might well ask next. We found some organic matter in the clay. About 1 ½ % carbon and about 15 hundredths of a per cent of nitrogen are found in our Missouri clay. They are still very tightly linked into the clay molecule even after you have oxidized the clay, and after electrodialysis takes out every inorganic ion that you possibly can. These clay studies brought carbon and nitrogen into a ratio of 10:1 right in the colloidal clay itself. That is the carbon-nitrogen ratio commonly given for well-weathered soils. But we can’t get much nitrogen or much carbon out of it. These organic elements seem to be a highly-fixed part of the clay.

Because of these discoveries, the clay had much later research consideration. A good number of men on the staff have contributed to this whole project. We began by taking the soil separates out and directed a concentrated study to the clay. Its separation and preparation were no small task. We dispersed it in water. The larger particles were settled out, and the coarsest clay was thrown out of the supernatant water by centrifuging. The remaining opalescent suspension was then electrodialized and the intense study of this clay began. It was on the basis of that attack and our increasing knowledge resulting therefrom that we are bold enough to talk about soil fertility and plant nutrition.

Because we have studied clay chemistry now for a long time we have moved to study plant chemistry in combination with that clay. In his first experience of bringing colloidal clay chemistry into combination with the biochemistry of the plant, Dr. Hans Jenny, now of California, met with disappointment in the plants so often that he was about to give up. But when near a complete disgust with our theories, he finally caught the vision that the clay might be the dynamic center of the soil for plant nutrition. He had thrown out two or three sets of plants before we rescued the situation. He had demonstrated the fact that it is a hard task to load the clay with enough fertility to feed a crop for its good health and growth. Subsequent trials in goodly numbers demonstrated quite clearly that the clay is the major center of the chemical dynamics in the soil which deal with the speedy process we might envision when we talk about growing a plant.

It was also discovered that the clay holds nearly the season’s supply of plant nutrients ready and exchangeable for the root when it comes along. We have in the silt of the soil some of the reserve fertility that can be broken out when we consider that the soil is “resting.” The research found that the clay within the plant root zone must hold approximately a season’s supply of fertility. That supply can be increased by absorbing more nutrients on the clay or by adding more clay. It is in that factor of a higher saturation of the clay or of more clay where the different fertilizer use on soils of different textures is determined. One can put a tremendous amount of fertilizers on a heavy clay soil and not see much difference in terms of any tests of the soil you use, but yet the plants will register it pronouncedly. And that fact holds true whether you are considering fertilizers in the form of nitrogen, phosphorus, and potassium, or in the form of magnesium and calcium as limestone.

 

The clay is the dynamic part of the soil in providing plant nutrition. More clay in the sand (visible through the clear glass jars in the bottom of the sand, left to right) carrying calcium (pH 4.4) gave better nutrition and protection against disease. “To be well-fed is to be healthy,” is a truth for soils as plant nutrition.

 

The fertility held within the root zone is not reachable by water. It is not in the free water of the soil, and is therefore not in the water-soluble condition when it moves to nourish the plant. We make fertilizers soluble so they will be speedily absorbed on the clay rather than be held in water and be “sucked in” with the water by the plant. We have learned from these clay studies that calcium is a major nutrient for most any crop we grow. It must be ready for the plant early in the plant’s life. It does more than appear in the plant ash. It is not “hitch-hiking,” nor are any other nutrient elements, as we find them when we burn or “ash” the plant. Calcium is the major one among the elements in the ash, while nitrogen is the foremost, when all the elements, both combustible and non-combustible are considered.

Our agricultural soils, in general, have less calcium when they are more acid, that is, have more hydrogen, and conversely, then as they have more calcium they have less hydrogen. This simple nutritional situation of the importance of calcium, and the way the soil behaves under acidity as mainly a calcium deficiency, has kept us in ignorance about what soil acidity really is; namely, a fertility deficiency rather than a bad environment. The roots make the clay more acid when the plants grow, since the hydrogen from the root and the calcium from the clay are exchanging places.

The legumes are taking tremendous amounts of calcium off the clay. It was discovered that the degree of the calcium saturation on the clay determined whether the nutrients moved from the soil into the plant root or vice versa, from the plant root back to the soil. We grew some legume hay crops of beautiful appearance, even when they were putting back into the soil a good share of what fertility originally was in the seed. When we grew a legume crop that was a so-called “hay” crop, but not a ”seed” crop, we had less nitrogen, less phosphorus, and less potassium in the hay and the roots combined than was in the seeds initially planted. It was from that day forward that we connected plant nutrition with the soil and also animal nutrition with the soil, because we couldn’t see the reason for trying to fool the cow and expect her to be a hay baler in terms of eating enough of that vegetative stuff to represent nutrition for herself when it had less of those three nutrient elements in it than the seed we planted.

The clay, then, is the seat of all of these activities. First, there is the absorption of the nutrients from any solution. That activity is involved in our use of soluble fertilizers. Acceptance of hydrogen from the plant root by the clay, and the exchange from the clay to the plant root of some of the cations that are nutrition from that source when the hydrogen accepted there is not, is the major clay activity. The clay is also the seat of the breakdown of the reserve silt minerals as this decomposition serves to restock the clay, which is especially noticeable while the soils are commonly said to be “resting.” Those are some of the basic facts, in summary about the soil’s clay factor as it plays a significant role in plant nutrition.

Root Biochemistry Connected With Clay Chemistry

Let us visualize next the happenings when the plant root comes in touch with the clay. We need to remind ourselves that the plant is carrying on a biochemical operation. It is not merely standing out of doors without being influenced very definitely by the soil. We discovered that when the same amount of fertility was adsorbed on less clay or when the clay was given a higher degree of saturation, then the plant root in contact with that clay experienced an increasing efficiency with which that fertility moved into it and into the crop. In that simple fact there is the basis for the practice of “banding” the fertilizer application in the soil in place of mixing it throughout a large soil volume. In limited soil volumes saturated with fertility rather than having that distributed all through the soils to have the clay as a competitor colloid against your plant root, there is the basis for the efficiency in applying fertilizers in bands. The plant root can find those soil saturated areas where it feeds itself to advantage. The clay has always been serving to remove a good deal of the fertilizer hastily from solution by absorbing it, and thus getting rid of the dangerous salt effects, which the plants always suffered when we drilled fertilizer–with the exception of superphosphate–along with the seeding. But as long as we used ordinary superphosphate or others containing a large amount of calcium, we could drill much more fertilizer with the seeding than one could without it. If potassium salts, or sodium salts were used with the seed, its germination and emergence were quickly disturbed. However, when plenty of calcium salts were mixed with them, then there was safety. Gypsum was that safety factor often without our recognition of that ”saving service” to the plant’s biochemical activities in the soil.

There is another interesting and significant fact we learned. We could study one ion and vary it on the clay to get a certain behavior with increasing saturation of the clay by that ion, provided the accompanying ions didn’t disturb that phenomenon. We learned that these different ions adsorbed on the clay behave differently even when the amounts exchangeable are constant; that they are held with different forces; that they move off variably into the atmosphere of their colloid; and that they exhibit different effects and different energies. Consequently consideration had to be given to the interrelationships of the different ions in the suite of them adsorbed on the clay. If we had a big divalent (calcium, magnesium) or a trivalent (iron, aluminum), or usually less soluble ion, its behavior was much different than if it was a monovalent one (hydrogen, nitrogen, potassium, sodium). Consequently, the monovalent ions have different effects on others in the suite than have the divalent ones. The monovalent ions as variables in the fertilizer can upset one’s thinking seriously if that has been more confined to the divalent and the trivalent ions required in such larger amounts in the plant root environment. The hydrogen is a particularly active one because that is Nature’s tool which the root sends in right close to the clay molecule to hustle the rest of the ions out as they might be clinging so closely to the clay as if intended to stay there. Monovalent ions are disturbers to the rest of them, particularly those of higher valances, in their movement into the plant roots as compared to the amounts exchangeable.

That disturbance behavior relative to the amounts of the inorganic elements is very significant in case we bring a big, organic molecule into contact with the clay molecule or into the suite of adsorbed ions. When you take a soil which is low in its supply of inorganic nutrient ions on the clay and then can get a big, organic molecule adsorbed there, that phenomenon changes, decidedly, the whole situation of relative activities of ions for movement into the plant root. Those inorganic ions then come into action to a higher degree than the exchangeable amounts suggest in the absence of the organic ion. A decided difference in the behavior of calcium was demonstrated when the clay was more highly saturated by it but accompanied by a little methyl blue. The degree of calcium saturation didn’t make much difference in the relative amount taken by the plant root.

This suggests that when we have been using fertilizers and trying to explain the effects, the organic matter in the soil has been the saving grace for the plants in many cases. A low degree of saturation of the clay by the inorganic ions in the less fertile soils may be improved decidedly when we get more organic matter into those soils. When we put organic matter back into the soil we modify this more commonly considered set of inorganic chemo-dynamics of the soil, which must have initially dominated there. With less organic matter in the soil, we have a different rate of fertility deliveries and a rearrangement of the suite of ions on the clay. Organic matter, then, shifts the ration of the crop about very decidedly. When adsorbed on the clay the organic molecule moves different ions into and out of action in different proportions. Therefore the soil is feeding the crop different diets. We ought, then, to expect different crop compositions in terms of some of the food compounds the crop manufactures.

In considering the plant root as a factor in modifying soil fertility in plant nutrition, we have found that if the root was a protein-rich one, it exhausted the inorganic fertility of the soil to a higher degree than any non-protein root. In other words, a legume is the quickest way we can take the inorganic fertility off the clay and deplete its fertility to a lower level than we can by any other crop. Sanborn Field, at the Missouri Experiment Station, tells us that fact with decided emphasis. The legume crops take the potassium off the clay more speedily than most other elements. On Sanborn Field the continuous clover cropping was tried beginning with the year 1888. After 14 years this effort to grow the red clover was abandoned under both manure and no manure treatments. After abandonment of these plots for some time they were planted to continuous cowpeas for about 20 years. Then they were left to grass for a period and later went to broom sedge, as a nurse crop for Korean lespedeza. This combination prevailed under the plot neglect until in 1950, when $70 an acre was spent for fertilizers and the plots put into continuous corn. The yields were 125 bushels of corn in 1950, 110 in 1951, 57 in 1952, and 73 bushels in 1953 when there was much talk about a drought. During those four years the corn yield gave an average of 91 bushels per acre.

On the basis of those yields as a sequel to the preceding history, one might well raise the point whether legumes have been soil-improvers. It suggests that we never really were certain (we just had a kind of blind faith) that the growing of legumes improved the soil. We know now from laboratory research and from such field records just cited that they can take the fertility out of the soil faster than the non-legumes. The first failures in the crops in that field era showed up in the continuous legumes. They couldn’t save themselves. The question may well be asked “Shall we go back to that plot and try to build up its fertility by seeding legumes, or shall we study the sciences of the soil and plant nutrition to work with the bi ology the best we can by using applied nitrogen and other fertilizers?” With present high costs of production and high taxes, what solution have we for some of these problems except higher yields per acre via fertilizers for economy of production?

The Crops’ Physiology Becomes a Deciding Factor

If the soil fertility is to function efficiently in plant nutrition, we must give attention to the physiological requirements of the crop we expect to grow. Do we know the proper ratios of the inorganic nutrients that ought to be moving off that clay into the plant root? As yet we do not, but researches are giving suggestions. By means of the colloidal clay we are trying to work out a concept of a balanced ration of calcium, magnesium, potassium, etc., on that clay, possibly for each different crop because different crops are synthesizing different organic compounds. We know that the legumes require more of the different elements than are required by the non-legumes. Legume tops and roots are running a bigger factory. They are creating collections of different proteins about which we don’t know much. What do we know about the plant’s nutrient needs during the different phases of the growing season? While all of the different phases of this problem are confronting us, our declining fertility is slipping down faster and the problem is becoming much larger.

The Soil’s Silt Fraction May Serve As Sustaining Fertility

The research on the clay at the Missouri Experiment Station suggests that the restocking of the clay, after exhaustion of its fertility supply by cropping, resulted from mineral breakdown in the silt fraction. After we have studied the clay so much, we are now studying the silt as a mineral reserve for crop nourishment.

This separate is of more service in areas of low rainfall and of less of soil development in the West than it is in the high rainfalls of the East. Probably the silt breakdown in many of the midwestern soils and in others on coming east farther, has been the major supply of fertility. Therefore, our crop production is holding up to what that annual silt breakdown allows. The clay is a weathering agent for minerals. Dr. E. R. Graham has done some very clever work to show that we can use these reserve minerals in contact with the dynamic clay and thus restock the clay. That is what limestone has been doing when we use it on the soil. That is what rock phosphate does too. When we have the Missouri River hauling lots of unweathered silt in from the West; then with rather dry winters; and with the wind blowing that unweathered silt out of the river bottom and deposit ing about a thousand pounds per acre of Missouri every year, it would seem to be good agricultural foresight to think about this reserve natural mineral material as a fertilizer which Nature is giving us very generously each year.

In the mechanisms of plant feeding, the plant nutrients, like calcium, on the colloid are exchanged for hydrogen. When the clay and humus become exhausted of nutrients (become saturated by hydrogen) the nutrients move from the silt mineral to the root through the colloid, while the hydrogen, or acidity, moves in the opposite direction. That serves to weather the reserve minerals and make their fertility active and available.

 

This silt fraction, composed as it is of minerals other-than-quartz, should bring all of us to consider more seriously the sustaining fertility in the soil. While we have learned much about soils, we scarcely have knowledge enough to maintain production by starter fertilizers only, particularly when we put them at the top of the soil where that is dried out during most of the time and then blame the drought for the failure of the fertilizers in doing what some folks commonly expect of them.

Organic Matter–The “Constitution” of the Soils

The most neglected and most important chemo-dynamic factor of the soil is the organic matter. Organic matter may be said to be the “constitution” of the soil. As for a definition of the word ”constitution” in that usage, we take its meaning when the doctor consoles the friends of a patient in serious illness by reminding them that “The patient has a good constitution.”‘ According to its meaning as used in medical practice, a good constitution is the capacity of the individual to survive in spite of the doctors rather than because of them. The organic matter in the soil has been the capacity for our soils and our crops to survive in spite of the soil doctors rather than because of them.

Your attention has already been called to the importance of the organic molecule when it is on the clay. There is also the tremendous significance of the organic matter as a season’s release of plant nutrition. This release is timed to increase during the growing season or become larger as the temperature goes higher. The microbial activities follow Vant Hoff’s Law and double their rate of decay of the organic residues with every 10 degree rise in Centigrade temperature. Nature has always been fertilizing with the organic matter which is dropped back to the soil from the previous plant generations which have died in place. Organic matter is still the most reliable fertilizer in terms of the nutrient ratios and of the time when maximum must be delivered.

Another aspect of organic matter about which we probably haven’t thought much is the value of some organic compounds in cycle. They may be dropped back as crop residues and the next crop’s roots may be taking them up, using them and dropping them back again. Plants need the various “ring” compounds in very small amounts to make some of the essential amino acids. They need the phenol ring in phenylalanine, one of the essential amino acids, essential for plant growth as well as for animals and ourselves. They need the indole ring, which is a phenol ring plus a side ring. It is the compound which gives the odor to feces when the digestion acts on the tryptophane of which that ring is a part. Tryptophane is the most commonly deficient amino acid, and is one of marked complexity.

Then there are also the sulfur compounds and the sulfur-containing amino acids. We might well wonder whether Man and his flocks have not been geared together so closely in their past history because some of those excreted organic compounds were put back into the soil, and were going through the cycle over and over again as a help in the survival of both man and beast. Now we are trying to divorce ourselves from animals, but perhaps we haven’t found the basis of safety for it. Organic matter must find a new and more important place in our minds as the neglected half of plant nutrition and soil fertility.

In terms of the inorganic half of that responsibility we have partly understood about one-half of that phase, namely, the major cations. We don’t know much yet about the cations of the trace elements. When it comes to those major inorganic ions which are cations, we have a good concept of their chemo-dynamics for plant nutrition. As for the anions like sulfates, nitrates, carbonates, and others, we do not yet know how they are handled by the microbes and the plant roots in the soil. We have much yet to learn when we have scarcely one-fourth of the field of soil fertility interpreted in terms of the basic principles of absorption, exchange, solubility, and what have you, when it comes to the problem of soil fertility and plant nutrition. There is enormous opportunity for a big research program ahead. However, we have charted our course now and believe we have analyzed the problems, though by no means outlined the solutions for all of them.

Conservation of Soil Fertility Demands Revision of Some Economics

We need help from observing and thinking minds to take these concepts about soil fertility and plant nutrition out into the fields for test, whether our concepts are on the right or the wrong track. Those who supply fertilizers have not discharged their responsibility completely when a car load of fertilizer has been delivered on the farm. They have the responsibility of making those goods serve properly in crop nutrition. Fertilizers must serve not just for increasing the crop bulk, but in terms of the necessary chemo-dynamics of soil fertility and plant nutrition which mean better nutrition for animals and man as well.

Our concern about soil fertility and plant nutrition naturally emphasizes their biological aspects. But even under demand for more food and need for a national agricultural policy, one dare not disregard some of the economics involved. Unfortunately, agriculture uses soil fertility as its biological capital. To date such capital is still an unknown in the money marts, the bankers’ vaults, and the political areas. According to present economics applied to agriculture, soil fertility capital is thrown into the bargain when we make a sale of agricultural products. Such values are not interpreted in dollars. The depletion of soil fertility, that is, the foundation for real food values in land values, is not yet considered. Consequently the. agricultural business does not have an accounting system for taxes on income, on lands, etc., set up to include fertility depletion allowances, allowed labor income, guarantee of perpetuity of capital assets, etc., to make agriculture and the soil under it, self-perpetuating. Soil exploitation and land ruin with time are therefore inevitable.

In spite of this lop-sided kind of economics for agriculture, when economics look toward guaranteeing self-perpetuation for most other forms of making one’s livelihood–even for the laborer by means of strikes–we expect increasing food delivery from the soil. There is no economic alternative except that the soil must be mined. That must be the result if food production by agriculture is to continue and to increase under the present economic disregard of fertility as the factor giving real value to the acres of land. No national agricultural policy for survival under high standards of living can come forth unless we finally realize that our national strength lies in the fertility of the soil and our future survival in the wise management and utmost conservation of it.