Mobilizing the Fertilizer Resources for Our Nation’s Soil

Foreword

Dr. Albrecht sent me the manuscript of this article last summer after I had sent him copies of my editorials in the April and July issues, 1943, in which it was argued that pulverized rock other than limestone also should have value as fertilizers and soil builders. We knew perfectly well that fertilizer manufacturers, and they have a lot of influence, oppose anything as a fertilizer which is not “water soluble.” For years they prevented the use of pulverized raw rock phosphate on that argument, but gradually, in the Middle West pulverized raw rock phosphate has been used in increasing amounts and with satisfactory results.

Naturally, fertilizer manufacturers will oppose the use of other pulverized rock as sources of potassium, phosphorous, iron, sulphur and all the other minerals so necessary to plant growth because of their alleged “insolubility.” Rocks like granite and others of igneous origin, are rich in these minerals, and soils formed by their natural disintegration are productive soils. Dr. Albrecht, in this article, comes to our rescue with proof that pulverized rock, whether disintegrated by natural means or mechanically, is the real source of permanent soil fertility. It is better for continuous cropping not to have the plant food in too soluble condition, because if it is, drainage gets more of it than plants. Of course “soluble” is a relative term. When crystals or particles of minerals are sufficiently small they go into “colloidal solution,” which means they stay in suspension in water without changing their chemical composition.

This article of Dr. Albrecht’s should be read and studied by all crushed stone producers, since it shows the soil mechanics by which mineral plant foods from pulverized rock are taken up by the plant, in spite of the fertilizer manufacturers’ contentions that “insoluble” rock dusts are not fertilizers or soil builders. Their application to soils will result probably in slower, but more permanent, recovery of soil fertility, which from now on will be one of our greatest national technical problems.

Nathan C. Rockwood

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The productivity of a soil is determined mainly by its delivery of ten chemical elements in effectively balanced amounts, since these nourish the plants directly and also enable them to use four additional elements coming from the air and water. Crop production depends on the successful management of the soil so that it delivers its plant nutrients or soil fertility to the crops. The fundamentals of the soil processes as they provide the raw materials to initiate and continue the manufacturing business of the growing plant, through which the sunshine power synthesizes the complex chemical compounds of vegetation, may well interest all of us.

All life depends on this natural chemical industry that draws on the soil for only about 5 or 10 percent of the plants’ makeup, while it draws on air and water for the remaining 95 or 90 percent. The decline of the fertility stores of the soil, that represent rock residues, is bringing these fundamentals into greater significance. It is well that we understand them while our efforts in conservation may still find sufficient of soil fertility left to be conserved. Fertility for future use is that which is in the rock and mineral reserves of the soil or that applied to the soil.

Chemical Elements Needed by Growing Plants

The bulk of all plants is combustible. They are therefore made up largely of carbon, so commonly combined with hydrogen and oxygen in the ratios by which the last two are found in water. This combination as carbohydrates in their various forms is the bulk of all vegetation. When to this chemical combination there is added some nitrogen, the compound becomes protein. For legume crops, these four elements, namely carbon, hydrogen, oxygen, and nitrogen, are supplied by the air and water. They are chemically combined by the energy of the sunshine. Carbohydrates in particular represent concentrated collections of chemical energy. As sugar, starch, cellulose, wood, and others, they represent fuel values in animal diets.

The protein also represents some energy collection, but more particularly it is the growth-promoting compound. Cell-multiplying capacity resides in it. If true proteins are to be produced–that is, if the nitrogen, and in some cases some phosphorus and some sulfur are to be coupled up with carbon, hydrogen and oxygen to make this life-carrying, body-builing substance–they cannot be built by sunshine using only air and water. The soil must contribute at least 10 elements. These 10 include calcium, so common in limestone; phosphorus, common in bones and phosphatic rocks; potassium, abundant in wood ashes and many rocks; magnesium, common in dolomite; sulfur; iron; boron; manganese; copper; and zinc. With the soil’s responsibility toward the plant, including as many as 10 elements, and with the shortage in any single element limiting the crop growth, is it any surprise that the problem of supplying the plants with their requisites may be a common one on some of our soil types?

Plant Composition Reflects Soil Fertility Supply

Every plant represents a skeletal structure for absorption of sunshine, intake of carbon dioxide from the air, and absorption of water and fertility from the soil. This structure can be roughly visualized as consisting of woody material much like a building’s skeletal frame. Every plant has it. Some have but little more than merely a woody framework. We can then imagine the plant’s manufacturing performances in constructing this woody mass largely of carbohydrates. Water, and carbon dioxide, both represented by the air and the weather, are the sources of this material.

Then, as the soil offers more of fertility to permit the manufacturing and synthesizing performances to move forward at greater intensity, the plants will contain higher concentrations of proteins, of minerals, and of other soil-borne substances to be put into abundant seed yield or larger tonnages of forage with high nutritional values for animal consumption. These are the manufacturing activities carried on within its woody frame structure. Roughly, we may view the plants as functioning to make products of value in animal body construction or in cell multiplication for their own growth from the fertility contributed by the soil. They make products mainly of fuel value from the contribution by air and water.

Vegetation can be classified, then, into two groups, the first being the woody, or the carbonaceous, group when the soil contributes little fertility and compels the plant to operate largely on water and weather. The second is the protein-containing and mineral-rich group, when the supply of soil fertility is large. Forest trees grow on soils of lower soil fertility, while legumes, such as alfalfa, demand higher soil fertility. The first of these two groups reflects the fuel value, and the second the nutritional service in body building, as we all know of alfalfa’s service for promotion of growth in young livestock.

Not only in the different plants are these differences found, but even within a single kind of plant there is a similar variation in composition according to the soil fertility. Soybeans, for example, become more woody in character if grown on a limited supply of soil fertility. When more generously nourished, they become rich in protein and rich in minerals as legumes are expected to be. The soil fertility supply determines the plant composition, irrespective of the plant’s pedigree or its parents as performers on some other soil.

Process of Mobilizing Fertility from Soil into Plants

Where within the soil are the plant nutrients stored, or where is the fertility retained? This is a question that must be answered if we are to appreciate the fertility problems of some of our soils. More baffling to many is the question, how can the plant get anything from the soil after water has been going through it so long to carry soluble materials away? One needs only to recall the universal practice of burying things in the soil to get rid of odors, or of filtering water through soil or charcoal to obtain clear water, to appreciate the natural phenomenon of adsorption by the soil. Filters are made of substances offering extensive surfaces by which materials in colloidal solution are taken out and held there safe from removal by more percolating water. The tremendous amount of surface in the clay fraction of the soil is the place on which plant nutrients are adsorbed. It is through this phenomenon of adsorption on its surface that clay serves in holding the fertility elements against loss by leaching and yet in readiness for delivery to the plant roots by exchange mainly for hydrogen given off by the plant.

Principles by Which Plants Feed on the Fertility of the Soil

Perhaps in the simplest way we can picture the plants’ getting nourishment from the soil largely as a business of barter or trade. The root, as a colloid with its extensive surface giving off carbon dioxide that provides hydrogen, is in intimate contact with the clay surface on which fertility elements, such as calcium, magnesium, potassium and others are adsorbed. The hydrogen from the root is a positively charged ion and is traded or exchanged for those of similar charge on the clay. These positive ions, or nutrient cations, on going into the root are synthesized into complexes and carried up into the plant to clear the way for others to follow. Thus, the plant gains nutrients, or “takes” the soil fertility from the clay to reduce the fertility supply there and in turn to increase the supply of hydrogen on the clay surface.

The performance may be illustrated by means of the diagrammatic sketch, Fig. 1, in which the hydrogen is designated as moving from the root to the clay along the lower sides of the circles representing clay or humus colloids, while the calcium is moving from the clay to the root along the upper sides of the circles.

Fig. 1.–The mechanism of plant feeding. Plant nutrients, like calcium, on the coiloid are exchanged for hydrogen. As the colloid clay and humus become exhausted, the nutrients move from the mineral to the root through the colloid while the hydrogen, or acidity, moves in the opposite direction.

 

Since it is the hydrogen ion that represents acidity, or sourness to our taste, the increasing concentration of hydrogen on the clay means increasing soil acidity. Plant growth and its removal of fertility from the soil is then truly a process that makes the soils more acid. In turn, the increasing soil acidity means an increasing degree of exhaustion of the fertility store from the soil. For plants growing on the more acid soils, then, we may expect that they are running a manufacturing business with an output of products with fuel–rather than growth–value. Acid soils are of low fertility and consequently of lower productivity. Their vegetation shifts toward that of mere fuel value.

Crusher set-up at Kentucky Stone Co., High Bridge, Ky., for production of agricultural limestone. Hammermill is V-belted to 100-hp. motor. Feed size is usually not more than 1-in. 

 

Acid Soils Bespeaks Fertility Problems

If the clay of our soils gives up its cationic supply of soil fertility in exchange for hydrogen from carbonic acid of root origin, the clay must do likewise to carbonic acid coming from the decaying organic matter within the soil. Percolating waters saturated with this acid, leach fertility from the soils and make them acid in a manner like that by the plant. Roots of cover crops can intercept this fertility and hold it as organic matter for the next crop. Missouri’s location, for example, in the region of higher rainfall and of longer and warmer seasons means that natural weathering has given her the more leached soils, or those soils of lower fertility. As an indication of this fact, we need only to recall that our virgin soils had long been depleted to the point of producing only a woody vegetation, or a timber cover, before they were put under our cultivation. That we should plan for a highly proteinaceous crop like alfalfa on such soil implies immediately that we must expect to provide the extra fertility, and to reduce the hydrogen store on the clay to the extent presented by the fertility differences between the natural woody crop and the introduced proteinaceous one. The very nature of our soils brings a fertility problem with it.

Amount of Fertility Delivered by the Clay

If the plant obtains its nutrients through surface contact between the root and the clay because of the hydrogen as the root offering in exchange, the amount of nutrients so obtained from the clay of the soil will depend on three factors. The first of these is the total amount of root surface the plant has to offer per unit volume of soil. Densely rooted plants get more fertility than those only sparsely rooted. Bluegrass with 66 square inches of root surface per cubic inch of soil can take more by exchange than soybeans with only 2.5 square inches in the same soil volume. The second factor is the amount of clay surface, rather than surface of silt or sand the soil has, since little of fertility is adsorbed on these latter mineral particles of larger size. The third and important factor is the degree of saturation of the clay by the nutrients, rather than by hydrogen. Much clay in a soil means more chance for the roots to make contact with nutrient-carrying surfaces. Here is the reason why heavy clay soils are appreciated for their productivity, even if often hated because of their intractability. Ancient civilizations on sandy soils have not been long-lived. Those on clay soil have persisted through centuries. Regions of older civilizations today are on soils of high clay content, because only such a soil would retain its productivity through those long periods of cultivation. But by far most effective in raising the productivity of the clay is the degree to which it is saturated with fertility rather than with hydrogen, or with infertility. This variation in degree of saturation is the hidden condition that is not recognized, and in it many of our soil problems originate. By it much can be done for their solution.

Agstone is occasionally placed in paper bags as shown In the foreground, but most of the production is handled in bulk for farm distribution.

 

Though many soils tend to be high in acidity, it is well to remember that if a soil can hold much hydrogen to make it acid, there is a large capacity to hold other positive ions that can be fertility, when we once decide to apply it. This fact offers hope for long continued use if our management puts the fertility into those soils. This acidity is also an agent to break down rock products applied, as soil treatment and mobilize their fertility for plant use.

Fertility Reserve in Some of Our Soils

Another glimpse at the diagrammatic sketch of Fig. 1 suggests that the clay and the silt minerals undergo interactions of exchange, much in the same manner as is true for the clay and the plant root. This is one of the fundamental facts pointing to the possible use of the reserve fertility in the mineral crystal of the soil. The clay plays an important part as intermediate agent between the plant root and the mineral crystal. The plant can get nutrients by direct root contact with the mineral crystal, but plant growth by this means has been found experimentally to be at a lower rate than when an acid clay serves as interceder between the plant and the mineral crystal. We can visualize the clay (a) as an acceptor of hydrogen from the plant root, (b) as a conveyor of it to the mineral in concentrations of significance for releasing the nutrients in the mineral by this acid effect, and then (c) as the deliverer in return to the plant of the nutrient set free from the mineral. The acidity in the clay is in reality of service rather than of detriment. Because the clay supply of fertility is rapidly exhausted, it is in the stock of minerals, particularly those of silt size particles in the soil, that the reserve fertility and future productivity of our soils must be found.

A careful study of the silt minerals of some different soils by Dr. E. R. Graham of the Department of Soils of the College of Agriculture of the University of Missouri, has given an interesting inventory of them with reference to this part of the soil body. As a soil is weathered more, or as it is older in its geological experience, these reserve minerals are mainly those resistant to weathering, such as quartz, for example. Unfortunately, quartz contains no plant nutrients. The windblown soils of the Missouri and Mississippi river bluffs are more youthful as they are nearer to the stream, and more weathered with greater distance from it. Accordingly, in going away from the stream there is more quartz and there is also less of other minerals, or those which carry the plant nutrients. Even these other minerals are more weathered and deliver less, for example, of calcium, which is a much needed nutrient in most soils.

Silt Minerals Represent Reserve Fertility

With more rainfall and higher temperatures as one goes from West to East and North to South in the United States, respectively, this reserve of silt is lower in its supply of fertility, or is higher in quartz. With less rainfall and cooler climates the soils are usually higher in minerals other than quartz, and in nutrients like calcium for example. Mr. Vanderford’s studies of six samples of the same soil type along the Missouri and Mississippi rivers extending from Sioux City, Iowa, to Mississippi, contained 53, 68, 71, 73, 79 and 80 percent of quartz and .90, .60, .44, .42 and .30 percent, respectively, of calcium in the silt in going from low to higher rainfall and to higher temperatures.

 

Quartz and Calcium in typical Soils

	Quartz %	Other than quartz %	Calcium %
Barnes silt loam, South Dakota	70	30	.90
Clarion silt loam, Iowa	66	34	.70
Cisne silt loam, southern Illinois	73.2	26.8	.40
Lufkin silt loam, Mississippi	87.8	12.2	.25

 

According to the data, these soils carry a calcium reserve–and possibly a corresponding reserve of other nutrients–in the silt fraction, or the rock fraction, that varies widely. Our less weathered soils bid fair to carry us into the future, because they have a fertility reserve in these minerals. This reserve also represents more productivity at the present time.

The clay and the organic matter are the agencies that are active in moving soil fertility from the mineral reserve in the rock residues of the soil to the crop roots. The fertility supply on the clay alone will serve but a few crops. The supply in the organic matter is also only temporal. It is in the rock and better fragments left as silt and sand sizes that the long-time, and better soil productivity resides. It is moved from these to the plant through the activity of the colloidal clay and the humus.

Water-insoluble materials like limestone or any rock products applied to the soil as fertilizer are also reserve materials handled in the same process. The hydrogen ion coming from the plant is passed to the clay and from it to the rock mineral to break it down. Its nutrient contents are taken by the clay and passed back to the plant. Lime rock applied to soil does modify the acidity some, but is really a fertilizer because of acidity.

Here in this soil mechanism is a case of truly “passing the ammunition” when soil fertility in the rock residues is passed by the colloid to the root and is being fabricated into the food to win the war. It is this passing of the ammunition by which we manage our soils wisely for maximum crop production with maximum soil and soil fertility conservation. Our understanding of how the rocks of the soil can feed the crops gives us the fundamentals in plant nutrition on which efficient agriculture and food production must really rest.