GRASS AND THE SOIL

Charles E. Kellogg

THE GREAT variety and complexity of the country scene appeals to most people whether they are professional naturalists or not. Each rural landscape has its own set of characteristics. Any one may be just a little different from the thousands of others, or very unlike any of them. Through science modern man tries to understand these landscapes to unravel their many interlocking relationships in order to discover principles that can be used to guide the great producing powers of nature to his own ends.

But the job is so big that scientists have had to divide it among them. Thus botanists, geologists, foresters, climatologists, horticulturists, agronomists, soil scientists, farmers, and others are each concerned with some part of the whole. Yet at some stages in scientific work the facts and principles discovered in these specific lines of inquiry must be brought together if principles of prediction value in the real world are to be developed. That is, plants growing in even the simplest farm, or garden, or forest are subject to all the influences of the environment acting together and they contribute to this environment as well.

Nothing illustrates this complexity better than grass. In some landscapes tall, luxuriant grasses grow naturally and help make black soils that are naturally productive for cultivated plants. The invasion of such landscapes by forest degrades these soils they lose part of their great producing potential for crops. And this may happen quickly not in terms of a man's lifetime perhaps, but in 200 years or so.

Yet in other places more productive soils are found under forest than under grass. Here invasion of the soil by grasses degrades it rapidly, within the period of one man's life or much less.

These are two extremes. But often it is by looking at the extremes that we discover principles of great importance to the soils between them, where differences are not so easily seen.

The Soil

Suppose we look at the soil itself. What is it? First of all, it is the natural medium in which plants grow. It is a mixture of mineral matter and organic matter, some of which is living. Things are being added to it and taken away from it. The soil on the very surface is not like that just beneath it; in fact, the soils in most places consist of a series of unlike layers, one over the other, each from a few inches to several feet in depth.

Then, too, the surface is gradually changing. Some soils are slowly being eroded, bit by bit, so that all the layers move down. To each layer a bit of the one beneath is being changed and added to its lower side as it loses its upper part by erosion or to another layer above it. Finally, new fresh minerals from the rock beneath are incorporated into the lower part of the lowest layer of the soil.

Other soils, of course, receive additions to the top instead of to the bottom. Along great rivers silty alluvium settles out of the water over the soil. Dust settles from the air perhaps just a little; often a great deal. Volcanoes add ash or cinders to soils, sometimes lowering their productivity for crop plants but more often increasing it.

When water enters the soil, air is forced out of the pore spaces. Then as the soil dries, air returns. In this process gasses like carbon dioxide escape and others like ammonia enter the soil to be absorbed.

The entering water, either as rain or irrigation water, brings soluble materials with it too usually just a little, but sometimes a great deal. The excess water beyond what the soil can hold seeps out into deep drainage and carries soluble materials away.

Then, of course, plants are growing on the soil, extracting nutrients, and producing organic matter from these soil nutrients and those from the air and water. Depending on the kind of vegetation, the total organic matter may be a ton or so per acre up to several hundred tons. Thus, in the living organic matter the soil has a great storehouse of nutrients. When the plants and animals die, the remains serve as food for micro-organisms. As it decomposes, the nutrients in it are made available to new plants.

Thus a soil changes between day and night, from season to season, and over long periods of geological time.

Yet soils are not quite so difficult to understand as this recital might suggest because many of the processes go together. Ignoring for the moment man's interference, a soil an individual set of soil characteristics that we call a soil type results from a particular combination of five genetic factors climate, vegetation, parent rock, relief, and time. Thus soils are not distributed promiscuously over the earth, but in an orderly discoverable geographic pattern. A given set of the five genetic factors everywhere produces the same set of soil characteristics the same soil type.

But to these natural types of soil must be added the changes caused by use often drastic changes for better or for worse, in terms of crop production. That is, many soils developed originally under forest in the humid temperate regions have been made ever so much more productive by careful husbandry, including the growing of grasses, the use of lime and manures, and improved drainage for hundreds of years. Other soils have been deprived of their essential cover of grass or trees and exposed directly to the sun, wind, and water, with serious degradation by erosion, blowing, burning of organic matter, and loss of structure.

Soil scientists have been and are now attempting to discover precisely what types of soil exist in the world, where they are, and how they respond to that whole group of practices we call "husbandry."

One cannot understand a soil by looking simply at one or two of its characteristics. Slope, depth, texture, color, structure, chemical composition, and many more must be seen in combination. Not only that, a soil must be seen in relation to those around it. A soil is three-dimensional. It occupies discreet areas of the earth. Around each area are boundary lines that separate it from the other soil types with different sets of soil characteristics. These boundary lines come in places where there is a change in one or more of the five genetic factors.

So a soil is a solid, the upper surface of which is the surface of the land. The lower surface is defined by the lower limits of biological forces, and the sides are the boundaries with other soil types. One cannot take a soil into the laboratory any more than he can a mountain or a river; but one may take samples of rock, water, or soil into the laboratory for important investigations to determine some of the characteristics of mountains, rivers, or soils.

Even further, a soil is a landscape with a characteristic climate and vegetation. Thus plants and soils are essential parts of one whole, each influencing the other and both reacting to the climate.

Soil Productivity

A central problem of inquiry in soil science is soil productivity for various crops, grasses, and trees and how to increase it or maintain it efficiently. The two principal aspects of soil productivity are its structure, or tilth, and its fertility, or content and balance of available plant nutrients.

Let us consider the fertility. Commonly, soil scientists attempt to express the amounts of nutrients available to plants in terms of "pounds per acre" of available phosphorus, potassium, calcium, and so on. These figures permit the comparison of soils only in the narrow sense, not as landscapes.

Suppose, for example, that we compare the black grassland soils (Chernozem) of eastern North Dakota with the light-colored forested soils of northern Michigan (Podzol). We shall see at once that the content of available plant nutrients is considerably higher in the Chernozem than in the Podzol. But to compare the total plant nutrients in, and available to, the biological cycles of the natural untouched landscapes, we shall need to add to the amount in the acre of soil that in the living matter in the trunks, branches, and leaves of the trees, in the animals, and in the other plants and microorganisms. This additional amount will be large for the forest and relatively low for the grasses. Of course, the nutrients tied up in living matter are not subject to much leaching not until the material dies and begins to decompose. It is mainly the material in the soil that is subject to leaching. Thus, of this total, more will be subject to leaching under grass than under forest.

Generally, the percentage of mineral plant nutrients in the organic remains from grass is higher than in those from forest. Thus more organic acids result from the decomposition of forest litter, even though the total of minerals supplied is somewhat greater. Per ton of dry matter produced, grasses return to the soil more bases, like calcium, potassium, and so on, than trees, other conditions being comparable; and, with nearly equivalent synthesis of organic matter, grasses produce more humus black, stable organic matter because of their chemical nature and their dense, fibrous root systems.

Then because of the relatively drier climate, the Chernozem soil is much less subject to leaching than the Podzol. Grass, side by side with forest in the moist Podzol region, holds less against leaching than the forest; grass side by side with forest at the boundary between the Chernozem and Podzol zones gives a darker soil, higher in organic matter and plant nutrients, than forest. The dark, fertile, granular surface soil is deeper under the grass and more suitable for crop plants than that under forest from the same rock material.

Thus, the figures selected for a comparison of an acre of Chernozem with an acre of Podzol vary greatly, depending upon whether we think only of the soil in a narrow sense or of the total landscape, including soils and plants together. In both scientific and practical work, both sets of comparisons are needed.

If the light-colored Podzols are fertilized, it is possible to have soil fertile for grass, despite the leaching. If this grass cover is maintained, the cultivated soil itself then takes on some of the physical and biological characteristics of the Chernozem of the black grasslands. But if we do not make up for the greater leaching in the Podzol landscape by the proper use of lime and fertilizers, the pastures and meadows are likely to be poor indeed, not only poor, but the soil may actually become less fertile under the grass than under the forest. (Young alluvial soils or others too young to have received normal leaching are exceptions.)

In practice a farmer on the Podzol soils, let us say in New England, will produce pasture more efficiently by using lime and fertilizer on a small area to develop a soil approaching the less-leached Chernozem in fertility than to use a far larger area for untreated pasture. That is, a hundred acres of untreated pasture will, ordinarily, give less return in the Podzol landscape than the same area with 20 acres of well-treated pasture and 80 acres of forest, to say -nothing of the fact that the long-time productivity of the soil will be better.