The Fineness Of Soil

If we take up a handful of mellow soil and look at it closely, we can see only a crumbling mass of particles, intermixed with black bits of decayed and decaying vegetation. There seems to be no life in it. Put a bit of this soil on a glass slide and look at it under a powerful microscope; a scene of constant activity is now revealed. Moulds, ferments, decays, bacteria, and other organisms are constantly at work, destroying, creating, changing the structure and the agricultural value of this soil. Currents of water pass through it; waves of heat quicken it. The tiny particles of rock are ground and worn smaller each year, and the plant foods are changed from one form to another. The soil has a flora and a fauna scarcely less complex than that which clings to its surface. Little is now known about the soil as compared with other agricultural subjects; it is remarkable that the soil, the foundation of agriculture and the beginning of all wealth, should have received so little minute study. We may expect the present deep interest in soil physics and soil bacteriology to greatly increase our knowledge of this most important factor in successful farming. Some of the significant facts about the nature of the soil, according to present knowledge, are considered in the following paragraphs.

It was stated before that the basis of most farm soils is rock, ground into "rock-meal" by Nature's millstones, the air, water, frost, ice and other elemental forces. At first the soil particles are very large, mere fragments of rock at the base of a cliff, but upon these wild morning-glories or mulleins may be able to grow. Some hundreds of years later these small rocks will be finer; perhaps they will average less than one-quarter inch in diameter, and they will be mixed with humus. The fining process goes on a few generations or centuries more, until the pieces of rock have been broken into such small particles that farm crops thrive upon them. Nearly every soil is constantly becoming finer. All soils that contain small rocks or pebbles receive from them each year many particles of soil by weathering, and the size of the rocks and pebbles is reduced that much. Even the rich prairie loam or alluvial clay, which is apparently all soil and contains no rocks or pebbles at all, is becoming finer. Weathering is much less active on such soil, however, than on gravelly and stony soils.

The number of individual particles in a fertile soil is astonishing to those who have not tried to count them under a microscope. A good corn soil has about 280,000,000,000 particles in an ounce, while the clay loams that are preferred for grass often have 400,000,000,000 particles in an ounce. These particles are of varying sizes and shapes, even in the same soil. Sometimes they are uniform and rounded, and pack together poorly, leaving large spaces between them, like marbles piled together. Sometimes they are uneven and jagged, packing together tightly, like the crushed rock of a macadamized road. The spaces between the soil particles differ in size and shape, according to the size and shape of the grains. I have met a farmer who could not quite see how a soil .could contain air at a depth of four feet, yet he admitted that there must be air at the bottom of his wheat bin. The trouble was he looked upon the soil as a solid mass, since he could not see the spaces between the grains with his naked eye as he could in wheat.

If he would think of his soil as a bin of wheat, with the kernels about one-millionth as large, he could see how it is that air and water pass freely through all ordinary soils, and to a great depth. It is of practical as well as of scientific interest to know about the size of the grains of a soil, and the size of the spaces between them. The value of a soil for certain crops depends quite largely upon just such factors. With the refinement of soil surveys and methods, soil experts assure us that they will be able to tell us with a fair degree of certainty that soils containing, for example, from 250,000,000,000 to 350,000,000,000 particles per ounce are adapted for potatoes; soils containing 350,000,000,000 to 450,000,000,000, for onions, and so on. At present we classify soils and judge their adaptability for certain crops in grosser terms ; we say potatoes do best on a sandy loam, and that an alluvial clay loam is excellent for onions. There are limits to the practical value of this information, for the fineness of the soil is but one of many factors that determine the adaptability of a certain soil for a certain crop; yet this one point is extremely valuable to know when selecting land for special crop farming. Fineness is Richness. — The fineness of the soil has a very important bearing upon its fertility. Other things being equal, the finer a soil is, the richer it is, because it contains more surface for the roots to feed upon. The rootlets of plants do not suck up particles of soil, as Jethro Tull supposed, in his now famous "Horse-hoeing Husbandry." They feed upon the film water upon the outside of the soil grains. This contains much plant food dissolved from the grains. The natural agencies that dissolve plant food from the soil — water, air, etc., act only on the outside of the particles.

Hence the more surface there is to the grains, the greater is the "pasturage," or feeding area for the rootlets, and the more rapid is the weathering. If a small stone is broken into six pieces, the pieces have several times more surface, in the aggregate, than the unbroken stone. It has been calculated that if every particle in one cubic foot of mellow soil could have all its surface spread out flat, the aggregate surfaces of all these grains would cover about one acre. The presence of small stones and pebbles in a soil is beneficial, making it lighter, more porous, and warmer. It would be a great calamity if all soils contained no pebbles and larger pieces of rocks. These are a store of plant food which is added to the soil from year to year. Yet the farmer should remember that, in general, fineness means richness. If a soil is lumpy, because of lack of humus or excessive moisture, its available feeding area is greatly reduced. This matter is considered more fully in succeeding chapters, where practical methods of making a soil fine and mellow are described.