A good soil should provide a congenial place for the development of plant roots. It should be deep enough to permit their normal spread.

It should carry the right amount of water, maintain a satisfactory temperature, and have adequate ventilation. It should have a chemical nature that will be congenial to plant roots and to useful organisms in

Published semi-monthly throughout the year by the New York State College of Agriculture at Cornell University. Entered as second-class matter October 13, 1911, at the post office at Ithaca, New York, under the Act of Congress of July 16, 1894.

the soil, and should supply the food required for plant development. These conditions should be maintained as evenly as possible. Not only are changes injurious in themselves, but one unfavorable condition will induce another.

That layer of soil in which the roots of plants are distributed and in which all the physical, chemical, and biological processes are most active is termed the root-zone. (Fig. 87.) For crops most commonly grown the root-zone should have a depth of at least three feet.

FIG. 87.- Section of soil showing natural structure and the distribution of grass roots. It shows, also, ideal plowing; the action of the moldboard in twisting the furrow slice; and the action of the jointer in turning under the edge of the furrow. Rather large spaces occur between the furrow slices adjacent to the subsoil. At the lower right hand side is an enlarged view of a section of the subsoil showing distribution of cracks, root cavities, and burrows of worms

TILTH

The factor that most often determines the adjustment of all these conditions of the soil is physical structure. Soil is a frame-work of particles of rock and organic matter through which many small spaces or pores are distributed. By their size, volume, and distribution, the spaces regulate the other properties of the soil. If the particles are small, the open spaces

FIG. 88. Types of root systems showing different forms and depths of penetration. Deep-rooted plants are an effective means of improving the structure of hard but well-drained subsoils

are correspondingly small. Spaces that are too small are likely to be continually full of water, which crowds out air and creates bad ventilation. Poor ventilation, in turn, smothers or prevents the development of roots and bacteria. This interferes with the food supply and is injurious in many other ways. If the spaces at the surface of the soil are too small the water that comes as rain does not find ready admission and is lost, so that the crop may later suffer from lack of water which should have been absorbed. Further, soil spaces that are too small, if clogged with water and consequently poorly ventilated, promote chemical and biological processes which are decidedly harmful. Thus, nitrogen in the form of nitrate fertilizer may be broken down to the condition of a free gas, so that it cannot be used by many kinds of plants. Excessive absorption of water due to very small spaces renders the soil cold, and from this arises a chain of injurious results. This brief explanation shows how important is that physical condition of the soil which gives the right size and proportion of pore spaces.

The physical nature of the soil is referred to as tilth. The most advantageous character and arrangement of the soil is termed good tilth, while the reverse condition is termed bad tilth. The maintenance of good tilth should be the first object of the farmer. Consequently he should understand that physical basis on which good tilth rests and the practical means by which it is regulated. This lesson is devoted to a brief explanation of the physical basis, of the things that most directly affect it, and of the action of the more common implements of tillage in their relation to these properties.

As has been indicated, good tilth is identified with the right size of pore space throughout the soil mass. These pores depend for their size on the size and arrangement of the particles of scil. The particles of soil are of many sizes. The term used to refer to the size or fineness of the individual particles of soil is texture. A fine-textured soil is one made up mostly of very small particles. A gravelly or a sandy texture is one made up of large particles.

The arrangement of the separate particles in a soil is termed structure. Soils may have a loose or a compact structure. Clay in a fine, friable condition easy to stir may be said to have a loose structure. If the separate particles are gathered in groups or kernels such a structure is termed granular. But the same soil when mixed and stirred in a wet condition becomes very dense, and such a soil is said to have a compact, or puddled, structure.

Texture of soil

Not only are soils made up of particles of different size, but each soil contains particles of many different sizes. These different sizes have

been classified into groups and each division has been given a name. The divisions most frequently used are as follows:

It will be noted that there are more divisions of the small particles than of the large ones. The finer the particles are, the greater is their effect on the properties of the soil in proportion to their volume. Sand is gritty and does not stick together, especially when dry. Silt is flourlike, but not gritty, and the particles have little tendency to stick together. Clay is powdery, and when wet usually becomes very sticky. When dry it hardens and bakes. The clodding of soil is evidence of the presence of clay, although it does not necessarily mean pure clay. A loam soil worked when wet may clod badly.

On these textural groups are based many of the descriptive terms in common use. (Fig. 89.) A sand soil is one made up essentially of sand particles with so small an amount of silt and clay that the latter are negligible. A clay soil is one in which clay particles prevail to such an extent that the soil acts like pure clay. A silt soil contains so much material of the size of silt that it acts essentially like pure silt. A loam soil is one made up of such proportions of all grades of material that no one of them is dominant. If soil is loamy but has one constituent predominant, it may be termed accordingly a sandy loam, a silt loam, a clay loam, or perhaps a silty clay loam. Thus, there may be a great variety of combinations of these materials, giving rise to many classes of soil based on their fineness.

Because of the influence of texture on the properties of a soil and consequently on its relation to plants, soils have sometimes been named according to the kind of crops to which they are suited. For example, wheat soils are generally of a clay-loam texture. The best grass soil is usually a heavy clay. Corn does best on a loam or a sandy loam, and early truck crops are grown on light sandy loam soil.