Way, in the period near 1850, established that the base exchange was due chiefly to the very finely divided materials in the soil. Prominent among these materials were the clays, and Way made synthetic materials by precipitating aluminum and silicon compounds that somewhat resembled the clays and possessed base-exchange properties. But these synthetic materials, which have long been exceedingly important for softening water, differed markedly from clays in many of their properties, such as the capacity to remain suspended in water without change. An understanding of base exchange, then, became largely an effort to understand the nature of clays. This search was successful after 1925.

We can separate soils into fractions of various particle sizes by shaking them through sieves and by suspending them in water. The coarse fractions are sands. The intermediate ones down to the limits of microscopic magnification are silt. Those below these limits are the clays, which settle very slowly in water.

If a soil after separation into fractions is reconstituted with omission of the clay, its cohesive properties will be greatly diminished, particularly if the soil is a loam or heavier in clay content. The clay, then, apparently is the fraction involved not only in base exchange but also in interaction with water in soil. In management, as an example, the clay interaction with water is determinative for many properties, such as those underlying the tendency of soil to erode and its features of internal drainage.

The basic principles about clays are to be found on an atomic scale. You might think it strange that this is the case for such gross features of a field as its lime requirement, its tendency to erode, the draft requirement for plowing, or the required spacing of drains. These properties of the whole field, though, are only reflections of the most minute parts. While the four features are quite different in external appearance, they might depend on the same (or on only a few) minute features.

KNOWLEDGE ABOUT CLAYS on the atomic scale advanced only after a long period of development in the basic physics of atomic structure. In principle, to work with phenomena on a particular scale, the measuring instruments must have features on that scale. Thus, for measuring a mile to the nearest foot, a measuring tape need only be divided into feet, which would make it unsuited for measurement of thickness for a plastic film.

The desired basic information about clays was the arrangement of the atoms. The measuring method that proved useful in finding the arrangement was the diffraction of X-rays.

X-rays are like visible light in that they can be resolved into different wavelengths by diffraction and refraction. In visible white light, the resolution into the different component wavelengths or colors by refraction is shown in a rainbow. This resolution can also be done with an ordinary window screen and a light by diffraction, as can be seen by looking at a distant light through the screen. The degree to which the various colors are separated depends on the fineness of the screen the smaller the separation of the wires, the greater is the color separation. The diffraction then can be used to measure the wire separation in the screen.

X-rays have wavelengths, equivalent to color in visible light, of about the same value as the separation of atoms in clays. The atoms in the clays, as in all crystals, irrespective of size, are arranged in an orderly and repeated manner much like the men in an army passing in review. This arrangement corresponds to the wires of the screen for visible light. Accordingly, the diffraction of the X-rays by the atomic arrangement in the clays can serve to measure the arrangement.

The kinds of atoms present in the clays are chiefly silicon, aluminum, and oxygen, with small amounts of the basic elements such as calcium, magnesium, sodium, and potassium, and the acid element hydrogen. They are the ones taking part in base exchange. This knowledge of composition came from the analysis by chemists of many pure clays collected from mineral deposits by geologists and mineralogists.

The diffraction of X-rays by a clay shows that the silicon atoms, which are relatively small, are each surrounded at the corners of a tetrahedron by larger oxygen atoms and that each oxygen atom is between two silicon atoms, which thus serve to link tetrahedrons together. The aluminum atoms, which are similar in size to silicon, are substituted, in part, in place of the silicon atoms. The tetrahedrons upon linking are formed into sheets, and two of these atomic sheets are joined by aluminum atoms between them.

This arrangement of the atoms in the clays has three features, which are the basic ones underlying many properties of soils. These features are: The atoms are arranged in sheets; the surfaces of the sheets are largely the surfaces of oxygen atoms; and the attraction between atoms in the sheets depends on the relative number of oxygen, silicon, and aluminum atoms.

As a result of the atomic arrangement in sheets, the clays can be split between the sheets and thus reduced to particles that are exceedingly thin. The clays therefore are finely divided; that is one of the necessary features for formation of a mud.

OXYGEN ATOMS on the surface result in attraction of water molecule through binding with the hydrogen atoms of the water. In other words, the sheets have a high tendency to be wetted with water. In one type of clay, as a matter of fact, water molecules on the sheet surfaces separate the sheets and cause the clays to swell. It is for this reason that many of the alkali soils have such poor drainage. It is the basic reason for the cracking of many soils during prolonged droughts.

If the attraction of the atoms is not balanced on the scale of atomic dimensions, other atoms must be present for the balance. These atoms can only be present on the surfaces of the sheets. On the surfaces they can readily be separated in the water layers that are also there. Accordingly, they are the atoms that can be exchanged with others the atoms of base exchange the atoms involved in the acid reactions of soils and the liming requirements.

As knowledge of clays developed, it was found that there are several ways of making the atomic arrangements in sheets and that clays with different arrangements differ considerably in such properties as the amount of their base exchange and their tendencies to swell in water.

A first question about most soils is, "What is the clay type?"

This question is accompanied by, "What is the soil texture; how much clay is present?" Finally, "What is the base-exchange capacity?"

A guiding method for learning more about clays in soils was to work on pure clays and from their properties to assess their degree of contribution to the general properties of the soil. Other soil components also affect these properties, and the part played by organic matter or humus can be great. Can the principles basic to the action of organic matter also serve as a guide through some of the complexities of soil?

Until the time of Liebig, the idea was held that humus was directly used by plants and as such contributed to soil fertility. Liebig showed that plant growth instead depends upon inorganic compounds. Organic matter is useful for fertility only as it is broken down with release of the constituent nitrogen and phosphorus into inorganic forms. Even today, though, some nonagricultural persons maintain that humus has direct attributes in fertility.

With the advance near the end of the last century in knowledge of bacteriology and the requirements of micro-organisms for growth, it became evident that humus was, in part, a product of the action of micro-organisms and, in part, their sustaining food. The release of the nutrient elements required the destruction by microorganisms of the organic matter from past crops or that of the native soil. From this point of view alone, the best practice would utilize the organic matter as rapidly as possible.

The organic matter has other properties. One is base exchange, or the capacity to hold nutrient elements, such as potassium, calcium, and magnesium, in saltlike combination much as do clays. Destruction of organic matter naturally reduces this exchange capacity of the soil.

The desirable effects of organic matter on the structure and, through these effects, on the physical properties of soil are of greatest importance. This is the discovery of no particular person but it is readily observed by all who are familiar with soils. It has to do with mellowness and friability, with the maintenance of a good tilth, and the preservation of a loose and uncompacted soil.

The question of principle would be to establish how organic matter contributes to desirable structure in soil.

The answer has not been found even by repeated inquiries into the nature of organic matter. Rather, it seems that something still is to be established about the interaction between organic matter and surfaces of clay minerals.

The most progress in this regard was the finding that some of the polymeric compounds related to the materials of plastics but containing more acid groups when added to clays or to soils have the desired action on structure. The principle, as vaguely formed, involves, in part, the presence of a number of acidic organic groups held together in one molecule that is resistant to attack by micro-organisms.

These acidic groups interact with the surfaces of the clay minerals. The natural materials possessing these properties, in part, are gums formed by bacteria. They are not very stable against further attack by bacteria, however, and have to be constantly renewed by supplying fresh organic matter for the bacteria to consume.

Fertility, the properties of clay, and the functioning of organic matter are examples of factors in soils for which basic principles have been sought.

Each principle is treated in chapters that follow. Other principles also are expressed, particularly with regard to the behavior of water in soils for which a basic understanding is well developed.

In the end, the search for principle is the only way by which we can gain information from one soil that is useful for farming another soil. Information about properties of the individual soils become an orderly and consistent part of knowledge about all soils.