Although I want to give modern science its full due, I should like to point out that soil conditioning is as old as agriculture itself. Anyone who feels that soil may only be modified by the addition of certain modern chemicals will do well to refer to the works of such ancient Greek and Roman writers as Columella, Pliny and Aristotle. They knew nothing about chemical soil conditioning, of course, but they were familiar with the effects of treatment. They recommended lime and marl to loosen soil, as well as organic matter of both animal and plant origin.
POSITIVE AND NEGATIVE
Basically, the effect of all such substances, as well as modern chemical conditioners, is to supply positively charged particles which attract the negatively charged clay particles. When these combine they form clumps or floccules. The chemistry of this process is quite complex, involving both organic and inorganic colloids. Rather than attempt to go into every possible combination and how to treat it, let us return to our Chapter Two discussion of pH. By maintaining the pH of a soil between 6.0 and 6.9 (whether we add lime to a too-
acid soil or sulfur to one that is too alkaline), we can solve most of the problems involved. The one major exception is in the case of soils where sodium colloids predominate-for example the drylands area of the Great Plains-but the addition of gypsum to such soils will convert the alkali carbonates into sulfates, reducing the harmful effects of sodium salts.
Wherever heavy clay exists, the most economical and effective way to condition soil is to add organic matter. Pound for pound it is not nearly so effective as modern chemicals, but dollar for dollar the cost is about one tenth that of chemical amendment. Where pH is so low that lime is needed, the use of ground limestone should also be a part of the conditioning. Organic matter, however, has advantages possessed neither by lime nor modern chemicals. (See related discussions in Chapter Eight.)
Organic matter improves tilth in every way. It provides a steady supply of fertility by the constant breaking down of plant and animal wastes, releasing the food elements they contain. Reduced to a more stable form-humus-it has been known to continue feeding for as long as half a century after application. Its spongelike structure blots up excess moisture and stores it against periods of drought. It forms crumbs with clay that open up passages in too-tight soils. Without generous supplies of organic matter, bacteria and fungi cannot thrive but when it is present they contribute their functions to good tilth.