Abstract: The physical properties of clay are of extreme importance in soil science. Plant growth, and hence crop production, within any environmental condition is largely controlled by soil structure which results from reactions involving clay. The active clay material in soil, particularly in combination with small amounts of organic matter, exerts a tremendous effect on soil properties. This effect maybe on structure (the arrangement of soil particles), or on consistence (the response of the soil to mechanical manipulation). Where structure is favorable soil grains are clumped together into effectively larger aggregates so that soils have a more open arrangement and water and air can move freely and roots function normally. Where structure is unfavorable, soils tend to be heavy and impervious, and both the physical and chemical properties of the soil become unfavorable for plant growth. Soils which are low in clay, such as sands and silts, exhibit a rather narrow range over which physical properties can change and may be unfavorable for plants, being droughty and lacking in fertility. Structure of soil may change through action of natural forces, management practices, or cropping systems, and it is of great importance that we understand how structure affects plants, and how it is formed and stabilized by reactions involving clay.
Combination of clay with relatively small amounts of certain organic compounds greatly changes the physical properties of the system and the nature of the combination and the mechanics of the soil structure-forming process are but little understood. Such changes greatly affect soil consistence and soil-water relationships as well as soil structure. The problem is made difficult since the results of any particular combination or change in clay characteristics must be interpreted not in terms of the clay system alone but in light of the resultant effect on the complex and dynamic system which constitutes a soil. Solution of such problems, however, may be of great importance to our future ability to produce food and fiber abundantly and efficiently from our limited soil resource.