Soil and Its Action on Plants 73 



tures on the limestone soils of Kentucky. Many herbivorous mam- 

 mals require an abundant supply of salt (NaCl) because sufficient 

 sodium must be taken in to maintain a proper ionic balance with the 

 large amount of potassium contained in their plant food. If an ade- 

 quate amount of salt is not available in the halophytes of salt meadows, 

 ruminants are forced to travel to "salt licks," or are excluded from the 

 region entirely. 



The amount of organic matter in the soil is of vital concern to 

 earthworms and other small invertebrates that use this material as a 

 source of food. A great number and variety of small insects and 

 spiders, of still smaller nematodes, and of microscopic Protozoa, which 

 live permanently in the soil, also depend upon this organic matter 

 for their nutrition. Thus the microfauna also is controlled by the 

 chemical qualities of the soil, as well as by its compactness, dryness, 

 and other physical characteristics. 



Soil and Its Action on Plants. An adequate discussion of the 

 nature of the soil, its changes in time and space, and its influence on 

 plants, and indirectly on animals through its effect on vegetation, 

 would require a whole book in itself. Nothing more than an intro- 

 duction to the subject can be given here. For a more extensive treat- 

 ment of soil itself the reader is referred to Lyon, Buckman, and Brady 

 (1952), Kellogg (1941), and the Yearbooks of Agriculture issued by 

 the U. S. Department of Agriculture. Excellent chapters on the 

 ecological relations of soils in relation to plants are to be found in 

 Costing (1948) and Daubenmire (1947). 



Besides its ecological importance as a substratum, soil has im- 

 measurably great economic importance. Fortunately we are rapidly 

 becoming aware of the critical value of the productive capacity of 

 soil as a support for civilization. Wolfanger ( 1950 ) has stated that 

 "the soil of a nation is its most valuable material heritage." The cru- 

 cial need for immediate soil conservation in almost all countries of the 

 world has been ably pointed out by Osborn (1948), Vogt (1948), 

 and others. A thorough understanding of the ecological relationships 

 involved is essential for the intelligent use of our existing soils, for the 

 prevention of further soil loss and degradation, and for the restoration 

 of the fertility of worn-out soils. 



Soil represents an extremely complex matrix consisting of minerals 

 derived from the parent rock of the area, organic matter of local origin, 

 and substances carried in by various agents. The physical nature of 

 a soil depends first of all upon its texture and structure. Texture is 

 determined by the size of the constituent particles, and structure is de- 

 pendent upon the aggregation of these particles in the undisturbed 



