TEXTURE AND STRUCTURE OF SOILS 



39 



gravity of a soil is useful because it is an indication of the 

 amount of pore space. 



38. Relation of structure to tilth. — The term " tilth" is 

 commonly used to denote the condition of a soil with refer- 

 ence to plant growth. When the physical condition of a 

 soil is favorable to plant growth, the soil is said to be in 

 good tilth ; when the physical condition is unfavorable, it is 

 said to be in poor tilth. A loam or clay soil to be in good tilth 

 must have the greater number of its particles in a granular 

 condition. The more sandy a soil the less the necessity for 

 a highly granular structure in order that it shall be in good 

 tilth. The greater the proportion of clay in a soil, the more 

 necessary is the granular structure. One of the great ob- 

 jects in soil management is to produce and maintain the 

 granular structure. 



39. Conditions and operations that affect structure. — 

 So far as the structure of a soil is concerned, something de- 

 pends on the inherent quali- 

 ties of the soil and something 

 on its treatment by the weather 

 and by man. These factors 

 may be enumerated as follows : 

 (1) texture,. (2) wetting and 

 drying, (3) freezing and thaw- 

 ing, (4) addition of organic 

 matter, (5) tillage, (6) roots 

 and animals, (7) lime. 



40. Relation of texture to 

 structure. — A coarse sand 

 admits only of the separate 

 grain structure. There is not sufficient cohesion to hold 

 the particles in granules, and there is no plasticity. With a 

 decrease in the size of the partieles, there is a greater tend- 

 ency to the formation of the granular structure, other con- 



Fig. 6. — Structure of a loam soil 

 in good tilth. (A) sand particle, 

 (B) pore space, (C) granule com- 

 posed of silt and clay particles. 



