. Soils and fertilizers . e less the necessity fora highly granular structure in order that it shall be in goodtilth. The greater the proportion of clay in a soil, the morenecessary is the granular structure. One of the great ob-jects in soil management is to produce and maintain thegranular 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 somethingon its treatment by the weatherand by man. These factorsmay be enumerated as follows :(1) texture, (2) wetting anddrying, (
. Soils and fertilizers . e less the necessity fora highly granular structure in order that it shall be in goodtilth. The greater the proportion of clay in a soil, the morenecessary is the granular structure. One of the great ob-jects in soil management is to produce and maintain thegranular 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 somethingon its treatment by the weatherand by man. These factorsmay be enumerated as follows :(1) texture, (2) wetting anddrying, (3) freezing and thaw-ing, (4) addition of organicmatter, (5) tillage, (6) rootsand animals, (7) lime. 40. Relation of texture tostructure. — A coarse sandadmits only of the separategrain structure. There is not sufficient cohesion to holdthe particles in granules, and there is no plasticity. With adecrease in the size of the particles, there is a greater tend-ency to the formation of the granular structure, other con-. Fig. 6. — Structure of a loam soilin good tilth. (A) sand particle,(B) pore space, (C) granule com-posed of silt and clay particles. 40 SOILS AND FERTILIZERS ditions being equal. This does not mean that a clay soil iseasier to keep in good tilth than is a loam soil, but underfavorable conditions the small particles have greater plastic-ity and cohesion and hence form granules more readily. 41. Wetting and drying. — As a soil becomes dry thereis a contraction of volume in which process lines of cleavageor cracks occur and clods are formed. If these clods beagain wetted and partly dried without working, they willseparate into smaller clods and finally a granular structurewill be produced. This is illustrated by the greater easewith which clods may be worked down after a rain andpartial drying, than when they remain perfectly dry. Landin need of drainage is usually in poor tilth, while after drain-age this condition gradually improves. 42. Freezing and thawi
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