Il6 PRINCIPLES OF AGRICULTURAL CHEMISTRY 



the cohesion between them. When the particles correspond in 

 size to silt, the wet soil may be a sticky mud, like clay, and is often 

 spoken of as a clay soil, but when dry it easily falls to powder. 



The behavior of soils upon drying is a matter of great practical 

 importance. Some soils in drying crumble easily, while others 

 form clods which are difficult to break down. This behavior 

 depends to a certain extent upon the amount of water present. 

 Some soils crumble easily when plowed in the right condition, but 

 when too much water is present they form intractable clods. 



Cohesion may be determined in the dry state, or the wet state. 

 For dry cohesion, the soil is mixed with water, molded into cakes 

 of uniform size, and dried. The amount of force required to crush 

 the cakes is then determined. This throws light on the liability 

 to form clods. 



For moist cohesion, the soil is mixed with water to 50 per cent, 

 of its water capacity, and the power required to separate a sec- 

 tion of the soil of a given area determined. This is related to 

 the plowing of the soil. 



The ease or difficulty of plowing or cultivating a given field 

 depends largely on the cohesion of the soil. A measure of this is 

 the draft of the plow. The draft of a plow on sandy soil may 

 be as low as 27 pounds per inch in depth of furrow, as against 

 100 pounds per inch for clay. This means that the former would 

 be light work for one horse, the latter heavy work for three 

 horses. 



Increasing the amount of organic matter in the soil has the 

 effect of decreasing the cohesiveness of clays, and increasing it 

 for sands. A dressing of lime also tends to decrease cohesiveness 

 of a soil. 



The state of dryness has an influence. Sand, lime, and humus 

 have little adhesion when dry, but considerable when wet. Clay, 

 under certain conditions of moisture, is very hard to plow. The 

 English practice of burning clays overcomes adhesion. When 

 clay is burned and then crushed, the particles no longer adhere 

 tenaciously when wet, and the mass is sandy-like rather than 

 clayey. 



