Soils and fertilizers . particles, byreason of their mi-nute size, tend tomake a soil plasticand may cause it tobecome hard, com-pact and cloddywhen dry. Siltdoes this to a muchless degree. Theextent to which asoil exhibits theseproperties dependson its content ofclay or silt. Soilscontaining muchclay or silt must not be plowed when wet or they will clay and silt serve to increase the water-holding powerof a soil, and clay especially increases the difficulty of tillage. The sand separates have the opposite properties ofclay, and in the order of their greater size of partic
Soils and fertilizers . particles, byreason of their mi-nute size, tend tomake a soil plasticand may cause it tobecome hard, com-pact and cloddywhen dry. Siltdoes this to a muchless degree. Theextent to which asoil exhibits theseproperties dependson its content ofclay or silt. Soilscontaining muchclay or silt must not be plowed when wet or they will clay and silt serve to increase the water-holding powerof a soil, and clay especially increases the difficulty of tillage. The sand separates have the opposite properties ofclay, and in the order of their greater size of soils are more easily worked, are not likely to puddleor to form clods, and do not hold a large amount of water,but on the contrary have a tendency to become dry. Sandysoils are termed light soils because they are easy to till ;clay soils are called heavy because they make a heavydraft on the plow. The absolute specific gravity, or weight of the particles ascompared with the weight of the volume of water which. Fig. 4. — Plan by which the soil class may beascertained from a mechanical analysis. 36 SOILS AND FERTILIZERS these particles would displace if they were immersed in it,does not necessarily correspond to these terms. Particles ofgreater and less specific gravity are scattered through both light and heavy soils and if we are to find the specificgravity of a soil we must have in the sample to be testedenough particles to give an average of all in the soil. 36. Chemical composition of soil separates. — The factthat one kind of mineral wears down to a small particlemore easily than does another indicates that there would bea preponderance of resistant minerals, like quartz, amongthe coarse particles and a large proportion of the moreeasily decomposed minerals, like the feldspars, among thefine particles. This is actually the case, and it indicatesa chemical difference in the separates. Analyses of sepa-rates made by the Bureau of Soils of the United States De-p
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