The Composition and Use of Fertilizers. 211 



cultural purposes depends largely upon the original material from 



which it was made, and upon the state of fineness to which it has 

 been reduced. 



Soil-Particles. — The fineness of division required in a good 

 agricultural soil is not easily realized. According to estimates 

 reliably made, a single ounce of good soil contains many thou- 

 sand-millions separate particles. If the surface of all the particles 

 in one cubic foot of soil could be spread out in one continuous 

 plane, they would cover an area of two or three acres; and it is 

 to be kept in mind that this great spread of surface is accessible to 

 the root systems of plants in each cubic foot of such soil. 



One classification of soils is based upon the size of soil-particles. 

 The particles of a soil may be separated into two general classes. 

 The first is called " skeleton " and consists of particles coarser than 

 one-fiftieth of an inch in diameter. The second class is called " fine 

 earth " and consists of all except the " skeleton." In agricultural 

 soils " skeleton " varies from five to twenty per cent. 



The " fine earth " is divided into six grades of soil-particles, 

 called (a) medium sand, (b) fine sand, (c) very fine sand, (d) silt, 

 (e) fine silt, (/) clay. 



Sand. — ■ The term sand, used in connection with soil-particles, 

 includes those particles whose size varies from one-fiftieth to one- 

 five-hundredth of an inch in diameter. These are subdivided into 

 three grades of sand. Soils characterized as sandy contain from 

 five to fifteen per cent, of silt and clay, as pure sand would have 

 no agricultural value. When mixed with clay, sand serves to 

 diminish the compactness of the clay, making it more porous and 

 permeable to air and water. Too large a proportion of sand rend- 

 ers soils too porous and open, so that they admit water, warmth, 

 and air too freely and there results excessively rapid evaporation 

 of soil moisture together with marked heating of the soil and a 

 consequent premature ripening and burning of the crops. 



Silt. — Soil-particles included under the term silt vary in size 

 from one-five-hundredth to one-five-thousandth of an inch. The 

 characteristics are intermediate between sand and clay. A silt soil 

 is more permeable to water than clay but less so than a sandy soil. 



Clay. — The word clay used in connection with soil-particles 

 has reference only to the size of the particles without regard to 

 chemical composition. It includes all soil-particles whose size is 

 less than one-five-thousandth of an inch in diameter. Arable soils 

 contain from three to fifty per cent, of clay. Used in its chemical 



