New York Agricultural Experiment Station. 55 



effect upon the formation of the green coloring-matter or chloro- 

 phjl of plants. 



5 The Relations of the Du^ferent Elements of Plant-Food 



to Soils. 



General Composition and Origin of vSoils. — Of the fourteen 

 elements necessary to perfect plant growth, ten come exclusively 

 from the soil, as previously indicated. These have already been 

 described, and we do not need to give further attention to them in 

 this place. The soil-derived elements, though forming on an aver- 

 age only about five per cent, of the whole vegetable kingdom, are 

 of the utmost interest and importance to the farmer; for, while the 

 atmosphere is in itself entirely beyond his control, he can, through 

 the medium of the soil, influence the amount of air-derived con- 

 stituents taken up by plants. 



Soils consist of decomposed rocks mixed in varying proportions 

 with organic matter called humus, formed by the decay of animal 

 and vegetable substances. The principal part of the soil was once 

 solid rock, and the first step toward the formation of soil was the 

 powdering of the rock. The conversion of .rocks into soil has been 

 accomplished by means of various agencies, such as heat and frost, 

 moving water and ice, chemical action of air and water, and the in- 

 fluence of animal and vegetable life. The value of a soil for agri- 

 cultural purposes depends largely upon the original material from 

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

 been reduced. 



Food Constituents and Mechanical Constituents of Soils. — 

 The constituents of soils can be divided into two general 

 classes, which we will call {a) food constituents and (b) meGhanical 

 constituents. 



(a) Food constittients include the ten soil-derived elements which 

 are essential to the development of plants. They may be divided 

 into two kinds, available and unavailable food constituents. 



The food constituents of the soil are available when they are 

 soluble ; that is, when they are in such forms as the plant can take 

 in and use. They are una/vaildble when they are in an insoluble 

 condition and can not be used at once by the plant. 



{b) The nnechanical constituents of the soil include (1) clay., (2) 

 sayid^ and (3) htimus. These act as a mechanical support to plants 

 and as indirect fertilizers. 



