210 BUREAU OF FARMERS' INSTITUTES. 



,(b) Relation to Fertilizers. — Silicon, in the form of silica and 

 silicates, is abundant in all soils, and does need special attention 

 in connection with, fertilizers. 



(c) Specific Functions in Plant Growth. — Tt is an unsettled 

 question as to what silicon does in plant growth. Some have 

 thought that its functions might be to give stiffness to slender 

 stems in the case of such plants as grasses, sedges, etc., but there 

 are some serions objections which interfere with the complete 

 acceptance of such a proposition. Others claim, on good grounds, 

 that silicon really performs no function in plant life, its common 

 presence being an accident and not a necessity. 



Manganese. 



Manganese is a common constituent of soils, though present in 

 small quantities. The weight of evidence at hand indicates that 

 it is an accidental but common constituent of plants, performing 

 no function in plant growth and being in no wise necessary for 

 plant nutrition. 



The Relations of the Different Elements of Plant Food 



to Soils. 



General Composition and Origin of Soils. — Of the 14 ele- 

 ments necessary to perfect plant growth, 10 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 

 tin; 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 

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

 ami 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 ami vegetable life. The value of a soil for agri- 



