8 GENERAL CONSIDERATIONS 



bility of a soil for different crops will depend very 

 largely on these percentages. 



Composition of Soils. — Tlie chemical composition of 

 soils naturally varies quite widely according to the 

 nature of the rocks from which they have been de- 

 rived and the climatic influences to which they have 

 been subjected. The smallest particles constituting 

 the clay are usually silicate of alumina, while the 

 sand is mostly silica. These two substances in vary- 

 ing proportions form the great bulk of most soils, 

 but they are both inert so far as supporting vegeta- 

 tion is concerned. A soil containing only these tw^o 

 substances would be absolutely sterile. Combined 

 with these in varying quantities are usually found 

 iron, carbonates and sulphates of lime and magnesia, 

 and minute quantities of phosphate of lime and of ni- 

 trates, carbonates, chlorides, and silicates, of potash 

 and soda. From a chemical point of view the fertility 

 of the soil mostly depends on the small amounts of ni- 

 trates and phosphates and of potash which it contains. 

 At least the other essential elements of plant food are 

 usually sufficiently abundant in all soils, but the quan- 

 tity of one or more of these three is often insufficient, 

 especially in soils that have been long in cultivation. 

 The solubility and therefore the availability of the 

 phosphates and potash in the soil depends largely on 

 the amount of vegetable matter which it contains. 

 The nitrates, which may be considered the most im- 

 portant of all the soil substances, are mostly derived 

 from decaying organic matter, so the amount of this 

 material in the soil may often be taken as an index 

 of its fertility. Bog soils, however, contain so large 



