28 SOILS 



No one kind of rock contains all the elements, 

 but all of the rocks from which fertile soil is made 

 contain at least seven of them nitrogen, potassium, 

 phosphorus, calcium, iron, magnesium, sulphur. 

 No plant can grow unless these seven are present 

 in the soil; they are the "plant foods," and con- 

 stitute from 80 to 90 per cent of most fertile soils. 

 The first four of these seven are much needed by 

 plants and so the soil is most likely to be exhausted 

 of them by continuous cropping; while the latter 

 three are usually so abundant that the farmer is 

 never concerned about how he may add them to 

 his soil. The nature and sources of these four 

 essential plant foods, nitrogen, potash, phosphorus, 

 and calcium, which are the necessary constituents 

 of fertilisers, are discussed in Chapters XI to 

 XIV. 



Besides these seven elements in the soil which 

 are absolutely necessary for the growth of plants, 

 a number of others are frequently absorbed Iby the 

 roots of plants and used by them. Of these the 

 most common are chlorine, silicon, aluminum, and 

 manganese. Numerous experiments have shown 

 that plants thrive as well without these as with 

 them, so they must be considered as accidental 

 or unnecessary elements. 



In considering the mineral contents of the soil 

 as a supply of food for the growth of plants, we 

 must not forget that the soil furnishes but a small 

 part of the material out of which plants are made. 

 We are so actively engaged in trying to keep up 

 the fertility of our soils by checking their wastes, 

 and by adding to them fresh supplies of the min- 

 erals mat our crops have taken from them, that we 

 are apt to think that the plant comes from the soil 

 alone. Yet over 90 per cent, of the crops that we 



