SOIL FERTILITY 315 



The first of these ten plant elements carbon is sup- 

 plied in the form of carbon dioxide by the atmosphere, and 

 hence forms no part of the soil's fertility. Hydrogen and 

 oxygen, which are the elements that compose water, are 

 taken up by the plant directly from the water of the soil. 

 The next four of the list, calcium, magnesium, iron and sul- 

 phur, are found in practically all soils in quantities sufficient 

 for ordinary plant growth. The fertility of the soil there- 

 fore depends chiefly on the supply of the last three, nitro- 

 gen, phosphorus and potassium. 



Rich or fertile soils are those that contain an abundance 

 of these three elements. Every crop removes some amount 

 of each of the seven elements supplied by the soil ; but since 

 calcium, magnesium, iron and sulphur are practically 

 inexhaustible, they do not have to be replaced in order to 

 maintain the soil's fertility. Nitrogen, phosphorus and 

 potassium, however, must constantly be returned to the soil 

 if it is not to become exhausted and the crops reduced. As 

 the strength of a chain is measured by its weakest link, so 

 the fertility of the soil is for most crops measured by the 

 plant element of which it has the least. 



Danger of loss of fertility. One of our chief agricul- 

 tural problems is to maintain the fertility of the soil. We 

 must all live from its products, no matter what our occupa- 

 tion. A large proportion of the tillable land of the United 

 States is now occupied. Our people must be fed from the 

 land ; there is no other source of supply. 



This means .that we should not only keep up the fertil- 

 ity of the soil, but actually increase it as time goes on. It 

 is estimated that our population is increasing five times as 

 fast as our food supply. This fact explains in part, at least, 

 the high cost of living. 



Much land has been rendered almost valueless by means 

 of single cropping, and by rental of land on short-term 



