SOIL FERTILITY AND PLANT GROWTH 237 



The first of these ten plant elements, carbon, is supplied 

 in the form of carbon dioxid 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 sulphur, are 

 found in practically all soils in quantities sufficient for ordi- 

 nary plant growth. The fertility of the soil therefore de- 

 pends chiefly on the supply of the last three, nitrogen, phos- 

 phorus 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 inex- 

 haustible, they do not have to be replaced in order to main- 

 tain the soil's fertility. Nitrogen, phosphorus, and potas- 

 sium, however, must constantly be returned to the soil if 

 it is not to become exhausted and the crops reduced. Like 

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

 the fertility of the soil is 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. Most of the tillable land of the United States is now 

 occupied. Our people must be fed from this 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, particularly in the East and South, has been 

 rendered almost valueless through careless methods of 



