LOSS OF FERTILITY 275 



necessary for the life of plants ; 7 of these come to plants 

 only through the soil, while 3 carbon, hydrogen, and 

 oxygen come also from air and water. The 10 necessary 

 elements are: carbon, hydrogen, oxygen, nitrogen, potas- 

 sium, phosphorus, sulphur, magnesium, iron, and calcium. 

 Chlorine may be necessary. These elements are all found 

 in ordinary fertile soil. Thus potassium is a part of clay; 

 calcium is a part of limestone; calcium and phosphorus 

 are present in calcium phosphate. Nitrogen and carbon 

 are parts of the organic matter, or humus, of the top soil. 

 Sulphur is usually present as a sulphate (cf. 221). 



While 10 soil elements are needed for plants, only 3 or 4 are used up 

 rapidly. These are nitrogen, phosphorus, potassium, and sometimes 

 calcium. It is often supposed that if any compounds of these neces- 

 sary elements are present, the soil is fertile. This is a mistake. The 

 elements must be in such compounds that the plant can absorb and 

 use them. One of the necessary conditions is that they must be in 

 compounds that are somewhat soluble; for it is the soil solution (cf. 

 295) that actually reaches the absorbing organs of the plant. How- 

 ever, these compounds must not be too soluble, or fertility will disap- 

 pear, because the rain will carry the compounds away. 



In speaking of crops we must not forget the forest; for trees are a 

 crop, as well as rice or wheat. 



299. Loss of Fertility. When we realize that plants 

 must be able to find certain elements in the soil, we can 

 see that the loss of these elements means the loss of 

 fertility. 



Fertility is lost, in the first place, by the growing of 

 crops; for crops are grown in order that they may take up 

 the constituents of the soil, and convert them into food 

 for man and animals. This loss of fertility cannot be 

 avoided. But there are many other causes of loss that 



