148 THE FIRST BOOK OF FARMING 



posed of several elements and that seven necessary 

 elements are taken from the soil. These seven are 

 nitrogen, phosphorus, potassium, magnesium, cal- 

 cium, iron and sulphur. 



Now a fertile soil must contain these seven ele- 

 ments of plant food and they must be in such form 

 that the plant roots can use them. 



Plant roots can generally get from most soils 

 enough of the magnesium, calcium, iron, and sul- 

 phur to produce well developed plants. But the 

 nitrogen, phosphorus and potassium, although they 

 exist in sufficient quantities in the soil, are often in 

 such a form or condition that the roots cannot get 

 enough of one or more of them to produce profit- 

 able crops. For this reason these three elements 

 are of particular importance to the farmer for, in 

 order to keep his soil fertile, he must so treat it that 

 these elements will be made available or he must 

 add more of them to the soil in the proper form or 

 condition. 



Nitrogen in the soil.\ Plant roots use nitrogen in 

 ' the form of nitric acid and salts of nitrogen called 

 nitrates. But the nitrogen of the soil is very largely 

 found in the humus With the roots cannot use. A 

 chemical change must take place in it and the 

 nitrogen be built into nitric acid and nitrates. This, 

 we have learned, is done through the aid of the 

 nitrifying germs. 



Phosphoric acid in the soil. Phosphorus does not 

 exist pure in the soil. The plant finds it as a phos- 

 phoric acid united with the other substances forming 



