1 88 NITROGEN AND ITS RELATION TO PLANTS 



FIG. 66. Roots of soy bean having 

 tubercle-bearing bacteria. 



bacteria-made compounds dissolve in the soil water and are ab- 

 sorbed into the plant by the roots. So much nitrogen-containing 



material is made by the root 

 bacteria of plants of the pea 

 family that the soil in which 

 they grow becomes richer in 

 nitrogen, and if plants which 

 cannot make nitrogen are 

 planted in such a soil, they 

 find there a store of it. A 

 crop of peas, beans, or clover 

 is equivalent to fertilizer and 

 helps to make the soil ready 

 for other crops. 

 Artificial fertilizers. Plants need other foods besides nitro- 

 gen, and they exhaust the soil not only of nitrogen, but also of 

 phosphorus and potash. There are many 

 other substances absorbed from the soil by the 

 plant, such as iron, sodium, calcium, magne- 

 sium, but these are used in smaller quantities 

 and the supply in the soil does not readily 

 become exhausted (Fig. 67). 



Commercial fertilizers generally contain ni- 

 trogen, phosphorus, and potash in amounts 

 varying with the requirements of the soil. 

 Wheat requires a large amount of phosphorus 

 and quickly exhausts the ground of that food- 

 stuff; a field which has supported a crop of 

 wheat is poor in phosphorus, and a satisfactory 

 fertilizer for that field must contain a large 

 percentage of phosphorus. 



The amount of fertilizer needed by the 

 farmers of the world is enormous, and the 



1 3 



FIG. 67. Water 



cultures of buck- 

 wheat : i, with all 

 the food elements ; 



2, without potash ; 



3, without nitrates. 



