CHAPTER XXIV 

 LEGUMES AND FERTILIZERS— SOIL AMENDMENTS 



Four important plant foods are likely to be exhausted by 

 cropping, leaching, and washing from agricultural soils. These 

 are nitrogen (N), phosphoric acid (P2O5), potash (K 2 0), and lime 

 (CaO). Besides these four ingredients, another important factor is 

 humus — a product from the decay of organic matter. 



Nitrogen is the most important plant food in the soil (1) because 

 it is most likely to be lacking; (2) because it is ordinarily the most 

 expensive to replace by the purchase of commercial forms; and (3) 

 because its absence from the soil reduces the vigor of growth and 

 also reduces the protein content of the plants. 



The problem of supplying nitrogen for gardens and fields becomes 

 more serious as the commercial sources of nitrogen become exhaus- 

 ted. Nitrate of soda from South America is already almost too 

 expensive to be used by farmers. This is true of ammonium 

 sulfate, dried blood, and meat scraps. The two latter and cotton 

 seed meal are so valuable as stock feeds that they are no longer 

 extensively used as fertilizers. 



One important source of nitrogen still remains as cheap as it 

 was before the world war, namely, atmospheric nitrogen. This is 

 obtained by the growth of legumes, such as clovers, beans, peas, 

 vetches and others. 



Value of Legumes. — It has been known for hundreds of years 

 that members of the clover family, when plowed under, would 

 greatly increase the growth of the crops which follow them. It has 

 been only a generation since the real reason for this has been 

 known. Since this reason was discovered the use of legumes to 

 improve the soils has greatly increased. 



The power of plants of this family to obtain nitrogen from the 

 air is not found in other plants. The gathering of the nitrogen 

 from the air is dependent upon the presence of special kinds of 

 bacteria to suit the various groups (Figs. 200 and 201). 



Soils already containing these bacteria are said to be naturally 

 inoculated. Others need to be inoculated by some artificial means 

 to make the gathering of nitrogen from the air possible. 



Methods of Inoculation. — Several practical methods of inocu- 

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