250 CROP PRODUCTION 



tiny bacteria that help the plant to grow by gathering 

 the free nitrogen from the air. 



Find out what farmers in your vicinity plow under 

 green crops to enrich the soil. Learn whether these 

 crops belong to the great family of legumes. 



Soil Fertility 



There are certain chemical compounds which are espe- 

 cially necessary for the growth of crops. The most 

 important of these are phosphoric acid, potash, and nitro- 

 gen. Nearly all crop plants take up large quantities 

 of these three materials, and it is often necessary to 

 replace this loss by the addition of special combinations 

 of chemicals containing them. Such combinations are 

 added for the purpose of fertilizing the soil, and as these 

 fertilizers are usually sold in large quantities commer- 

 cially, they are commonly called commercial fertilizers. 

 Most soils contain certain amounts of these substances 

 which were originally present in the formation of the 

 soil or have been developed through the action of bac- 

 teria or chemical compounds on the humus or other 

 soil material. 



In order to be available for the growth of crops, all 

 of the materials that enter into the plant must be either 

 in a Hquid form or dissolved in water. A soil may con- 

 tain considerable amounts of potash, phosphorus, or 

 other elements which are locked up so far as plants are 

 concerned, because they are not in the soluble form. 

 One cannot always tell from the mere fact that a soil on 

 chemical analysis shows large amounts of these essential 

 elements that it is necessarily in a fertile condition. 



