PLANT FOOD AND SOIL FERTILITY 27 



plant use by certain bacteria, chiefly those occurring on the 

 roots of such crops as clover; certain products rich in nitrogen 

 that are sold as commercial fertilizer. 



Value of organic matter. In the early days of farming 

 the soil was treated as if it were inexhaustible; crops were 

 taken off year after year without a thought of returning to 

 the soil any equivalent of the plant food removed. As a 

 consequence, the yield became less and less until finally the 

 farms became unprofitable. They were rightly called " worn- 

 out " farms. and were often abandoned. Abandoned farms 

 are still found in some parts of the Eastern States. One 

 practice, which more than anything else brought about a 

 decrease in yield, was the removal of the organic matter. 

 This practice made farming hard for two reasons; it reduced 

 granulation and the water-holding capacity of the soil, and 

 depleted the store of nitrogen. Many worn-out farms have 

 been brought back to fertility by being liberally supplied 

 with organic matter. 



Making the nitrogen of organic matter available for plant 

 use. The presence of bacteria in fertile soils has been 

 referred to several times. Bacteria are very small plants, so 

 small that the aid of a good microscope is necessary to see 

 them at all. It would require many thousands in line to 

 span an inch. But they make up for their small size by 

 their great numbers. It is estimated that a cubic centimeter 

 of rich soil will contain from 500,000 to 5,000,000. 



Some of these bacteria are able to convert the complex 

 and insoluble compounds of organic matter containing nitro- 

 gen into simple, soluble compounds of nitrogen, chiefly 

 nitrates. Nitrates, being soluble, are readily absorbed by 

 plants. 



Bacterial action is required in order to make available for 



