138 SOILS AND FERTILIZERS 



quantities of nitrogen from the soil. Even when the nitro- 

 gen-fixing bacteria are present, leguminous plants maj' draw 

 much of their nitrogen from the nitrates in a soil that is 

 rich in these substances. As a result, less nitrogen is taken 

 from the air and if the crop is removed the quantity of nitro- 

 gen remaining in the soil may be no greater than before the 

 legume was planted. 



177. Soil inoculation for legumes. — After it had been 

 discovered that leguminous plants acted as hosts for bacteria 

 that draw nitrogen from the soil air, the idea at once pre- 

 sented itself that soils not containing these bacteria could be 

 inoculated with them, and thus be made much more suitable 

 to the growth of legumes. It has been found to be practi- 

 cable to accomplish this inoculation by spreading on the land 

 soil from a field on which the kind of legume it is proposed 

 to plant has grown successfully. The fact that inoculation 

 by means of soil from other fields may possibly transmit 

 weed seeds and fungous diseases, and that it also necessitates 

 the transportation of a great bulk and weight of material has 

 led to numerous efforts to inoculate soil by means of pure 

 cultures of bacteria. This has been fairly successful in re- 

 cent years, but the surest way is by the use of soil. However, 

 pure cultures may be obtained from most of the agricultural 

 experiment stations and from the U. S. Department of Agri- 

 culture, Washington, D. C. 



It must be borne in mind that when soil is used for inocula- 

 tion it must come from a field that has produced a good crop 

 of the same kind of legume that is to be planted on the inoc- 

 ulated field, also that the soil must not be allowed to become 

 very dry, as that is likely to kill the bacteria. The inoculat- 

 ing soil is applied after plowing and is harrowed in. 



If inoculation is to be successful, the soil on which the 

 legume is to be planted must be of a nature favorable to the 

 legume, otherwise growth will not be normal in spite of 



