334 SOILS 



hundred or more pounds of nitrogen; unde> other 

 conditions it may add little if any nitrogen to the 

 soil except that which it has drawn from the soil. 

 In order that a legume may gather nitrogen from 

 the air there must be "nitrogen-fixing bacteria" 

 in nodules on its roots. If the legume is grown in 

 a soil that has never been used for that crop, or not 

 for several years, there may be none of these bac- 

 teria in the soil. If there are none the crop will 

 not thrive, or very few nodules will be found on 

 the roots, and when there are no nodules nitrogen is 

 not gained. If a leguminous plant is dug up and 

 no nodules can be found on the roots, one may be 

 reasonably sure that the plant is not gathering from 

 the air the plant food that costs fifteen cents 

 a pound in commercial fertilisers, but that it is 

 living on the nitrates in the soil. 



Inoculating With Old Soil. If no bacteria are 

 present, they must be supplied. A few of them 

 often cling to the seeds of the crop, sometimes 

 enough to inoculate the soil quite thoroughly 

 after one or two crops of the legume have been 

 grown in it. Usually, however, it is best to in- 

 oculate with soil taken from a field on which that 

 particular crop has been grown successfully. This 

 soil contains millions of the germs; when it is 

 broadcasted or drilled in, the bacteria are spread 

 and will find the roots of the leguminous crop 

 when it is planted. From 400 to 800 Ibs. of soil 

 is sufficient. It is best to take the soil several 

 inches below the surface and in a part of the field 

 on which the plants had many nodules the year 

 previous. The practice of sprinkling old soil over 

 a new field has given luxuriant crops of legumes 

 after failures to get a satisfactory stand. 



Inoculating With Artificial Cultures. Another 



