128 



BACTERIA AND SOIL FERTILITY 



The average results for a number of years, stated as pounds per 

 acre, of nitric nitrogen in the £rst six feet is given below. 







Season 





Crop 

















Summer 



Fall 



Spring 



Alfalfa 



'^9-3 



29^^^ 



44.9 



Oats 



10.4 



^3-3 



42.1 



Corn 



17-3 



33-3 



27.4 



Potatoes 



113.7 



IQI.1 



196.3 



Fallow 



72.8 



106.9 



143-0 



This fallow soil would contain about 100 pounds more avail- 

 able nitrogen in the spring than would the soil producing al- 

 falfa, oats, or corn. Other soils may contain more or less than 

 this amount, but usually where there has not been sujfficient winter 

 rains to wash the nitrates from the soil fallow contains more the 

 next spring than similar cropped soil. At times this is due 

 mainly to the season's accumulation of nitrates and only occa- 

 sionally to more rapid nitrification. Now, less water is required 

 to produce a pound of dry matter on a rich soil than on a de- 

 pleted soil. Moreover, rapid bacterial growth would mean the 

 formation of large quantities of acid which in turn would lib- 

 erate more phosphorus. Therefore, corps following fallow may 

 have a better supply of both nitrogen and phosphorus, and a good 

 supply of available phosphorus promotes root growth in the young 

 plant. Both of these factors would greatly help the plant in 

 resisting drought. 



Moreover, Lipman found that wheat grown on soil well sup- 

 plied with nitrates contains a higher protein content, and the pro- 

 tein content of the wheat grown on the soil of the arid west 

 where summer fallowing is extensively practised is very high in 

 protein. 



We have further found that the average nitrifying powers 

 of a cultivated soil is about twice that of adjoining virgin soil, 

 and altho the nitrate content of alfalfa soil is low, yet it promotes 



