BOOTS 37 



In order to ascertain how much nitrogen is produced by any 

 given crop it is necessary to make chemical analyses of care- 

 fully selected and weighed samples of the clover, alfalfa, or 

 other crop studied. In this way it was found in one series of 

 experiments that a single crop of alfalfa yielded 95 pounds, 

 red clover 102 pounds, and crimson clover 134 pounds to the 

 acre. As about two thirds of this nitrogen is taken from the 

 air by the plant, turning under by plowing any of these legu- 

 minous crops adds greatly to the available nitrogen of the soil. 

 A ton of barnyard manure contains only about 10 pounds of 

 nitrogen. Therefore an acre of alfalfa plowed under might 

 add to the soil as much nitrogen as could be gained from 16 

 tons of manure, though the manure would add other desired 

 substances to the soil. A crop of corn of 50 bushels per acre 

 would remove from the soil (in both grain and fodder) about 

 74 pounds of nitrogen per acre. A wheat crop of 25 bushels 

 per acre would remove from the soil about 48 pounds of 

 nitrogen per acre. Experiments continued for a series of years 

 upon worn-out land, treated year after year with lime and 

 leguminous crops plowed under, show great gains in fertility. 

 In one set of experiments lasting from 1902 to 1907 the soil 

 which had been thus treated produced a little more than six 

 times as large a crop of oats as a similar untreated area. On 

 common prairie land in Illinois the value of the nitrogen 

 gained by the root tubercles of a single crop of alfalfa was 

 found to amount to $25.80 per acre, reckoning the nitrogen 

 as worth the usual fertilizer price for it, 15 cents per pound. 

 What part of this improvement is due to better cultivation is 

 a matter still under discussion. The poorer the land is in 

 nitrogen the more effective is this process of "green manur- 

 ing " with leguminous crops. Provided the tubercle bacteria 

 are present, clover can make a vigorous growth without any 

 soluble nitrogen in the soil at the beginning. 1 



1 On the general subject of maintenance of fertility by plowing under 

 leguminous crops, see Hopkins, Soil Fertility and Permanent Agriculture, 

 chap, xvi, and Part III. Ginn and Company, Boston. 



