Part II.] MAINTENANCE OF SOIL FERTILITY. 37 



The Nitrogen Supply. 



But while the composition of the crop may offer a sugges- 

 tion as to the balance between the mineral elements of plant 

 food required, the case is different when we come to nitrogen. 

 For example, when we analyze equivalent crops of corn and 

 wheat, as grown under equally favorable conditions of soil and 

 climate, we find that the corn has secured twice as much 

 nitrogen from an acre of land as the wheat; but if we grow 

 the two crops on land of the same character a nitrogenous 

 fertilizer may produce twice as great a proportionate increase 

 in the wheat as in the corn. And while neither of these crops 

 can be grown to perfection on a nitrogen-hungry soil without 

 the addition of some source of nitrogen, a perfect clover crop 

 may be grown on such a soil if only the mineral elements — 

 phosphorus, potassium and especially lime — are provided in 

 available form and in sufficient abundance. 



The superior ability of corn over wheat to obtain its nitrogen 

 supply is explained by the fact that the corn crop is grown 

 during the hot months, when the process of nitrification, by 

 which the inert organic nitrogen in the soil is converted into 

 available form through the action of the nitrifying micro- 

 organisms, is most active, while the wheat is grown chiefly 

 during the cooler months; but clover, as is well known, owes its 

 advantage to the action of the bacteria which inhabit the 

 nodules on its roots. 



In Broadbalk Field, at the Rothamsted Experimental Station, 

 wheat has been grown continuously since 1843. Starting with 

 an average yield of 17 bushels per acre for the first ten years 

 the yield has fallen to 10 bushels for the ten years 1903-12, 

 averaging 12.6 bushels for the sixty-one years 1852-1912. 

 When a mineral fertilizer, made of 342 pounds acid phosphate, 

 200 pounds sulphate of potash and 100 pounds each of the 

 sulphates of soda and magnesia, has been used the yield has 

 been 14.5 bushels, a gain for this large dressing of minerals of 

 less than 2 bushels of wheat. When to this dressing of minerals 

 43 pounds of nitrogen has been added in ammonium salts the 

 average yield has been increased to 23.2 bushels. Doubling the 

 ammonium salts has increased the yield to 32.1 bushels, and 



