106 AGRONOMY 



Manures. The word manure comes from the Latin word manus 

 meaning " hand." The reason for the derivation is seen when 

 it is known that to manure originally meant to dig or culti- 

 vate by hand. Thus in Defoe's " Robinson Crusoe," written 

 in 1719, we find the expression, " The ground that I had 

 manured, or dug up, for them was not great." Digging about 

 the plant was early found to make it more thrifty, and the 

 old farmer's maxim that " tillage is manure " is true in a 

 more literal sense than he perhaps imagines. When it was 

 found that adding various matters to the soil had the same 

 effect upon the plants as cultivation, these substances soon 

 gained the name of " manures." 



Green manures. Frequently the cultivator plows under a 

 growing crop for the purpose of adding humus and its con- 

 tained nitrogen to the soil. Such additions are known as 

 green manures. Barley, turnips, and similar crops, and even 

 weeds, may be used for this purpose, but clover, alfalfa, and 

 other legumes are usually relied upon. These latter bear 

 upon their roots numerous small nodules containing bacteria 

 which are capable of fixing atmospheric nitrogen in the soil, 

 and are therefore especially valuable for such purposes. 

 Legumes actually leave the soil in better condition than they 

 find it. 



Nitrification. Undoubtedly the most important single ele- 

 ment of plant food is nitrogen. This element is not found 

 in combination in the rocks because it is too inert to readily 

 combine with other elements, and the large quantities in the 

 air are not available to ordinary plants, though some species 

 are believed to be able to absorb nitrogen from the ammonia 

 in the air through their leaves. Small amounts of both am- 

 monia and nitric acid may be added to the soil by being brought 

 down from the air in rain water or snow and converted into 

 plant food by the soil bacteria, but the amount thus added to 

 the soil is too insignificant to make it important as a source 



