HUMUS IN THE SOIL 39 



to prevent the surface of the soil from baking; it absorbs 

 ammonia (which contains nitrogen) from the air, and 

 thus aids in fertilizing the soil; it also serves to some 

 extent as plant food. 



Cropping Land. When land is cropped year after year 

 without being manured, it loses much of its humus, as 

 well as much of its mineral plant food. Such a soil is 

 said to be " run down " and it rarely produces good crops. 

 It can be restored to a fertile condition only by the appli- 

 cation of the humus and the plant food that it lacks. 

 This may cost for a time as much as the crops from the 

 land are worth. It is, therefore, very unwise to crop 

 land long without restoring the humus and the plant food 

 that are removed in the crops. 



The Best Fertilizer The best way for the farmer or 

 gardener to supply his land with humus is to use plenty 

 of stable or barnyard manure. Not only does this ma- 

 terial produce humus, but, in its early stages of decay, 

 it absorbs some water from the air, and, as this water 

 contains ammonia, it adds some nitrogen to the soil from 

 the air. It also tends to warm the soil, for it gives out 

 heat in decaying and also absorbs some heat from the air. 



Plowing Crops Under. Another way of adding humus 

 to the soil is by plowing under unharvested crops, such 

 as clover, rye, turnips, or buckwheat. This is often a 

 cheaper way of supplying the soil with humus than put- 

 ting on stable manure, but, except in the case of clover 

 (Lesson 9), these crops do not add fertility to the soil, 

 because they take as much plant food out of the soil as 

 they return to it. They are, therefore, less valuable than 

 stable manure. 



