104 



The organic matter is also the source of supply for large 

 amounts of humus which is so essential to soils in promot- 

 ing the absorption and retention of moisture, and in the im- 

 provement of the mechanical condition and physical char- 

 acteristics of the soil. 



Humus is a material of somewhat complex composition 

 and is the result of the partial decompositi(m of organic 

 matter (whether vegetable or animal) in the soil. 



Its color varies from brown to black, owing to the stage 

 of decomposition it has reached, and the dark color of our 

 most fertile soils is due to the presence of considerable 

 proportions of this substance. 



Very light, quickly drained soils, to which the term 

 "thirsty" is often applied, are especially benefited by the 

 addition of large quantities of humus forming material, 

 which enable the soil by its increased absorptive and re- 

 tentive capacity to withstand drought much more readily. 



It also improves to a marked degree the texture of stiff, 

 difficulty tillable soils, enabling them to be Avorked more 

 easily, at the same time that it permits of more thorough 

 aeration by virtue of their increased porosity. 



Gases, such as carbonic acid, are more readily retained, 

 and in conjunction with the water with which they are 

 brought in contact, render the solution of certain forms of 

 plant food comparatively easy. 



It is also a most important agent in effecting the decom- 

 position and disintegration of the mineral constituents of 

 the soil, and in supplying some of the conditions most es- 

 sential to nitrification. 



In addition to the cow pea, which is the favorite crop 

 for plowing under in the South, the clovers and alfalfa (or 

 lucerne) rank very high as crops adapted to green manuring. 



