96 ELEMENTS OF AGRICULTURE 



plant food. As humus decays, it also liberates carbon 

 dioxid (carbonic acid gas). This acts on the minerals 

 of the soil, making them soluble and ready for plant use. 

 Another extremely important function of humus is that 

 it encourages the growth of the bacteria that fix free 

 nitrogen from the soil air, rendering it available as a plant- 

 food. Dark-colored soils usually contain considerable 

 humus. Such soils are usually fertile. 



The more air in the soil, the more rapidly the humus 

 is decomposed. If a soil is saturated with water, the oxi- 

 dation practically stops and organic matter accumulates. 

 This is the way that peat and muck are formed. For 

 crop-production, a moderate rate of decomposition is to 

 be preferred. If too rapid, the supply is exhausted; if too 

 slow, the plant does not receive enough food. 



104. Humus of Arid and Humid Soils. In humid regions 

 the soil is usually much darker colored than in the arid 

 regions. The surface soil is much darker than the subsoil 

 because of the presence of more humus. In arid regions 

 the soils are so well aerated that the organic matter is 

 rapidly decomposed, leaving no difference in color of soil 

 and subsoil. The subsoils in humid regions are not very 

 productive, but in arid regions there is not much difference 

 between soil and subsoil. This is a great convenience, 

 for it makes it possible to level fields for irrigation. 



Some analyses have shown that, on an average, there 

 is about four times as much humus in humid soils as in 

 arid ones. At first thought, this would indicate a lack of 

 nitrogen in arid regions. But the humus in arid regions 

 is so much richer in nitrogen that the total amount present 

 is not much less. 



