52 SOILS AND FERTILIZERS 



Partially decomposed organic matter is of benefit to soils 

 in a number of ways and it may also be injurious. These 

 properties will be discussed later. The term " humus" 

 has been somewhat loosely used with reference to the sub- 

 stances of this class. It will not be used in this book. 

 Organic substances represent a wide range of intermediate 

 products of decomposition. They profoundly affect the 

 properties of soils and are always present in arable soils. 



Final products of decomposition of organic matter are 

 water and gases. The latter may unite with some of the 

 inorganic matter of the soil to form purely inorganic sub- 

 stances, and these are as a rule readily available to plants. 

 They differ from the substances of the other two classes in 

 that none of them is injurious to crop production. 



51. Beneficial effects of organic matter. — There are 

 many ways in which organic matter may benefit soils, either 

 directly or indirectly. Soils differ somewhat in the effect 

 that organic matter may have on some of their properties. 

 An example of this has been cited in the effect of organic 

 matter on a heavy soil in a humid region as compared with 

 its results in a light soil in a semi-arid region. Another 

 example is to be found in the results that follow the plowing 

 under of green-manures. In some soils and under certain 

 conditions this may be temporarily injurious, although it 

 is usually a very beneficial practice. 



An enumeration of the beneficial effects of organic matter 

 in soil is necessarily open to criticism on account of the dif- 

 ferent responses of different soils, but with some modifications 

 the following will hold. 



(1) It increases the tendency towards the formation of 

 granular structure. 



(2) On account of the porous nature of organic matter 

 the pore space of the soil is increased and aeration improved. 



(3) It increases the water-holding capacity of soils. 



