112 



SOILS AND FERTILIZERS 



humus. When the vegetable matter has been only 

 partially removed, the crops at first may be smaller, 

 but in a few years returns will be greater than if all 

 of the vegetable matter had been burned. 



120. Physical Properties of Soils influenced by Humus. 

 The physical properties of a soil may be entirely 

 changed by the addition or the loss of humus. The 

 influence of humus upon the weight, color, heat, and 

 water-retaining power of soils is discussed in the chap- 

 ter on the physical properties of soils. Soils reduced in 

 humus content have less power of storing up water and 

 resisting drought. This fact is illustrated in the follow- 

 ing table : 31 



PER CENT WATER 



121. Humic Acid. In the absence of calcium car- 

 bonate or other alkaline material, the vegetable matter 

 of soils through processes of decay may form organic 

 acids destructive to the growth of some crops. The com- 

 position and physical properties of these organic acids 

 have never been determined, and the indefinite term 

 ' humic acid ' has been applied to them. Succinic acid 



