SOIL IMPROVEMENT 161 



slowly. The value of humus to a soil can scarcely be over- 

 estimated because it improves the soil in so many ways. The 

 addition of humus to a soil is one of the surest ways to im- 

 prove it. Decaying humus adds mineral salts and nitrogen- 

 carrying amino acids. It improves the physical condition by 

 making the soil looser and increasing the water-holding 

 capacity and the aeration. 



Plants respond to applications of animal manures which 

 cannot be explained by the above, and, in fact, has never 

 been clearly determined. It has been suggested that the 

 micro-organisms of the soil may be aided by manure and that 

 the free-living nitrogen-fixing bacteria (Azotobacter) may 

 increase the nitrogen supply to the plants. Recently it has 

 been found to supply certain hormones which may be very 

 important. 



Humus is a constituent of all soils but it accumulates very 

 slowly under favorable conditions. Heavy annual applica- 

 tions of manure adds humus only slowly. Best results are 

 had by its slow natural addition to the soil. It is very easy 

 to destroy the humus by excessive cultivation or by the loss 

 of the top soil through erosion. 



Humus is one of the most important factors in soil conser- 

 vation. Cultivation increases the rate of decomposition of 

 humus, which decreases the absorption of rainfall, thereby 

 increasing the amount of the soil carried away. Both types 

 of humus loss have taken place from many farm soils, where 

 cultivated crops predominated, with the loss in productivity 

 becoming so great that the farms have been abandoned. The 

 rotation of crops in which humus-adding crops are used in 

 place of cultivated ones has been helpful for their influence 

 on erosion. 



Compost, an artificial humus, is made, usually, by mixing 

 plant material, sand, a high phosphate fertilizer, and some- 

 times manure, in a pile to decompose for a year or more. It 

 is a means for a gardener who has the space, to make use of 



