virgin soil is formed from the natural growth and decay of vegetation 

 during the past ages. In cultivated soils it is maintained and can be 

 increased by the application of barnyard manures and by plowing 

 under green crops or any vegetable growth. 



It must also be remembered that cropping lessens the amount of 

 humus in soil, and by continued use, that which is not consumed by 

 being made into plant food compounds, in a measure, becomes inactive. 



The following table given by Snyder shows the influence of different 

 systems of farming upon the humus content and other properties of the 

 soil: 



It will be seen from the foregoing table that, as the humus con- 

 tent decreased, the weight of the soil increased, and that with the 

 decrease in humus, there was a corresponding decrease in nitrogen and 

 phosphoric acid. The decrease in the water-holding capacity of the 

 soil is also marked, indicating the necessity of maintaining an abun- 

 dance of live humus in the seed-bed. 



Humus or organic matter is the main immediate source of nitrogen in 

 the soil. Nitrogen-fixing bacteria, which have the power to gather 

 nitrogen from the air, require organic matter in some form. Productive 

 soil contains countless millions of living forms which may be properly 

 called soil laboratory workers. These living organisms flourish on the 

 organic matter forming and transforming both organic and inorganic 

 elements into plant food compounds. Upon the number and activity 

 of these organisms depend the amount of available fertility. If humus 

 or organic matter is absent or deficient, a corresponding deficiency 

 is reflected in the crop. The availability of other elements, such as 

 potash and phosphorus, also depends upon the nature and amount of 

 the humus in the soil. Again, humus increases the absorbing and 

 retaining qualities of moisture in the soil. Humus, also, in a measure, 

 regulates the temperature of the soil ; besides, it improves the physical 

 condition to a marked degree. 



