Farm Manure. 



143 



for the slight gain in phosphoric acid, but it was assumed that 

 no other concentrates or fertilizers were purchased. The de 

 cidedly smaller loss of nitrogen on the second farm and the actual 

 increase of nitrogen on the stock and dairy farms are due to 

 fixation of nitrogen from the growth of clover. The figures rep- 

 resent pounds of fertilizing material lost or gained on the farm 

 in 1 year. 



Effect of Style of Farming on Fertility. 



Green manuring. The lowered crop producing power of a soil 

 is in many instances due to the rapid decrease in the amount of 

 humus which it contains. Humus is formed in most cases from 

 the plants which have previously grown on the field and have 

 later become a part of the soil. It may be produced from animal 

 or vegetable material added as manure. Virgin soils are rich in 

 humus, but continued cropping with no provision for maintaining 

 the supply may result in its being decreased from one-third to 

 one-half in a period of not more than 15 years. Humus is of im- 

 portance because it is a storehouse of plant food, especially nitro- 

 gen. Most of the nitrogen of the soil is contained in the more 

 or less decomposed organic matter present. 



Plowing under green crops, grown for that purpose, is one of 

 the oldest means of increasing the humus content of soil. By 

 this practice, not only is the soil enriched with carbonaceous mat- 

 ter derived from the air, but a considerable amount of nitrates 

 which would have been formed by nitrification during the growth 



