38 MANURES FOR FRUIT TREES 



temperature, tilth, permeability, absorptive power, 

 weight and colour of soils, and directly or in- 

 directly controls to a high degree their supply of 

 water, nitrogen, phosphoric acid, and potash. 

 Humus, when not present in excessive proportions 

 to make the land " sour," increases the fertility 

 of a soil — and the loss of humus means a decline 

 of fertility. Humus contains nitrogen with potash 

 and lime as humates and phosphates — and the 

 decline of humus means a loss of nitrogen and 

 other fertilising elements. A virgin soU, of good 

 quality, contains about 4 per cent, of humus ; 

 and after twenty years of cropping would show 

 about 2 '5 per cent, of humus — therefore, there 

 has been a loss of IS per cent., equivalent to 

 about 3 '500 lbs. per acre ; this is due to the 

 decomposition of the humus by the bacteria of 

 nitrification and denitrification. The nitrifying 

 bacteria feed upon the humus, yielding nitrates, 

 which may be washed out of the soil in the 

 drainage, and the denitrifying bacteria complete 

 the work by feeding upon the nitrates, pro- 

 ducing nitrogen gas, which escapes into the air. 

 Nitrification is one of the most important 

 functions for rendering the inert nitrogenous 

 compounds available to plants. Under in- 

 judicious management or cultivation of the soil 



