Fertilizers and Shade-crops 197 



Humus usually exists only in small quantities in arid 

 soils, and often is entirely absent when clean cultivation 

 without manuring has been practiced. It may also be 

 present in small quantities along with partially decayed 

 organic matter. Much of this valuable material is lost 

 by constant clean cultivation and the consequent burning 

 of the organic materials, before it reaches the stage in 

 which humifaction may take place. A large part of the 

 loss of nitrogen could be prevented, and many of the 

 benefits conserved, by shading the soil. 



While it is no doubt true that humus aids in improving 

 the physical condition of the soil, it is probable that much 

 of the beneficial effect, such as giving fiber and improving 

 tilth, as well as increasing the water-holding capacity, 

 is due in a much larger degree to the partially decayed 

 organic matter. 



The Leguminous Green-crops 



Several leguminous crops are known to be of great im- 

 portance in orchard management, but their value needs 

 to be still further emphasized. By leguminous plants is 

 meant the members of the family that includes the beans, 

 peas, cowpeas, clovers, alfalfa, and vetches. These are the 

 only plants of agricultural importance, so far as now 

 known, that have the power of taking nitrogen from the 

 air and converting it into combinations that other plants 

 can use. Thus they actually add plant-food to the soil; 

 and nitrogen is the most expensive element to supply in 

 commercial form as well as the most difficult to keep, 

 since it is readily dissipated in gaseous form. A certain 

 species of bacterium lives on the roots of vigorous legumes 



