SOIL MANAGEMENT. 53 



Light-coloured soils are notoriously low in productivity, a 

 condition largely attributable to the small humus content of 

 these soils. The reverse applies in the case of dark soils, 

 where the humus content is usually high and the productivity 



The formation of humus is due to the activities of soil 

 micro-organisms, and is facilitated by good aeration, proper 

 drainage, high temperatures, and by the presence of lime. 



While the prime object of green-manuring is to increase 

 the humus content of the soil, it serves at the same time a 

 number of additional useful purposes. The various useful 

 functions of green-manuring may be summarised briefly, as 

 follows : — 



(1) It adds easily decomposable organic matter to the 

 soil. 



(2) It increases moisture retention bv the soil. 



(3) It improves tilth, giving " body " to sands, and ren- 

 dering clays more friable. 



(4) It stimulates the activity of useful soil micro- 

 organisms. 



(5) It stimulates the production of available mineral plant 

 food from the soil's reserves. 



(6) It increases the amount of available plant food in the 

 surface soil, because the plant foods taken un by the crop from 

 various parts of the soil during the entire period of its growth 

 are left in the surface soil when the crop is ploughed down, 

 and are soon set free in available form. 



(7) It increases the nitrogen content of the soil if the 

 crop used is a legume, and the necessary nodule bacteria are 

 present. 



(8) Green manure crops serve to some extent as cover 

 crops by preventing leaching and conserving the available 

 plant food. 



The yield of dry matter of crops used for green-manuring 

 varies considerably, both in the above-ground parts and in the 

 roots. At the Delaware Experiment Station, the weights of 

 dry roots of various crops per acre in the first foot of soil were 

 found to be : — 



Red Clover, 1,202 pounds; 

 Vetch , 600 pounds ; and 

 Soy Beans, 756 pounds. 

 From this it will be seen that when the above-ground 

 parts are removed the residual effect of the roots will vary 

 with the crops grown, and consequently the extent of humus 

 depletion will also vary. 



