n6 TILLAGE MOVEMENTS OF SOIL WATER [chap. 



Of course, the conservation of soil moisture is 

 not the only good effect brought about by the surface 

 cultivation during the summer : the aeration of the 

 soil, the mechanical distribution of the nitrifying 

 bacteria that is effected, the warmth of the surface 

 layers due to their dryness, all combine to render 

 nitrification active, and to bring into a form available 

 for the plant the reserves of nitrogen in the humus of 

 the soil. This point will be dealt with more at length 

 later : for the time, it will be sufficient to remind the 

 reader how a turnip crop with its frequent spring and 

 summer cultivations is almost independent of any 

 nitrogenous manure, though it removes something like 

 ioo lbs. of nitrogen per acre : whereas a wheat crop, 

 removing less than half that quantity of nitrogen per 

 acre, often requires the application of a nitrogenous 

 manure, because it is grown on undisturbed soil in 

 the cooler season of the year. 



The saving of soil moisture which can be effected by 



hoeing is illustrated by one of King's experiments, 



when, during a dry period, the soil on a piece of land 



kept cultivated to a depth of 3 inches was sampled from 



time to time down to a depth of 6 feet, samples being 



taken simultaneously from an adjacent piece of land 



where the surface was kept smooth and firm. On the 



cultivated land there was a daily loss equivalent to 



14I tons of water per acre, which was increased on the 



uncultivated land to 17-6 tons per acre; the difference 



during the 49 days over which the trial was spread, 



amounting to 17 inch of rain saved by the cultivation. 



The value of surface cultivation is well seen in other 

 trials of King's, where the water content down to a 

 depth of 4 feet was compared on two adjacent pieces of 

 land, one stirred to the depth of 3 and the other to \\ 

 inches only. The 3-inch soil mulch, taking the whole 



