SOIL MANAGEMENT. 33 



hydrates and ash, and to decrease the protein content of the 

 crop. Besides these effects it is apt to encourage lodging in the 

 cereals and to predispose them to disease, e.g., rust. Injudi- 

 cious irrigation often has the effect of promoting vegetative 

 growth at the expense of fruiting. Losses due to frost may 

 sometimes be lessened by irrigating when frost is expected. 



DEAINAGE. 



Land drainage is the quick removal of the excess of free 

 water from the pores of the soil. 



Effects of Drainage. — (1) Gravitational water is 

 removed by drainage from the pores of the soil and replaced by 

 air, thus ensuring a sufficiency of oxygen. A deficiency of 

 oxygen restricts plant growth, and therefore the crop yield. 

 Moreover, this deficiency results in an increased activity of 

 anserobic bacteria, with the result that nitrates may be reduced 

 to injurious nitrites or even to nitrogen gas, which is then lost 

 from the soil. Harmless ferric compounds may also be changed 

 into harmful ferrous compounds. 



Further, by drainage the water is lowered, thus enabling 

 crops to root deeper, and, what is more important, abnormal 

 fluctuations of the water-table are reduced. Crops, which may 

 be thrifty with a water-table at a reasonable depth, are in many 

 cases destroyed should the water-table rise considerably for any 

 length of time. A water-table fluctuating widely in its dis- 

 tance from the surface is often more inimical to plant growth 

 than one which is relatively stationary, even if comparatively 

 shallow. 



(2) In a water-logged soil the crumb structure becomes 

 broken down, and the tilth of the soil suffers in consequence. 

 Under these conditions aeration, too, is poor. Drainage in- 

 duces the formation of granules, thus improving the tilth, and 

 incidentally allows longer periods for tillage operations after 

 rains or irrigation. 



(3) Drainage does not remove available water. In fact, 

 by reason of the improved tilth of the soil and the deeper pene- 

 tration of the roots of plants, a larger amount of capillary water 

 is made available, and it is found that on drained land crops 

 withstand dry weather better than on land intermittently 

 water-logged. 



The fluctuations of soil moisture are much greater near the 

 surface than deeper down, where the moisture content is rela- 



