THE SOIL. 47 



sarily high. Further, infiltration, like subsoil water, is 

 deficient in gases dissolved in it. The effects of too high a 

 water table are: (1) The plant roots are unhealthy and 

 food is slowly prepared on account of the lack of oxygen 

 in the soil. (2) The solution of plant food in the soil is 

 rendered very dilute and the plants, more especially if 

 young, will be unable to absorb a sufficient volume from 

 which to extract the required amount of food. (3) Salts 

 and acids (waste products) will accumulate in the soil. 

 (4) The amount of water evaporated and the high specific 

 heat of water, renders the soil cold and thereby retards 

 plant growth. Wet soils can only be improved by lowering 

 the water table by drainage and, if the water is from infil- 

 tration, by lowering the source from which the subsoil is 

 being supplied with water. Heavy waterings are occa- 

 sionally advisable to destroy obnoxious insects, but other- 

 wise frequent and light waterings give the best results ; 

 heavy waterings at long intervals cause an unequal 

 growth of the crop. 



The great advantage of irrigation is that the water 

 supply can be regulated. The quantity of water required 

 by a crop varies with the particular conditions of the 

 cultivation. Thus level land requires less water than uneven 

 soil ; salt soil must be kept pretty moist in order to keep 

 the salt solution dilute and harmless ; sandy soil requires 

 more water than heavy soils ; crops in small beds require 

 less water ; green fodder crops require a moister soil than 

 the cereals. In summer, except at high Nile, more water 

 is required than in winter. In order to avoid the washing 

 1 1 way of plant food, bare land is only watered under 



