46 EGVI'TIAN AGRICULTURE. 



and the consequent increased tenacity and impermeability 

 of the soil. 



In digging a well it is observed that the soil gets moister 

 and moister until eventually a point is reached at which the 

 spaces between the soil particles are entirely filled with water. 

 Here the soil is said to be saturated with water and the 

 level of this saturated soil is called the "water table." 

 The level of the water table is constantly shifting ; it rises 

 when irrigation is applied, when the Nile is high and 

 when neighbouring lands are flooded ; it falls as the land 

 dries, as the Nile falls and as the outlet of its water is 

 lowered. The water below the water table is always 

 moving onwards and downwards until it discharges itself 

 into a drain or sea. As Avater and air cannot occupy the 

 interspaces of the soil at the same time it follows that 

 below the water table there is not much air and plant roots 

 cannot thrive. Hence in low-lying lands the depth for plant 

 roots is restricted by the proximity of the water table 

 to the surface level. A wet soil is not necessarily quite 

 saturated with water, and, in fact, in dry countries a retentive 

 soil is to be preferred to a dry one. Again a soil too dry for 

 berseem may be a good soil for barley, a soil too dry for rice 

 a good soil for maize. On the other hand a saturated soil 

 except the saturation is with moving water is bad for 

 all crops and no plant root can be healthy if it dips too 

 far below the water table. To allow a full root develop- 

 ment, the water table at high Nile should not rise nearer 

 to the surface than about 1J metre. Land near high 

 canals is often kept constantly wet, and the plant food 

 solution constantly dilute, but the water table is not neces- 



