102 EGYPTIAN AGRICULTURE. 



vertically. The soil becomes more compressed instead of 

 forming a hard crust as when only the surface is rapidly 

 dried. The reduction of surface evaporation prevents the 

 too rapid rising of water and plant food in the soil, and 

 the consequent deposition of plant food on the surface of 

 the soil. Fassing prevents excessive hardening of the soil 

 crust, and the deep cracking which proceeds when land is 

 allowed to dry without ^protection. When irrigation water 

 is applied to land it washes the finer soil particles into and 

 blocks up the air spaces. The soil on drying becomes very 

 firmly packed and forms a hard crust which is about as 

 porous as a brick. Slowly dried soil sinks when contracting, 

 rapidly dried land contracts horizontally. At first, the cracks 

 are about four centimetres deep and do little damage. With 

 further drying, the cracks get deeper and cause the rupture 

 of many of the plant roots. They limit the spreading of the 

 roots of the plant to a very small volume of soil and in 

 this way are of the greatest possible hindrance to the 

 growth of the young plants. The deepest cracks, as in 

 the basin lands, do great damage to the oldest and deepest 

 rooted plants. The primary reasons for fassing may be 

 stated as being: (1) to avoid the necessity of frequent 

 irrigation, and consequent dilution of the plant food, 



(2) to keep the surface open so that air may easily enter, 



(3) to prevent the soil from deep cracking, whereby plant 

 roots may be killed or plants may be isolated on clods of 

 soil. 



Whilst emphasising the essential results of fassing, of 

 course due weight must be given to the incidental advan- 

 tages already mentioned. Land which has been heavily 



