50 EGYPTIAN AGRICULTURE. 



of the upper layers of the soil, arid the subsequent crack- 

 ing, heavy soils require to be continually fassed. The 

 more heavily a soil is watered the greater is its temporary 

 increase in volume, and, on drying, the larger and deeper 

 are the cracks. 



A soil is said to be pervious or "permeable" when it 

 allows water to drain easily through. Soils through which 

 water cannot pass are impervious or impermeable. It must 

 be noticed that a "porous" soil is not necessarily a pervious 

 one, for the pores, although abundant, may be too small 

 to allow of the passage of water. This may be seen in 

 the case of the Facus soil (see p. 39) the porosity of w r hich 

 in the loose state amounts to 52 %. and vet it is the 



/ \J / */ 



most impervious soil. Soils with few, large pores sandy 

 soils are much more permeable and more easily drained 

 than clay soils. The impermeability of the fine clay soils 

 is due partly to the small size of the interspaces and partly 

 to the colloid bodies which swell and close these spaces. 

 Clay soils are made more porous by the addition of humus, 

 of sand, and of lime, etc., which flocculate the finest clay 

 material. A soil with particles of about an equal size is more 

 porous than a mixed soil where the small particles fill up 

 the larger interspaces and the soil is more closely packed. 

 By loosening the packing, cultivation increases the per- 

 meability of the soil. The firmly packed subsoil is usually 

 [ess porous than the soil unless, of course, the subsoil is of 

 a more sandy nature. The Facus soil which is permeable 

 in laboratory experiments is probably quite impervious 

 in its naturally compressed state. Generally the perme- 

 ability and tenacity vary inversely. Most salts increase 



