22 EGYPTIAN AGRICULTURE. 



of a weathering agent, for it collects the products of the 

 weathering of rocks in its catchment area and brings them 

 down in suspension as Nile mud. 



AIR, or more properly the oxygen of the air, is an agent 

 which is constantly decomposing mineral and organic 

 matter. It oxidizes many substances with which it is in 

 contact but, as a weathering agent, its full effect is only 

 utilised through the medium of living bodies. In the soil 

 it supplies the small organisms with oxygen and these use 

 it to decompose a great variety of bodies. The great 

 importance of these organisms will be brought forward 

 when considering the biological properties of the soils. 



Variations in temperature, causing alternate expansion 

 and contraction, contribute to the decomposition of rock 

 material. 



Cultivation, by exposing the soil to the action of the 

 atmosphere, indirectly assists in the weathering of the soil 

 particles and the preparation of plant food. When the 

 weathering agents are decomposing the hard rock surface 

 of the earth to form new soil, and when they are acting 

 on and further decomposing the particles of the existing 

 soil, they are preparing plant food and assisting the farmer. 



In Egypt, except on irrigated lands, the weathering- 

 agents have only a feeble action, and it is fortunate that 

 the supply of new soil is independent of that produced 

 from the hard rocks of the country. The soil is practically 

 all all n riff I soil, and the particles of which it is composed 

 have been carried by water to their present position. In 

 the upper reaches of the Nile valley, the rain and other 



