Of greatest importance for agriculture in Egypt are the Nile sediments of
the Holocene, which to a large extent originate in the volcanic highlands of
Ethiopia. These sediments are up to 8 m thick in the Nile valley and about
10 m thick in the Nile delta. The grain size of this mostly fine material de-
creases from south to north and from the valley scarps to the centre of the
valley. In Wadi Halfa, for example, the relation between fine sand (grain size
0.02–0.2 mm), silt (grain size 0.002–0.02 mm) and clay (grain size < 0.002
mm) is 34 : 38 : 28, whereas it is 17.7 : 26.9 : 55.4 in Cairo (Schamp 1977: 18).
A study of the change in soil texture from the middle to the outer parts of
the Nile valley shows a drastic decrease in the content of silt and clay from
37.5 per cent to 20.8 per cent and from 32.1 per cent to 9 per cent respectively.
Likewise, the nutrients that are important for the growth of plants and trace
elements decrease: namely the organic matter N, Fe, Mn, Zn and Cu (Kishk
1985: 15–23). According to Kishk the deficiencies of the latter at the rims of
the Nile valley are caused by the fact that no siltation has occurred there since
the construction of the Aswan High Dam (see Chapter 6).