222 WHEAT SOILS OK TlIK ALKX AXOKl A DINISION. 



PriYsrcAL Deterioration of Soils. 



In considering the deterioration of the Alexandria soils, it 

 is evident that there are two main directions in which such 

 deterioration may have proceeded, v'xz., physical and chemical. 

 The varieties of physical deterioration are more or less closely 

 associated with changes in soil texture. The general effect of 

 rain or irrigation on a cultivated soil is, as I have pointed out 

 elsewhere.* to carry the finer silt and clay particles from the 

 surface down to the lower soil levels, and so gradually to denude 

 the surface soil of silt and clay, and to increase their pro])ortion 

 in the subsoil. This process also tends to impoverish the surface 

 soil in plant food, for it is chiefly in those silt and clay particles 

 that the plant food constituents, in a form available to the crops, 

 reside: hence the improvement, chemical as well as ])hysical. 

 effected in poor soils l)y admixture of the silt deposited by 

 overflowing rivers. 



In the soils now inider discussion, the above effect of water 

 has not been strongly marked all round, but it is nevertheless 

 apparent to some extent, e.g., when we compare the virgin soil 

 No. 2, which contains 41 ])er cent, of silt and clay, with the 

 cultivated soil No. i, which contains only 34 per cent.; or the 

 virgin, soil No. 14, containing 32 per cent, of silt and clay, with 

 the cultivated soil No. 13, which has only 25 per cent. Similarly, 

 No. 16 has 46 per cent., but No. 15 only 29 per cent., and No. 

 18 has 24 per cent., but No. 17 only 16 per cent. In two cases 

 this rule seems to be c(~»mpletely reversed, /.r., as bet\veen Nos. 

 5 and 6, and again between Nos. 11 and 12. In both these case.- 

 the cultivated soil has not lost, but gained in silt and clay, and. 

 as we shall afterwards see, in both cases the loss of plant food 

 has been less general than with aluKxst all the soils of the series — 

 in fact, in some res])ects in these two s])ots, the cultivated soil 

 has more of .some elements of ]jlant fot)d than the corres])onding 

 virgin soil. This may ]iossi1^1y l)e due to deep ]jloughing return- 

 ing the finer soil particles to the surface. 



From the above, it follows that the effect of frequent rain 

 or irrigation on a soil inclined to be light and sandy (and there- 

 fore already less suited for wheat culti\'ation) is to render it 

 still lighter and sandier. 'i"o what extent such a purely physical 

 rdteration of the soil may go beft)re the wheat crops begin to 

 be seriously affected is a matter for direct investigation, mider 

 South African conditions, for the l(Kal climatic conditions, as 

 Dr. Wiley justly i><)ints out. constitute an im])ortant factor. 



PossiiiiLiTN' oi- Brack. 



It will have been observed, from the description of the 

 various samples taken, that, with one exception, brack or alkali 

 has not been observed on the lands represented. It was never- 



*" Soils of Cape Colony," 192. 



