50 REPOET OF CHEMICAL LABORATORY 



from their geographical position, not only receive more moisture, Imt, hy reason of tlieii' 

 higher proportion of clay resulting from this condition, are better able to retain the moistiri'e 

 received. Azaza soils, here considered very light, would in a country of normal i-ainfall he 

 held to be rather heavy than otherwise. 



Near Khartoum, where the rainfall is only a few inches, the prevailing type of soil is a 

 light azaza. This statement does not include the soils directly bordering on the river, 

 which may be subjected to flooding from time to time. These, as a rule, are distinctly 

 heavier. South of Khartoum, as the rainfall increases, the decomposition of the silt and the 

 formation of clay take place to an increasing extent, until, in the neighbourhood of Wad 

 Medani, with an average rainfall of about 16 inches, the proportion of clay amounts to 

 50 per cent, and even, in some cases, to as much as 60 per cent. Soils in humid regions 

 containing such a high proportion of clay would be considered almost impervious and 

 extremely difficult to work ; but the fact that in arid countries even higher proportions may 

 exist in good arable soils has already been pointed out. Thus Means' calls attention to a 

 sample from Egypt in which the clay (below -005 nun.) amounted to about 75 per cent, and 

 the silt ('005 to '05 mm.) above 12 per cent., making a total of 87 per cent., of heavy material. 

 This "according to the mechanical analysis should be an almost impermeable clay; but in 

 the field the soil was found to be easily drained, perfectly amenable to cultivation and 

 favourable to plant growth." The same remarks apply to Gezira soils and to Sudan soils 

 generally, provided theij are -properly drained. Good drainage is essential to all soils ; but the 

 disastrous eii'ect of inefficient drainage on heavy soils has been especially evident in certain 

 lands, improperly worked, near Khartoum, and in the irrigation basins in Dougola Province, 

 in course of construction. 



Means {Joe. cit.) explains the ease with which these essentially heavy soils may be 

 worked by the "cementing action of lime and magnesia and iron compounds which join 

 together the fine grains of silt and clay and form larger aggregates, thus giving the soil 

 a lighter appearance than a mechanical analysis would indicate. The fact," he states, 

 " has been very clearly brought out in mechanical analyses of soils from American desert 

 lands where calcium and magnesium carbonates were abundant. In New Mexico, certain 

 soils were classed in the field as sandy loams, but upon subjecting them to mechanical 

 analysis, when water acts upon the soils for several days, the cementing material was 

 dissolved, the aggregates broken down, and the soil was found to contain enough clay to be 

 classed as a loam or clay loam. The field examinations of Egyptian soils show this 

 cementing process to be developed to a high degree, and soils in the field seem lighter than 

 would be indicated by the mechanical analysis." 



The writer was formerly of the same opinion as to the cause of the above efi'ect, but 

 recent experience has shaken his belief in this respect. It has been found, for instance, that 

 the extent to which the clay particles are held together appears to be independent of the 

 amount of earthy carbonates present. It has already been mentioned that the attempt to 

 apply the United States Bureau of Soils method to Sudan soils had not met with success, 

 disintegration of the clay aggregates not being effected by the prescribed agitation with 

 water, and often still less so when ammonia was added. It was found, however, that 

 subsoils often gave results more nearly approximating the truth, and yet the proportion 

 of earthy carbonates in these was not appreciably less — in some cases, in fact, it was 

 even greater. The following are instances of such a condition : — 



' "The rcclamatiou of alk.ali lands in Egyjrt." liiillrtin of I'.S. Jiiirtan of Soils No. 21. Jirj,f. of 

 AgricuUure. 



