SOILS OK THK GH/AHX 53 



heating of soil evuu to a luodurate degree results in a greater solubility in water ol tlie 

 constituents essential to plant life. 



The fact that a treatment so far from drastic as the simple puddling of the soil by 

 means of a soft camel-hair brush, in the presence of an extremely dilute cold solution 

 of sodium carbonate (not sufficient to dissolve the humus), suffices at once to disintegrate the 

 clay aggregates and bring the former into suspension, points rather to the conclusion that 

 the clay has simply been flocculated by the heat and dryness, and that, in the soil, it 

 is usually not held together entirely, or even to any very great extent, by cementing 

 material such as calcium carbonate. It is for this reason that the treatment by puddling in 

 the preparation of the sample for mechanical analysis is held to be a sound one and the 

 substitution of the very small amount of sodium carbonate for the ammonia usually 

 employed, entirely unobjectionable. 



There is further evidence that it is not necessary to assume cementation of the clay in 

 the soil aggregates. It has already been noted (vide page 44) that when subsoils containing 

 rather high proportions of salts are treated for the mechanical analysis, flocoulation of the 

 clay occurs ; and that when the excess of salt is removed, these floccules do not readily 

 break up on the addition of water, even in the presence of sodium carbonate. The floccules 

 may appear to do so at first, but if the liquid be allowed to stand for some hours, 

 precipitation of the clay usually recurs. 



It is obvious that the intense baking and drying to which arid soils are subjected may 

 cause a similar flocculation of the clay, and the aggregates so formed may be expected to be 

 much more resistant to disintegration than those obtained by precipitation from suspension 

 in water. 



Chemical coiitposition of Gezira soils. Extended chemical examinations have as yet 

 only been made of the tract in the Gezira which it has been proposed to irrigate, and which 

 is indicated in the sketch map on page 51. The results of examination of individual samples 

 are detailed in the tables on pages 56 to 59. The variations in chemical character are more 

 clearly shown in the table of averages on page 55, which should be studied in connection 

 with the rainfall map. 



As would be expected, the proportion of water-soluble salts in the surface soil, and 

 especially in the subsoils, is greater towards the northern end of the tract where rainfall and 

 consequent leaching are least. The same is true of the proportion of calcium carbonate. 

 Organic matter, as expressed by the content of humus and of nitrogen, is, on tlie other hand, 

 greater towards the southern end where there has been a greater amount of cultivation and 

 of plant residues left in the soil. Phosphates readily soluble in acid are also less where 

 there has been least rain and wliere, consequently, rock decomposition has taken place to the 

 least extent. Acid-soluble potash compounds are found to vary but slightly, but in the 

 same direction. 



Speaking generally, the soils, lioth hardohe and a:aza, in tiiis tract may be said to 

 be fairly well supplied with potasli and with phosphates. They are. liowever, like most 

 Egyptian and Sudan soils, markedly deficient in organic matter and in nitrogen, and the 

 results from their cultivation will be largely dependent upon treatment with respect to this 

 deficiency. Rotation with a leguminous crop such as berseem and the ploughing of a fair 

 proportion of the crop into the soil is wliat is especially indicated. Without sucli a rotation, 

 rapidly diminishing crops may be predicted with confidence. Since such leguminous crops 

 have not been grown on most Gezira soils, inoculation of the seed first planted should make 

 a very marked difference in the yield. This experiment was tried on a small scaler lu^ar 

 Khartoum, with results which exceeded all expectations. 



