THE MECHANICAL ANALYSIS OF ABID SOILS 45 



The remaining sand in the beaker is separated into two portions, by means of an 

 appropriate wire sieve.' To the portion remaining on the sieve is added that which 

 was retained by the 1 mm. sieve in order to obtain the entire proportion of coarse 

 sand, 2 mm. to '2 mm. in diameter. The fine sand is determined by difference. 



All figures should, oViviously, be expressed on the dry soil. 



In concluding these notes it is desired to call attention to the fact that while 

 the above method is especially recommended for arid soils, it is equally suitable for 

 those of humid regions. Further, it is held that a method of this kind should invariably 

 be used since otherwise the comparison of humid with arid soils becomes impossible. 

 It must be recollected also that the treatment by acid not only results, at times, 

 in the complete solution of the larger particles, but invariably, in the extraction of 

 material from the finer constituents, so that as a method of "mechanical" analysis 

 it cannot but be considered scientifically unsound. 



Soils of the Gezira 

 A Preliminary Note 



The " Gezira " or "island" is that roughly triangular section of the Sudan which lies 

 between the White Nile and Blue Nile, with Khartoum at its apex, at the junction of these 

 two rivers. It has long been held that the main hope of agricultural development of the 

 Sudau lies in the irrigation of this district. The writer takes a more sanguine view, but, 

 in any case, it appears evident from a study of the rainfall map on pages 46 and 47 that the 

 portion south of, say, the 15th parallel, may be capable of profitable development by rainfall 

 cultivation, and there is no little evidence to indicate that even so valuable a product as 

 Egyptian cotton may be successfully grown, as a rain crop, if modern methods of cultivation 

 be api^lied. Very good crops of dura are now produced, even with the primitive methods of 

 the natives, and it appears likely that wheat may displace this largely when the native 

 cultivator has learned to realise the advantage to be gained. The newly established Gezira 

 railway is already having a marked effect on the output of rain-grown grain from the Gezira, 

 and while this is at present largely dura there is also a certain amount of other grain and of 

 cotton, as well. The latter is grown from native seed and therefore of poor quality, but 

 experiments have been made with Egyptian seed, with most promising results. 



Since many appear to be slow to believe in the possibilities of the Gezira from the point 

 of view of rainfall cultivation, a comparison with similar semi-arid districts in other 

 countries may not be out of place. In these districts of low rainfall a method of so-called 

 " dry soil farming" is practised, the distinguishing features of which are (a) deep ploughing 

 to ensure the entrance of the rain into the soil, and (fe) frequent harrowing in order to 

 preserve a loose mulch on the sui'face and thereby check excessive evaporation. It is 

 obvious that season of rainfall in such dry climates is more important than its actual 

 amount. 



In the United States (Utah and California), 590 kilogrammes of wheat per acre are 

 stated to have been obtained with a total annual rainfall of about 10 inches. The average 

 annual rainfall over the whole of these wheat-producing districts is only about 14| inches. 



' The sieve found to pass particles most nearly approximating 0-2 mm. in diameter was of 80 meshes to the 

 linear inch. This will, of course, depend in part upon the size of wire used. 



