109 



The specific gravity of the silt represented by No. 1 is 2 '03 on the 

 basis of the dry specimen as above described, so that a cubic foot would 

 weigh about 127 pounds. One acre of land, covered with the silt to a, 

 depth of half an inch, and thus receiving a deposit of 1,815 cubic feet, or 

 115 Cape tons, of the silt, would therefore be enriched to the extent of: 



3,314 pounds of lime, 

 1,086 pounds of potash, 

 507 pounds of phosphoric oxide. 



In round figures, each such acre would receive about one ton and a 

 half of lime, half a ton of potash, and a quarter of a ton of phosphoric 

 oxide. The commercial value of the fertilising constituents, in an avail- 

 able form, added to each acre of land upon which the silt is deposited 

 to half an inch in depth is at least 30; the pecuniary value of these 

 silt deposits, therefore, bears comparison, and very favourable comparison, 

 with the cost of manuring in the ordinary way. 



Many other instances may be quoted of lands having acquired fer- 

 tility by the accretion of similar river-borne silts. It is, for example, 

 largely due to the fact, already commented on," that its soil has been 

 built up of old alluvial deposits that the Oudtshoorn Division is so fertile ; 

 the rich silts and clays brought down from the Karroo by the Olifanta 

 and Gamka Rivers have, in great part, contributed to this. It is also 

 common knowledge that these rivers occasionally overflow their banks on 

 their seaward course through the Mossel Bay and Riversdale Divisions, and 

 thus contribute greatly to the fertility of the adja,cent farm lands. Fur- 

 thermore, 011 certain farms in the Division of Britstown, traversed by a 

 tributary of the Brak River, the practice, when the river comes down in. 

 flood, of constructing checks or weirs to control the passage of the water 

 and retain the silt, so as to ensure the deposition of the transported silb 

 on the lands, is said to have resulted in such a continual enrichment of 

 the latter as to render any other mode of fertilising needless. This ig 

 scarcely to be wondered at, and so valuable an accretion to the land surely 

 deserves the expenditure of time and trouble in retaining it. 



It is, of course, well known not to confine our examples to South 

 Africa that this system of silting has been practised in the Nile Valley 

 from ancient times, and sandy soils, comparatively worthless before, have 

 become rich fields, and have remained productive for thousands of years. 

 Another case in point is that of the Rio Grande, the application of 24 

 inches of water from which adds nearly one quarter of an inch of soil 

 to the field, in the form of river sediment, and supplies every acre with 

 1,821 pounds of potassium sulphate, 116 pounds of phosphoric oxide, and 

 107 pounds of nitrogen. With regard to this, King observes: f 



" Four years of irrigation at this rate would add an inch of soil to the field, and 24 

 years would cover it six inches deep with a sediment containing three times the amount of 

 potash found in the average clay soil, and the same percentage of phosphoric acid, and a 

 high percentage of nitrogen." 



QUEENSTOWN. 



(Officially collected.) 



No. Field Cornetcy. Farm or place. Collector. 



1. Gwatyu. J. Fronemann, junr. St. C. O. Sinclair. 



o' 



6 ' 



~t . . , 



* See pages 91 and 93. 

 f " Irrigation and Drainage," 2nd ed., 1902, p. 259. 



