BASIC SLAG AND SWEDES.

King Country Chronicle, Volume VIII, Issue 687, 18 July 1914, Page 7

BASIC SLAG AND SWEDES.

From University College of North Wales, Bangor, cornea a bulletin upon the purchase of basic slag and the manuring of swedes with basic slag, superphosphate and mineral phosphate.

During the last year or two it Bays there has been a good deal of controversy as to the best basis for the valuation and purchase of basic slag. This manure contains phosphate which is insoluble in pure water, but it is soluble to a considerable extent in weak acids. Owing to this fact plants are able to make use of it; their roots exude a solution of carbon di-oxide (carbonic acid gas) and, aa this is acid, the phosphate, though insoluble in pure water, ia disolved and is taken up by the plant. As a rough test .of the extent to which plant roots are able in a reasonable time, to make use of the phoephate in the slag the "Wagner" test has been devised. This consists in finding th 9 amount of phosphate dissolved by a 2 per cent solution of citric acid when shaken up for, a certain length of time unoer fixed conditions. Indifferent kinds of basic slag the amount thus dissolved varies very m-ach. For instance, in one of the slags tested, more than ninetenths of the phosphate was soluble in citric acid, in the other only about half of the phosphate was thU3 soluble. It, therefore, is an important imt-" ter to settle whether this test is a real'measure of the value of the slag. The manure is valued because it sup plies phosphate to crops, and it is obvious that, if only part ot the phosphate of the slag can be used by plants, the manure ought to be valued according to that part only The supporters of the citric acid test claim that it gives an approximate idea of the percentage) of phosphate which plants will make use of in a reasonable time and that slag ought to be valued according to the amount of citric soluble phosphate and that the percentage of total phosphate may he disregarded. The opponents of the test say that it is no measure uf the availability of the manure to the cops. Very few experiments to settle this poir-t have been carried out, though it obviously is a point of very considerable commercial and prac'ical importance. FINDINGS I—That the amount of citric sluble phosphate in basic Blag ia at least a better guide to to its value than the amount of total phosphate, and that farmers in buying basic slag should insist on an analyst showing the amount of citric soluble phosphate. 2—The fineness of grinding of the slag does not necessarily give an indication bs to the citric solubility of the phosphate, 3—Basic slag may safely be used to replace superphosphates aa a manure fo? swedes on heavy soils or on light soils that are deficient in lim<?. 4 —Mineral phosphate, which supplies phosphate in a cheap form, has a considerable value when used direct, provided it is very finely ground, and possibly it may prove a very economical manure on poor, sour soils, which would not give a satisfactory return for an expensive manure.