PHOSPHATIC MANURES.

Otago Witness, Issue 3330, 9 January 1918, Page 8

PHOSPHATIC MANURES.

Both in the CommonwealtSi and New Zealand we have frequently seen it demonstrated that a dressing of manure containing phosphates has a markedly good effect on practically any kind of crop, iindicating that there is a deficiency of .phosphates in the average soil. The source of our phosphates may be varied, but the two main classes are manures containing soluble phosphates—that is to say, phosphates soluble in water and available to plants at once, and manures acting in a slow manner, or the insoluble phosphate—that is, insoluble in water. The chief source of soluble phosphate in the D*ominion is superphosphate, which is obtained by acting on mineral phosphate with sulphuric acid. 'lt is an acid manure, 'find the percentage of phosphates depends on the quality of the rock phosphate treated. So that a high-grade super, if the cost of carriage is considerable, is the cheaper source of phosphates, as one gets more phosphates to the ton. The buyer should beware of low-grade phosphate manure, except at a low-grade price, as, apart from the lack of phosphates, there are frequently many impurities. All the phosphates of a mineral phosphate are not acted upon in the process of manufacture, and sometimes one may light upon a damp pasty super. This should be avoided; it is too acid, and probably won't run freely in the drill, besides being harmful to the machine. Animal refuse and bones contain phosphates (insoluble), and are slow in action. By mixing lime with a highclass super we have the so-called basic superphosphate, the lime acting as a drier, and the surplus lime with the ordinary functions of lime. Basic slag is a byproduct in the manufacture of steel from phosphatic iron ores. When the ore is at a high temperature lime is introduced, and the phosphates unite with the lime, forming basic slag, which is run into pits. The slag is ground to a very fine state of division and sold as manure. High-grade slag containing 30 to 40 per cent, phosphates are the best from a farmer's point of view. The impurities in low-grade slags are more or less injurious. It is necessary to get a guarantee when buying basic slag in regard to the percentage of phosphates, a guarantee as to fineness, and as to the solubility of phosphates in a 2 per cent, solution of citric acid. Slag should be kept dry, or its fineness will be destroyed.