FERTILISER VALUES

Wairarapa Times-Age, 20 October 1939, Page 3

FERTILISER VALUES

POSITION OF PHOSPHATIC MANURES CONFUSION OVER CONTENT. DIFFERENT TERMS IN USE. There is no need to ciwell upon the importance of phosphatic manures to this country, as the remarkable responses from this form of topdressing are well known and thoroughly appreciated, states Mr N. Lamont, of the Department of Agriculture, Masterton. Indeed, with the exception of three or four, more or less clearly defined areas where potash responses are obtained, no other form of topdressing ,is required, although in certain districts such as the Wairarapa, the use of lime can be recommended in order to obtain the maximum benefit from phosphates. The effectiveness of phosphates has naturally enough led to the presence on the market of a fair variety of fertilisers which supply this ingredient and buyers are often somewhat confused and even misled by the quotation of phosphate content in different terms. We may take it that the essential ingredient of a phosphatic manure is phosphoric acid or phosphorous pentoxide but in all the fertilisers in general use, this phosphoric acid is combined with lime in one of three ways. In chemical language these are termed mono-calcic phosphates, di-calcic phosphates and tricalcic phosphates —or more simply, “one lime,” two lime,” and “three lime” phosphates; the difference being, as indicated by the terms, due to the amount of lime with which the phosphoric acid is combined. It will be at once apparent that confusion can arise if it is not clearly stated in what form the phosphate exists when a particular fertiliser is quoted as having a certain “phosphate” content. All three forms of phosphate are represented in commerce: — (a) The three-lime phosphates are the ground phosphate rock manures obtained from Nauru and Ocean Islands and from Northern Africa. The phosphate in slag is also in the three-lime form.

(b) The two-lime form is that in which phosphate is found in basic super. (c) The one-lime form is the familiar super-phosphate. The practical significance of the difference between these three forms lies in the different availability of each; the “three-lime” form being relatively insoluble and unavailable to plants, the “two-lime” being intermediate and the “one-lime” being readily soluble and quickly available for plant food. It is not the purpose of this article to discuss with reasons, the relative merits of phosphate applied in these different forms and no more will be said than that in a dry district such as this one, the soluble “ione-lime” form, that is superphosphate, is always to be recommended as a most shitable source of phosphoric acid. All that is desired at present is to assist in clarifying the position in regard to that very vague term “phosphate content.” In commerce, “phosphate content” usually means either phosphoric acid content or “three-lime” phosphate content, and it is between these two that most confusion arises, as manures are rarely quoted in terms of their content of “one-lime” or “two-lime” phosphate. For instance, a ground rock phosphate, such as some of the North African phosphates, may be quoted as having a “phosphate content” of say 70 per cent. To the uninitiated this sounds extremely good as compared with ‘say basic slag as 17-20 per cent, but when it is appreciated that the one figure refers to the “three-lime phosphate” and the other to actual phosphoric acid the degree of difference is greatly reduced; as 70 per cent rock phosphate is equivalent to only about 32 per cent phosphoric acid. The position is more confusing in regard to super-phosphate. Although the phosphate in super is not the three-lime form, the custom exists of describing it as if this were so. The figures 44-46 per cent mean that super contains the equivalent of 44 to 46 of “three-lime” phosphate whereas its actual phosphoric acid content is about 20 per cent. It will be noted, incidentally, from these figures that phosphoric acid content may be converted into “three-lime”, phosphate content by simply multiplying by 2 1-5.

It would bo highly desirable if all phosphatic manures were described in terms of their phosphoric acid content and a good deal of confusion would be avoided thereby. However, while the use of different terms remains, the distinction should be dearly understood and due allowance made. Having done so, questions of solubility and availability can be given attention as further considerations of importance before finally deciding what manure to employ. Generally speaking, the insoluble nature of ground rock phosphates more than discounts their extra phosphoric acid content over superphosphate, and the latter is a much more desirable Source of phosphate. This statement will certainly be more acceptable also, when it is appreciated that the former, described as having “70 per cent phosphate.” have only 30 per cent of phosphoric acid as compared with 20 per cent, in the same terms, in the case ' of suns.“phosphate.