BASIC SLAG AND ROCK PHOSPHATE.

New Zealand Journal of Agriculture, Volume XXIII, Issue 5, 21 November 1921, Page 302

BASIC SLAG AND ROCK PHOSPHATE.

B. C. ASTON,

F.1.C., F.N.Z.lnst., Chemist to the Department

In view of the statement that the United Kingdom quota of Nauru Ocean phosphate has been sold for a year to a Welsh basic-slag company for the purpose of grading up its slag (see Journal for June last, page 340) the situation in regard to basic slag has been. closely watched by the Department, and a considerable number of samples have been analysed. So far there is no evidence that' mixtures of slag and rock phosphate are being sold ■to farmers as genuine basic slag, or that the citric-acid solubility of the phosphate is unduly low. - Although the bulk of the basic slag on the New Zealand market at present is registered with a somewhat lower grade : than the pre-war fertilizer (15-17 per cent, phosphoric acid (P. 205 0 5 ), as against 17-20 per cent.) there have been only very small importations of the low grade 9-10 per cent. slag. Further samples .are being examined, and in the meantime there is no necessity for the farmer to purchase basic slag of lower grade than, say, 15 per cent, phosphoric anhydride (P 205 0 5 ) and 75-80 per cent, -citric-acid solubility, and he should insist on this specification as the minimum guaranteed composition. ■’ ’■■■■■ The London manager of a firm of basic-slag vendors has stated in a letter published in a Wellington paper that the use of rock phosphate has been condemned by the Canadian and United States Government Departments..'. This is evidently an incorrect presentation of the facts which, as far as these countries are concerned, are also set out in the Journal for June, page 337. The climatic conditions of those countries are, moreover, unlike those of New Zealand, which generally are much more favourable to the use of insoluble, phosphates than those of the United States or Canada (see last month’s Journal, page 198). It would be incorrect to say that the fertilizing-value of finely, ground rock phosphate in New Zealand is unknown and unproven. Current usage in Southland and the Department’s experiments in the North Island have proved its value. ; . , ,

The British Journal of the Ministry of Agriculture for September, 1921, page 484, in an editorial note states as follows :

. Many experiments , show that finely ground rock phosphates win ao the work of the basic slag and encourage the growth of clovers in the same way, especially on old grassland which is well provided 'with organic matter, and where the rainfall is abundant. Recent experiments in Essex, where the conditions are comparatively unfavourable, show that the action of rock phosphates compares favourably with that of basic slag. In America the use of ground rock phosphates has become very general. Actual trials with these particular phosphates from the • Pacific islands ■ have as yet only been reported- from New ■ Zealand; but there is , every reason to suppose that they-will be even more effective, than the ordinary

rock phosphates. . ..v. > The Ministry of Agriculture has no hesitation in recommending farmers to try the , mixture of the Nauru and Ocean Islands phosphate and slag for application to grassland at the rate of 4 cwt. to 6 cwt. per acre according to grade, especially upon heavy soils, peaty soils, and all situations where the rainfall is not too light. ■•‘-On arable land the mixture cannot always take the place of superphosphate for. th A turnip crop; but a dressing of 4 cwt; per acre will form a good basis of continuously acting phosphatic manure for the whole, of the rotation. A further 2 cwt. per acre of superphosphate, drilled with the seed for the turnip crop, will give it that initial start for which superphosphate is so valuable. . . Further experiments have been started to 'ascertain more ■ accurately the applicability of the Nauru and Ocean Islands phosphate to particular soils and crops. Sufficient knowledge already exists, however, to enable the Ministry to recommend with confidence Nauru and Ocean Islands phosphate to the farmer who wishes to improve his grassland and to lay a good manurial foundation for his arable land. •• • ' “ ; . The quantities recommended in the above extract are, of course, intended for the soil conditions of the United Kingdom, and are not necessarily applicable for New Zealand. This Department’s officers are conducting further held experiments with finely ground phosphate rock, and the results will be published in due course. With regard to the poisonous ; nature of basic slag, the necessity of allowing ample time for the rain to wash the slag off the herbage before depasturing stock on it-is evidently a disadvantage to its use, and, although it is one which •in most cases can be avoided, the fact that such poisonings may occur is evidently not universally known, and therefore must be pointed out.