CAB129-37 — Page 320

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N has been of little value, even when placed; but P2Os has given nearly 80 lbs. additional nuts when broadcast alone, and when placed it has given a further 68 lbs., making 148 lbs. in all. The effect of P2O, was also shown to be reduced by the presence of N, particularly when the fertilisers were placed, so that not only did N have little effect alone, but it reduced the effect of P2O. There was thus considerable justification on the first year's results for eliminating N from the fertiliser prescription for groundnuts at Kongwa.

At Urambo, on the grey soil type, the exploratory experiment was to some extent vitiated, since the fertiliser was placed by hand too close to the seed and planting conditions were adverse. Nevertheless, on the red soil type a useful effect of placed P2O, was obtained, while K2O appeared to be effective on the grey soil.

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At Namanga, on the heavy red soil type, planting was very late, and the seed germinated very poorly (possibly owing to a long delay in humid conditions at Dar-es-Salaam in transit, the seed having been decorticated at Kongwa), so that stands were very poor. The local staff sought to remedy this defect by planting up the gaps with fiwi bean. This crop was harvested separately, and weighed green at harvest, and the results give some indication of soil conditions. In the exploratory experiment a strong effect of PO, was shown. Results for the groundnut crop in the experiments at Namanga are not available owing to errors after harvest.

Experiments on types of phosphate

As stated above, it was assumed at the outset (and later proved) that phosphate would be of value for increasing groundnut yields. It was, therefore, of importance to discover what type of P2Os would yield the best economic return. This would not only be affected by the availability of the phosphate in the fertiliser to the groundnut crop on the soils to be developed, but also by the cost, including the cost of transport. The second type of experiment, of which six examples were brought to harvest at Kongwa, was aimed at this problem. It compared a wide range of processed and rock phosphates with superphosphate, the materials all being broadcast by hand on the plots.

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The materials tested were superphosphate at approximately 1, 2 and 4 cwt. per acre (0.2, 0.4 and 0.8 cwt. P2O, per acre) and triple superphosphate, ammonium phosphate, a mixture of sulphate of ammonia and triple super- phosphate, a nitrophosphate (prepared by treating phosphate rock with nitric acid instead of sulphuric acid, as in the case of superphosphate), Uganda rock phosphate from Tororo, a soda phosphate prepared from the Uganda rock phosphate (by heat treatment with soda ash), Algerian rock phosphate from Gafsa (a standard material in the manufacture of phosphatic fertilisers as well as in research), Christmas Island rock phosphate, Saphos (Egyptian) rock phosphate, and a mixture of equal weights of Gafsa rock P2O with superphosphate. In this last material the availability of the originally water soluble P,O, in the superphosphate is reduced by a reaction (called reversion) with the rock phosphate. All these materials were applied at rates calculated to give 0.5 cwt. P.Os per acre; and differences in N content were balanced up with sulphate of ammonia.

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These materials cover a wide range in the availability of the phosphate they contain. The superphosphate, triple superphosphate, ammonium phosphate, and sulphate of ammonia/triple superphosphate mixture have over 90 per cent. of their P.O. in water soluble forms, and practically all of it is soluble in 1 per cent. citric acid, a standard reagent in the testing of phosphatic materials. The Gafsa/superphosphate mixture has about two-fifths of its phosphate water soluble, and nearly two-thirds citric soluble. The PO in the nitro-phosphate is one-tenth water soluble and three-fifths citric soluble. None of the others has any appreciable content of water soluble POs, and their citric soluble P.O. ranges from over 70 per cent. in the soda POS to 17 per cent. in the original Uganda tock Phosphate. These points are tabulate section 1097

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