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The S.A type of superphosphate can probably be produced in Fanganyika, which shouldgarthen mereke its profitability; but this is some extent offset by rather higher storage and transport costs per acre owing to the greater bulk needed.
It is too early to attempt the corresponding calculation for groundnuts in other areas.
The fertiliser requirements of other crops likely to be grown (such as sunflower) are probably very different from those of groundnuts, particularly in regard to nitrogen. Pending the result of investigations, it may be expected that these crops will require a fertiliser containing a higher proportion of nitrogen than is contained in the fertiliser at present in use; probably a 1:1 ratio of N:P as highly concentrated as 25:25 (which could be secured approximately by mixing ammonium phosphate with ammonium nitrate) would be suitable. (Ammonium nitrate in the prilled form is an entirely practicable fertiliser material under Kongwa conditions.) With the industrial development of new types of phosphate, even more concentrated materials may be possible, but their higher cost may not offset the saving in transport and storage charges.
Investigations are proceeding into the placement drilling of cheaper phos- phatic materials, which may further increase the profitability of the use of fertilisers.
Liming at Kongwa should be confined to the limited areas where acidity or Ca deficiency are demonstrated by analysis.
5. SOIL AND WATER CONSERVATION
Although in the present division of responsibilities conservation is not part of the duty of the Scientific Department, some experience has accumulated over the period under review as a result of operations on the Kongwa Experiment Station and elsewhere.
The conservation system at present in use at Kongwa is based on the necessity of removing flattened bush and roots from the cultivated area, of preventing erosion, and of conserving as much water as possible by inducing it to percolate into the ground. Accordingly, level contours were laid out at 9-foot vertical inter- vals and the clearing debris together with some earth was pushed on to the contour lines, forming the so-called windrows. Wherever it has been correctly applied it can be claimed that this system has prevented major erosion damage during the crop season 1947-48. There has, however, been some tendency for wash .to occur within the contour strips, particularly where slight inequalities of the ground concentrate the rain water, or where occasional extremely heavy storms at the rate of 1 to 2 inches per hour over periods of up to 45 minutes, are experienced. Although recording gauge data are not available, it seems that the general storm intensity is of the order of one inch per hour, and that falls at a rate over two inches per hour are distinctly uncommon. These heavier storms appear to be in all instances of considerably less than one hour's duration. It should be noted that the foregoing data apply to Kongwa only: in the Western and Southern Provinces, where total annual falls are higher, storms have been experienced of considerably longer duration.
It would seem advisable to plan in future for the gradual replacement of the windrows by properly constructed broad-based terraces, while intermediate terraces should be put in, giving a general vertical contour interval of 41 feet or less.
These might well be developed by ploughing, and should be of such construction that farm operations can proceed over them. The present windrows have the further disadvantage that they are a breeding-ground for weeds, par- ticularly species of Ipomoea and Cynodon plectostachyum which spread by stolons or runners. They may also become the source of insect infestations, particularly of the blue plant-sucking bug, Calidea dregei, referred to in sections 8 and 14**
Other measures necessary are the location and grassing-up of minor gullies. In areas of particularly steep slopes (over 3 per cent.) it may well be an advan- tage to develop variable grade terraces, discharging into the larger gullies, in