CAB129-37 — Page 321

Page 321

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MaterialsPofobigh gitrif by tow water soluble P.O. content may be of special importance. Soils are known, and are believed to be common in the wetter tropics, which react with water soluble P.O. in ways that greatly reduce its availability to crops. Citric soluble (but water insoluble) phosphate, which is generally available to crops, reacts more slowly with the soil, and may thus be of higher efficiency on such reactive soils.

The results at Kongwa indicate broadly that the effectiveness of the materials decreases with decreasing water or citric solubility. The average increase of yields from highly water soluble phosphates was from 60 to 80 lb. per acre, much as in the broadcast phosphate treatment of the exploratory experiment at Kongwa (above). The lowest average increase (2 lb.) was from Uganda rock, Nitrophosphate was an exception to this general result, giving a far smaller increase in yield than would have been expected from the citric solubility.

The design employed in these experiments was inefficient statistically, and the results varied widely from one site to another. Further, the materials were broadcast, which has now been shown to be less effective than placement drilling, at least for water soluble fertiliser. The results cannot be regarded as giving more than a general indication of the relative usefulness of the materials.

At Urambo the results of the corresponding experiment on the grey soil were inconclusive. On the red soil there was a general increase in yield with superphosphate of about 60 lb., but other materials appear to have been less effective. At Namanga the fiwi bean weights from the phosphate experiment show considerable general improvements with phosphate which were again broadly related to the citric solubility values of the materials.

In the 1948-49 season an improved design has been adopted for experiments. on these questions, and all materials are being placement drilled.

Experiments on rate of application of standard fertiliser

The standard fertiliser used for groundnuts at Kongwa was a mixture of sulphate of ammonia and triple superphosphate containing 6 per cent. N and 30 per cent. P.Os. Experiments were set out to supplement those already described by testing the effect of varying the rate of application of this standard material. The fertiliser was applied with the placement drill at the standard rate of 13 cwt. per acre (0.5 cwt. P.O, per acre) and at one quarter and one half of this rate.

The average results of seven experiments at Kongwa were as follows:-

Table 2.-Average results of seven experiments on rate of application of standard fertiliser, groundnuts, Kongwa, 1947-48

170

No fertiliser Quarter rate

Half rate

Full rate

Yield kernels,

Yield difference kernels,

lbs. per acre

lbs. per acre

678

654

724

817

-24

+46

+139

The reduction in yield at the lowest rate was even more marked in some of the individual experiments, and there is some indication that adverse and beneficial effects were working in opposition. This may well be related to the reduction by nitrogen of the effect of phosphate (and vice versa), which was shown in the Kongwa exploratory experiments (page 106); and it illustrates the inadvisability of testing fertiliser mixtures when the actions and interactions of the components are not fully understood.

At Urambo, on both red and grey soils, the fertiliser in the corresponding experiments gave improvements in yield which rose with the rate of application, The fiwi bean but again there was some indication of adverse interactions. figures from the plots of the experiment at Namanga varied in the same way, showing marked improvements in yield.

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