American experiments (for example Colwell and Brady, 1945) have shown that on certain soils very deficient in lime (Ca) considerable increases in yield may be obtained by dusting the plants with gypsum (calcium sulphate) at the early flowering stage. These increases may be far greater than those obtained by broadcasting ground limestone before planting or by combine drilling limestone with the seed. It is known that developing groundnut pods, in their early stages, have structures like root hairs (Pettit, 1895, Reed, 1924), and that they absorb plant foods and water independently of the main root system; and it has been shown that in some varieties of groundnut, at least, Ca is of the highest importance for kernel formation (Burkhart and Collins, 1941). It may be that under the relatively humid conditions of the south-eastern United States gypsum has special advantages for the groundnut crop, being rapidly absorbed by the nuts when applied to the surface of the soil just as their development is beginning.
This question was examined by means of a series of experiments in which limestone was applied at 1, 4 and 8 cwt. per acre, while on other plots gypsum was applied at rates giving equivalent Ca. Each of these treatments was applied in three ways: broadcast before sowing, drilled with the seed, and spread over the plants at first flowering six to eight weeks after sowing. All plots received a uniform dressing of phosphate.
At Kongwa, where acutely Ca deficient or acid soils are rare, and at Urambo on the red soil, no benefit was obtained from either lime or gypsum, and some treatments reduced yields. On the grey soil at Urambo the crop was some- what improved by low rates of gypsum, but the most marked increases, from 781 to over 900 lb., were from 4 cwt. of limestone, which also seemed to have its best effects when broadcast before sowing..
At Namanga fiwi bean showed large responses to limestone and smaller ones to gypsum applications.
It seems, therefore, that there is no case for general applications of limestone at Kongwa, but that limestone may be required for parts at least of the Urambo and Southern Province areas, where the matter is under further examination.
Recommendations on fertiliser policy at Kongwa
With present costs of P and N the results of the majority of the fertiliser experiments on groundnuts at Kongwa lead to the conclusion that P.O, appears to be the only plant food whose large-scale application to the groundnut crop at Kongwa is likely to be economic, and that N often reduces the effective- ness of P2O. The average benefit from the placement drilling there of 0.5 cwt.. P.O as water soluble granular fertiliser was 148 lbs. kernels per acre. (It is not yet known whether this can be still further increased by raising plant population.) At an assumed future price of £41 Os. Od. per ton of groundnuts, f.o.b. Dar-es-Salaam, as a basis, and deducting £6 Os. Od. per ton for decortication, bags, railing and handling charges, this quantity is worth just over 46 shillings. The cost of the fertiliser required to produce this result is shown in the following table:-
Table 3.-Economic Data on the Use of Phosphate on Groundnuts at Kongwa, 1947-48 Season
171
P205
Material
content
Rate per acre (cwt.)
Cost per ton at Kongwa E.A. Shs.
Cost per acre
Profit (+) or
E.A. Shs. loss (-)
Per cent. profit (+) or loss (-)
on
outlay,
Per cent.
Triple super
46
1.10
615.
34
12
+35
Sulphate of ammonia triple
super mixture
30
1.67
775
...
65
-19
-29
Dried superphosphate ex
South Africa
20
2.50
248
31
15
+48
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