Compost Making in Rhodesia
by J. M. Moubray
In 1939, when I last wrote a few notes for An Agricultural Testament, compost making in Rhodesia was in its infancy. Now it has become , general. The usual procedure now adopted is to break down the vegetable wastes by spreading them in stock-yards or pens. Here they absorb and get well mixed with the animal wastes both solid and liquid, and are then removed to the compost heaps. In this part of the country growth is very rank. When tall grass and reeds were moved straight to the compost heap, the stems took a considerable time to break down, but by being first trampled down the stems are broken and the fungi and bacteria are then able to attack both from the inside and outside at once.
In the five years that have passed since 1939 little change in procedure has taken place with the exception of passing all raw material through the stock-yards. I still build the heaps some fifteen feet wide and three feet high and up to any length (Plate XII). Two turnings are sufficient and at the end of three months the breakdown is complete. In the dry weather, if the heaps are fairly moist when built, a good wetting with the hose- pipe each time the heaps are turned is sufficient (Plate XIII). Material from the outside of the heap is always turned inside. I cut a good deal more hay than I used to do and if some of this is a bit coarse or gets a wetting, it does not matter, as what the cattle do not eat goes to the compost heap. Our veldt is improving with mowing, as when the coarse grasses are kept down and in check the finer and more valuable grasses get a better chance to develop.
Plate XII. Compost-making at Chipoli, Southern Rhodesia.
Plate XIII. Compost-making at Chipoli, Southern Rhodesia. Watering the heaps.
We are learning that under conditions in many parts of Mashonaland nitrification is very rapid. Under favourable moisture conditions a green crop ploughed in leaves little visible organic matter at the end of three months. Partly for this reason, if the compost is not quite broken down when applied to land for crops like maize, we get better results.
The nitrogen content of compost has been found to be quite stable. I have found the loss of nitrogen in a heap which has stood for some months in the dry weather to be negligible. Mr. van Vuren, who has done so much in the Union of South Africa for municipal compost, has found much the same to happen with him. I now spread out some of my compost in a thin layer. In the hot sun this gets quite dry in a day or two. I then grind it in a hammer mill, sack it, and it can be kept in such a manner for an indefinite period. In this way it has probably lost some 40 per cent of its moisture content and is so correspondingly richer in humus. If, instead of broadcasting rough compost, a cupful of the ground material is applied round the plant in the field for such crops as tobacco or tomatoes, a considerable economy is effected.
I add ground raw rock phosphate to all my compost heaps. It is probable that some of the inorganic phosphorus is changed during the fermentation into organic forms. If this is so, and some of the best American opinion considers such a change takes place, it is all to the good, as in its organic forms phosphorus is not locked up and so made unavailable to the plant, as it does not combine with iron and alumina.
As regards cost of making compost, assuming that bedding of some sort has to be provided for the stock-yards and that the work of cutting and carting such bedding is debited against the stock account, then I think most farmers in this part of the world will agree that a sum of Is. or 2s. per ton will cover the cost of compost making. That is, of course, apart from the cost of raw rock phosphate or similar material added to the heap.
The effect of compost on fruits, vegetables, and field crops in Rhodesia is now so well known that further propaganda is unnecessary. A neighbouring farmer, to give one example, used it on bananas and found that in two seasons he not only doubled the size of the bananas, but doubled the numbers held in the bunch besides greatly improving their flavour.
The trouble now is that we cannot make enough compost. With labour becoming more difficult various mechanical devices for handling and turning compost are coming into use. An ordinary dam scoop with the bottom elongated by means of steel fingers acts very well in moving the material to make the compost out of the stock-yards, and in turning the heap itself. I find nothing to beat hand labour. Once a native gets into the work he will do a large tonnage per day and nothing mixes the material so well as hand labour. If the material is fairly damp and requires little wetting, then two natives, working side by side, keep pace with the hose- pipe; but if it is very dry, then one turner only is used, so that more water can be applied as it is thrown over.
In Rhodesia compost has been found to control the parasitic plant, witchweed (Striga lutea), which attaches itself to the roots of the maize. Witchweed used to be a major problem, but on my farm it is now negligible.
It is now being accepted that, in the same way, good applications of compost will eliminate eelworm. This pest had begun to assume very serious proportions in tobacco lands, to such an extent that infested lands were considered unsuitable for further tobacco crops.
Organic farming is coming more and more to the fore in Rhodesia. Itis at last being recognized that many of our troubles were due to lack of humus in the soil.
Green cropping is taking a larger and larger part in the rotation and the chief plant used is the legume, san hemp (Crotalaria juncea), this on good soil grows eight to nine feet high and ploughs in very well with a tractor-drawn disc plough. If a light dressing of compost, containing a good proportion of animal wastes, is added to the soil for such a leguminous green crop, more seed is formed. This may be due to the plant growth substances which originate in the animal and perhaps further supplies are formed during decomposition in the compost heap.
Compost and, in fact, all organic matter appears to have considerable effect on the mycorrhizal growth. I speak now, in particular, of the orange tree, of which I have many thousands growing on this farm, Chipoli. If the hair-like feeder roots of a healthy tree are carefully exposed, they will be seen to be covered with a mould-like growth, but if the same is done to an unhealthy orange tree, showing signs of decline, then this is found to be absent.
And now to give what I consider to be one of the best examples of chemicals versus organics. There are in this Mazoe valley two orange groves, both of considerable extent, planted about a quarter of a century ago, of the same variety of orange, the Valencia Late. The trees grow on the same type of good red soil, well drained and irrigated in the dry weather. In fact, conditions are about as similar as they could be. One grove has been fed almost exclusively on artificials -- superphosphate, muriate and sulphate of potash, nitrate of soda, and sulphate of ammonia, this last in large proportion. Cultivation is more or less clean, little weed growth being allowed and little or no organic matter applied. The trees in this grove are now practically finished; new growth has all but ceased. The trees are full of dead wood and the crop of oranges they now carry is sub-economic. The foliage is sparse and of an unhealthy colour. In the other grove the only fertilizer used has been raw rock phosphate and bone, but since the start of the war bone has been unprocurable. A heavy green crop of legume is grown during each rainy season, this is broken down and disked in, and the soil is covered with old grass, trash of all kinds, ground nut haulms, and so forth. Irrigation is then applied, when a rank growth of grass and weeds of all sorts comes up through the mulch. This is eaten off in situ by cattle and sheep whose droppings fall on the vegetable wastes. With the advent of the rains what remains on the surface and has not been assimilated by the soil bacteria is disked in and the cover crop is at once planted. One has only to look at the trees to see that they thrive. They carry heavy crops of good-quality fruit, the foliage is a dark green, the trees carry no dead wood, and regularly put out a thick new growth.
This example of two treatments is, I think, almost unique. It shows the culminative effect of a treatment of chemicals and of organics over some twenty-five years. These groves are open to inspection by any and all, and the owners will confirm the treatment under which they are grown.
What is the explanation? The accumulation of the sulphate ion in the chemically treated grove must be considerable. Is it this that has prevented the mycorrhizal connection functioning, or is it the lack of humus, or both that have been slowly killing the trees?
One fact emerges and on this there need be no further argumentthe orange tree under the conditions described will not thrive for any lengthy period on chemical food alone, but it will do so on organic food. Whether the healthy trees would have been more healthy still if chemicals had been added to the organics, or whether the sulphate ion would have been too much for the mycorrhiza I cannot tell. To prove this conclusively would require another quarter of a century and that is a good deal more than is left to me.
Shamava, Southern Rhodesia.
27th July 1944.
Next: Appendix C. The Utilization of Municipal Wastes in South Africa
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