In the Highlands of Kenya every 1-week delay in planting maize after the onset of the rains reduces the grain yield by approximately 0-6 t/ha. No satisfactory explanation has been found for this phenomenon. A physiological growth study was made on four dates of planting per year over 4 years. Early growth rates and maximum crop growth rates showed a progressive decline with delay in planting which resulted in smaller plants at 5 weeks post-emergence and at 50 % tassel emergence in maize planted later. There was a strong relationship between the size of plant at tasselling and the final grain yield and a highly significant relationship (r = 0-94) between the size of plant at 5 weeks post emergence and the final grain yield. Other environmental studies showed that soil temperature at 7-5 cm, coupled with a soil moisture stress factor, largely controlled the dry-matter production rate during early growth, and consideration of the mean value of these two variables over the first 5 weeks of growth accounted for 70 % of the variation of dry matter at 5 weeks post-emergence. It was further shown that 82 % of the variation in final grain yield caused by date of planting could be accounted for by consideration of the mean value of these two variables during the first 5 weeks of growth. account for the pronounced 'time of planting' effect in the Highlands of Western Kenya. He In the tropics it has been widely observed that concluded that low yields of maize planted late maize planted at the onset of the rains gives con-were largely due to a progressive deterioration in siderahly greater yields than later plantings soil physical conditions as the season advanced. (Hemingway, 1955;Goldson, 1963; Akehurst & Furthermore, in an artificial watering trial he Sreedharan, 1965;Allan, 1972). This has become found that increasing amounts of water applied known as the 'time of planting' effect.during the period from planting to 5 weeks postMany varied explanations have been put for-emergence caused corresponding reductions in ward to account for the 'time of planting' effect early growth, number of kernels initiated and final in East Africa. Hemingway (1955) suggested that grain yield. As a result of these experiments, and yields of maize planted late were less because young a study of rainfall distribution patterns, he maize plants were more severely attacked by fun-suggested that poor soil aeration was the most gal leaf diseases than maize planted early, whereas important soil physical factor causing yield reducBirch (1960) suggested that crops planted early tions in maize planted late. However, this hypoyielded more because they were able to take ad-thesis was not based on actual measurements of vantage of the flush of soil nitrogen mineralization soil aeration, and Cooper (1975) recently showed that occurs during the wetting and drying cycles that the aeration status of Kitale soils under at the onset of the rains. In certain areas with short young maize never limits growth, even if the rainy seasons, maize planted late will suffe...

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SummaryPrevious work has shown a strong relationship between the mean soil temperature during the first 5 weeks of growth of a maize crop, and the final grain yield, warmer soils leading to greater yields. Trials were laid down in 1975 and 1976 to establish how early in the development of a maize crop higher soil temperatures would lead to increased yields. Soil temperatures were raised by polythene mulching applied at planting with six times of mulch removal: at crop emergence, 1, 2, 3, 4 and 5 weeks after emergence. Raised soil temperature led to a greater rate of development and leaf area production during early growth. Greater leaf area was due to greater leaf emergence rate rather than increase in leaf size, since increase in soil temperature was associated with a decrease in individual leaf size. This trend was reversed from leaf number 15 onwards resulting in no differences in leaf area, leaf weight or total dry matter at tasselling. In spite of this, yield differences were observed. Increase in soil temperature during germination alone had a beneficial effect on final grain yield, and this effect increased with duration. Increasing soil temperature for longer than 3–4 weeks from emergence caused no further yield increase. Yields increased from 133 and 172 g/ plant to 220 and 238 g/plant in 1975 and 1976 respectively. Yield increases were associated with more grains per plant rather than greater grain size. The period during which increased soil temperature led to increased yields coincided with the period when the apical meristem was below ground level. The mechanism involved is not yet clear.

Large expanses of vertisols currently used, especially in Africa, for low-intensity live-stock or rain-fed agriculture could beneficially be brought under irrigation. They generally have favourable chemical characteristics with relatively few instances of salinity or alkali hazard. Whilst slow rates of water movement through the soils may limit water application and restrict artificial drainage, these vertisols are capable of sustained productivity, under irrigation, of a wide range of crops.

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