The effect of some tillage methods and cropping sequence on rainfall conservation in a semi-arid area of Eastern Kenya

Abstract

To the east and south of the central highlands of Kenya lies an area whose agricultural production is based on a low and unreliable rainfall. There are two cropping seasons in a year of roughly equal duration. The rainfall duration of each season seldom exceeds two months and heavy rain usually occurs during the first month from the start of rains. This, combined with structurally weak and rapidly capping, sandy clay or sandy loam soils, results in the loss of considerable amounts of rainfall as runoff. With little or no rainfall occuring after two months from the onset of the rains, it means that crops of three to four months duration have to

reach maturity mainly with moisture stored in the soil, Hence crop failures are quite frequent. In this research project, selected tillage practices and cropping sequences were investigated to determine their'effectiveness in conserving rainfall. The research project was carried out at Katumani Agricultural Research Station, Machakos, which is situated within the target area. Minimum, conventional, cloddy and tied-ridge tillage

methods were tried. It was expected that the different methods would have different effects on the infiltration capacity of the soil and hence on the proportion of rain which would be lost as runoff and that which would infiltrate into the soil. The cropping sequences included maize/maize, beJ3,ns/maize and bare fallow/maize, and it was expected that the d~fferent sequences would conserve varying amounts of residual moisture at the end of the first season. The residual moisture would be carried over to aug - ment of rainfall received during the second season, and so help to obtain a better crop yield. The project was carried out during the long (April) and the short (November) rainy seasons of 1976 and the long rains of 1977. The data presented show that with low rainfall similar to thereceived during the long and the short rains of 1976(200mm or less) the deeper layers of the soil profile under the different tillage methods remained dry throughout the rainy season, because only a few of the daily showers were large enough to wet the soil deeper than 15-20cm. However significantly higher moisture contents were obtained under tied-ridges as compared to the other tillage methods following a wet period in Field M, hence demonstrating the superiority of tied ridges in conserving rainfall. Crop establishment was satisfactory on the minimum and the conventionally tilled seedbeds but less so on the cloddy and the tied ridge seedbeds. Subsequent growth was satisfactory on minimum, conventional and the tied ridge seedbeds but, adversely affected on the cloddy seedbed.Since the rain stopped too early for maize to reach maturity in either the long or the short rain of 1976 the dry matter was harvested. Thei drymatter yields were lowest on the cloddy seedbed and significantly higher on tied ridges in the long rains but not superior in the short rains of 1976. During the 1977 long rains, which were substantially above average (507.3mm) maize on tied ridges gave significantly higher grain yields while that minimum ,tillage gave the lowest. There was no difference between the maize yields from the conventional and the cloddy seedbeds. Bean yields were lowered signifi cantly on the cloddy seedbed compared to other seedbeds during the relatively dry long rains of 1976, but were highest on the cloddy seedbed during the wet long rains of 1977. Bean yields from the tied ridges were lowest during the long rains of 1977 though the differences were not significant probably

because the tied ridges were too wet for the beans. With such low rainfall as occurred during the long rains of 1976 the water content of the soil profile under the different cropping sequences did not increase significantly. Thus at the end of the long rains, and after a maize or bean crop or after a bare fallow, the soil profile was still at or below the permanent wilting point. However, the profiles under previous crops (maize or beans) were slightly drier than the profile under a previous bare fallow. Although the differences in drymatter yields of the following maize crop during the short rains of 1976, were not significant, the differences in cob number and grain yield were highly significant, with preceding fallowing giving higher cob number and grain yield than preceding cropping. No firm conclusions could be reached, since the research project extended over only three seasons of which two were virtual crop failures. Nevertheless, the results obtained suggest that the tied ridge tillage method may be a very promising method of rainfall conservation, especially as most of the cropping seasons are usually intermediate between the relatively dry seasons of 1976 and the wetter long rains of 1977. Similarly, the evidence from the

short rains of 1976 suggests that fallowing could store sufficient water in the soil which might significantly improve crop yields of the following season. It is therefore recommended that further trials should be carried out on the tied ridge method of tillage and the use of fallow period in the cropping sequence before firm recommendations for farmers could be made.