Water and nitrogen management for risk mitigation in semi-arid croping systems

[摘要] English: This study was conducted with three main objectives which were firstly to characterize the smallholder farming system of semi-arid southern Zimbabwe and its rainfall pattern. The second objective was to quantify the crop yield and soil water benefits derived from in situ (single and double ploughing, ripping and planting basins) and inter-field (dead level contours and infiltration pits) soil water management techniques in southern Zimbabwe using field trials established on the farmers' fields. The on-farm study also explored the effect of combining in situ soil water management technologies (single and double ploughing, ripping and planting basins) and nitrogen fertilizer (0, 10 and 20 kgblha) application on crop yield under semi-arid conditions. An on-station experiment was established to assess the effect of combining in situ soil water management techniques (single ploughing, ripping and planting basins) and mulching (0, 0.5, 1, 2, 4, 8 and 10 tha') on maize, cowpea and sorghum yields, and soil water dynamics. The third objective was the evaluation of tillage systems on the farmers' fields over three growing seasons (2005/06, 2006/07 and 2007/08). The smallholder farmers appraised the tillage systems at the end of the last growing season of the study. Simulation modeling was then used to assess the long term effect of using the basin tillage system over a 69 year period. The daily rainfall data was collected from five meteorological stations located in the Mzingwane catchment of the Limpopo basin. The analysis revealed that neither the total annual rainfall, based on the July-June calendar, nor the start nor end of growing season has changed significantly over the past 50-74 year period in southern Zimbabwe. The analysis indicated that the length of the growing season decreases along the Bulawayo to Beitbridge transect. The growing season starts during the first eight days of December at all stations except at Filabusi where the season starts during the last week of November. The number of wet days per growing season has also not changed along the Bulawayo to Beitbridge transect. There are better chances of getting rainfall during the January-March period compared to the first half of the growing season. Our study revealed that there were no in situ soil water management techniques practiced by smallholder farmers in either Gwanda or Insiza districts during 2006. In Insiza district, the graded contours were the only structures constructed between fields while in Gwanda the dead level contours and infiltration pits were found on most farms particularly in wards 17 and 18. The dead level contours were being promoted by a Non-Governmental Organization (NGO) called Practical Action. Smallholder farmers in Insiza district used both manure and inorganic fertilizer as soil fertility amendments. However, in Gwanda district the majority of smallholder farmers used neither manure nor inorganic fertilizer for fear of crop burn. Farmers who used manure and fertilizer in Gwanda district had been exposed to how much manure and fertilizer is applied in semi-arid areas through interaction with agriculture extension officers and researchers. However, there is need for wider promotion of training and demonstrations on soil water and fertility management in the semi-arid smallholder farming areas. The on-farm experimentation assessed the effect of integrating soil water and nitrogen management under smallholder farming conditions. The study assessed the effect of single and double conventional ploughing, ripping and planting basins combined with nitrogen ferti Iizer on maize yields, surface runoff and soi I water dynam ics. Resu Its of the on-farm experimentation showed that the double conventional ploughing combined with nitrogen fertilizer outperformed the other three tillage systems regardless of the rainfall pattern in Insiza and Gwanda districts. Nitrogen fertilizer increased maize yields and water use efficiency in each season regardless of the tillage system used under smallholder farming conditions. The planting basin system had higher maize crop establishment at most farms during the period of experimentation. The on-station experiment showed that mulching had a significant influence on maize grain production across the three tillage systems in a season with below average rainfall. There were no significant maize yield differences across the three tillage systems tested at the on-station experimental site. Delayed planting in the conventional system resulted in reduced cowpea yields in a season with below average rainfall. Planting basin system gave lower sorghum yield as a result of reduced plant stand which was caused by rodent attack that was experienced at crop establishment stage. The on-station experiment indicated that sorghum and cowpea can be grown at the 0.9 m x 0.6 m spacing of the basin system without significantly reducing yield compared to the conventional system. The soil water dynamics were similar under single and double ploughing, ripper and basin tillage systems. The on-station experiment also showed similar soil water dynamics in the conventional, ripper and basin systems under mulched conditions at Matopos (clay soil) and Lucydale (sandy soil). The basin system had more soil water during the November-December period when the growing seasons started. Surface runoff measurements indicated that planting basins significantly reduce surface runoff water losses from cropped or uncropped field. However, the reduced surface runoff and higher initial soil water content was not translated into higher yields under the basin tillage system compared to the other tillage systems. r The study on the soil water contribution of dead level contours and infiltration pits indicated that these inter-field structures have no significant effect on soil profile water content in seasons with below average rainfall. During seasons that receive daily rainfall events of more than 40 mm, the dead level contours and infiltration pits collect more rainwater than the dead level contour only. Lateral soil water movement occurred after rainfall events of 60-70 mm particularly downslope of the contour with significant changes in soil water being observed at 3 m from the contour. The dead level contours and infiltration pits supplied soil water to the 0.25-0.45 m layers of the 0.6 m profile measured in this study. The evaluation of the in situ tillage systems by smallholder farmers revealed that labour demand and crop yields are major factors considered by smallholder farmers in semi-arid southern Zimbabwe when selecting a technology for adoption. The majority of the farmers achieved the highest yields under the double ploughing system, hence they ranked it as the most appropriate tillage system to use under their conditions. Availability of cereal and legume seed is one of the major challenges being faced by all households sampled in Gwanda and Insiza districts during the period of our study. The long term assessment of the basin system through simulation modeling revealed that basins give only marginal maize yield benefits over the conventional system regardless of the nitrogen level used. The long term simulation also indicated that crop failures can be experienced in both conventional and basin systems due largely to uneven distribution of rain events through the growing season.

[发布机构] University of the Free State