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A 3-year field experiment was undertaken at two sites in semi-arid Tanzania with the objective of analysing the potential for improving the productivity of maize based cropping systems through combinations of water conservation and nutrient management. The soils at the two sites differed mainly in their degree of weathering, clay content of the argillic horizon and the overall soil fertility level. The rainfall regime during the experimental period included years with drought, normal rainfall, and a very wet year.
Tied-ridging enhanced the response to rainfall and fertilizer, the soil supply of plant available N, the positive effect of inter-cropping on total grain yield, and the harvest index. The combined response to annual rainfall and, N and P fertilizer under tied-ridged conditions could be summarised as simple empirical functions accounting for 98% of the variance in yields. Considerable yield increases above "farmers' practices" (i.e. flat cultivation and no fertilizer) could be realised by combining tied-ridged tillage with inputs of mineral N and P fertilizer, reaching maize grain yield levels of six times the prevailing yield under farmers' practices of approximate 1 Mg ha-1. Similarly, the rainfall-grain productivity could be tripled reaching levels of 6 kg ha-1 mm-1 at high fertilizer inputs in years of near-normal rainfall. The effect of tied-ridging was both positive and negative, with the positive effect dominating at near-normal rainfall (500¿600 mm) whereas the negative effect dominated at annual rainfalls above 700¿900 mm. However, the negative effect could be partly mitigated through fertilizer inputs. The site with the more weathered soil displayed a higher sensitivity to moisture deficits and excesses and a narrower rainfall range with positive effect of ridging. Under near-normal to slightly dry rainfall conditions, tied-ridging without nutrient inputs resulted on an average in 34 and 42% grain yield increase at the two sites.
A benefit¿cost analysis supports tied-ridging as a potentially profitable technology, especially at the site with better soil quality, slightly higher rainfall and market integration. Thus, a subsistence oriented household will maximise profitability of a maize¿cowpea inter-cropping system by using tied-ridging and no fertilizer input whereas a market oriented more resourceful household will maximise profitability from a combination of tied-ridging and high fertilizer inputs. The scarcity of cattle manure and the total cost of application are prohibitive to cattle manure becoming a viable and significant option of nutrient inputs for most farmers in the area. Inter-cropping with cowpea is an important economic factor, currently constrained by diseases and low crop densities.
The study highlights the importance of analysing soil and water conservation technologies within a farming systems perspective, and supports a focus on soil and water conservation practices such as tied-ridging in a semi-arid environment with annual rainfall of 500¿700 mm. However, recognising potential drawbacks of the tied-ridged technology, it is proposed also to investigate alternative practices improving in situ moisture conservation at minimum soil disturbance.
Author Keywords: Water harvesting; Tied-ridging; Maize; Semi-arid; Small-holder; Benefit
