Impact of green manure and mineral fertilizer inputs on soil organic matter and crop productivity in a sloping landscape of Sri Lanka

Fields and Homegardens (representative of tropical smallholder extensive and intensive systems) were compared for soil quality and productivity of maize and mungbean in a hilly region of Sri Lanka (twinned 30 farms with Fields and Homegardens, each) over 2 years. The inclination categories were Flat (0–10%), Moderate (10–30%) and Steep (>30%). On half of the farms, green manure was added using Gliricidia leaves (3 tonnes ha−1 per season). Soil organic matter contents of Homegardens were at least 30% greater than in Fields, most pronounced in the Flat category. After 2 years of continuous maize and mungbean cropping in the major and minor rainy seasons, respectively, SOM generally declined; the greater depletion rate in Homegardens was positively correlated with crop yields. With recommended fertilizers (NPK), maize yield was always significantly higher in Homegardens than in Fields Flat and Moderate, this hold true at Steep lands only without NPK. Maize yield with NPK was on average two times higher than with Gliricidia alone, a respectable 4.5 tonnes ha−1 at Flat and Moderate, and 3.5 tonnes ha−1 even on Steep lands, seemingly the chemical soil fertility was more lacking than the physical properties. Gliricidia alone had no impact on yield in Fields but just in Homegardens, yield was increased by almost 30% in comparison to Zero fertilizer, an indication that chemical fertility was not directly supported by Gliricidia. Mungbean yields were generally less influenced by treatments, just the beneficial short term impacts of Gliricidia in Fields were relatively much higher than in maize. Conclusively, the long term input of organic manure in Homegardens was translated into a higher productivity in comparison to extensively managed Fields. However, the annual green manure supply supported yields mostly at marginal input levels and was overridden by mineral fertilizers independent of the inclination. Although, the usefulness of Gliricidia in degraded Steep lands was proven, the great risk of erosion remains in this landscape. Arable cropping there should be replaced by access to NPK on low inclined land once socio-economic conditions allow it.

► Long and short term green manure impacts mostly more pronounced at low inclinations.
► Continuous high input of organic matter reverses soil degradation according to SOM and maize yield.
► Recommended mineral fertilizers boosted yield levels much more than green manure.