Uncovering the Footprints of Erosion by On-Farm Maize Cultivation in a Hilly Tropical Landscape

A hilly region in Sri Lanka was considered to be degraded by erosion driven by intensive tobacco production, but what are reliable indicators of erosion? In addition to determining soil chemical and physical traits, maize was cropped with Nitrogen, Phosphorus and Potassium (NPK, PK) recommended mineral fertilization and without fertilizer (ZERO) in two major seasons(October–January in 2007/2008 and 2008/2009—Seasons 1 and 2 respectively) on 92 farms at inclinations ranging from 0% to 65%. In a subset of steep farms (n = 21) an A horizon of 6 cm rather than of 26 cm was strong proof of erosion above 30% inclination. Below the A level, the thickness of the horizon was unaffected by inclination. Soil organic matter contents (SOM) were generally low, more so at higher inclinations, probably due to greater erosion than at lower inclination. Maize yields decreased gradually with increasing inclination; at ZERO, effects of climate and soil moisture on yield were easier determined and were probably due to long-term erosion. However, despite an initial set of 119 farms, an exact metric classification of erosion was impossible. NPK strongly boosted yield. This was a positive sign that the deficits in chemical soil fertility were overriding physical soil weaknesses. The study illustrated that chemical soil fertility in these soils is easily amenable to modifications by mineral and organic manures