Soil quality effects of accelerated erosion and management systems in three eco-regions of Tanzania

Soil erosion can adversely influence soil quality, especially in tropical soils. Thus, a multi-location field experiment was conducted on eight major agricultural soils with different degrees of erosion, in three eco-regions in Tanzania. The objective was to assess the impact of topsoil depth (TSD) and management on soil properties. Three eco-regions comprising of humid at Kilimanjaro, sub-humid at Tanga and sub-humid/semi-arid at Morogoro were selected. There were a total of eight locations within three eco-regions comprising two at Kilimanjaro (e.g., Kirima Bore and Xeno Helena), two at Tanga (Mlingano 1 and Mlingano 2) and four at Morogoro (Misufini 1, Misufini 2, Misufini 3, and Mindu). The soil management treatments consisted of farmyard manure (FYM), N and P fertilizer, tie-ridging and farmers' practice. Plant nutrient content was generally lowest on severely eroded and the highest on least eroded soil classes. Soil pH decreased with increasing severity of erosion on soils with higher content of Ca+2 in the sub-surface. In general, there occurred a decline in soil organic carbon (SOC) and P with the decrease in TSD. The SOC content decreased on severely eroded soil class by 0.16%, 0.39% and 0.13% at Misufini 1, Mlingano 1 and Kirima Bore, respectively, compared to slightly or least eroded soil class. Corresponding decline in available P at these sites was 41%, 62% and 61%, respectively. Application of FYM significantly increased soil pH at some sites. Soil content of SOC, N, P, K and Mg were significantly increased by FYM application. Significant effects of N and P fertilizers on SOC and P were observed at most sites. In comparison with farmer's practice, FYM application increased SOC by 0.55%, N by 0.03%, P by six fold and K by two-fold. Nitrogen and phosphorus fertilizers had comparable effects for SOC and P only at some sites. The results indicate that FYM is a better soil input than N and P fertilizers in improving soil quality. The data show that SOC, N and P are most adversely affected with accelerated erosion and that FYM fertilizer applications have the potential to improve fertility of eroded soils. (C) 1999 Elsevier Science B.V. All rights reserved.