SOIL LOSS AND CASSAVA YIELD UNDER RIDGE TILLAGE IN HUMID TROPICAL CLIMATE OF SUMATERA, INDONESIA

Erosion is one of problem threatens the soil resource caused soil degradation. Applying the ridges system and organic fertilizer contributes to suppressing runoff and erosion. This study aims to determine the effect of applying different ridges system and the organic fertilizer on the runoff, soil erosion, nutrients loss of soil, nutrient enrichment ratio, starch content, growth, and yield of cassava on 12.5% slope land. The research was conducted at the field experiment, University of Lampung, under a humid tropical climate. This study used a Randomized Complete Block Design, which was arranged factorially with two factors, ridges system and organic fertilizer (organonitrofos) and four replications. Factor one is consists of G1 (ridges in the same direction with the slope) and G2 (ridges in the opposite direction with the slope), while factor two is consists of P0 (Organonitrofos 0 tons/ha) and P1 (Orgononitrofos 20 tons/ha). The results showed that the application of the ridges in the opposite direction with the slope significantly reduces the runoff 42.63%, soil erosion 61.68%, and suppress nutrient loss of N-total by 61.85%, P-available by 57.92%, K-total by 80.41% and C-organic by 61.22%, compared to ridges in the same direction with the slope. However, ridges systems do not give a significant difference to starch content, growth, and cassava yield. Physical obstacles formed by ridges in the opposite direction with the slope will force water to infiltrate into the soil so that the runoff rate, erosion, and nutrients loss can be reduced significantly. In reverse, the organic fertilizers do not affect all variables. The ridges system and the organic fertilizer did not significantly affect the starch content, growth, and yield of cassava. That caused by the application of synthetic fertilizers at the same dose (standard dose for planting cassava) in each experimental unit that fertilizer already enough to grow optimally.