Climbing and Bush Beans’ Cultivation Effects on Runoff, Soil Properties and Soil and Nutrient Losses in Bufundi Sub catchment, Uganda
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Climbing beans were developed by International Centre for Tropical Agriculture (CIAT) and are heavily being promoted over bush beans in humid highlands of South Western Uganda; characterized by steep degrading hill-slopes. Climbing beans yield two to four times more than bush beans and are characterized with heavy vegetation cover and longer maturity period of 200 days over bush beans. These two types of beans are the major legume cover crops heavily cultivated at the middle landscape position in Bufundi catchment. The catchment is heavily experiencing limited arable land due to declining soil fertility attributed to increased soil erosion. Several studies have been conducted on these two types of beans on various thematic areas however; the empirical contribution of each of the two types of beans in controlling runoff, soil and nutrient losses down the catchment is not clear and documented. Therefore, this study was carried out to assess the effects of climbing and bush beans‟ cultivation on runoff, soil and nutrient losses and soil properties in Bufundi catchment, Southwestern Uganda. Specific objectives included determining runoff; examining soil and nutrient losses as well as evaluating the effect of these bean types on soil properties. The experiment adopted runoff approach and conducted at the middle land scape position at three farmers‟ terraced sites with slope range of 23 -29 % which were representative of those terraces commonly found in the study area. A total of 18 runoff trap plots measuring 2 x 9 m were established. Climbing and bush beans were planted and replicated thrice at each farmer‟s site. Half the number of plots was planted with climbing beans and the other half with bush beans for two rainfall seasons in 2012. Before planting and after harvest, a total of twelve composite soil samples were taken at depths of 0-15 cm and 15- 30 cm within the runoff plots and characterized in the laboratory for soil pH, Soil Organic Carbon, total nitrogen, available phosphorus and exchangeable potassium and soil structure. Runoff was measured using a measuring cylinder after every rain event and the measured volume poured in clean bottles for soil and nutrient losses analysis. Precipitation received per storm event was recorded using a non-recording rain gauge. Soil and nutrients (Total nitrogen, Phosphorus and Potassium) lost were analyzed in the laboratory. Runoff, soil and nutrients data were statistically analyzed using one way Analysis of Variance in Genstat software version 13 and the means differentiated using 5 % L.S.D. A significantly lower (p< 0.05) annual runoff in climbing beans (36 m3 ha-1yr-1) than bush beans (248 m3 ha-1yr-1) was observed. Runoff was highest at a slope of 23 -25 % (379 and 48 m ha-1yr-1) than at 26 -29 % (137 and 18 m3 ha-1yr-1) from bush and climbing beans, respectively. Annual soil loss significantly varied at p<0.05 between bush (548 kg ha-1yr-1) and climbing (121 kg ha-1yr-1) beans. Similarly, soil nutrient losses were significantly (p<0.05) much higher in bush (TN= 1.87 Kgha-1yr-1; TP=0.6 Kgha-1yr-1; TK=0.12 Kgha-1yr-1) than climbing (TN= 0.49 Kgha-1yr-1; TP=0.1Kgha-1yr-1; TK=0.03Kgha-1yr-1) beans. Climbing beans contributed to higher increase of SOC, soil structure and high net decrease in soil N over bush beans at the end of the two seasons. Therefore, cultivating climbing beans are more environmentally friendly and beneficial crop husbandry strategy in soil and water conservation as part of the integrated watershed management approach however, their nitrogen uptake is higher than bush beans. Climbing beans should be promoted at middle slope position/above over bush beans in order to have a double benefit of soil erosion control and increased yield however, if bush beans are to be planted, then IWM practices should be promoted and implemented in the catchment by local government, farmer groups and SWC NGOs operating in the catchment.