Soil loss and nutrient losses due to erosion in a maize-legume Based cropping system in Gatanga (Kenya).
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In Central Kenya highlands, accelerated soil erosion is caused mainly by intensive agricultural •. activities. The current conventional agricultural practices where the crop residues are completely harvested alongside with the crop, leave the soil bare during the onset bf the subsequent rains. The study, therefore, addressed the use of selected green manure cover crops (GMCC) as intercrops with Zea mays for erosion and nutrient loss control targeting the onset of the rainy season. This is the time when the ground is usually unprotected and most of the soil and nutrients are lost due to erosion. This study was initiated in 1998 short rains on farmers' fields in Gatanga Division, Kenya. Workshops involving farmers and researchers were held to select legumes on the qualities of what farmers considered the best attributes and train the fanners on monitoring erosion and data collection. The four systems tested consisted of the following: pure stand of maize (Zea mays) (Tl), maize intercropped with Mucuna pruriens (velvet bean) (T2), maize intercropped with Vida benghalensis (purple vetch) (T3), and maize plus Lablab purpureus (dolichos lablab) (T4). Therefore, four runoff plots measuring 2 m wide by 4 m long were installed adjacent to each other on a single catenal position on each of the six farms to monitor erosion. Measurements taken included soil loss, percent crop cover and analysis of nutrients present in the original soil and eroded sediments. Results of soil loss showed considerable variability depending on the nature of the rainy season and/or treatment. During the short rain season (SR) of 1998 no soil loss was recorded as the rainfall was very low and the erosivity of storm events sub-critical. The cumulative soil loss across the treatments during the 1999 long rain season (LR) was 58.64, 61.7, 61.56, and 61.19 t ha-I for Tl, T2, T3 and T4 respectively. Although T2 (maize intercropped with mucuna) had the lowest soil loss, the result was not significantly (P::;O.05) lower than other treatments. On the XIV other hand, the treatment effects exhibited significant (P~0.05) effects on soil loss during the 1999 SR. The lowest (0.35 t ha') and the highest (3.3 t ha') soil losses were recorded from T2 and Tl that had average cover of 43.2 and Olyo respectively. This cover (T2) was a result of the post-harvest crop cover from the previous season. At the onset of the 1999 SR, the average cover from the previous season was 0, 43.2, 9.0 and 11.4% for TI, T2, T3 and T4, respectively. There was significant difference (P~0.05) in percent crop cover between treatments at this point in time. The cumulative soil loss for the 1999 SR ranged from 12.24 to 20.31 t ha'l for T2 and T1 respectively. Data of nutrient loss via sediments suggests that prolonged soil erosion from field plots may result in infertility and hence unproductivity of the soil in question. Nutrient levels of sediments were found to be significantly (P~0.05) different from the original soil. Nutrients in sediments were compared with the original soil and the enrichment ratio (ERI) for major nutrients (i.e. organic C, total N, available P, Ca and K) was found to be greater than I. There was no significant difference (P~0.05) in nutrient losses between the treatments. The pH of the eroded sediments was slightly higher than that of the original soil, as the sediments were richer in bases than the original soil. This indicates that prolonged soil erosion from a field makes it unfertile and thus unproductive as observed from the field during the period of study. Therefore, soil management practices, such as the use of biological soil conservation measures, which in addition improve the levels of soil organic matter, nitrogen and phosphorus should be encouraged for sustained productivity of these soils. I The ratio of a nutrient element in eroded soil material to that in original soil or "field soil" is known as the enrichment ratio (ER). . xv