Effect of legume integration and application of organic fertilizers on soil nutrient status and kale yield in Kabete, Kenya
Kale production in Kenya is being hindered by declining soil fertility among other factors. A study was done on ways of improving soil fertility, quality and yield of kale (Brassica oleracea var. acephala) in Kabete - Kenya, through use of farmyard manure and Minjingu rock phosphate with integration of chickpea (Cicer arietinum) and white lupin (Lupinus albus cv. Amiga) either as an intercrop or in rotation. The study was carried out for two seasons during the long rains of 2014 and short rains of 2014. Experimental layout was a Randomized Complete Block Design with a split plot arrangement replicated thrice. The main plots were cropping systems; (Crop rotation, Monocropping and intercropping). The split plots were organic fertilizers (Farmyard Manure and Minjingu rock phosphate) whereas the test crop was kale. Soil organic carbon, N, P, and mineral N were tested at an interval of 1, 2 and 3 months of kale development. Kale N, P, K and yield were measured at an interval of 1, 2 and 3 months on kale leaves. The determination of nitrogen fixed by legumes under different organic fertilizers and cropping systems was done using the extended difference method where wheat was sown as a reference crop and sampling done at the late pod fill stage of the legumes. As part of the study, once the legumes were harvested, they were put in litterbags incorporated into soil for determination of their decomposition and mineralization rates. The study was done to address the asynchrony of nutrients between legume nutrient release and kale nutrient uptake. The decomposition and mineralization rate of the legumes was done by taking weights after every retrieval and chemical analysis done in the laboratory at an interval of 0,15,30,45,60,75,90,105 and 120 days. The decay formula was adopted (y =y0*e-kt) for determining the rate of decomposition and mineralization. The half life’s for the legumes was determined (time when half of the nutrients are released) by calculating using t1/2=In (2)/k. The cropping system white lupine-kale rotation (FYM) (0.59%) significantly increased the amounts of soil available nitrogen in the 1st season. Soil available phosphorus was significantly higher in white lupine/kale intercrop (MRP) (21.83ppm) at the 3rd month of sampling. Mineral nitrogen was significantly highest where white lupine-kale rotation (FYM) at the 3rd month of sampling. During the short rain season, there was a significant (P≤ 0.05) increase in the level of organic carbon under white lupine-kale rotation (FYM) (3.16%). Soil available nitrogen was significantly increased where white lupine/kale intercrop (FYM) (0.614%) at the 3rd month of sampling. The concentration of potassium in the kale leaves during the first season were increased where monocrop (FYM, MRP and Control) (4.00%, 4.87%, 4.17%); chickpea/kale intercrop (FYM, MRP and Control) (3.80%, 4.15%, 3.75%); lupin/kale intercrop (FYM, MRP and Control) (3.90%, 3.70%, 4.00%) the increment was recorded at the 2nd month of sampling. A lower level of potassium concentration was recorded in the 1st and 3rd month of sampling. During the second season, the trend was similar to the first season. The kale % N concentration was significantly (P≤ 0.05) increased in lupin/kale intercrop (FYM, MRP and Control) (1.63%, 0.97% and 1.27%); chickpea/kale intercrop (FYM, MRP and Control) (1.53%, 1.20%, 1.00%); monocrop (FYM, MRP and Control) (1.26%, 1.53% and 1.40%) during the second month of sampling. Season two had an increase in nitrogen concentration, chickpea-kale rotation (FYM, MRP and Control) (3.47%, 3.17%, and 2.18%). Phosphorus concentrations during season one was increased in the order , monocrop (FYM, MRP and Control) (50ppm, 63ppm and 47ppm); chickpea/kale and lupin/kale intercrop (FYM, MRP and control), the same trend was recorded in season two where, chickpea-kale rotation(FYM, MRP and control); white lupine-kale rotation(FYM, MRP and Control) .There was a significant (P≤ 0.05) effect of cropping systems with organic fertilizers where, white lupine-kale rotation (FYM) (3.68t/ha).Number of nodules was significantly affected by application of organic fertilizers in the different cropping systems, white lupine-kale rotation (Control (32); white lupine/kale intercrop (Control) (31) and the chickpea/kale intercrop (MRP) (27). Lupin significantly (P ≤ 0.05) higher amounts of N2 (62-86.13 kgha-1) as compared to chickpea (50.4 - 82.16 kgha -1) in both seasons with significantly higher amounts fixed in white lupine/kale intercrop (FYM, MRP and CNTRL), chickpea/kale intercrop (FYM, MRP and CNTRL). White lupin-kale rotation (CNTRL, FYM) (3076.8kgha-1, 2348.9kgha-1) yield was higher as compared to white lupine/kale intercrop (MRP) (1798.44kgha-1), chickpea had significant high yields chickpea-kale rotation (MRP, FYM) (1024kgha-1, 845 kgha-1) in season one whereas in the second season, white lupine/kale intercrop (CNTRL, FYM and MRP) (3324.5 kgha-1, 2346.9 kgha-1 and 1987.3 kgha-1); Chickpea/kale intercrop (MRP, FYM) (1287 kgha-1 and 768 kgha-1 ) had significant higher yields more than those of season one. The fresh weights (tha-1) across treatments and cropping systems increased, white lupine-kale rotation (CNTRL, FYM, MRP) (15.37 tha-1, 13.53 tha-1 and 13.13 tha-1), chickpea-kale rotation (MRP, FYM and CNTRL) (9.31tha-1, 6.05 tha-1 and 6 tha-1) while intercrops recorded lower biomass during both seasons. The lowest green matter content was recorded under white lupine/kale intercrop (FYM) (2.01 tha-1). For the decomposition and mineralization of legume residues, weights dropped after every retrieval and chemical compositions of the residues also declined with time. After 120 days of incubation, the white lupin had.24% of N, 16 %P, 16% K of the initial weight, whereas for chickpea was 19% N 15% P and 9 % K. A 50 % in weight loss of the legumes was found after 9 days of incorporation in the soils for the two legumes. A 50 % loss in N, P and K release of the legume residues was found at day 18 (N) 23 (P) and 11 (K) for chickpea and day 14(N), 11 (K) and 25 (P) for white lupin. Use of organic methods for enhanced soil fertility and improved kale quality and yield, can always be achieved by the small scale farmers. Integration of legumes (lupine) in a kale production systems and use of organic fertilizers (farm yard manure) led to increased yields of kale an improvement in the nutrient status of the kales. Application of organic fertilizers is a better solution for improved soil nutrients. Use of MRP with a lupin as an intercrop, more nitrogen was fixed as compared to chickpea. Determination of the mineralization rates of chickpea and lupine residues shows the farmer’s adequate time for application of the residues for maximum synchrony between nutrient release and nutrient uptake.