Soil fertility decline is a major factor limiting crop production in smallholder farms in sub-Saharan Africa. Coastal Kenya farmers are aware of the problem of declining soil fertility caused by continuous cropping without returning nutrients, soil erosion, burning of plant residues, and short fallow intervals in food crops production. This research examined intercropping of forage legumes as potential nutrient sources in maize and cassava production systems. The overall objective was to study the effects of harvest frequency and developmental stage at harvest of an annual (dolichos) and perennial (clitoria) forage legume on: i) biomass and nutrient accumulation ii) rate of nutrient release from the respective legume residues, and iii) availability of residue derived nutrients to maize and/ or cassava. The design for the experiments was a split-plot with five replications, consisting of cropping system (main plot) and harvest management (subplot) factors. Main plot treatments were clitoria or dolichos grown as monocultures or as an intercrops with two tropical food crops (cassava and maize). Clitoria was first cut at 2 mo after planting, and subsequently every 6 or 10 wk after the first cut, while dolichos was cut at 2 and 4 mo after planting the legume. The harvested foliage was applied as surface mulch. A third experiment monitored the decomposition and nutrient release from clitoria and dolichos residues. Following each legume harvest, three replicates of nylon mesh bags containing legume residue as intact plants were placed on the ground surface within their respective cropping system and legume management plots, and then retrieved at 0, 1, 2, 4, 8 and 16 wk in the field. Nonlinear regression equations for percentage of original dry weight, N, P, K, Ca, and Mg remaining at each retrieval date were determined by cropping system and legume cutting strategy and data was fit to single, double, and asymptotic models of NLIN procedure. Results from the maize/cassava production experiments revealed that the legumes accumulated nutrients (2-yr avg.) ranging from 50 to 101kg N ha-1, 4 to 8kg P ha-1, 33 to 83 kg K ha-1, 7 to 32 kg Ca ha-1, and 5 to 7 kg Mg ha-1 during the long rain season, with dolichos producing the greater values. Unlike the long rain season, nutrient contents of clitoria were greater than dolichos during the short rain season, with values ranging from 36 to 54 kg N ha-1, 3 to 5 kg P ha-1, 26 to 40 kg K ha-1, 5 to 8 kg Ca ha-1, and 5 to 7 kg Mg ha-1.Intercropping reduced nutrient contents of the legumes to < 80% of the monoculture, and was most pronounced for clitoria intercropped with cassava. Delayed harvest of dolichos from two to four mo after planting increased dolichos nutrient accumulation between two and fourfold, but harvest management was not significant for clitoria nutrient accumulation. As expected, fertilizer inputs increased maize and stover yields by 70% over maize grown without fertilizer inputs. Intercropping maize with clitoria increased grain and stover nutrient accumulation by values ranging from 50 to 80%, compared to maize monoculture without fertilizer inputs, while intercropping maize with dolichos yielded less grain and stover. Cassava monoculture resulted in the greatest tuber yields (9 Mg ha-1), while intercropping of cassava with a legume reduced tuber yield by an average of 21%. Overall, positive nutrient balance results occurred for maize monoculture systems supplied with fertilizer, and maize/legume intercropping systems with maize stover returned to the system and P supplied to the legume as inorganic fertilizer. Generally, the presence of cassava in a cropping system resulted in negative N and K balances, supporting the need to review N and K replenishment recommendations for maize and/or cassava cropping systems in the region.Results of the decomposition and nutrient release study revealed that an asymptotic model best described clitoria and dolichos dry matter disappearance across different harvest management strategies, while legume residue decomposition rates were unaffected by cropping system. The rate coefficients for dry matter disappearance of clitoria and dolichos residues were 0.2 and 0.5 wk-1, respectively. Similarly, nutrient release from clitoria and dolichos residues best fit an asymptotic model. The k-values obtained for dolichos showed the greatest variation (0.2 to 2.5 wk-1) compared to those obtained for clitoria (0.3 to1.0 wk-1). Nitrogen release was generally slowest in clitoria and in dolichos 4 mo cut foliage. Across harvest management strategies, the general order of nutrient release was K > P > Mg > N, while that of dolichos cut at 2 mo was K>Mg> N>P and K>N>P>Mg for dolichos cut at 4 mo. Calculations of the area by time equivalent ratios (ATER) based on totals of long and short rain seasons' yields of maize, legume, and cassava revealed that intercropping of clitoria or dolichos with maize or cassava resulted in the most efficient way of utilizing land area and time, with the longer cutting intervals of legume indicating a more efficient use of space and time. The ATER for clitoria intercropping with maize and/and or cassava ranged from 1.2 to 1.6, while that of dolichos ranged from 1.1 to 2.0.Generally, greater values of ATER were observed where the basis for the comparisons was the no fertilizer input control, implying that advantages of intercropping would be noticeable in situations where fertilizer inputs are a constraint, as is the case in coastal Kenya. These results demonstrated the legumes potential to supply nutrients for maize grain and cassava tuber production, and indicated that biological efficiency may be improved by intercropping maize with legumes. The need to develop N and K replenishment recommendations for cropping systems with cassava is suggested. Further research aimed at reducing nutrient losses by synchronizing nutrient release with principal crop demand is recommended.
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Evaluation of Forage Legumes for Soil Fertility Improvement in Maize/Cassava Production Systems.
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