Soil Nutrient Status and Maize (Zea Mays L.) Performance under Contrasting Legume-Maize Cropping Systems and Soils in Central Rift Valley, Kenya
Onwonga R.N., Lelei J.J., Friedel J.K., Freyer B

The study investigated effect of low cost inputs on soil available N and P and maize performance under contrasting legume-maize cropping system and soils. Experiments were conducted during the short (SRS) and long rain season (LRS) of 2003 and 2004 in Njoro and SRS of 2004 SRS and LRS of 2005 in Molo with soils classified as mollic Phaeozems and mollic Andosols, respectively. Three cropping systems (CS) with or without diammonium phosphate fertilizer (DAP as control), minjingu phosphate rock (MRP), lime (L) and farm yard manure (FYM) applied were; tested in Molo using a randomized complete block design (RCBD): (i) maize in rotation with natural fallow (NF- M and NF-M(DAP)), (ii) maize in rotation with cowpea (CP-M and CP-M(L, MRP) and (iii) maize/cowpea intercropping in rotation with crotalaria (CR-/CP(L, MRP) and CR-M/CP(L, MRP, FYM). CR was either incorporated as green manure or removed and FYM applied instead. The experimental setup in Njoro was a split plot fitted to RCBD. The main plots were two CS; sole maize and maize/bean intercrop preceded by CR, lablab (LB), garden pea (GP) and NF. The sub-plots were residue (i) incorporation and (ii) removal with FYM applied instead. MRP was applied to all treatments in Njoro. Soil available N and P were monitored with maize growth and grain yield determined at maturity. Soil available N and P were significantly (P<0.05) higher in CR – M/CP(lime, MRP) and CR – M/CP(lime, MRP, FYM) in Molo. For Njoro, soil available P was higher in the rotation than intercropping system and did not vary significantly with the premaize legume and residue management. Legume residue incorporation resulted in significantly (P<0.05) higher amounts of available N in soil than FYM application in Njoro. Soil available N did not differ significantly between rotation and intercropping in Njoro but was significantly higher following LB. The SRS legumes increased soil available N and P and maize yield in the LRS in comparison to NF in both sites. Maize grain yield ranged from 1.48 to 3.85 t ha-1 in Molo and was higher in intercropping system. In Njoro, maize grain yield ranged from 2.2 to 4.6 t ha-1 and was higher in the rotation system. In both sites use of low cost inputs increased availability of N and P in soil and subsequently maize yield. The levels contrasted with cropping system and soil type, and thence site specific research cognizant of the soil limitations recommended.

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