DESMODIUM EFFECT ON SOIL FERTILITY, STRIGA CONTROL AND MAIZE PRODUCTION IN BUSIA AND SIAYA COUNTIES, WESTERN KENYA.

Abstract

Low soil fertility, stemborer and Striga weeds limit production of maize in western Kenya. Desmodium in the “Push-Pull” technology has been found to reduce Striga and stemborer but its contribution to soil fertility rehabilitation is not well understood. Field trials were conducted in Busia and Siaya counties, western Kenya, to test the hypotheses that (1) inorganic nitrogen, soil carbon, 13C isotope discrimination ( 13C) and biological nitrogen fixation (BNF) in maize-Desmodium intercropping system are affected by Desmodium species and sampling time and (2) maize yield, the degree of Striga suppression and economic benefits of intercropping maize with Desmodium are affected by different Desmodium species and the cutting regime of the Desmodium. Maize was intercropped with Desmodium uncinatum (Jacq.) DC., cv Silverleaf (D. uncinatum) or Desmodium intortum (Mill.) Urb. cv Greenleaf (D. intortum), and treatments with sole maize with or without urea were included for comparison. The first two Desmodium cutting events were fixed at every start of every season and 4 weeks later while the third cutting was varied and conducted at 9, 12 or 18 weeks after planting maize in each season. Maize biomass, mineral nitrogen, soil carbon,  13C and biological nitrogen fixation parameters were determined over time in both maize-Desmodium intercrop with Desmodium cut at 18WAP and sole maize systems. To study the contribution of Desmodium to soil phosphorus (P) rehabilitation, a greenhouse experiment was conducted in two phases to assess the extent to which Desmodium spp fertilized with Busumbu phosphate rock (BPR) could increase soil available P, P uptake and biomass yield of maize crop planted after Desmodium spp. Treatments included sole maize and two Desmodium spp : (D intortum and D.uncinatum) with or without BPR grown in the first phase of the experiment followed by sole maize with no P application in the second phase of the experiment. Reference treatments with soluble P (KH2PO4) were included. Results showed that D. intortum was superior to D. uncinatum in producing the highest biomass, fixed N, soil carbon, mineral N and N concentration at different sampling times. However, despite these positive attributes, D. intortum resulted in somewhat lower cumulated (over four seasons) maize grain yields. Maize shoots 13C values in Desmodium intercropping and sole maize systems did not differ significantly, an indication of comparable environmental effect on both systems. Varying the time of cutting Desmodium had little effect on maize yield and net benefits. Average net benefits from Desmodium intercropping over the four seasons were increased by 1290 and 918 $ ha-1 relative to the maize monocrop in Busia and Siaya respectively. D. intortum and D. uncinatum equally reduced Striga counts, an indication that the two species may demonstrate comparable phytochemical attributes. Greenhouse results showed that maize dry matter yields, plant P concentration and available P in soils were higher in maize following Desmodium spp compared to maize following maize wheather BPR was applied or not. D. intortum was however a more potent solubilizer of BPR than D. uncinatum. The present study extends the recommendation of the Desmodium-maize intercropping system to farmers who apply P fertilizers and can benefit from increased soil nitrogen, soil carbon and crop yield after the system becomes well established. Nevertheless, a further multi-locational and multi-seasonal evaluation of Desmodium-maize intercropping system would be required to investigate whether the system is also advantageous in terms of yield stability and sustainability, across locations and seasons.