Mulch effects on soil moisture and nitrogen, weed growth and irrigated maize productivity in a warm-temperate climate of South Africa

Low soil fertility and weeds reduce maize yields on most smallholder (SH) irrigation schemes in South Africa (SA). While cover cropping can increase maize productivity, benefits from different types of mulch are not well understood, leading to challenges in selecting the most appropriate cover crop species to grow. Field experiments investigating the effects of oat (Avena sativa), grazing vetch (Vicia dasycarpa), faba bean (Vicia faba), Lupin (Lupinus angustifolius) and forage pea (Pisum sativum) mulch on maize grown at two fertilizer levels (0 and 60 kg N/ha) were undertaken in the 2007/08 and 2008/09 seasons. Plots where maize was grown without mulch were included as controls. Treatments were arranged in a randomised complete block design with three replications. Grazing vetch and forage pea mulch significantly (P < 0.01) improved soil inorganic N at 0–5 cm soil depth, with a mean of 84 mg N/kg compared to a mean of 64 mg N/kg in other treatments at planting. Oat and grazing vetch mulch significantly (P < 0.05) reduced weed biomass compared to plots with lupin mulch and the control. Lack of maize fertilization tended to reduce maize yields but not for maize grown on grazing vetch mulch which yielded 7477 kg/ha compared to 2409 kg/ha for unfertilized maize grown on oat mulch. From an economic perspective, grazing vetch mulch resulted in the highest returns with a benefit:cost (B:C) ratio of 1.9 for the treatment with no fertilizer inputs during cover crop growth and maize growth. Grazing vetch is an attractive cover crop for SH farmers in irrigation schemes in warm-temperate climates.

Research highlights▶ Oat and grazing vetch mulch reduce soil temperature and conserve soil water. ▶ N release by grazing vetch mulch partly compensates lack of fertilization in maize. ▶ Oats and grazing vetch mulch restrict weed growth and lower weed species diversity. ▶ High economic returns are gained with minimal fertilizer and grazing vetch mulch. ▶ No-till plus winter cover cropping is a viable technology for SH irrigation farmers.