Tropentag, September 19 - 21, 2012 in Göttingen "Resilience of agricultural systems against crises"

An Assessment of Soybean-Maize Rotations and Biological Nitrogen Fixation on Smallholder Farmers' Fields in Malawi

International Institute of Tropical Agriculture (IITA), Malawi, Malawi

Abstract

Soils in cereal-based smallholder cropping systems in southern Africa are often characterised by nitrogen deficiencies which result in low yields. Soybean (Glycine max) production can address this problem by biological nitrogen fixation (BNF) which, if crop residues are returned to the soil, benefits the following cereal crops.
This study was conducted to quantify atmospheric nitrogen fixation by soybean crops under different external input levels and to assess the rotational benefits of soybean on subsequent maize crops under smallholder farmers' management techniques in central Malawi.
Soybean biomass samples from 150 plots and reference maize and broadleaf weed species were collected at mid pod filling stage from 60 on-farm trials in the 2009 and 2010 growing seasons in Dowa, Mchinji and Salima districts. Soybean treatments included farmers' usual practices, improved variety, inoculation and fertiliser application. Samples were analysed for ¹⁵N contents and BNF was quantified using the natural abundance method. In 2010, 53 farmers compared yields of maize planted after the zero-input soybean plots of the 2009 season with maize planted after maize. Farmers used their own inputs and practices but similar in each plot. Soil sampling, field measurements and structured questionnaires generated data on plot history, crop management, yields and farmers' perceptions on rotation.
The soybean plots yielded on average 1202 kg/ha. At mid pod-filling stage the crop accumulated an average biomass dry weight of 2761 kg/ha containing on average 103 kg nitrogen per ha. Depending on the reference plant used, soybean fixed 47.3 % (for maize) or 56.5 % (broad leaf weeds) of this nitrogen from the atmosphere. There was no relationship between nitrogen fixation and fertiliser application or inoculation. Average maize yields were 1629, 2941 and 4365 kg/ha in Dowa, Mchinji and Salima respectively and maize crops planted after soybean showed 22.4, 56.2 and 33.0 % higher yields compared to continuous maize in these sites.
This study indicates that under current crop management practices, farmers experience significant yield gains from planting maize after soybeans. This can partly be explained by BNF which results in reduced nitrogen depletion of the soils.

Keywords: Crop rotation, natural abundance method, nitrogen fixation, soil fertility, soybean

Contact Address: Daniel van Vugt, International Institute of Tropical Agriculture (IITA), Malawi, Chitedze Research Station Po Box 30258, Lilongwe, Malawi, e-mail: d.vanvugtcgiar.org