Understanding the role of soybean (Glycine max (L.) Merrill) cultivation by smallholder farmers in northern Uganda

With an increasing  population, food production also needs to increase in Uganda. However, continuous crop cultivation and a low use of external inputs is expected to cause nutrient depletion resulting in diminishing crop yields. Nitrogen is an important nutrient for crop growth and is commonly limiting agricultural production. Leguminous crops have the ability to fix nitrogen from the atmosphere in a symbiotic relationship with rhizobia (bacteria) in the soil. In Oyam, a district in northern Uganda, the N2Africa project disseminated cultivation technologies for soybean, such a leguminous crop. It was unclear how the crop was performing in this region and whether the disseminated technologies were used. The aim of this research was to understand the role of soybean in smallholder farm households and how the disseminated cultivation technologies (improved varieties, inoculants, P-fertilizer) were used in variable farm systems. To understand the diversity of farmer households, a rapid farm characterization was done using three focus group discussions. Afterwards, a more detailed household survey and field survey were done with 20 selected households in the first growing season of 2018. The farmers were divided in three categories based on their farm size and livestock ownership: large area – medium livestock (LA-ML) farmers, medium area – high livestock (MA-HL) farmers and small area – low livestock (SA-LL) farmers. Soybean was cultivated on 41% of the total farmland in the first season of 2018, and was therefore the crop cultivated by most farmers in this season. Farmers of all types allocated various proportions of land to soybean, and SA-LL farmers never allocated more than 55% while MA-HL and LA-ML farmers allocated up to almost 100%. Soils of MA-HL farmers were poorer than the soils of the other farmers in terms of nitrogen and organic carbon. There was no difference between soil characteristics between fields to which soybean was allocated. Farmers did not use any chemical fertilizer, manure or inoculants on soybean, and the use of biocides was very minimal on soybean. The use of improved soybean varieties was common for LA-ML and MA-HL farmers. Fields with soybean as main crop required more labour than the other fields where mainly cereals, cassava, sunflower and other legumes were cultivated. There were no differences between farm types in the percentage of nitrogen derived from biological nitrogen fixation, or the amount of N-fixed. However, in soybean the fixation was not sufficient to prevent nutrient depletion without the use of external inputs, considering an outflow through grain harvest of around 80 kg N ha^-1 and an average fixation of 9.1 kg N ha^-1. The average soybean yield was 1695 kg ha^-1, with the highest on fields of SA-LL farmers (median yield of over 2000 kg ha^-1). There were sufficient markets for selling soybean, in which farm cooperatives played a large role. The share of soybean in household income was diverse (17-100% for LA-ML farmers, 0-93% for MA-HL farmers and 0-79% for SA-LL farmers). Farmers cultivated soybean mostly because of its high income potential, consumed little and were positive about cultivating and selling the crop. Soybean cultivation has been beneficial for farmers in Oyam, and the N₂Africa project seemed to have increased these benefits although the technologies disseminated by N₂Africa were not fully put into use. The imminent problem of nutrient depletion is not likely to be solved with the currently used soybean cultivation methods, but there is plenty of space left for improvements in the future.