Maize (Zea mays L.) is an important smallholder crop in Uganda. Yields are low because of low soil fertility and little fertilizer use. Yield response to nutrient application and economically optimal rates (EOxR, where x = N, P, or K) and N use efficiency (NUE) were evaluated. Twenty‐two trials were conducted in four agroecological zones. Yield was consistently increased with N application. Mean maize yield with no N applied (N0) was 1.79 Mg ha−1 and increased by 120% with N application. Mean EONRs were 45 to 24 kg ha−1 N with fertilizer use cost to grain price ratios (CPs) of 10 to 30. With N applied, the mean increase in yield due to P application was 0.28 Mg ha−1 and mean EOPRs were 9 to 1 kg ha−1 P with CPs of 10 to 50. Yield was not increased with K application. Profitability was greater for N than P application. Mean aboveground biomass N with 0 and 150 kg ha−1 N applied was 46.3 and 94.3 kg ha−1, respectively. Mean N concentration and N harvest index at the EONR were 1.60 and 63.8%, respectively, and higher than for N0. Mean recovery efficiency, partial factor productivity, and agronomic efficiency declined with increasing N rate and were 66%, 86 kg kg−1, and 41 kg kg−1, respectively, at the EONR. Fertilizer N use can be very profitable, with high NUE, for smallholder maize production in Uganda, and the financial capacity of smallholders to use fertilizer will increase with reduced CP.
Sorghum [Sorghum bicolor (L.) Moench] is important for smallholder production in semiarid parts of Uganda. Grain yields are low because of low soil fertility. Little fertilizer is used. Yield response to N, P, and K application, economically optimal rates for N, P, and K (EONR, EOPR, and EOKR, respectively), and N use efficiency (NUE) were evaluated at 11 site‐seasons. Mean sorghum yield with no N applied (N0) was 0.69 Mg ha−1 and was consistently increased by a mean of 230% with N application. Mean EONRs were 34 to 18 kg ha−1 N with fertilizer use cost to grain price ratios (CPs) of 10 to 30, respectively. Mean EOPRs were 11 to 2 kg ha−1 P with CPs of 10 to 50, respectively. Sorghum did not respond to K application. Net economic returns were greater for N than P application. Mean aboveground biomass N with 0 and 90 kg ha−1 N applied was 31.3 and 75.9 kg ha−1, respectively. Grain N concentration, N harvest index, and internal NUE at the EONR were 1.67%, 53.2%, and 31.8 kg kg−1, respectively, and higher than for N0. Mean recovery efficiency, partial factor productivity, and agronomic efficiency declined with increased N rate and were 135%, 79 kg kg−1, and 52 kg kg−1, respectively, at the EONR. The profit potential of fertilizer N use is high for smallholder sorghum production in Uganda. Policy interventions to reduce fertilizer cost and improve grain marketing efficiency will enable smallholders to increase fertilizer use for substantial increases in sorghum production.
This research investigated empirically the qualities of improved soil management practices (ISMPs) most likely to lead to land user adoption. Land users' perceptions of soil‐related constraints were investigated in two hillside districts of eastern Uganda. The research looked at land user rationale for adopting and adapting specific ISMPs at the end of a two‐year period of increased advice and support. Land user engagement with soil management improved markedly after this period of support and multifunctionality, that is, provision of a number of different products or benefits, was seen to be a common characteristic of those ISMPs taken up by land users. It is argued that in the search for ‘best‐bet’ ISMPs, multifunctionality may be a particularly relevant and easily measurable indicator of likely adoption of a practice by land users. The research also demonstrates the value of supporting land users in their efforts to adapt ISMPs to fit with their own circumstances. Copyright © 2011 John Wiley & Sons, Ltd.
Application of iron (Fe) -rich amendments to soils has been proposed as a means of decreasing phosphorus (P) losses from soils. However, anoxic conditions following soil saturation are known to increase Fe and P solubility in soils, thus cancelling out the potential benefits. Our aim was to evaluate the effects of continuous oxic, continuous anoxic and alternating anoxic/oxic conditions on P exchangeability and Fe forms in soil amended with Ca(OH) 2 and FeSO 4 . We incubated amended and unamended soils under these conditions for 8 weeks and measured Fe forms and P exchangeability. Under oxic conditions, addition of Ca(OH) 2 and FeSO 4 resulted in a strong decrease in P exchangeability and an increase in oxalate-extractable Fe. Mo¨ssbauer analyses suggested that an unidentified Fe oxide (D1oxide) with a strong sorbing capacity for P was precipitated. Under continuously anoxic conditions, P exchangeability and oxalate-extractable Fe increased with or without the amendments. Mo¨ssbauer analyses suggested that there was a partial dissolution of the D1oxide phase, precipitation of another unidentified Fe oxide (S3) and a reduction of structural Fe 3þ in phyllosilicate, thereby increasing soil negative charge. These transformations resulted in a strong increase in rapidly exchangeable P. Alternating anoxic and oxic periods induced the dissolution and precipitation of iron oxides and the increase and decrease in P exchangeability. Implications of the results for limiting P losses from grassland soils are discussed.
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