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24Sub-Saharan Africa (SSA) is plagued by low productivity and little research is available 25 on the attainable responses and profitability to applied nutrients under variable 26 environments. The objective of this study was to determine the attainable maize grain 27 response to and potential of profitability of N, P and K application in SSA using Both response to fertilizer and value cost ratio (VCR) are highly variable and no more 32 than 61% cases for N, 43% for P and 25% for K attain VCR of 2 or more. Also, based 33 on the recent AfSIS data, VCR exceeds 1 in just 67% (N), 57% (P) and 40% (K) of the 34 cases, even when best management practices are applied on a research farm, and 35 interest rates are zero. Chances for profitability are highest when soil organic carbon is 36 1 -2% and control maize grain yield is 1 -3 t ha -1 but also depends on relatively static 37 soil properties (primarily texture and mineralogy) that are not under farmer control. We 38 conclude that return on investment of macronutrient fertilizer is highly variable and can 39 be substantially increased by helping farmers decide where to apply the fertilizers. 40Consequently, farmers need access to information on factors influencing economic 41 returns of fertilizer use in order to make the right decisions.

teriorate the soil physico-chemical conditions of the often physically and chemically fragile moist savanna soils Nutrient depletion is a major constraint to crop production for in the long term, especially in the case of ammonium moist savanna soils, and inputs of nutrients are required to overcome sulfate [(NH 4 ) 2 SO 4 ] (Pieri, 1992). This observation, tothis constraint. The impact of sole and combined applications of organic inputs (OIs) [fresh tree prunings, Pueraria phaseoloides (Roxb.) Recherche Agronomique, B.P. 1026, Lomé , Togo; J. Diels, N. Sanginga, and O. Lyasse, RCMD, IITA, Ibadan, Nigeria, c/o L.W. Lam-al., 1997), most trials in the West African moist savanna bourn and Co., 26 Dingwall Rd., Croydon CR9 3EE, UK; R. Merckx, deal with manure or crop residues as a source

Legume-maize rotation and maize nitrogen (N)-response trials were carried out simultaneously from 1998 to 2004 in two distinct agro-ecological environments of West Africa: the humid derived savannah (Ibadan) and the drier northern Guinea savannah (Zaria). In the N-response trial, maize was grown annually receiving urea N at 0, 30, 60, 90 and 120 kg N ha -1. In Ibadan, maize production increased with N fertilization, but mean annual grain yield declined over the course of the trial. In Zaria, no response to N treatments was observed initially, and an increase in the phosphorus (P) and sulphur (S) fertilizer application rate was required to increase yield across treatments and obtain a response to N applications, stressing the importance of non-N fertilizers in the savannah. In the rotation trial, a 2-year natural fallowmaize rotation was compared with maize rotated with different legume types: green manure, forage, dualpurpose, and grain legumes. The cultivation of some legume types resulted in a greater annual maize production relative to the fallow-maize combination and corresponding treatments in the N-response trial, while there was no gain in maize yield with other legume types. Large differences in the residual effects from legumes and fallow were also observed between sites, indicting a need for site-specific land management recommendations. In Ibadan, cultivation of maize after the forage legume (Stylosanthes guianensis) achieved the highest yield. The natural fallow-maize rotation had improved soil characteristics (Bray-I P, exchangeable potassium, calcium and magnesium) at the end of the trial relative to legume-maize rotations, and natural fallow resulted in higher maize yields than the green manure legume (Pueraria phaseoloides). In Zaria, maize following dual-purpose soybean achieved the highest mean yield. At both sites, variation in aboveground N and P dynamics of the legume and fallow vegetation could only partly explain the different residual effects on maize.

The yield increases often recorded in maize following grain legumes have been attributed to fixed-N and 'other rotation' effects, but these effects have rarely been separated. Field trials were conducted between 2003 and 2005 to measure these effects on maize following grain legumes in the northern Guinea savanna of Nigeria. Maize was grown on plots previously cultivated to two genotypes each of soybean (TGx 1448-2E and SAMSOY-2) and cowpea (IT 96D-724 and SAMPEA-7), maize, and natural fallow. The plots were split into four N fertilizer rates (0, 30, 60 and 90 kg N ha -1 ) in a split plot design. The total effect was calculated as the yield of maize following a legume minus the yield following maize, both without added N and the rotation effect was calculated as the difference between rotations at the highest N fertilizer rate. The legume genotypes fixed between 14 and 51 kg N ha -1 of their total N and had an estimated net N balance ranging from -29.8 to 9.5 kg N ha -1 . Positive N balance was obtained only when the nitrogen harvest index was greater than the proportion of N derived from atmosphere. The results also indicated that the magnitude of the fixed-N and other rotation effects varied widely and were influenced by the contributions of the grain legumes to the soil N-balance. In general, fixed-N effects ranged from 124 to 279 kg ha -1 while rotation effects ranged between 193 and 513 kg ha -1 . On average, maize following legumes had higher grain yield of 1.2 and 1.3-fold compared with maize after fallow or maize after maize, respectively.