Integrated soil management for the savanna zone of W. Africa: legume rotation and fertilizer N

Integrated soil management with leguminous cover crops was studied at two sites in the northern Guinea savanna zone of northern Nigeria, Kaduna (190 day growing season) and Bauchi (150 days). One-year planted fallows of mucuna, lablab, and crotalaria were compared with natural grass fallow and cowpea controls. All treatments were followed by a maize test crop in the second year with 0, 30, or 60 kg N ha^–1 as urea. Above ground legume residues were not incorporated into the soil and most residues were burned early in the dry season at the Kaduna site. Legume rotation increased soil total N, maize growth in greenhouse pots, and dry matter and N accumulation of maize. Response of maize grain yield to 30 kg N ha^–1 as urea was highly significant at both sites and much greater than the response to legume rotation. The mean N fertilizer replacement value from legume rotation was 14 kg N ha^–1 at Kaduna and 6 kg N ha^–1 at Bauchi. W ith no N applied to the maize test crop, maize grain yield following legume fallow was 365 kg ha^–1 higher than natural fallow at Bauchi and 235 kg ha^–1 higher at Kaduna. The benefit of specific legume fallows to subsequent maize was mostly related to above ground N of the previous legume at Bauchi, where residues were protected from fire and grazing. At Kaduna, where fallow vegetation was burned, maize yield was related to estimated below ground N. The results show that legume rotation alone results in small maize yield increases in the dry savanna zone.     

Effect of phosphorus application in legume cover crop rotation on subsequent maize in the savanna zone of West Africa

The benefit of planted fallow with legume cover crops may be limited on P deficient soil. A trial was conducted at two P deficient sites in northern Nigeria to test the hypothesis that application of P to legume cover crop fallow can substitute for N application to subsequent maize. Mainplots consisted of leguminous fallows followed by unfertilized maize, or native (mostly grass) fallows followed by maize with 0 or 40 kg N ha⁻¹ (Kaduna) and 0, 30 or 60 kg N ha⁻¹ (Bauchi). Three rates of P (0, 9, and 18 kg ha⁻¹) were applied to fallow sub-plots as single superphosphate. In the first year, dry matter accumulation of lablab (Lablab purpureus) responded to P application, while mucuna (Mucuna cochinchinensis) dry matter did not. Lablab mulch dry matter during the dry season was significantly increased by previous season P application while mucuna was not. Previous fallow vegetation was a significant factor for maize growth in the second year but the interaction with P applied to the fallow was not significant at P < 0.05. Substantial and similar yield increases were achieved with application of N fertilizer to maize and from application of 9 kg P ha⁻¹ to previous lablab. Depending on local economic circumstances, a good use of expensive inorganic fertilizer might be to apply P sources to cover crop legumes to profit from additional N benefits as well as residual effects of applied P. This revised version was published online in June 2006 with corrections to the Cover Date.     

Towards a Revised Coefficient for Estimating N2O Emissions from Legumes

The Intergovernmental Panel on Climate Change (IPCC) standard methodology to conduct national inventories of soil N₂O emissions is based on default (or Tier I) emission factors for various sources. The objective of our study was to summarize recent N₂O flux data from agricultural legume crops to assess the emission factor associated with rhizobial nitrogen fixation. Average N₂O emissions from legumes are 1.0 kg N ha⁻¹ for annual crops, 1.8 kg N ha⁻¹ for pure forage crops and 0.4 kg N ha⁻¹ for grass legume mixes. These values are only slightly greater than background emissions from agricultural crops and are much lower that those predicted using 1996 IPCC methodology. These field flux measurements and other process-level studies offer little support for the use of an emission factor for biological N fixation (BNF) by legume crops equal to that for fertiliser N. We conclude that much of the increase in soil N₂O emissions in legume crops may be attributable to the N release from root exudates during the growing season and from decomposition of crop residues after harvest, rather than from BNF per se. Consequently, we propose that the biological fixation process itself be removed from the IPCC N₂O inventory methodology, and that N₂O emissions induced by the growth of legume crops be estimated solely as a function of crop residue decomposition using an estimate of above- and below-ground residue inputs, modified as necessary to reflect recent findings on N allocation.     

Legume cover crops and mulches: effects on nitrate leaching and nitrogen input in a pepper crop (<Emphasis Type="Italic">Capsicum annuum</Emphasis> L.)

There is not sufficient knowledge concerning the risks involved in NO₃–N leaching in relation to the use of cover crops and mulches. A 2 year field experiment was carried out in a pepper (Capsicum annuum L.) crop transplanted into different soil management treatments which involved the addition of mulch of three different types of winter cover crops (CC) [hairy vetch (Vicia villosa Roth.), subclover (Trifolium subterraneum L.), and a mixture of hairy vetch/oat (Avena sativa L.)], and an un-mulched plot. At the time of CC conversion into mulch, the hairy vetch/oat mixture accumulated the highest aboveground biomass (5.30 t ha⁻¹ of DM), while hairy vetch in pure stand accumulated the highest quantity of N (177 kg ha⁻¹) and showed the lowest C/N ratio (12). The marketable pepper yield was higher in mulched than in conventional (on average 33.5, 28.9, 27.7 and 22.2 t ha⁻¹ of FM for hairy vetch, subclover, hairy vetch/oat mixture, and conventional, respectively). Generally, the NO₃–N content of the soil was minimum at CC sowing, slightly higher at pepper transplanting and maximum at pepper harvesting (on average 15.2, 16.8, and 23.3 mg NO₃-N kg⁻¹ of dry soil, respectively). The cumulative leachate was higher during the CC period (from October to April) than the pepper crop period (from April to September), on average 102.1 vs 66.1 mm over the years, respectively. The cumulative NO₃–N leached greatly depended on the type of mulch and it was 102.3, 95.3, 94.7, and 48.2 kg ha⁻¹ in hairy vetch, subclover, hairy vetch/oat mixture, and conventional, respectively. A positive linear correlation was found between the N accumulated in the CC aboveground biomass and the NO₃–N leached during pepper cultivation (R ² = 0.87). This research shows that winter legume cover crops, especially hairy vetch in pure stand, converted into dead mulch in spring could be used successfully for adding N to the soil and increasing the yield of the following pepper crop although the risks of N losses via leaching could be increased compared to an un-mulched soil. Therefore when leguminous mulches are used in the cultivation of a summer crop, appropriate management practices of the system, such as a better control of the amount of irrigation water and the cultivation of a graminaceous or a cruciferous catch crop after the harvesting of the summer crop, should be adopted in order to avoid an increase in NO₃–N leaching.     

Nitrogen contributions from decomposing cover crop residues to maize in a tropical derived savanna

In cover cropping systems in the tropics with herbaceous legumes, plant residues are expected to supply nitrogen (N) to non-legume crops during decomposition. Field experiments were carried out to (i) determine the effects of residue quality on decomposition and N release patterns of selected plants in cover cropping systems, (ii) relate the pattern of residue N release to N uptake by maize in cover cropping systems. To study decomposition, litter bags were used and monitored over two maize growing seasons. The residues studied were mucuna (Mucuna pruriens (L.) DC. var. utilis (Wright) Bruck), lablab (Lablab purpureus (L.) Sweet), and leaves and rhizomes of imperata (Imperata cylindrica (L.) Raueschel). Mucuna and lablab decomposed rapidly losing more than 60% of their dry weight within 28 days. In contrast, imperata decomposed slowly with only 25% of its dry matter lost in 56 days. At 28 days, mucuna had released 154 kg N ha^-1 in in-situ mulch systems and 87 kg N ha^-1 in live- mulch systems representing more than 50% of its N. More than 64% of N in lablab was released within 28 days amounting to 21 to 174 kg N ha^-1. Imperata rhizomes mineralized 4 to 14 kg N ha^-1 within 14 days, and subsequently immobilized N until 112 days whereas imperata leaves immobilized N throughout the study period. Decomposition and N release rates from the plant residues were most strongly correlated with the (lignin+polyphenol)/N ratio, N content, lignin/N ratio, polyphenol/N ratio, C/N ratio and lignin content of the residues. Relative to the controls, herbaceous legume residues increased maize dry matter yield and N uptake during the two cropping seasons. At 84 days, the maize crop had utilized 13 to 63 kg N ha^-1from mucuna representing 13 to 36% of N released, whereas 16 to 25% of N released from mucuna was recovered by the maize crop at 168 days. The first maize crop recovered 9 to 62 kg N ha^-1 or 28 to 35% of N released from lablab. However, at 168 days, N uptake by maize in antecedent live-mulched lablab was 32% higher than the quantity of N released, whereas imperata residues generally, resulted in net reduction of maize N uptake.     

How winter cover crops and tillage intensities affect nitrogen availability in eggplant

This study evaluates the fate of nitrogen (N) content in winter cover crops under different tillage intensities. Field trials were conducted over a 2-year period in a Mediterranean environment adopting a cover crop–eggplant sequence. The treatments were: three cover crops (hairy vetch, oat and oilseed rape); three tillage intensities (residue left on soil surface, shallow green manure and deep green manure). The measurements included: cover crop and eggplant characteristics, N mineralization from cover crops, soil inorganic N and soil CO₂ emission. At cover crop termination, N accumulated in the cover crops was 207, 77 and 77 kg N ha^?1 in hairy vetch, oat and oilseed rape, respectively. Tillage intensity affected biomass decomposition and N mineralization from cover crop residues which were slower when residues were left on soil surface (54 and 71%, respectively) than when incorporated into the soil (66 and 79%, respectively). Hairy vetch showed a greater ability to supply N to eggplant (151 kg N ha^?1), due to the fast decay of its residues, consequently, the N balance index was always high after hairy vetch throughout eggplant cultivation. N mineralized by cover crops was positively correlated with total soil CO₂ emission and soil inorganic N. Placing cover crop residues on soil surface enhances synchronization between N mineralized and eggplant N demand in hairy vetch, while in oat it appears to mitigate the shortage of soil inorganic N for the following vegetable. These findings may also be extended to other summer vegetables which have similar requirements to the eggplant.     

Reducing Nitrate Leaching to Groundwater in an Intensive Dairy Farming System

Dairy farming is one of the main contributors to nitrate leaching to groundwater, particularly on soils that are susceptible to leaching, such as light well-drained sandy soils. In the Netherlands, as in many other European countries, these soils are predominantly used for dairy farming. A prototype dairy farming system that has been implemented in practice in 1989 has continuously been adapted since then to meet environmental standards (i.e. the EU-standard of 50 mg NO₃⁻ l⁻¹) without reducing milk production intensity (11900 kg ha⁻¹). After an initial decline in nitrate concentration from 193 mg l⁻¹ to 63 upon implementation, it subsequently ‘stabilized’ at a level higher than the environmental standard: 55 mg l⁻¹. The goal of this paper is to examine causes of excessive nitrate leaching. This was done by relating measured nitrate concentrations with management characteristics such as N balances, cropping patterns and grazing intensities. Special attention was paid to aspects that were supposed to be conducive for leaching: crop rotation of grass and maize and grazing. No evidence was found for enhanced nitrate leaching due to the rotation of grass with maize compared to permanent cultivation. This could be ascribed to the reduction in fertilization levels in first and second year maize with 90 and 45 kg N ha⁻¹, respectively to account for the expected N release from the ploughed-in grass sod. Triticale was found to lead to higher leaching than grass or maize which is attributed to its poor growth in the period that it should function as catch crop in maize. Grazing contributed to a nitrate increase of about 30 mg NO₃⁻ l⁻¹ on grassland. As grazing management and intensity is already strictly optimized in order to restrict nitrate leaching, this result underpins the need to develop sustainable grazing methods on soil that is susceptible to nitrate leaching.     

The effect of nitrogen source and crop rotation on the growth and yield of processing tomatoes

Four crop rotation and management systems were studied in 1994 and 1995 in relation to growth and yield of irrigated processing tomatoes (Lycopersicon esculentum Mill.). The four treatments were three four-year rotation systems [conventional (conv-4), low input and organic] and a two-year rotation system [conventional (conv-2)]. The four-year rotation was tomato-safflower-corn-wheat(or oats+vetch)/beans, and the two-year rotation was tomato-wheat. Purple vetch (Vicia sativa L.) was grown as a green manure cover crop preceeding tomatoes in the low input and organic systems. Nitrogen was supplied as fertilizer in the conventional systems, as vetch green manure plus fertilizer in the low input system and as vetch green manure plus turkey manure in the organic system. Tomato cv. Brigade was direct-seeded in the conventional systems and transplanted to the field in the low input and organic systems. In both years the winter cover crop was composed of a mixture of vetch and volunteer oats with N contents of 2.2% in 1994 and 2.7% (low input) or 1.8% (organic) in 1995. In 1994 yields were higher in conventionally grown tomatoes because a virus in the nursery infected the transplants used in the low input and organic systems. In 1995 tomatoes grown with the low input and conv-4 systems had similar yields, which were higher than those of tomatoes grown with the conv-2 and organic systems. N uptake by the crop was greater than 200 kg N ha^–1 for high yield (> 75 t ha^–1) and uptake rates of 3 to 6 kg N ha^–1 day^–1 during the period of maximum uptake were observed. The lower yield with the organic system in 1995 was caused by a N deficiency. The main effect of the N deficiency was a reduced leaf area index and not a reduction of net assimilation rate (NAR) or radiation use efficiency (RUE). Nitrogen deficiency was related to low concentration of inorganic N in the soil and slow release of N from the cover crop + manure. A high proportion of N from the green manure but only a low proportion of N from the manure was mineralized during the crop season. In the conventional systems, the estimated mineralized N from the soil organic matter during the crop season was around 85 kg ha^–1. A hyperbolic relationship between N content and total dry weight of aboveground biomass was observed in procesing tomatoes with adequate N nutrition. Lower yields with the conv-2 than with the conv-4 system were due to higher incidence of diseases in the two year rotation which reduced the NAR and the RUE. Residual N in the soil in Oct. (two months after the incorporation of crop residues) ranged between 90 and 170 kg N ha^–1 in the 0–90 cm profile.Department of Vegetable Crops.     

A comparison between legume technologies and fallow, and their effects on maize and soil traits, in two distinct environments of the West African savannah

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 fallow–maize rotation was compared with maize rotated with different legume types: green manure, forage, dual-purpose, 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 fate of nitrogen from incorporated cover crop and green manure residues

Nitrogen retention and release following the incorporation of cover crops and green manures were examined in field trials in NE Scotland. These treatments reduced the amounts of nitrate-N by between 10–20 kg ha^-1 thereby lowering the potential for leaching and gaseous N losses. However, uptake of N by overwintering crops was low, reflecting the short day-lengths and low soil temperatures associated with this part of Britain. Vegetation that had regenerated naturally was as effective as sown cover crops at taking up N over winter and in returning N to the soil for the following crop. Incorporation of residues generally resulted in lower mineralisation rates and reduced N₂O emissions than the cultivation of bare ground, indicating a temporary immobilisation of soil N following incorporation. Emissions from incorporated cover crops ranged from 23–44 g N₂O-N ha^-1 over 19 days, compared with 61 g N₂O-N ha^-1 emitted from bare ground. Emissions from incorporated green manures ranged from 409–580 g N₂O-N ha^-1 over 53 days with 462 g N₂O-N ha^-1 emitted from bare ground. Significant positive correlations between N₂O and soil NO₃ ^- after incorporation (r=0.8–0.9; P<0.001 and r=0.1–0.4; P<0.05 for cover crops and green manures, respectively) suggest that this N₂O was mainly produced during nitrification. There was no significant effect of either cover cropping or green manuring on the N content or yield of the subsequent oats crop, suggesting that N was not sufficiently limiting in this soil for any benefits to become apparent immediately. However, benefits of increased sustainability as a result of increased organic matter concentrations may be seen in long-term organic rotations, and such systems warrant investigation.     

Nutrient evolution in soil and cereal yield under different fertilization type in dryland

Under semiarid conditions the response of crops to synthetic fertilizers is often reduced. Organic fertilizers can be used to provide a continuous source of nutrients for the crops. The soil nitrogen and crop yield in a rotation of durum wheat (Triticum durum)–fallow-barley (Hordeum vulgare)–vetch (Vicia sativa) were studied during 4 years when synthetic fertilizer (chemical), compost (organic) or no fertilizer (control) were applied in a field with high initial contents of soil NO₃–N (> 400 kg N ha⁻¹), phosphorus (22 mg kg⁻¹) and potassium (> 300 mg kg⁻¹). Changes in soil organic matter, phosphorus and potassium were also measured. During the crop period, chemical fertilization significantly increased the content of soil NO₃–N in the first 0.30 m of soil with respect to organic fertilization and the control. The yield of wheat and barley was not increased after applying chemical or organic fertilizer with respect to the unfertilized plots. The estimated losses of nitrogen were similar for the three types of fertilization, as well as the uptake of nitrogen for the total biomass produced. The initial levels of organic matter and phosphorus were maintained, even in the plots that were not fertilized, while the potassium decreased slightly. Thus, the rotation and burying of crop residues were enough to maintain the crop yield and the initial content of nutrients.     

Population dynamics of mixed indigenous legume fallows and influence on subsequent maize following mineral P application in smallholder farming systems of Zimbabwe

Developing soil fertility management options for increasing productivity of staple food crops is a challenge in most parts of Sub-Saharan Africa, where soils are constrained by nitrogen (N) and phosphorus (P) deficiencies. A study was conducted to evaluate the response of indigenous legume populations to mineral P application, and subsequently their benefits to maize yield. Mineral P was applied at 26 kg P ha⁻¹ before legume species were sown in mixtures at 120 seeds m⁻² species⁻¹ and left to grow over two rainy seasons (2 years). Application of P increased overall biomass productivity by 20–60% within 6 months, significantly influencing the composition of non-leguminous species. Dinitrogen fixation, as determined by the N-difference method, was increased by 43–140% although legume biomass productivity was apparently limited by nutrients other than P and N. Crotalaria pallida and C. ochroleuca accounted for most of the fixed N. Improved N supply increases the abundance of non-leguminous species, particularly Conyza sumatrensis and Ageratum conyzoides. However, abundance of common weed species, Commelina benghalensis, Richardia scabra and Solanum aculeastrum, declined by up to18%. Application of P did not significantly influence productivity of those legume species that reached maturity within 3 months. There was increased N₂-fixation and biomass productivity of indifallows as influenced by specific legume species responding to P application. Compared with natural (grass) fallows, indigenous legume fallows (indifallows) increased subsequent maize grain yields by ~40%. Overall, 1- and 2-year indifallows gave maize grain yields of >2 and 3 t ha⁻¹, respectively, against <1 t ha⁻¹ under corresponding natural fallows. Two-year indifallows with P notably increased maize yields, but the second year gave low yields regardless of P treatment. Because of their low P requirement, indigenous legume fallows have potential to stimulate maize productivity under some of the most nutrient depleted soils.     

Comparing the effectiveness of radish cover crop, oilseed rape volunteers and oilseed rape residues incorporation for reducing nitrate leaching

The soil water and N dynamics have been studied during two long fallow periods (between wheat or oilseed rape and a spring crop) in a field experiment in Châlons-en-Champagne (eastern France, 48°50 N, 2°15 E). The experiment involved frequent measurements of soil water, soil mineral N, dry matter and N uptake by cover crops. Water and N budgets were established using Ritchie's model for calculating evapotranspiration in cropped soils and a model (LIXIM) for calculating water drainage, N leaching and N mineralisation in bare soils. During the first autumn and winter, a radish cover crop (grown from September 1994 to January 1995) was compared to a bare soil. During the second period (July 1995 to April 1996), a comparison was carried out between (i) oilseed rape volunteers, (ii) bare soil with two types of oilseed rape residues incorporated into the soil (R0 and R270 residues) and (iii) bare soil without residues incorporation. R0 and R270 residues came from two preceding oilseed rape crops which received two rates of N fertilizer (0 and 270 kg N ha^-1).Soil mineral N content was markedly reduced by the presence of radish cover crop or oilseed rape volunteers during autumn. The calculated actual evapotranspiration (AET) did not differ much between treatments, meaning that the transpiration by the cover crop or volunteers was relatively low (100–150 L kg^-1 of dry matter). Consequently, nitrate leaching was reduced during the rest of the winter and spring as well as nitrate concentration in the percolating water: 45 vs. 91 mg NO₃ ^- L^-1 for radish cover crop and bare soil, respectively. The incorporation of oilseed rape residues to soil also exerted a beneficial but smaller action on reducing the nitrate content in the soil. This effect was due to extra N immobilisation which reached a maximum of about 20 kg N ha^-1 in mid-autumn for both types of residues. Nine months after the incorporation of the oilseed rape residues, and comparing to the control soil without residues incorporation, N rich residues induced a significant positive N net effect (+ 9 kg N ha^-1) corresponding to 10% of N added whereas for N poor residues no net effect was still obtained at the end of experiment (–3 kg N ha^-1, not significantly different from 0).To reduce nitrate leaching during long fallow periods, it is necessary to promote techniques leading to decrease mineral-N contents in the soil during autumn before the drainage period, such as (i) residue incorporation after harvest (without fertiliser-N) and (ii) allowing volunteers to grow or sowing a cover crop just after the harvest of the last main crop.     

Utilization of liquid hog manure to fertilize grasslands in southeast Manitoba: impact of application timing and forage harvest strategy on nutrient utilization and accumulation

Liquid hog manure (LHM) is used to improve productivity of grasslands in western Canada. However, application of manure to meet crop N requirements can result in excessive accumulation of P, especially in grazing systems. A three-year study was carried out to assess the impact of timing of liquid hog manure application and harvest strategy on nutrient utilization and accumulation by grasslands in southeast Manitoba. Liquid hog manure was applied annually at a full rate of 142 ± 20 kg available N ha⁻¹ in spring (Single application) or as two half rate applications of 70 ± 6 kg available N ha⁻¹, one in fall and one in spring (Split application). Two harvest strategies, haying and grazing, were employed to export nutrients from grasslands. Spring-applied manure averaged 8.9% dry matter, 5.7 g total N L⁻¹, 1.5 g total P L⁻¹, and 2.1 g total K L⁻¹ and fall-applied manure from the same source averaged 3.9% dry matter, 4.4 g total N L⁻¹, 0.7 g total P L⁻¹, and 2.2 g total K L⁻¹. Manure application based on grass N requirements resulted in at least two times more P and K applied than recommended for Manitoba grasslands. Nutrient (N, P, and K) export from grasslands was five times higher when grass forage was harvested as hay than through grazing. Average nutrient utilization when forage was harvested as hay was 153 kg N ha⁻¹, 18 kg P ha⁻¹, and 123 kg K ha⁻¹ and was higher in the years with increased precipitation. Grazing was not effective in removing nutrients from grasslands as indicated by lower N, P, and K utilization efficiency (% applied nutrient) in grazed (30% for N, 7% for P, and 18% for K) relative to hayed (75% for N 32% for P, and 103% for K) paddocks. Nutrient accumulation was impacted by a combination of harvest strategy and timing of manure application. Both single and split applications increased soil extractable nutrients, but soil extractable nutrients were higher in grazed relative to hayed paddocks following single manure application. After 3 years of manure application, the amount of Olsen-P (62 kg ha⁻¹) exceeded that required for optimal forage growth. However, soil levels did not exceed the soil Olsen-P regulatory threshold (60 mg kg⁻¹) that restricts manure P applications in Manitoba. An analysis of P balance, for this particular soil, indicated that a surplus of 18.9 kg manure P ha⁻¹ (in excess of forage P exported as hay or weight gain) increased the soil Olsen-P concentration by 1 mg kg⁻¹. Nutrient utilization and accumulation will be impacted by timing of manure application and harvest strategy employed as well as amount of precipitation received during the growing season.     

Effect of the integrated use of legume residue,poultry manure and inorganic fertilizers on maize yield,nutrient uptake and soil properties

Identification of a sustainable integrated soil fertility management option in the tropics will not only salvage the degraded soils but also enhances the attainment of the goal of food security. This study was conducted in 2004 and 2005 on a degraded tropical Alfisol in south western Nigeria to evaluate the effect of legume residue, poultry manure and inorganic fertilizers on maize yield, nutrient uptake and soil properties. The treatments consisted of two rates of poultry manure (0 and 5 t ha⁻¹), three rates of N fertilizer (0, 50 and 100 kg N ha⁻¹ applied as urea), three rates of P fertilizer (0, 30 and 60 kg P ha⁻¹ applied as single super phosphate) and two soybean treatments (with or without incorporation of legume residue) in various combinations as a factorial experiment in Randomized Complete Block design with three replicates. Results showed that poultry manure alone led to significant increase in maize yield (60%) and soil organic matter (45%). In contrast, legume residue incorporation gave significantly lower increase in yield (7%) and soil organic matter (11%). However, the combined application of poultry manure and legume incorporation led to 72% increase in maize yield as opposed to 63 and 10% increase recorded when manure alone or legume alone were incorporated, respectively. Optimal maize yield was achieved when manure application was integrated with P fertilizer application. The interaction of P fertilizer and legume incorporation indicated that soil phosphorus and maize P concentration were significantly increased with the application of the P fertilizer and legume incorporation. Hence, the application of P fertilizer alone is most likely to be economical compared with its integration with legume incorporation.     

The effect of managing improved fallows of Mucuna pruriens on maize production and soil carbon and nitrogen dynamics in sub-humid Zimbabwe

Mucuna pruriens has emerged as a successful forage or green manure legume for use in the smallholder animal-livestock systems of Zimbabwe. The efficiency of N recovery from mucuna residues in subsequent maize crops can be low and the loss of nitrate nitrogen from the soil profile prior to maize N demand is proposed as a reason for this. An experiment was established in the 1999–2000 wet season at seven on-farm sites in a communal farming district of Zimbabwe (average rainfall 650–900 mm) on acidic (pH < 5), and inherently infertile soils with texture ranging from sandy/sandy loam (n = 5) to clay (n = 2). Improved fallows of mucuna grown for 19 weeks produced between 4.7 and 8.5 t/ha dry matter (DM) at the sandy/sandy loam sites and between 9.5 and 11.2 t/ha DM at the clay sites. This biomass was then either cut and removed as hay, or ploughed in as a green manure. Weedy fallow treatments, which represent typical farmer practice, produced 3.3–6.3 t/ha DM. A maize crop was then grown on these same sites in the following 2000–2001 wet season and the dynamics of soil N and C and maize production were investigated. Where mucuna was green manured, a positive linear response (r² = 0.72) in maize yield to increasing mucuna biomass (containing 101–348 kg N/ha) was found. On the sandy sites, and where no P fertiliser was applied to the previous mucuna phase, a maize grain yield of 2.3 t/ha was achieved following the mucuna green-manure system; this was 64% higher than the maize yield following the weedy fallow and 100% higher than the maize yield following the mucuna removed hay system. Apparent nitrogen recoveries in the range of 25 to 53% indicate that there are large quantities of nitrogen not utilised by the subsequent maize phase. The loss of 73 kg/ha of nitrate N from the soil profile (0–120 cm) early in the wet season and prior to maize N demand is proposed as a reason for low N recovery. No change in labile C (measured with 333 mM KMnO₄) was detected through the soil profile at this time and it is suggested that labile C movement occurred between the sampling times.     

Effect of incorporation of crop residues and organic manures on adsorption/desorption and bio-availability of phosphate

Transformations of applied phosphorus (P) to unavailable residual soil P is the major cause of limited P supply in most of the P-deficient soils. The effect of the incorporation of crop residues (rice straw [RS] and wheat straw [WS]) and organic manures (farmyard manure [FYM] and green manure [GM]) on P release in soil and its bio-availability to various summer and winter crops was investigated in laboratory and screen house experiments. Surface (0–0.15 m) soil samples collected after 32 years of differential fertilization to maize–wheat–cowpea fodder crop rotation, were examined for adsorption/desorption behavior of P, after incubating with organics of varying C:P ratios. Incorporation of crop residues increased P adsorption maxima as well as resistance to P release in soils. Increased buffering capacities in crop residue-incorporated treatments decreased P desorption in soil, whereas the incorporation of organic manures decreased P sorption, maximum buffering capacity (MBC), bonding energy, and increased P concentration in soil solution. Although the incorporation of crop residues decreased P release in soil its bio-availability in the soil–plant system was crop-specific and varied with the time of incorporation of organics. Raya showed increased P uptake with incorporation of both RS and WS, whereas in the case of berseem increased P uptake occurred only with wheat straw. Phosphorus uptake in rice, maize, and soybean decreased with the incorporation of both RS and WS. Incorporation of crop residues 1 day before the sowing of summer crops decreased P uptake, whereas incorporation 3 weeks prior to the sowing of winter crops improved P bio-availability. Incorporation of organic manures with a narrow C:P ratio, however, improved P uptake in all the crops under investigation, in both the seasons. The results thus emphasized that adsorption parameters calculated from the examination of soil samples should not be used independently for making fertilizer P recommendations. Crop effects (root exudates) and their interaction with P reaction products in soil and synchronization in P release from organics and crop uptake need to be considered to understand the virtual behavior of P bio-availability in the soil–plant system.     

Nitrogen use efficiency by maize as affected by a mucuna short fallow and P application in the coastal savanna of West Africa

Maize is the primary food crop grown by farmers in the coastal savanna region of Togo and Benin on degraded (rhodic ferralsols), low in soil K-supplying capacity, and non-degraded (plinthic acrisols) soils. Agronomic trials were conducted during 1999–2002 in southern Togo on both soil types to investigate the impact of N and P fertilization and the introduction of a mucuna short fallow (MSF) on yield, indigenous N supply of the soil, N recovery fraction and internal efficiency of maize. In all plots, an annual basal dose of 100 kg K ha^–1 was applied to the maize crop. Maize and mucuna crop residues were incorporated into the soil during land preparation. Treatment yields were primarily below 80% of CERES-MAIZE simulated weather-defined maize yield potentials, indicating that nutrients were more limiting than weather conditions. On degraded soil (DS), maize yields increased from 0.4 t ha^–1 to 2.8 t ha^–1 from 1999 to 2001, without N or P application, in the absence of MSF, with annual K application and incorporation of maize crop residues. Application of N and P mineral fertilizer resulted in yield gains of 1–1.5 t ha^–1. With MSF, additional yield gains of between 0.5 and 1.0 t ha^–1 were obtained at low N application rates. N supply of the soil increased from 10 to 42 kg ha^–1 from 1999 to 2001 and to 58 kg N ha^–1 with MSF. Application of P resulted in significant improvements in N recovery fraction, and greatest gains were obtained with MSF and P application. MSF did not significantly affect internal N efficiency, which averaged 45 kg grain (kg N uptake)^–1. On non-degraded soils (NDS) and without N or P application, in the absence of MSF, maize yields were about 3 t ha^–1 from 1999 to 2001, with N supply of the soil ranging from 55 to 110 kg N ha^–1. Application of 40 kg P ha^–1 alone resulted in significant maize yield gains of between 1.0 (1999) and 1.5 (2001) t ha^–1. Inclusion of MSF did not significantly improve maize yields and even reduced N recovery fraction as determined in the third cropping year (2001). Results illustrate the importance of site-specific integrated soil fertility management recommendations for the southern regions of Togo and Benin that consider indigenous soil nutrient-supplying capacity and yield potential. On DS, the main nutrients limiting maize growth were N and probably K. On NDS, nutrients limiting growth were mainly N and P. Even on DS rapid gains in productivity can be obtained, with MSF serving as a means to allow farmers with limited financial means to restore the fertility of such soils. MSF cannot be recommended on relatively fertile NDS.     

Prediction of nitrogen availability in soil after crop residue incorporation

Controlled study was conducted with five farm-based organic residues to predict the expected nitrogen availability with time, based on N concentration and C/N ratio of the residues under two moisture availability situations. Two kg of red loamy sand soil representing Alfisol and related soil was incubated with the residues to supply 40 kg N ha^–1 at 28 ± 1°C at field capacity (FC) and 50% FC moisture conditions. Results obtained indicated that N content in soil released from residues increased with the time allowed for decomposition reactions at both FC and 50% FC moisture regimes. Relatively higher amount of N release was observed in case of legume residues like mungbean (Vigna radiata),L. leucocephala and perennial pigeonpea (Cajanus cajan), although the rate of N release was more with low N concentration residues. Sorghum stover with C/N ratio of 72.1 resulted in immobilization of N upto 90 days. Nitrogen release was higher at FC than 50% FC. The critical N concentrations of the residues at which N neither released nor immobilized decreased with the length of incubation. At 50% FC the critical concentrations were higher than at FC.     

The role of legume fallows in intensified upland rice-based systems of West Africa

Traditional upland rice-based cropping systems in West Africa rely on periods of fallow to restore soil fertility and prevent the build-up of insect pests and weeds. Demographic growth and increased demand for land is forcing many farmers to intensify their rice production systems. Declining fallow length and increasing number of crops before leaving the land to extended fallow result in a significant yield reduction. Promising cropping system alternatives include the use of site specific, weed-suppressing, multi-purpose cover legumes as short duration fallows. Constraints to rice production related to intensification were determined in 209 farmers' fields in four agro-ecological zones during 1994 and 1995. Nitrogen accumulation and weed suppression were evaluated in 54 legume accessions, grown for six months during the dry season, under a range of hydrological and soil conditions in 1994/95. Their effect on the yield of upland rice was determined in 1995. To increase benefits from improved fallow technology, the timing of legume establishment in relation to rice and the effect on crop and weed growth of removing, burning, mulching, or incorporating fallow residues prior to the rice crop were determined. Intensified land use resulted in a significant plot-level yield reduction that was highest in the derived savanna and the bimodal forest zones where it was associated with a doubling of the weed biomass in rice and a significant reduction in soil N supply. Legume fallows appear to offer the potential to sustain rice yields under intensified cropping. Legume biomass was in most instances significantly greater than in the weedy fallow control and several legume species suppressed weed growth. Nitrogen accumulation by legumes varied between 1–200 kg N ha-1 with 30–90% Ndfa. Rice grain yield following legume fallows increased by an average of 0.2 mg ha-1 or 29% above the weedy fallow control. Relay establishment substantially increased legume biomass. However, seeding of the legume at 28 days or earlier significantly reduced grain yield due to interspecific competition. Incorporating or mulching of fallow residues provided no significant yield advantage as compared to burning. Absolute effects varied as a function of site, legume species, and management practice.