A critical analysis of challenges and opportunities for soil fertility restoration in Sudano-Sahelian West Africa

Since the 1970s, research throughout West Africa showed that low soil organic matter and limited availability of plant nutrients, in particular phosphorus and nitrogen, are major bottlenecks to agricultural productivity, which is further hampered by substantial topsoil losses through wind and water erosion. A few widely recognized publications pointing to massive nutrient mining of the existing crop–livestock production systems triggered numerous studies on a wide array of management strategies and policies suited to improve soil fertility. Throughout Sudano-Sahelian West Africa, the application of crop residue mulch, animal manure, rockphosphates and soluble mineral fertilizers have been shown to enhance crop yields, whereby yield increases varied with the agro-ecological setting and the rates of amendments applied. In more humid areas of Western Africa, the intercropping of cereals with herbaceous or ligneous leguminous species, the installation of fodder banks for increased livestock and manure production, and composting of organic material also proved beneficial to crop production. However, there is evidence that the low adoption of improved management strategies and the lack of long-term investments in soil fertility can be ascribed to low product prices for agricultural commodities, immediate cash needs, risk aversion and labour shortage of small-scale farmers across the region. The wealth of knowledge gathered during several decades of on-station and on-farm experimentation calls for an integration of these data into a database to serve as input variables for models geared towards ex-ante assessment of the suitability of technologies and policies at the scale of farms, communities and regions. Several modelling approaches exist that can be exploited in this sense. Yet, they have to be improved in their ability to account for agro-ecological and socio-economic differences at various geographical scales and for residual effects of management options, thereby allowing scenario analysis and guiding further fundamental and participatory research, extension and political counselling.     

Managing soil fertility diversity to enhance resource use efficiencies in smallholder farming systems: a case from Murewa District,Zimbabwe

Smallholder farms in sub-Saharan African exhibit substantial heterogeneity in soil fertility, and nutrient resource allocation strategies that address this variability are required to increase nutrient use efficiencies. We applied the Field-scale resource Interactions, use Efficiencies and Long-term soil fertility Development (FIELD) model to explore consequences of various manure and fertilizer application strategies on crop productivity and soil organic carbon (SOC) dynamics on farms varying in resource endowment in a case study village in Murewa District, Zimbabwe. FIELD simulated a rapid decline in SOC and maize yields when native woodlands were cleared for maize cultivation without fertilizer inputs coupled with removal of crop residues. Applications of 10 t manure ha⁻¹ year⁻¹ for 10 years were required to restore maize productivity to the yields attainable under native woodland. Long-term application of manure at 5 and 3 t ha⁻¹ resulted in SOC contents comparable to zones of high and medium soil fertility observed on farms of wealthy cattle owners. Targeting manure application to restore SOC to 50–60% of contents under native woodlands was sufficient to increase productivity to 90% of attainable yields. Short-term increases in crop productivity achieved by reallocating manure to less fertile fields were short-lived on sandy soils. Preventing degradation of the soils under intensive cultivation is difficult, particularly in low input farming systems, and attention should be paid to judicious use of the limited nutrient resources to maintain a degree of soil fertility that supports good crop response to fertilizer application.     

The use of organic substrates with contrasting C/N ratio in the regulation of nitrogen use efficiency and losses in a potato agroecosystem

The effect of the application of organic amendments of contrasting C/N ratio, combined with mineral N fertilizer, on the N budget of a potato crop was evaluated. The hypothesis was that the combination of substrates of contrasting chemical composition can improve the synchronization between N crop requirements and N availability in the soil, increasing yields and reducing losses to the environment. Potato plants were cultivated in microplots (0.225 m²) in the tropical uplands of Venezuela, using mineral N (labeled with ¹⁵N), poultry manure (C/N = 12), and rice hulls (C/N = 90) as N sources. Four treatments with the same total dose of N (25.0 g N m⁻²) were applied: (1) MF: mineral fertilizer; (2) P + F: poultry manure and mineral fertilizer; (3) R + F: rice hulls and mineral fertilizer and (4) P + R + F: poultry manure, rice hulls and mineral fertilizer. Labeled and non-labeled N was measured in drained water and plant and soils compartments, and a N budget was established for each treatment. The ratio of N crop uptake to N losses was proposed as an ecological indicator of N use efficiency. The highest value (3.0) of this ratio was obtained in the treatment combining the two organic substrates of contrasting quality, P + R + F, followed by the R + F (2.0) and P + F (1.8) and the lowest ratio was obtained in the MF treatment (0.9). The P + R + F combination may represent a good soil amendment to obtain a high yield with a lower environmental impact, at least in the short-term. The generalization of these results to other soils and climates is discussed.     

Factors affecting the use of fertilizers and manure by smallholders: the case of Vihiga,western Kenya

Sub-Saharan Africa faces huge food supply challenges due to increasing human population, limited opportunities to increase arable land, and declining yields associated with continuously declining soil fertility. To cater for their food requirements, smallholders use only modest levels of inorganic fertilizers and rely to a large extent on manure, which is generally of low quality. To explore factors influencing fertilizer and manure use at the farm level, 253 farm households in Vihiga district of western Kenya were sampled. A pair of Tobit models was used to relate amounts of manure and fertilizer used to household variables. The results indicate that the use of both manure and fertilizer reciprocally influence each other and are strongly influenced by household factors, and also imply that manure and fertilizer uses are endogenous. Policy changes are required to (1) reduce the burden on farming alone in rural areas; (2) promote the use of higher-cost, higher-value inputs such as fertilizers; (3) improve access to input and output markets; and (4) encourage farmer education so as to promote sustainable soil fertility management. Improved understanding of the biophysical and socioeconomic environment of smallholder systems can help target sustainable soil fertility interventions more appropriately.

Pathways of dissolved unreactive phosphorus loss under long-term crop straw and manure application

Dissolved unreactive phosphorus (DUP) loss can increase the risk of water organic and inorganic compound pollution. Because DUP loss under different fertilizers has not been evaluated, selecting a fertilizer that decreases losses but maintains yields remains challenging. Therefore, we compared DUP losses via overland flow, leaching, and sediment in lysimeter plots (8 m?×?4 m?×?0.6 m) with mineral and organic fertilizers for two years. The six treatments, with three replicates, included no fertilizer (CK), mineral fertilizer (NPK), pig manure only (OM), pig manure combined with NPK (OMNPK), crop straw only (RSD), and crop straw combined with NPK (RSDNPK). The results showed that leaching accounted for 54–89% and was the major route for DUP loss. Compared with NPK (0.074 kg ha^?1 year^?1), in the OM and OMNPK, the annual DUP losses via leaching were substantially increased by 11.1% and 8.6%, respectively, whereas they were decreased by 13.2% and 6.2% in the RSD and RSDNPK treatments, respectively. There was a significant positive linear relationship between the soil P surplus and the annual total DUP loss loading (r?=?0.80, p?=?0.05). Compared with the NPK (43.44 kg ha^?1 year^?1), the values of the soil P surplus under the RSD and RSDNPK treatments were greatly reduced by 122.6% and 14.5%, thereby decreasing the annual total DUP loss by 34.5% and 26.4%, respectively. In conclusion, pig manure application can increase DUP leaching risk, whereas the combination of mineral fertilizers and crop straw residues can decrease the DUP loss without compromising the crop yield.

     

长期定位连施化肥的增产培肥效应研究

河西走廊灌区质地中壤的灌漠土上,21年的长期定位试验表明：无有机肥条件下连年施用化肥比无肥处理增产粮食19．6％-300．5％,比有机肥增产16．2％～101．6％,对粮食增产贡献率平均达50％。连续平衡施用化肥,土壤有机质含量连续19年下降了24．9％～27．4％;土壤全N比初始年提高13．7％～46．1％,速效N含量呈稳中下降的动态：土壤全P含量比初始年提高4．4％～5．1％,速效P含量提高16．3％-19．1％;土壤K素库处于长期亏缺状态。速效K含量呈稳步降低的动态。化肥养分具有明显的间接培肥效应,在生物循环中以有机肥为载体下年输入土壤的N、P、K化肥养分分别占当年投入量的27．1％～31_3％、23．8％～27．0％和33．2％;化肥增产的秸秆有机c输入下年土壤的达567．3～626．7kg／hm。,证明有机肥中相当一部分养分来自化肥及其化肥的生产物。     

Conservation tillage,local organic resources and nitrogen fertilizer combinations affect maize productivity,soil structure and nutrient balances in semi-arid Kenya

Smallholder land productivity in drylands can be increased by optimizing locally available resources, through nutrient enhancement and water conservation. In this study, we investigated the effect of tillage system, organic resource and chemical nitrogen fertilizer application on maize productivity in a sandy soil in eastern Kenya over four seasons. The objectives were to (1) determine effects of different tillage-organic resource combinations on soil structure and crop yield, (2) determine optimum organic–inorganic nutrient combinations for arid and semi-arid environments in Kenya and, (3) assess partial nutrient budgets of different soil, water and nutrient management practices using nutrient inflows and outflows. This experiment, initiated in the short rainy season of 2005, was a split plot design with 7 treatments involving combinations of tillage (tied-ridges, conventional tillage and no-till) and organic resource (1 t ha⁻¹ manure + 1 t ha⁻¹ crop residue and; 2 t ha⁻¹ of manure (no crop residue) in the main plots. Chemical nitrogen fertilizer at 0 and 60 kg N ha⁻¹ was used in sub-plots. Although average yield in no-till was by 30–65% lower than in conventional and tied-ridges during the initial two seasons, it achieved 7–40% higher yields than these tillage systems by season four. Combined application of 1 t ha⁻¹ of crop residue and 1 t ha⁻¹ of manure increased maize yield over sole application of manure at 2 t ha⁻¹ by between 17 and 51% depending on the tillage system, for treatments without inorganic N fertilizer. Cumulative nutrients in harvested maize in the four seasons ranged from 77 to 196 kg N ha⁻¹, 12 to 27 kg P ha⁻¹ and 102 to 191 kg K ha⁻¹, representing 23 and 62% of applied N in treatments with and without mineral fertilizer N respectively, 10% of applied P and 35% of applied K. Chemical nitrogen fertilizer application increased maize yields by 17–94%; the increases were significant in the first 3 seasons (P < 0.05). Tillage had significant effect on soil macro- (>2 mm) and micro-aggregates fractions (<250 μm >53 μm: P < 0.05), with aggregation indices following the order no-till > tied-ridges > conventional tillage. Also, combining crop residue and manure increased large macro-aggregates by 1.4–4.0 g 100 g⁻¹ soil above manure only treatments. We conclude that even with modest organic resource application, and depending on the number of seasons of use, conservation tillage systems such as tied-ridges and no-till can be effective in improving crop yield, nutrient uptake and soil structure and that farmers are better off applying 1 t ha⁻¹ each of crop residue and manure rather than sole manure.     

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.     

Combining hill-placed manure and mineral fertilizer enhances maize productivity and profitability in northern Benin

Throughout much of Sub-Saharan Africa (SSA), maize production is characterized by low productivity due to the scarce availability and use of external inputs and recurrent droughts exacerbated by climate variability. Within the integrated soil fertility management (ISFM) framework, there is thus a need for optimizing the application of fertilizers and manure to better use the limited nutrient resources and increase crop yield and farmer income. An on-station experiment was conducted in Northern Benin over a 4-year period to evaluate the effect of hill placement of mineral fertilizer and manure on maize yields and soil chemical properties. The treatments consisted in the combination of three rates of manure (0 (NM), 3 (3M) and 6 (6M) Mg ha^?1) and three levels of fertilizer (0% (NF), 50% (50F) and 100% (100F) of the rate recommended by extension (76 kg N + 13.1 kg P + 24.9 K ha^?1)). On average across the fertilizer rates, hill-placement of manure significantly improved soil organic carbon content, available P and exchangeable K after 4 years by up to 124, 166 and 77%, respectively, compared to the initial values. As a result of the nutrient inputs and improved soil properties, yields increased steadily over time for all manure and fertilizer combinations. Value-cost ratios and benefit–cost ratios were >2 and generally as good or even better for treatments involving 50F compared to NF or 100F. Although applying half the recommended rate of fertilizer without manure as currently done by many farmers appears to make economic sense, this practice is unlikely to be sustainable in the long term. Substituting 50F for 3M or complementing 50F with 3M are two possible strategies that are compatible with the precepts of ISFM and provide returns on investment at least as good as the current practice. However, this will require greater manure production, made possible in part by the increased stover yields, and access to means of transportation to deliver the manure to the fields.     

Farm typologies,soil fertility variability and nutrient management in smallholder farming in Sub-Saharan Africa

Farm typologies are a useful tool to assist in unpacking and understanding the wide diversity among smallholder farms to improve targeting of crop production intensification strategies. Sustainable crop production intensification will require the development of an array of nutrient management strategies tailored to farm-specific conditions, rather than blanket recommendations across diverse farms. This study reviewed key literature on smallholder farm typologies focusing on three countries (Kenya, Malawi and Zimbabwe), to gain insights on opportunities for crop production intensification, and the importance of developing farm-specific nutrient management practices. Investigations on farm typologies have done well in highlighting the fundamental differences between farm categories, with 3–5 typologies often adequate to represent the wide differences in resource endowment. Resource-endowed farmers have ready access to large quantities of manure and mineral fertilizers, which contribute to higher soil fertility and crop productivity on their farms. Resource-constrained households use little or no manure and mineral fertilizers, and have limited capacity to invest in labour-demanding soil fertility management technologies. These farmers often have to rely on off-farm opportunities for income that are largely limited to selling unskilled labour to their resource-endowed neighbors. The variability in management practices by farmers has resulted in three main soil fertility classes that can be used for targeting soil fertility management technologies, characterized by potential response to fertilizer application as: (1) low-responsive fertile fields that receive large additions of manure and fertilizer; (2) high-responsive infertile fields that receive moderate nutrient applications; (3) poorly responsive degraded soils cultivated for many years with little or no nutrient additions. The main conclusions drawn from the review are: (1) resource constrained farmers constitute the widest band across the three countries, with many of the farmers far below the threshold for sustainable maize production intensification and lacking capacity to invest in improved seed and fertilizer, (2) farm sizes and livestock ownership were key determinants for both farmer wealth status and farm productivity, and (3) soil organic carbon and available P were good indicators for predicting previous land management, that is also invariably linked to farmer resource endowment.     

我国有机肥料的资源、利用、问题和对策

针对化肥施用量不断增加、有机肥施用比例逐渐下降的现实,介绍我国有机肥料资源和利用现状,论述有机物对土壤、作物的作用;提出发展我国有机肥料的对策（推行平衡施肥、开发利用城市有机肥源、积极发展绿肥、秸秆还田）,虽然施用化肥仍是补充土壤中氮磷钾营养元素的重要手段,但强调施用有机肥,对于资源利用和防治环境污染都具有重要意义。     

Nutrient recycling potential in rice–vegetable cropping sequences under in situ residue management at mid-altitude subtropical Meghalaya

Low native soil nitrogen (N) and very low phosphorus (P) coupled with apathy of farmers towards use of fertilizer are the major constraints limiting rice productivity in the North Eastern Hill (NEH) Region of India. Field experiment was undertaken in narrow valley wetland during Kharif and pre-Kharif (rainy) seasons of 2003, 2004, and 2005, respectively, at Umiam (950 m MSL), Meghalaya to evaluate the nutrient recycling and production potential of rice (Oryza sativa L.)—vegetables cropping sequences under low input in situ residue management under rainfed conditions. After rice, five vegetable crops, viz. tomato (Lycopersicon esculentum Mill), potato (Solanum tuberosum L.), frenchbean (Phaseolus vulgaris), cabbage (Brassica oleracea L. var. capitata), and carrot (Daucas carota L.) were grown during pre-kharif season on temporary raised and sunken beds. Minimum tillage was given to both rice and vegetables. No external input including fertilizer, organic manure, pesticides, etc. were applied to either crops. Only the economic parts of the crops were harvested and the rest of the biomass of the entire crop and weed residues were chopped and incorporated into the soil. Among the five cropping sequences, rice–tomato gave the highest rice equivalent yield (214.40 q/ha), followed by rice–carrot (206.4 q/ha). Highest net return (rupees [Rs.] 66,635 ha⁻¹) was recorded in rice–carrot, closely followed by rice–tomato (Rs. 66,139 ha⁻¹). Rice–cabbage and rice–frenchbean were found to be on a par with each other in terms of yield and income. The highest amount of NPK recycling was recorded for the rice–potato sequence. Weed biomass production in the different sequences ranged from 37.5 q/ha for rice–tomato to as high as 50.6 q/ha in the case of rice–fallow. Soil fertility in terms of available NPK status was found to be stable in all the crop sequences except rice–cabbage, where fertility status declined slightly. The soil biological properties such as population of Rhizobium, bacteria, phosphorus-solubilizing microorganisms, and earthworm activity were all found to be remarkably higher in the experimental field compared to other adjacent plots that are managed inorganically. It was concluded that vegetables such as tomato, carrot, potato, etc. could be profitably grown after rice without the addition of fertilizer and manure, if residues are managed effectively under temporary raised beds without deteriorating soil fertility.     

Effects of mulch and mineral fertilizer on crop, weevil and soil quality parameters in highland banana

The decline of cooking banana production in parts of East Africa hasbeen associated with a loss of soil fertility and increased pest pressure.Previous work indicated that the use of mineral fertilizers at recommendedratesis not financially viable on plantations where pest pressure is high. Theobjectives of this study were to determine the effects of half the recommendedrates of mineral fertilizers and organic mulch on banana yield, soil and foliarnutrient status, soil water and pest damage in the central region of Uganda.Thestudy was conducted in a four-year old banana plantation infested with weevils.Treatments consisted of mineral fertilizer alone, mulch alone, a combination ofmulch with fertilizer, and a control (no mulch from outside the plot, ormineralfertilizer applied). Fruit yield was generally low (mean of 7.4 Mgha^–1 yr^–1) and increases above thecontrol ranged from 1.1 to 2.0 Mg ha^–1 yr^–1. Banana yield was significantly higher in the solemulch treatment than in the control. However, there were no significantdifferences in yield among the treatments that received external inputs. Thus,there was no demonstrable yield advantage of combining mineral fertilizer withmulch. At the end of the trial, soil K concentrations in the two treatmentsthatwere mulched, were almost twice those of the control, but this difference wasnotstatistically significant. Concentrations of Ca and Mg did not differ withtreatment. Soil available P significantly increased when mineral fertilizer andorganic mulch were combined. Treatments that received organic mulch were lowerin foliar concentrations of N and Mg, and higher in K. Soil water content wasgreater in the surface soil layer in the treatments that received mulch,presumably due to less surface run-off and evaporation, since there was nochangein surface soil porosity. The effect of organic and inorganic amendments had noimpact on weevil damage, which remained above 5% at the end of the trial. Weconclude that where weevil damage is over 5% of the cross section near thecollar, banana production may not be economically increased through use ofeither mineral fertilizer and/or organic mulch.     

Nitrogen and residue management effects on agronomic productivity and nitrogen use efficiency in rice–wheat system in Indian Punjab

Development of a sustainable and environment friendly crop production system depends on identifying effective strategies for the management of tillage and postharvest crop residues. Three-year (2004–2007) field study was initiated on two soil types to evaluate the effect of straw management (burning, incorporation and surface mulch) and tillage (conventional tillage and zero tillage) before sowing wheat and four nitrogen rates (0, 90, 120 and 150 kg N ha⁻¹) on crop yields, N use efficiency, and soil fertility in the northwestern India. Effect of tillage and straw management on nitrogen transformation in soils was investigated in a laboratory incubation study. In sandy loam, grain yield of wheat with straw mulch-zero-till (ZT) was 7% higher compared to when residues were burnt-ZT but it was similar to straw burnt-conventional till (CT), averaged across 3 years. In silt loam, grain yield of wheat with straw mulch-ZT was 4.4% higher compared to straw incorporated-CT, but it was similar to straw burnt-CT. Response to N application was generally observed up to 150 kg N ha⁻¹ except in 2004–2005 on sandy loam where N response was observed up to 120 kg N ha⁻¹, irrespective of straw and tillage treatments. In sandy loam, RE was lower (49%) for straw burnt-ZT than in other treatments (54–56%). In silt loam, RE was higher in straw mulch-ZT compared with straw incorporation-CT (65 vs. 58%). In sandy loam, AE was higher in straw burnt-CT and straw mulch-ZT compared with the other treatments (19.2 vs. 16.9 kg grain kg⁻¹ N applied). In silt loam, AE was lower in straw incorporation-CT than in other treatments (16.0 vs. 17.6 kg grain kg⁻¹ N applied). Rice yield and N uptake were not influenced by straw and tillage management treatments applied to the preceding wheat. Recycling of rice residue (incorporation and surface mulch) compared with straw burning increased soil organic carbon and the availability of soil P and K. There was more carbon sequestration in rice straw mulch with zero tillage (25%) than in straw incorporation with conventional tillage (17%). Soil N mineralization at 45 days after incubation was 15–25% higher in straw retention plots compared with on straw burnt plots.     

Fractions of organic carbon in soils under different crop rotations, cover crops and fertilization practices

We investigated the long-term effects (13–48 years) of crop rotations, cover crops and fertilization practices on soil organic carbon fractions. Two long-term experiments conducted on a clay loam soil in southeastern Norway were used. From the crop rotation experiment, two rotations, one with two years grain + four years grass and the second with grain alone (both for 6 years), were selected. Each rotation was divided into moderate fertilizer rate (30–40 kg N ha^–1), normal fertilizer rate (80–120 kg N ha^–1) and farmyard manure (FYM 60 Mg ha^–1 + inorganic N at normal rate). Farmyard manure was applied only once in a 6-year rotation, while NPK was applied to every crop. The cover crop experiment with principal cereal crops consisted of three treatments: no cover, rye grass and clover as cover crops. Each cover crop was fertilized with 0 and 120 kg ha^–1 N rates. Soil samples from both experiments were taken from 0–10 cm and 10–25 cm depths in the autumn of 2001. The classical extraction procedure with alkali and acid solution was used to separate humic acid (HA), fulvic acid (FA) and humin fractions, while H₂O₂ was used to separate black carbon (BC) from the humin fraction. The rotation of grain + grass showed a significantly higher soil organic carbon (SOC) compared with grain alone at both depths. Farmyard manure application resulted in significantly higher SOC than that of mineral fertilizer only. However, cover crops and N rates did not affect SOC significantly. Organic carbon content of FA, HA and humin fractions accounted for about 29%, 25% and 44% of SOC, respectively. The rotation of grain+grass gave a higher C content in HA and humin fractions, and a lower C in the FA fraction as compared with the rotation with grain alone. Farmyard manure increased HA and humin fractions more than did chemical fertilizers. Clover cover crop increased the C proportion of humin more than rye grass and no cover crop. No significant differences in C contents of FA, HA and humin fractions were observed between N rates. Effects of cover crop and N rates as well as fertilization with NPK on black carbon (BC) content were significant only at 10–25 cm depths. Farmyard manure increased the BC fraction compared with chemical fertilizers. Clover crop also enhanced the accumulation of the BC fraction. Application of 120 kg N ha^–1 resulted in a significant increase of the BC fraction.     

Effect of farmer management strategies on spatial variability of soil fertility and crop nutrient uptake in contrasting agro-ecological zones in Zimbabwe

Variability of soil fertility within, and across farms, poses a major challenge for increasing crop productivity in smallholder systems of sub-Saharan Africa. This study assessed the effect of farmers’ resource endowment and nutrient management strategies on variability in soil fertility and plant nutrient uptake between different fields in Gokwe South (ave. rainfall ~650 mm year⁻¹; 16.3 persons km⁻²) and Murewa (ave. rainfall ~850 mm year⁻¹; 44.1 persons km⁻²) districts, Zimbabwe. In Murewa, resource-endowed farmers applied manure (>3.5 t ha⁻¹ year⁻¹) on fields closest to their homesteads (homefields) and none to fields further away (outfields). In Gokwe the manure was not targeted to any particular field, and farmers quickly abandoned outfields and opened up new fields further way from the homestead once fertility had declined, but homefields were continually cultivated. Soil available P was higher in homefields (8–13 mg kg⁻¹) of resource-endowed farmers than on outfields and all fields on resource constrained farms (2–6 mg kg⁻¹) in Murewa. Soil fertility decreased with increasing distance from the homestead in Murewa while the reverse trend occurred in Gokwe South, indicating the impact of different soil fertility management strategies on spatial soil fertility gradients. In both districts, maize showed deficiency of N and P, implying that these were the most limiting nutrients. It was concluded that besides farmers’ access to resources, the direction of soil fertility gradients also depends on agro-ecological conditions which influence resource management strategies.     

Multiple benefits of manure: The key to maintenance of soil fertility and restoration of depleted sandy soils on African smallholder farms

Manure is a key nutrient resource on smallholder farms in the tropics, especially on poorly buffered sandy soils, due to its multiple benefits for soil fertility. Farmers preferentially apply manure to fields closest to homesteads (homefields), which are more fertile than fields further away (outfields). A three-year experiment was established on homefields and outfields on sandy and clayey soils to assess the effects of mineral nitrogen (N) fertilizer application in combination with manure or mineral phosphorus (P) on maize yields and soil chemical properties. Significant maize responses to application of N and manure were observed on all fields except the depleted sandy outfield. Large amounts of manure (17 t ha⁻¹ year⁻¹) were required to significantly increase soil organic carbon (SOC), pH, available P, and base saturation, and restore productivity of the depleted sandy outfield. Sole N as ammonium nitrate (100 kg N ha⁻¹) or in combination with single superphosphate led to acidification of the sandy soils, with a decrease of up to 0.8 pH units after three seasons. In a greenhouse experiment, N and calcium (Ca) were identified as deficient in the sandy homefield, while N, P, Ca, and zinc (Zn) were deficient or low on the sandy outfield. The deficiencies of Ca and Zn were alleviated by the addition of manure. This study highlights the essential role of manure in sustaining and replenishing soil fertility on smallholder farms through its multiple effects, although it should be used in combination with N mineral fertilizers due to its low capacity to supply N.     

Evaluation of current fertilizer practice and soil fertility in vegetable production in the Beijing region

A survey on current fertilizer practices and their effects on soil fertility and soil salinity was conducted from 1996 to 2000 in Beijing Province, a major vegetable production area in the North China Plain. Inputs of the major nutrients (NPK) and fertilizer application methods and sources for different vegetable species and field conditions were evaluated. Excessive N and P fertilizer application, often up to about 5 times the crop requirement in the case of N, was very common, especially for high-value crops. Potassium supply may have been inadequate for some crops such as leafy vegetables. Urea, diammonium orthophosphate ((NH₄)₂HPO₄) and chicken manure were the major nutrient sources for vegetable production in the region. Over 50% of N, 60% of P and nearly 90% of K applied originated from organic manure. Total N application rate for open-field Chinese cabbage from organic manure and inorganic fertilizers ranged from 300 to 900 kg N ha^–1 on 78% of the farms surveyed. More than 35% of the surveyed greenhouse-grown tomato crops received > 1000 kg N ha^–1 from organic and inorganic sources. A negative K balance (applied K minus K removed by the crop) was found in two-thirds of the surveyed fields of open-field Chinese cabbage and half of the surveyed fields of greenhouse-grown tomato. Plant-available N, P and K increased with increasing length of the period the greenhouse soils had been used for vegetable production. Similarly, soil salinity increased more in greenhouse soils than in open-field soils. The results indicate that balanced NPK fertilizer use and maintenance of soil quality are important for the development of sustainable vegetable production systems in this region.     

Variable grain legume yields,responses to phosphorus and rotational effects on maize across soil fertility gradients on African smallholder farms

Promiscuous soyabean varieties have potential to contribute significantly to income generation, food security and soil N budgets on smallholder farms. One of the major factors limiting this potential is farmers’ preference to allocate nutrient resources to food security cereal crops on the most fertile fields, leaving grain legumes to grow on residual fertility on infertile fields. Two experiments were conducted to: (i) compare the current farmer practice with targeting manure and single super phosphate (SSP) to soyabean in a three-year rotation cycle on two fields with different soil fertility: an infertile sandy soil and a more fertile clay soil; and (ii) assess the effects of variability of soil fertility within and across farms on productivity of soyabean and groundnut. In the first experiment, soyabean (<0.2 t ha⁻¹) and maize yields (<0.7 t ha⁻¹) without fertilizer were poor on a degraded sandy soil. Both crops responded poorly to SSP due to deficiency of other nutrients. Manure application significantly increased soyabean and maize yields, led to yield stabilization over three seasons and also significantly increased the proportion of N₂ fixed by soyabean (measured using ¹⁵N natural abundance) from 60% to 83%. On the sandy soil, P was used more efficiently and gross margins were greater when SSP and manure were applied to maize in a maize–soyabean rotation. Soyabean and maize yields without fertilizer inputs were larger on clay soil with moderate fertility (0.4–0.7 t ha⁻¹ and 2.0–2.3 t ha⁻¹ respectively) and were significantly increased by application of SSP and manure. Within rotations, P recovery was higher when manure and SSP were applied to maize (43 and 25%) than when applied to soyabean (20 and 19%). However, application of manure to soyabean on the clay was more profitable than application to maize for individual crops and within rotations. In the second experiment, soyabean and groundnut yields were largest (∼1 and ∼0.8 t ha⁻¹ respectively) on plots closest to homesteads on wealthy farms, which were more fertile due to good past management. Yields were poor (< 0.5 t ha⁻¹) on other fields which previously had received little nutrient inputs. Soyabean and groundnut yields correlated well with available P (R ² = 0.5–0.7) and soil organic C (SOC) contents (R ² = 0.4–0.6). For smallholder farmers to maximise benefits from legume production they need to focus attention on the more fertile plots, although production should be optimized in relation to maize. Targeting nutrients to maize as currently practiced by farmers was more efficient and economic under poor soil fertility conditions, whilst potential exists to increase income by targeting manure to soyabean on the more fertile soils.