Organic Matter Management as an Underlying Cause for Soil Fertility Gradients on Smallholder Farms in Zimbabwe

Management of spatial and temporal variability of soil fertility within fields and farms is one major challenge for increasing farm-level crop productivity in smallholder agriculture. A study was conducted across 120 on-farm field sites in three agro-ecological regions of Zimbabwe to identify management factors influencing the formation of within-field/farm soil fertility gradients. Using farmer participatory research approaches, host farmers were put into three classes according to resource endowment, namely, Resource-endowed, Intermediate and Resource-constrained farmers. Each host farmer identified the most (rich) and least (poor) productive field or field section, which were then studied over two years. Farmer criteria for defining soil fertility ranged from colour through elements of soil structure to crop response following external nutrient inputs. The fertility ranking of fields consistently matched with laboratory indices, with rich fields containing significantly more soil organic carbon (SOC) and nutrients than the corresponding poor fields. Fertility gradients were evident within and across farms belonging to different farmer classes. The mean SOC content for rich fields were >6.0 g kg⁻¹ compared with <4.6 g kg⁻¹ for the designated poor fields. Rich fields belonging to Resource-endowed farmers had 16–28% more SOC than those belonging to their resource-constrained counterparts, suggesting differences in organic matter management. Differences in SOC and fertility status between rich and poor fields were wider in two of the study areas which had more than 70 years of cultivation in contrast to the third site which had been under smallholder farming for only 20 years, suggesting that the observed fertility gradients are a cumulative effect of years of differential management practices by different farmer classes. Analysis of potential benefits from in situ organic biomass inputs suggested that the processes of organic matter capture and utilization discriminated against Resource-constrained farmers. About 50% of in situ biomass, preferentially maize stover, was lost in three dry season months, and up to 72% of potentially recyclable N is lost from poor fields managed by Resource-constrained farmers. In contrast, Resource-endowed farmers incorporated more than 1.5 t C, 25 kg N and 5 kg P ha⁻¹ season⁻¹ because of their access to draught power during the early dry season. Such inputs could make a difference on these nutrient-depleted soils. Intermediate farmers represented a diverse transitional group whose size and variability could be indicative of the dynamism of technology usage. It was concluded that management of soil fertility gradients to increase crop productivity on smallholder farms hinges on increasing the capacity and efficiency with which organic matter is generated and utilized by different farmer weaclth groups across temporal scales.     

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.     

Diagnosis for ecological intensification of maize-based smallholder farming systems in the Costa Chica,Mexico

Enhanced utilization of ecological processes for food and feed production as part of the notion of ecological intensification starts from location-specific knowledge of production constraints. A diagnostic systems approach which combined social-economic and production ecological methods at farm and field level was developed and applied to diagnose extent and causes of the perceived low productivity of maize-based smallholder systems in two communities of the Costa Chica in South West Mexico. Social-economic and production ecological surveys were applied and complemented with model-based calculations. The results demonstrated that current nutrient management of crops has promoted nutrition imbalances, resulting in K- and, less surprisingly N-limited production conditions, reflected in low yields of the major crops maize and roselle and low resource use efficiencies. Production on moderate to steep slopes was estimated to result in considerable losses of soil and organic matter. Poor crop production, lack of specific animal fodder production systems and strong dependence on animal grazing within communal areas limited recycling of nutrients through manure. In combination with low prices for the roselle cash crop, farmers are caught in a vicious cycle of cash shortage and resource decline. The production ecological findings complemented farmers opinions by providing more insight in background and extent of livelihood constraints. Changing fertilizer subsidies and rethinking animal fodder production as well as use of communal lands requires targeting both formal and informal governance structures. The methodology has broader applicability in smallholder systems in view of its low demand on capital intensive resources.     

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.     

Appropriate technologies to replenish soil fertility in southern Africa

In southern Africa, soil nutrient reserves are being depleted because of continued nutrient mining without adequate replenishment. The consequent downward spiral of soil fertility has led to a corresponding decline in crop yields, food insecurity, food aid and environmental degradation. The central issue for improving agricultural productivity in southern Africa is how to build up and maintain soil fertility despite the low incomes of smallholder farmers and the increasing land and labour constraints they face. Under this review five main options namely: inorganic fertilizers, grain legumes, animal manures, integrated nutrient management and agroforestry options appropriate to smallholder farmers are presented. Issues addressed in the use of inorganic fertilizers are reduction in fertilizer costs, timely availability and use efficiency. Legumes can be used to diversify farm system productivity but this requires P and lime application to support better legume growth and biological nitrogen fixation (BNF) as well as development of markets for various legume products. Manure availability and quality are central issues in increasing smallholder farm productivity and increasing its efficiency through proper handling and application methods. Integrated nutrient management of soil fertility by combined application of both inputs will increase use efficiency of inputs and reduce costs and increase profitability; but the challenge is often how to raise adequate amounts of either inorganic or organic inputs. Issues such as quality of inputs, nutrient balancing, labour to collect and transport organic inputs and their management need to be optimized. These are the challenges of adoption as are the scaling up of these options to millions of small-scale farmers.     

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.     

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.     

Farm nitrogen flows of four farmer field schools in Kenya

Re-use of nutrients within farming systems contributes to sustainable food production in nutrient limited production systems. Re-use is established when nutrients pass through several farm compartments before they leave the farm via marketable products. In this paper re-use of nitrogen is examined as an indicator for sustainable soil fertility management. Re-use (RU, kg farm⁻¹) was defined as the amount of nitrogen that was translocated within one farm divided by the sum of transitions between farm compartments within a farm. In 2002, a total of 101 farms belonging to 4 farmer field schools in Kenya were analysed using the NUTMON (now known as MonQI) toolbox. The farms were distributed over 4 farmer field schools located in two agro-ecological zones. RU was positively related to the net farm income and to crop yields. However, data were scattered and often local farm conditions veiled the relation between nitrogen management strategies and farm performances. The results of this paper demonstrate that different agro-ecological zones with diverse production constraints have developed different in-farm nitrogen management strategies that are best adapted to the local conditions, but may have different environmental impacts. An erratum to this article can be found at     

Recycling of nutrients in Japanese mint - assessment of soil fertility and crop yield

The efficiency of mint-residue, composted alone and amended with starter nutrients, microbial culture and soil suspension (hereafter termed amended compost) was compared with farm yard manure and inorganic fertilizer on the yield of Japanese mint (Mentha arvensis L.) and improvement of soil fertility. Herbage, essential oil yield, nutrient uptake of Japanese mint and soil available nutrients were significantly enhanced due to application of amended compost as compared to nonamended compost, farm yard manure and inorganic fertilizer. Organic fertilized soils maintained significantly higher available nutrients throughout the crop growth period as compared to inorganic fertilized soils. No additional improvement in yields and soil fertility was recorded with combined application of compost and inorganic fertilizer in 1:1 ratio as against addition of compost alone. Advantage of such combinations was recorded in case of farm yard manure. Results of the study suggested possibilities for nutrient recycling through composted mint-residue for supplementing the fertilizers requirement of Japanese mint.     

A GIS-based fertilizer decision support system for farmers in Northeast China: a case study at Tong-le village

Application of computer technology such as Geographical Information Systems (GIS) to help farmers optimize fertilization, improve soil fertility and protect soil from erosion is currently of considerable interest as a topic of research. The objective of this study was to develop a GIS-based software package that can be used to help farmers select fertilizer application rates and manage soil nutrients. The ease of integration and programming in current digital spatial technology made it feasible to combine a fertilization model with a GIS platform to develop a GIS-based Fertilizer Decision Support System (FDSS) that enables farmers to determine precise fertilizer recommendations through an interactive computer interface. This paper outlines the development and test application of a GIS-based FDSS at Tong-le Village of Baiquan county, Heilongjiang province of Northeast China. The FDSS uses a farm field as the base mapping unit and incorporates data from field samples, farmer surveys and remote sensing, as well as expert knowledge in agriculture, soil science and computer science to develop a spatial database and soil/crop management system. The FDSS is a crop fertilization and management software developed on the SuperMap platform, and will help farmers and managers of agricultural production units to increase their fertilizer utilization efficiency, and thus their net profit.     

Modeling farm nutrient flows in the North China Plain to reduce nutrient losses

Years of poor nutrient management practices in the agriculture industry in the North China Plain have led to large losses of nutrients to the environment, causing severe ecological consequences. Analyzing farm nutrient flows is urgently needed in order to reduce nutrient losses. A farm-level nutrient flow model was developed in this study based on the NUFER (NUtrient flows in Food chains, Environment and Resources use) model, and was used to analyze nitrogen (N) and phosphorus (P) flows, use efficiencies, and losses for nine representative farm types in the North China Plain. Data from 401 farms were evaluated for the years 2012–2015. The analysis showed that mixed farms were more efficient in nutrient utilization than crop-based or landless livestock farms. The efficiencies of N and P used in crop production were highest for mixed dairy farms, reaching 67% for N and 68% for P. Consistently, mixed dairy farms had the lowest N and P surpluses and losses in crop production. Mixed swine farms were 5 and 9% higher in N and P efficiency in livestock production than landless swine farms, respectively. Losses of N and P from the animal manure management chain were 20–42% lower for mixed swine and 69–78% lower for mixed poultry farms than for landless farms of the same animal type. This is at least partially due to more frequent manure removal. Integrated crop-livestock production using livestock wastes as crop fertilizer was shown to be the most sustainable model in nutrient use for the agriculture industry in the North China Plain.     

Balanced pasture fertilization in the Basque Country: I. Phosphorus and potassium budgets on dairy farms

Dairy farming is the main agricultural activity of the Basque Country. A dairy farm is characterized as a system with soils and crops, forage, cattle and manure as main components, and in such a system, nutrient cycling is very important to maintain soil fertility and optimize forage production. To quantify nutrient transfers in the cycle, a simple system was developed and has been applied to seventeen farms to examine its ability to achieve a balanced P and K fertilization. These farms have provided data on inputs (fertilizer, feeds, concentrates), pasture and manure management, and outputs (milk production), and soil samples have been taken from farm pastures. Phosphorus and K in excreta and uneaten pasture is used with a relatively high efficiency as suggested by the relatively high efficiency of P and K utilization by the pasture that usually ranges from 70 to 90%. Concentrate feeding (3000 kg cow^–1 yr^–1) represents one of the main P and K inputs in Basque Country dairy farms, averaging 26 and 66 kg ha^–1, respectively. Besides, release of K in the soil through slow liberation from non-exchangeable sites was estimated as 30 kg ha^–1. Thus, a high efficiency in excreta recycling would diminish substantially P and K mineral fertilizer needs. Farm nutrient budgets appear to be a convenient tool for determining nutrient shortages and surpluses at farm level, and thus they are considered as a first step to support a better management of maintenance fertilization of permanent pastures.     

Assessing phosphorus management among organic farming systems: a farm input,output and budget analysis in southwestern France

Organic farming is gaining interest worldwide due to its low environmental impact. However, questions still remain about its long-term sustainability, particularly in terms of nutrient management. There is debate about the ability of organic farming systems to compensate for nutrient exports due to crop and animal production. Stockless systems are considered as the most critical and they are generally associated with negative farm-gate nutrient budgets. In this study, we examined the farm-gate nutrient budgets of 23 organic farms located in southwestern France, with special focus on stockless farming systems. Phosphorus (P) was taken as a case study due to the issue of its critical management in organic farming systems. The farms were characterised on the basis of interviews with farmers and the soil nutrient status was assessed through soil sampling. Results showed that none of the farms imported rock phosphate fertiliser. On the contrary, most farms imported organic fertiliser and/or compost and manure, the latter from neighbouring farms or urban areas. As a consequence, stockless farm P budgets were not necessarily negative and options existed from achieving better nutrient cycle closure. However, soil P test was low to moderate in many cases. These results suggested that P management in organic farming systems is not simply related to the mixed versus specialised characteristics of the farms and that nutrient cycling should be addressed and assessed at a larger, e.g., district, scale.     

Major nutrient balances in small-scale vegetable farming systems in peri-urban areas in China

Balances of major nutrients such as nitrogen (N), phosphorus (P), and potassium (K) in small-scale farming systems are of critical importance to nutrient management and sustainable agricultural development. Mass balances of N, P, and K and some of their influencing factors were studied for two years from July 2003 to July 2005 on small-scale vegetable-farming systems in two contrasting peri-urban areas (Nanjing and Wuxi) of the Yangtze river delta region of China. This balance approach considered organic fertilizer inputs (cow manure, pig manure, and human biosolids), inorganic fertilizer inputs (urea, composite fertilizer, and phosphates), irrigation water, and atmospheric deposition; and considered outputs by vegetables. Input via organic fertilizers was significant for all element balances in the Nanjing area. Inorganic and organic fertilizer, particularly inorganic fertilizer, contributed major nutrient inputs to the system in the Wuxi area. Compared with nutrient output by vegetables, there were significant surpluses of N and P on two vegetable farm systems. Furthermore, N surplus in the Nanjing area was higher than that in the Wuxi area with an inverse relationship to P surplus. In contrast, the general trend of K balances was negative on both sites; hence, the nutrient use efficiency was significantly lower for N and P than K. The nutrient imbalance may be attributed to the differences between fertilizer types and management modes driven by social economic status differences among farmer households. The large N and P net excess creates an environmental threat because of potential losses to ground or surface waters, whereas negative K balance creates soil fertility risks. The results highlight researchers’ and farmers’ need to develop rational fertilization technology to optimize nutrient management on vegetable farmlands to promote sustainable agricultural development in peri-urban areas.     

Factors influencing manure application by farmers in semi-arid west Africa

Animal manure is one of the principal sources of nutrients for soil fertility maintenance and crop production in semi-arid west Africa. Farm-level decisions concerning the use of manure are governed by socioeconomic and institutional factors, as much as they are by agronomic and ecological concerns. This paper analyses the cultural and socioeconomic factors affecting manure use decisions in parts of semi-arid West Africa using a logit model. The results show that the major factors that positively influence farmers' manuring decisions are the farmer's own herd size, contractual arrangements between herders and farmers for manure, seasonal migration and its effect on livestock investment and the proportion of cultivated land owned by the farmer. Factors found to negatively affect manure use are farm size, distance of fields to the homestead, the proportion of cultivated land recently under fallow and land-labour ratio. The paper concludes by suggesting strategies for removing the constraints to efficient manure utilisation and soil fertility improvement in semi-arid West Africa.     

Effects of long-term fertilization and mulch on soil fertility in contour hedgerow systems: A case study on steeplands from the Three Gorges Area,China

The use of contour hedgerows is widely advocated to sustain crop production and reduce soil loss on steeplands in the Three Gorges Area of China. However, little is known about the effects of soil management on soil fertility within these systems, or about the spatial gradients in soil nutrients that may develop in terraces formed behind the vegetative barriers. Therefore, we carried out a study on the effects of various long-term soil management practices on soil fertility and spatial variation of fertility between hedgerows. At a site in the Three Gorges Area, China, we applied five treatments to a contour hedgerow system: control (no fertilizer and manure); chemical fertilizer (CF); chemical fertilizer and mulch (CF + MU); pig manure (PM); and mulch, pig manure, and chemical fertilizer (CF + PM + MU). Soil samples were collected from the topsoil horizon (0–20 cm) of the selected five treatments in 2006 after 11 crop cycles, and physical and chemical properties were analyzed. The results showed that chemical fertilizer clearly improves nutrient status of the topsoil, while pig manure also increased the amount of soil organic matter. This increase in organic matter was associated with an increase in soil aggregate stability, a reduction in bulk density, and reduced penetration resistance of the soil. Mulch with pig manure and chemical fertilizer was the best management practice for improving soil quality and crop yields in the Three Gorges Area. Further, mulch and pig manure addition also decreased the magnitude of the spatial variation, but did not offset the soil fertility gradients because tillage resulted in significant movement of soil. More favorable soil properties were found at the lower positions within each alley, regardless of the management practice applied.     

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.     

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.

Interpreting fertilizer-use efficiency in relation to soil nutrient-supplying capacity, factor productivity, and agronomic efficiency

Although efficient use of N remains a critical constraint to productivity in irrigated lowland rice, a comprehensive database does not exist for the efficiency of on-farm management of N and other nutrients. In 1994, IRRI initiated its Mega Project on Reversing Trends of Declining Productivity in Intensive Irrigated Rice Systems in selected rice production domains of five tropical Asian nations to improve on-farm fertilizer-use efficiency and to monitor long-term productivity trends as related to fertilizers and other inputs. Data are reported here for the first crop cycle, the 1994–95 dry season. The indigenous soil N supply (INS) was estimated by aboveground crop N uptake and grain yield (GY) in plots without applied N established in farmers' fields under otherwise favorable growth conditions. The fertilizer N rate each farmer applied to his/her field surrounding these plots was recorded; GY was also measured in that area. In each domain, GY in unfertilized plots varied considerably among farms, as the range between maximum and minimum values within each domain was at least 2.8 t ha-1, thus of comparable magnitude to mean GY for these plots. Fertilizer N rates varied from 36–246 kg ha-1 across all domains, but their lack of relationship to INS contributed to relatively low fertilizer N efficiency and high variability in efficiency among farms. Mean agronomic efficiency (GY/applied N rate) for each domain was only 6–15 kg grain kg-1 N, while values for individual farmers ranged from 0 to 59 kg grain kg-1 N. Initial data on P and K fertilizer management also suggest highly variable applications at suboptimal efficiency. These results indicate the potential for greater fertilizer efficiency from improved congruence between the indigenous soil supply and applied fertilizer, and emphasize the need for field-specific nutrient management. Although agronomic efficiency and partial factor productivity (GY/applied N rate) can each be used to describe the efficiency of fertilizer applications, a complete analysis of nutrient management should include both terms, grain yield, fertilizer rates, and native soil fertility.