The role and function of organic matter in tropical soils

Soil organic matter (SOM) has many functions, the relative importance of which differ with soil type, climate, and land use. Commonly the most importantfunction of OM in soil is as a reserve of the nitrogen and other nutrients required by plants, and ultimately by the human population. Other important functions include: the formation of stable aggregates and soil surface protection; maintenance of the vast array of biological functions, including the immobilization and release of nutrients; provision of ion exchange capacity; and storage of terrestrial carbon (C). This paper considers the quantity and quality of SOM of soils in the tropics, which are estimated to contain one quarter of the C in the global pool in terrestrial soils, and supports strongly the use of analytical methods to characterizing labile SOM to develop valuable insights into C dynamics. As in other regions, the transformation of tropical lands for agriculture exploits SOM, and in particular nutrient reserves. The process of exploitation is accelerated in the tropics by the necessity to increase agricultural production, largely through agricultural intensification, to overcome inadequate nutrition, to satisfy population growth, and to cope with the limited reserves of arable land. Poverty has an overriding influence on the exploitation and degradation processes. Areas at greatest risk of land degradation are the infertile acid soils of the tropics, which, invariably, are cultivated by the poor. Soil organic matter has a central role in sustainable land management, but perspectives on the roles of SOM differ widely between farmers, consumers, scientists and policy-makers. Some consider SOM as a source of nutrients to be exploited, whereas others can afford to utilize it as a key component in the management of the chemical, biological, and physical fertility of soils. Still others see SOM as a dumping ground for excess nutrients and toxins, or as a convenient store for fossil fuel emissions, particularly CO₂. Farmers need sustainable land management systems that maintain OM and nutrient reserves. Nevertheless, many available practices, whether based on indigenous or scientific knowledge, do not meet social and economic criteria that govern farmer behaviour. Much scientific knowledge about the various roles of SOM does not reach farmers and other decision-makers in a form that can be used easily. The biggest challenge to researchers is to engage with clients to pinpoint gaps in knowledge and utilize new and existing information to devise decision support Systems tailored to their needs. This revised version was published online in June 2006 with corrections to the Cover Date.     

Organic matter turnover and management in low input agriculture of NE Brazil

The storage and release of nutrients by soil organic matter (SOM) is the primary determinant of soil fertility in low-input agriculture of semiarid NE Brazil. Traditional shifting cultivation systems have utilised the SOM built up during the fallow phase to supply nutrients for a cultivation phase of some 4–6 years. In this paper we analyse the turnover, stabilisation and quality of organic matter in landuse systems of NE Brazil. This analysis relies on a review of our own and literature data as well as farmers' perceptions recorded in a survey of 240 farms. Components critical for the understanding of SOM balances and transformations are residue inputs under native and agricultural vegetation, rates and controls of SOM mineralisation under cultivation, controls on SOM accretion under fallow vegetation, and the quality of SOM with respect to nutrient supply. While all these factors are known in outline, the detail of understanding that would be required for fine tuning management systems to be sustainable under present (and increasing) production pressure is still lacking. In particular quantity and quality of organic matter inputs from different vegetation types, controls on SOM stabilisation under different cultivation regimes, and the rates and synchrony with plant demand of nutrient release from mineralising organic matter need to be investigated in further detail. This revised version was published online in June 2006 with corrections to the Cover Date.     

Composting of Disposal Organic Wastes: Resource Recovery for Agricultural Sustainability

One of the major problems of agricultural soils in the tropical regions of the Pacific is the low organic matter content. Because of the hot and humid environment, the soil organic matter (SOM) is minimal due to rapid decomposition. Composted organic material is being applied on agricultural fields as an amendment to provide nutrients and enhance the organic matter content for improving the physical and chemical properties of the cultivated soils. In addition land application of composted material as a fertilizer source effectively disposes of wastes that otherwise are buried in landfills. In our soil program at the University of Guam, we are evaluating the use of organic material as an alternative to synthetic fertilizers. Its goal is to develop management strategies and use available resources for improving crop production while conserving resources and preserving environmental quality. Our case study project is designed to improve soil fertility status by using composted organic wastes and assessing how the nitrogen and other essential nutrients contribute to long-term soil fertility and crop productivity without application of synthetic fertilizers. In our pilot project, compost is produced from wood chips, grinded typhoon debris mixed with animal manure, fish feed, shredded paper and other organic wastes. Mature compost is then applied on the field at the rates of 0, 5, 10 and 20 t/ha as a soil amendment on the eroded cobbly soils of southern Guam. Corn is planted and monitored for growth performance and yield. The effect of land application of composted material on the SOM content and overall soil quality indices are being evaluated in this pilot study.     

Social, economic and policy dimensions of soil organic matter management in sub-Sahara Africa: challenges and opportunities

In recent years and in some situations the status of soil organic matter (SOM) has deteriorated considerably due to long periods of continuous cultivation and limited external inputs in the form of mineral fertilizers. Deterioration of SOM varies by agro-ecological zones, by soil types and by cropping patterns. It is more intense in East Africa, followed by coastal West Africa and Southern Africa and least intensive in the Sahel and Central Africa. It is also more serious in areas under low-input agriculture irrespective of the prevailing cropping system. The major consequence of the decrease in SOM in the tropics is lower agricultural productivity with a direct negative effect on food security. While biophysical dynamics of SOM have been extensively covered in the literature, social considerations have not received similar attention. This paper examines the social, economic and policy factors associated with the management of tropical soil organic matter. Empirical data from a range of environments in Africa show that SOM improvement options yield a positive return to land as well as labour. However, there are a number of constraints. Social constraints are related to the large quantities of organic matter that are required (case of farmyard manure), the competitive uses for the material (case of crop residues), land and labour requirements, and gender-related issues. From a policy stand point, unsecured tenure rights together with price distortions and other market failures may be important constraints. Challenges for sustainable management of SOM are identified. These include management conflicts, land tenure arrangements, the divergence in goals between individuals and society, land and labour requirements, inadequate support systems for land users, profitability issues, the role of subsidies, and the absence of national action plans. A number of opportunities are identified that could enhance the improvement or maintenance of SOM. These include: exploring the need and potential role of community-based SOM management practices; development of an integrated plant nutrient management strategy involving both organic and inorganic inputs; and development of concrete national action plans. It is argued that because externalities of SOM improvement or maintenance extend beyond the farmer's fields, SOM investment may require cost sharing between individuals and the society. Policies on subsidies need to be reconsidered. Research priorities are identified that require closer collaboration between scientists from a variety of disciplines. This revised version was published online in June 2006 with corrections to the Cover Date.     

Soil organic carbon (SOC) management for sustainable productivity of cropping and agro-forestry systems in Eastern and Southern Africa

In Eastern and Southern Africa, the shifting from the no-external input agriculture (shifting cultivation through slash and burn) to intensified agricultural systems has resulted in widespread agro-ecosystems with high soil organic carbon and nutrient depletion. This is quite evident in farming systems with reduced fallow period or those that practice continuous cropping without or with little inputs. Long-term experiments indicate that losses of up to 0.69 t carbon ha⁻¹ yr⁻¹ in the soil surface layers are common. Such losses are commonly reported by farmers engaged in participatory community-based research development projects. This calls for judicious strategies for recapitalization or replacement of these depletions. Such strategies include direct SOC replenishment through addition of organic materials, notably manures, wastes, residues and plant litter; biomass transfer; incorporation of improved fallows in the farming systems. Indirect contribution to SOM replenishment or recapitalization may be achieved through inorganic fertilizations and amendments; legume integration in the production system; and combined inorganic and organic inputs. Research is required to identify inputs that help meet both nutrients availability while contributing to SOC build up at the same time. This revised version was published online in June 2006 with corrections to the Cover Date.     

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.     

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.     

Smallholder farmer management impacts on particulate and labile carbon fractions of granitic sandy soils in Zimbabwe

Crop production in maize-based smallholder farming systems of Southern Africa is hampered by lack of options for efficiently managing limited and different quality organic nutrient resources. This study examined impacts of farmers’ short- and long-term organic resource allocation patterns on sizes and quality of soil organic matter (SOM) fractions. Farmers’ most- (rich) and least- (poor) productive fields were studied for two seasons under low (450–650 mm yr⁻¹) to high (>750 mm yr⁻¹) rainfall areas in Zimbabwe, on Lixisols with ∼6% clay and 88% sand. Rich fields received 0.5–14 Mg C ha⁻¹ compared with <4 Mg C ha⁻¹ for poor fields, and the differences were reflected in soil particulate organic matter (POM) fractions. Organic inputs were consistent with resource endowments, with well-endowed farmers applying at least five times the amounts used by resource-constrained farmers. Rich fields had 100% more macro-POM (250–2,000 μm diameter) and three times more meso-POM (53–250 μm) than poor fields. Application of high quality (>25 mg N kg⁻¹) materials increased labile C (KMnO₄ oxidizable) in top 60 cm of soil profile, with 1.6 Mg C ha⁻¹ of Crotalaria juncea yielding labile C amounts similar to 6 Mg C ha⁻¹ of manure. Labile C was significantly related to mineralizable N in POM fractions, and apparently to maize yields (P < 0.01). Farmers’ preferential allocation of nutrient resources to already productive fields helps to maintain critical levels of labile SOM necessary to sustain high maize yields.     

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.     

Available technologies to replenish soil fertility in East Africa

Low inherent soil fertility in the highly weathered and leached soils largely accounts for low and unsustained crop yields in most African countries. But in particular, the major nutrients, nitrogen (N) and phosphorus (P), are commonly deficient in these soils. This scenario of nutrient depletion is reflected in food deficits and hence the food aid received continuously, specifically in sub-Saharan Africa. Undoubtedly, substantial efforts have been made in the continent to replenish the fertility of degraded soils in attempts to raise crop yields, towards self-sufficiency and export. Such efforts consist of applications of both organic and inorganic resources to improve the nutrient status of soils and enhanced nutrient uptake by crops, provided that soil moisture is adequate. Overall, positive crop responses to these materials have been obtained. Thus in the East African region, maize (staple) yields have been raised in one growing season from below 0.5 t/ha without nutrient inputs, to 3–5 t/ha from various nutrient amendments at the smallhold farm level. However, in spite of the positive crop responses to nutrient inputs, farmers are generally slow to adopt the soil fertility management technologies. In this paper we review the impact of some technologies, focussing the use of nutrient resources of different characteristics (qualities) in relation to improved crop yields, with an overall goal to enhance technology adoption. Thus, inorganic resources or fertilizers often give immediate crop responses, but their use or adoption is rather restricted to large-scale farmers who can afford to buy these materials. Organic resources, which include crop residues, water hyacinth and agroforestry shrubs and trees, are widely distributed, but they are generally of low quality, reflecting the need to apply large quantities to meet crop nutrient demands. Moreover, most organics will add N mainly to soils. On the other hand, phosphate rocks of varying reactivity are found widely in Africa and are refined elsewhere to supply soluble P sources. The recently developed soil fertility management options in East Africa have targeted the efficient use of N and P by crops and the integrated nutrient management approach. Some people have also felt that the repackaging of inputs in small, affordable quantities, such as the PREP-PAC described in this paper, may be an avenue to attract smallhold farmers to use nutrient inputs. Nonetheless, crop responses to nutrient inputs vary widely within and across agroecozones (AEZs), suggesting specificity in recommendations. We highlight this observation in a case study whereby eight soil fertility management options, developed independently, are being tested side-by-side at on-farm level. Farmers will be empowered to identify technologies from their own choices that are agronomically effective and economically friendly. This approach of technology testing and subsequent adoption is recommended for technology development in future.     

The humus balance model (HU-MOD): a simple tool for the assessment of management change impact on soil organic matter levels in arable soils

With this paper we present a simple model for the assessment of management impact in arable farming systems on soil organic matter (SOM) levels. The humus balance model (HU-MOD) is designed for application by farmers and extension workers in practice as a tool for management support. To enable practice applicability, HU-MOD bypasses the need for data on soil parameters and can be run with simple management data. HU-MOD is based on a simplified model on carbon and nitrogen pools and fluxes in the soil–plant system. The model proved to be an applicable simple tool for the comparison of management systems in arable farming with regard to the impact on SOM levels. Even though an absolute quantification of SOM level changes is not possible due to the methodical approach bypassing the need for any data on soil parameters, the model may be used to assess a positive or negative impact of a management system or management period compared to a reference and thus may be used to assess the impact of management changes, or to analyse a specific impact for different management periods on a defined spatial unit.     

The significance of available nutrient fluxes in N and P budgets for maize cropping on a Rhodic Kandiustox: a study with compost,NP fertilizer and stubble removal

Nutrient budgets may be useful tools for nutrient management of crops especially if they estimate the nutrient fluxes available from a variety of sources including organic and inorganic fertilizer, crop residues and soil organic matter. The aim of the present study was to develop a budget of available nutrients by determining the contribution of mineralized nutrient fluxes and fertilizer input relative to nutrient losses and removal in harvested products in the overall N and P balances. N and P inputs and outputs and available N and P fluxes in the soil were estimated for 3 consecutive maize crops where inputs and outputs were altered by NP fertilizer, compost and stubble removal on a Rhodic Kandiustox. A sensitivity analysis of calculated and measured nutrient budget items was conducted to identify the main factors affecting the accuracy of the nutrient balance calculations. Mineral fertilizer rate was the major factor for maize nutrient budgets as shown by its contribution to N and P balances. Without mineral fertilizer application, soil organic matter (SOM) mineralization was the most important within-season nutrient input. In the case of N, shoot uptake was the main output followed by denitrification. Phosphorus adsorption by the soil was the major P output from the available pools followed by shoot uptake. SOM mineralization maintained the pools of available N and P if stubble of the previous crop was returned. Mineral fertilizer application, which produced surplus balances of N and P, would however, be needed to attain high yield, even with stubble return. The available N and P from compost were not significant inputs in the nutrient balances until year 3. Total N and resin extractable P in soil after five crops supported the calculated nutrient balances indicating the importance of available nutrient fluxes in calculating N and P balances.     

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.     

Overcoming soil nutrient constraints to crop production in West Africa: Importance of fertilizers and priorities in soil fertility research

The Alfisols, Oxisols, Ultisols and Inceptisols which dominate the soils of West Africa have sustained crop growth for a very long time. As a consequence, their fertility has become perilously low and the task of increasing or even maintaining the productive capacity of these soils has become perhaps the greatest challenge to agricultural scientists in this latter half of the 20th century.Water is useful not only for the growth of plants but also for the efficient use of costly inputs such as fertilizers. On the other hand, fertilizers increase the water-use efficiency. Such interactions must be closely studied so as to maximize the impact of inputs of agricultural production.The nutrients in the soil are always in a state of flux, with additions and subtractions. Monitoring the dynamics of the nutrients would promote their efficient use by crops and prolong the productive life of the soils.Several models currently exist for the study of organic matter dynamics in soils. These models should be applied to the West African situation since it is vital to develop management practices that can promote efficient use of nutrients released during mineralisation of soil organic matter.Judicious fertilizer use in West Africa should be promoted as this practice will enhance agricultural production while protecting the fragile environment.     

Management of soil organic matter in semiarid Africa for annual cropping systems

Ensuring sustainable agriculture in semiarid Africa requires the implementation of methods to balance nutrients and to conserve soil organic matter (SOM). There is an urgent need to improve the management of all types of SOM input. In this paper; the authors review a wide range of agricultural practices and discuss their advantages, limitations and feasibility. They distinguish ‘traditional systems’ such as traditional fallow, parkland and manuring from ‘improved systems’ such as ‘improved fallow’, forest fallow, alley cropping, cover crops and application of composted manure. Biomass production (BMP) for ‘improved systems’ is mainly linked: (i) for agroforestry, to the tree species used in forest fallow, to the synchronization of nutrient supply by the soil with the cereal demand in alley cropping, and generally to the efficiency of the root system and its development with the depth; (ii) for cover crops, BMP is mainly linked to the initial soil fertility and to the ecological zone: establishment and management of cover crops are not yet fully mastered under some conditions such as an annual rainfall below 800 mm and/or on very clayey soils; (iii) for manure, BMP is mainly linked to the improvement of fallow in order to ensure sufficient forage resources. Because semiarid Africa is mainly a livestock zone, the authors emphasise manure: constraints, quality indicators and tools used to encourage its production, are analysed. Thus it was concluded that the intensification of manure production and its rational use in semiarid African regions, threatened by drought and malnutrition, is very important: this cannot be separated from the production of plant biomass, whose possibilities have been examined above. This revised version was published online in June 2006 with corrections to the Cover Date.     

Influence of plant density on variability of soil fertility and nutrient budgets in low input East African highland banana (Musa spp. AAA-EA) cropping systems

The productivity of East African highland (EAH) banana cropping systems is declining, particularly in areas with low inherent soil fertility. Soil fertility management requires knowledge of nutrient flows at the interface between the soil surface and the soil system. The magnitude of soil fertility dynamics and nutrient depletion was studied for a short-term banana plant density trial in three contrasting agro-ecological sites of Rwanda (Kibungo low rainfall with medium soil fertility, Rubona high rainfall with low soil fertility and Ruhengeri high rainfall with high soil fertility) using nutrient stock and partial nutrient balance calculations. Plant density did not influence significantly nutrient mass fractions in plant parts (fruit, leaves and pseudostems) but nutrients contained through shredded leaves and pseudostems and those removed through bunch dry matter increased with plant density. Plant density responses to variation in soil fertility and partial nutrient balances seemed to depend on diversity in climate and soil type. Partial N and K balances (kg ha^?1 year^?1) were estimated to be strongly negative at Rubona and Ruhengeri while Ca and Mg were positive at Kibungo and Ruhengeri but negative at Rubona. This study showed that partial nutrient balances associated with soil nutrient stocks can provide the first order of magnitude of nutrient depletion in low input EAH banana cropping systems. This brings attention from agricultural researchers and farmers to develop options that can improve the productivity of these systems, where resource availability for improved nutrient management is scarce.     

Soil fertility status and nutrient input–output flows of specialised organic cropping systems: a review

The management of specialised organic crops for vegetable and fruit production require the use of imported fertilizers. A wide range of fertilizers is currently available to organic farmers. These include bulky organic materials with a relatively low nutrient concentration commonly used as base dressing (e.g. composts, solid animal manures) and complementary commercial organic fertilizers with relatively high nutrient concentrations to adjust nutrient supply to crop requirements (e.g. feather meal, hoof and horn meal, vinasse, meat and bone meal, etc.). Nutrient imbalances are a major threat affecting the long term sustainability of horticultural and fruit cultivation systems. Major reasons for these imbalances are the biased element composition of base as well as complementary fertilizers in relation to the nutrient offtakes via harvested products. Gaseous nitrogen losses during manure management and gaseous as well as leaching nitrogen losses after application are major reasons for such nutrient imbalances, as they lead to a relative increase in the concentration of other elements. Conceptual weaknesses in the fertilizer approach in organic farming exist, namely the preferred application of slow release nitrogen fertilizers often rich in phosphorus. This review suggests that the current soil fertility approaches based on application of solid fertilizers and simultaneously a low rate of N inputs via N₂ fixation do not foster balanced nutrient levels. The key challenge is to design cropping systems with a higher share of N inputs via biological N₂ fixation, and to find fertilizers with a nutrient stoichiometry better suited to match the overall specific offtakes of fertilized crops.     

Possible options for mitigating methane emission from rice cultivation

Studies focused on mitigating CH₄ emission from rice paddy fields are summarized and the possibilities and limits that the options might be applied to world's rice cultivation are discussed. The mitigation options are water management, soil amendments, organic matter management, different tillage, rotation, and cultivar selection. Altering water management, in particular promoting midseason aeration by short-term drainage, is one of the most promising strategies, although these practices may be limited to the rice paddy fields where the irrigation system is well prepared. Improving organic matter management by promoting aerobic degradation through composting or incorporating into soil during off-season drained period is another most promising candidate. There are several formidable obstacles to adopt the mitigation options into local rice farming, including limited applicability to different types of rice fields, increasing cost and labor, negative effects on rice yield and soil fertility, and time requirement for practical application. Further studies to verify the mitigation options should focus on the feasibility for local farmers. This revised version was published online in August 2006 with corrections to the Cover Date.     

Soil organic carbon management for sustainable land use in Sudano-Sahelian West Africa

Judged by their negative nutrient balances, low soil cover and low productivity, the predominant agro-pastoral farming systems in the Sudano-Sahelian zone of West Africa are highly unsustainable for crop production intensification. With kaolinite as the main clay type, the cation exchange capacity of the soils in this region, often less than 1 cmol_c kg⁻¹soil, depends heavily on the organic carbon (Corg) content. However, due to low carbon sequestration and to the microbe, termite and temperature-induced rapid turnover rates of organic material in the present land-use systems, Corg contents of the topsoil are very low, ranging between 1 and 8 g kg⁻¹ in most soils. For sustainable food production, the availability of phosphorus (P) and nitrogen (N) has to be increased considerably in combination with an improvement in soil physical properties. Therefore, the adoption of innovative management options that help to stop or even reverse the decline in Corg typically observed after cultivating bush or rangeland is of utmost importance. To maintain food production for a rapidly growing population, targeted applications of mineral fertilisers and the effective recycling of organic amendments as crop residues and manure are essential. Any increase in soil cover has large effects in reducing topsoil erosion by wind and water and favours the accumulation of wind-blown dust high in bases which in turn improves P availability. In the future decision support systems, based on GIS, modelling and simulation should be used to combine (i) available fertiliser response data from on-station and on-farm research, (ii) results on soil productivity restoration with the application of mineral and organic amendments and (iii) our present understanding of the cause-effect relationships governing the prevailing soil degradation processes. This will help to predict the effectiveness of regionally differentiated soil fertility management approaches to maintain or even increase soil Corg levels. This revised version was published online in June 2006 with corrections to the Cover Date.     

Role of manures and crop residue in alleviating soil fertility constraints to crop production: With special reference to the Sahelian and Sudanian zones of West Africa

In the West African semi-arid tropics (WASAT), continuous cultivation leads to drastically reduced levels of soil organic matter. Such reductions in the level of soil organic matter have resulted in decreased soil productivity. The addition of organic materials either in the form of manures or crop residue has beneficial effects on the soils' chemical and physical properties. For many of the countries in this region, the amounts of nutrients in crops and crop residue are often several orders of magnitude higher than the quantity of the same nutrients applied as fertilizers. The return of the crop residue for soil fertility improvement cannot be overstressed. It is essential that more information on the rates of organic matter decomposition as well as the many reactions between products of organic matter decomposition and the soil under WASAT conditions be made available.