Increased attention is now being paid to developing Integrated Plant Nutrition Systems (IPNS) that maintain or enhance soil productivity through a balanced use of mineral fertilizers, combined with organic sources of plant nutrients. IPNS are ecologically, socially and economically viable, and they can lead to sustainable increases in both soil productivity and crop yields. They focus on the seasonal or annual cropping system (rather than an individual crop), on the management of plant nutrients in the whole farming system, and on the concept of village or community areas rather than individual fields.
At farm level, IPNS aim to optimize the productivity of nutrient flows that pass through the farming system during a crop rotation. This means application of external plant nutrient sources and amendments, efficient processing, and recycling of crop residues and on-farm organic wastes that limit plant nutrient losses. In the process, IPNS empower farmers by increasing their technical know-how and decision-making capacity, and promote changes in land use, crop rotations, and interactions among forestry, livestock and cropping systems in support of agricultural intensification.
At village or farming community level, IPNS take into account plant nutrient sources outside cropped areas, including those in irrigation water and flood sediments, livestock manure, and forest litter and organic material that is physically transferred from forest and pastures. IPNS promote rationalization in the transfer of organic matter and plant nutrients from non-cropped to cropped areas, and the mobilization of unused nutrient resources or the saving of valuable nutrient sources diverted as domestic fuel, raw materials for building or for industrial purposes.
Plants obtain nutrients - primarily nitrogen, phosphorus and potassium, but also micronutrients or trace elements - from seven main sources:
Natural soil reserves, in quantities that depend on soil composition and stage of weathering. Only a minor portion is released to plants each year.
Mineral fertilizers, manufactured in liquid or solid form, have a higher plant nutrient content and a lower bulk than organic sources of plant nutrients.
Organic sources, including bone meal and blood, manure, slurry, compost and sewage sludge, can enhance soil water retention and physical condition.
Biological nitrogen fixation is promoted by some micro-organisms able to convert nitrogen in the air to ammonia for use as their nitrogen source.
Aerial deposition, including nitrates in rainwater, ammonia as a gas or dissolved in rainwater, sulphur in acid rain, salts and chlorine in marine spray.
Irrigation, flood and ground water supply nutrients, either naturally or because fertilizers have been added to the irrigation water.
In practice, the quantity of nutrients available for recycling via plant and animal residues is rarely sufficient to compensate for the amounts removed in agricultural products, even in low-productivity farming. In addition, losses inevitably occur, even in the best-managed systems. Consequently, mineral fertilizers have to play a key role in areas where increased agricultural production is required.
Farmers apply plant nutrients only if their effects on crop yields are profitable. The decision to apply external plant nutrients is generally based on economics - price and affordability - but is also conditioned by availability and the production risks involved. The pursuit of higher production has to be balanced against the need to maintain soil fertility and avoid soil degradation. The profitability of adopting IPNS should, however, be viewed over the long term, since improved efficiency in crop nutrient use tends to become apparent only after several seasons.
A number of economic and institutional factors must also be taken into account: the price relationship between plant nutrients and the crops to which they are applied, together with the market outlook for these crops, determines how profitable it will be to use fertilizer; income and the availability of credit decide whether farmers can afford to buy plant nutrients; and lack of security of land tenure can reduce the incentive for farmers to use fertilizers.
Small-scale farmers with few resources are compelled to look for short-term results when applying plant nutrients. The removal of access constraints to markets and production technology, and protection from risks, would allow farmers to use plant nutrients economically and in ways that support sustainable crop production.
Advice on efficient plant nutrition. Providing good advice to farmers and enhancing their decision-making capacity is the best way to promote the sustainable intensification of agriculture.
Advice at plot level should be based on local on-farm experiments that provide information on the impact on crop yields of combined nutrient doses, timing of nutrient supply, and sources of nutrients. The experiments should be simple and are of two types: site-specific trials and validation trials. The data collected from the trials are used to derive nutrient response curves for the predominant cropping systems and conditions. These trials are usually managed by public sector researchers because they require a fair degree of control of variability, as well as a high level of supervision.
Farmer participation in information gathering and decision making is minimal. However, researchers can interact with the farmers and learn about their methods of production. Important considerations in planning site-specific trials are population definition, choice of sites, choice of treatments, experimental designs, and management of the experiment. Sites for the trials should be representative of the farming conditions of the area being studied.
Farm level advice must be based on understanding of the entire farm and its crop rotation system, including production objectives, available resources, home consumption needs, market opportunities and climatic conditions. In addition, advisers need to identify centres of decision making, the organization of land use and crop rotations, and competing demands for scarce resources such as organic material, labour and capital.
Once all potential plant nutrient sources have been identified and possible technologies selected, farmers themselves can determine the correct mix of these technologies to optimize their resource use and meet their production objectives. The new alternatives may involve an increase in labour requirements or financial investments, modification of the farm structure or adaptation of land use and crop rotations. Arrangements will, therefore, need to be made for a complementary supply of external inputs. A technological package which entails the use of plant nutrients from different sources should be tested, through a network of local pilot farms, to verify its appropriateness to the farmers' situation and to evaluate farmers' acceptance.
Village level advice is required in many traditional low input/output farming systems, where plant nutrition management relies on local nutrient resources collected in areas other than those farmed. Improving the transfer of these materials (e.g., litter from forests and fodder from pastures) can enhance the plant nutrient content in the cropped area. Some examples of improved management practices include cultivation of leguminous forest species, alley cropping of forests with pasture crops, mineral fertilization of pastures, regulation of bush fires and development of systems for allocating manure produced by collective herds.
Soil and water conservation can greatly reduce losses of plant nutrients through runoff and leaching. In addition, water harvesting techniques and the development of irrigation will lead to increased efficiency of nutrient use. These investments require the cooperation of the entire village and must therefore be considered while developing efficient plant nutrition management systems.
Poor plant nutrient management by individual farmers results in a decrease in soil fertility because of heavy mining, erosion, silting and deforestation. Similarly, an oversupply of nutrients may pollute drinking water, and poor management of organic wastes is a potential heath hazard. Thus the involvement of the entire farming community is essential in developing a sound plant nutrient management system at village level.
At the village level, farmers' groups can create favourable conditions for the procurement of inputs and access to credit. Farmers' associations, trade connections between these associations and the input sector, as well as traders in agricultural products and banks, should therefore be encouraged to support improved plant nutrition management by farmers.
Research support. Advice on quantities of nutrients to be applied may be based on empirical results from field experiments, on soil or plant analysis, on a nutrient balance-sheet approach, on mathematical models of nutrient dynamics, or on a combination of methods. In the absence of more detailed information, knowledge of the quantities of nutrients removed by crops at the desired yield level provides a starting point for estimating nutrient requirements.
Field experiments are valuable in providing quantitative information on the supply of nutrients from soil and organic residues, and the short-term effects on crop yield from mineral fertilizers supplied in different forms and quantities. This provides a vital basis for advising farmers and for testing and improving advice systems.
Wherever possible, long-term field experiments should be set up so that residual effects of fertilizers and organic sources on crop growth and soil properties can be studied and taken into account when formulating nutrient recommendations. Such experiments can also provide information on interactions between nutrient applications and other agricultural activities, and on the likelihood of unforeseen problems, such as environmental contamination or deficiency of a micro- or secondary nutrient.
Published November 1998