Integrated Nutrient Management for Crop Production
Clive A. Edwards and Thurman Grove
All agricultural systems must have sources of nutrients if they are to produce crops. Prior to the discovery of inorganic fertilizers in the nineteenth century, soil fertility and nutrient supply were maintained by returning organic matter to the soil and through regular rotations and fallow periods. The work of Liebig, summarized in his book Organic Chemistry in its Applications to Agriculture and Physiology (1840), and the experiments of Lawes and Gilbert in the mid-1800s at Rothamstead, England, led to a progressive expansion in the use of inorganic fertilizers containing nitrogen, phosphorus, potassium, and other minor nutrients. Inorganic fertilizers enabled farmers to grow crops in much closer sequence and ultimately in monocultures, and they facilitated the separation of crop and animal production. As a result, crop production in many areas is highly dependent on inorganic sources of nutrients. Parallel to the development of inorganic fertilizers, a progressive expansion occurred in the breeding of high-yielding crop varieties that respond well to high inputs of inorganic nutrients.
The combination of inorganic fertilizers and new crop varieties has greatly increased crop yields. There has been, for instance, an almost threefold increase in crop yields in Europe and the United States since World War II, and yields have more than doubled in developing countries where the green revolution has taken place. At the same time, use of animal manures and other organic sources of nutrients has steadily decreased, which has often
created organic waste disposal problems for intensive animal production systems in developed countries. Decreased use of organic inputs, increased use of inorganic nutrients, and reduced rotations have ultimately led to the growing of crops such as maize in monoculture. These, in combination with heavy soil cultivations, have led to extensive wind and water erosion of poorer soils in the United States and many other parts of the world.
Clive Edwards is professor, Department of Entomology, The Ohio State University. Thurman Grove is program officer for agroecology and environment, Winrock International Institute (formerly with the Agency for International Development).
A considerable degree of polarization has arisen between conventional high-production farmers who depend on inorganic fertilizers for nutrient supply and those who avoid using them for what they perceive to be environmental and ethical reasons. The latter, commonly called organic or biodynamic farmers, base their crop production on organic sources of nutrients and rotations.
In many developing countries, where soils are poorly structured and low in base fertility, and where the availability of inorganic fertilizers is limited, crop production has depended on periodic clearing of the forest and cropping for only 1 to 3 years—a practice commonly known as slash-and-burn agriculture. Traditionally, this method involved cropping the area only once over the 12- to 15-year “rotation.” In recent years, population pressures have reduced the interval between cropping phases, and this method has begun to fail.
THE LESSON FROM INTEGRATED PEST MANAGEMENT
The experience with arthropod pest, disease, and weed control has been remarkably similar to that of nutrient provision. Prior to World War II, pests were controlled mainly by rotations and the use of cultural techniques. The development of extremely effective insecticides, fungicides, and herbicides in the 1940s transformed agriculture and led to virtually complete dependence on pesticides. Not until the 1960s was it realized that extensive use of broad-spectrum pesticides, often applied over large areas from the air, had led to major environmental problems.
Beginning with the introduction of the concept of integrated pest management (Stern et al., 1959), there was a systematic movement toward the use of improved pesticide formulations and localized applications of minimal amounts of pesticides, combined with appropriate cultural and biological control techniques. This trend still continues, in both developed and developing countries, and it has led to significant decreases in the amount of pesticides used on many crops.
Integrated pest management holds a clear lesson for nutrient provision and management. If sustainable agriculture and natural resource management is to be promoted on a global basis, similar principles must be developed for the provision of nutrients. The use of minimal amounts of inorganic fertilizers—applied as a “topping off” only when necessary, placed in
the crop row where they will contribute only to crop growth and not to weed growth, and timed for crop needs—can be combined with plant and animal organic inputs to provide an integrated nutrient management program based on principles similar to those employed in integrated pest management programs. Although agroecosystems around the world differ greatly in soil fertility, soil structure, organic matter status, and climate, the need for pest, disease, and weed control and nutrient supply are common to all systems, and the same principles can be applied to all systems to minimize off-site inputs and maximize conservation of natural resources on a global basis.
There is increasing evidence that the critical inputs for pest, disease, and weed control are the encouragement of all forms of biodiversity and the availability of organic matter. Both inputs are critical to increasing the diversity of soil organisms, which in turn are very important to providing alternative prey and hosts for insect pests and diseases, and increased competition for weeds. Increased diversity also builds up large populations of natural enemies of insect pests, diseases, and weeds, so that they are held in check by biological pressure and do not reach serious levels.
There are many ways to increase biodiversity through cultural practices, including rotations, undersowing, alley cropping, and strip and contour cropping. There are also many ways to provide organic matter, such as plowing crops and crop residues into the soil and using animal manures and a wide range of other organic wastes from industrial sources. These practices can be complemented by the use of minimal amounts of pesticides, used in optimal ways and combined into integrated management programs.
Biodiversity and availability of organic matter are also the critical factors in the availability of nutrients for crop growth. Biodiversity of crops and cropping patterns provide a broad nutrient base and promote highly active soil microflora and fauna, which can spur the breakdown of dead plant and animal materials and the release of the nutrients they contain. In particular, legumes can be the main sources of the nitrogen essential for crop growth, and rhizosphere organisms, such as vesicular arbuscular Mycorrhizae, can increase the availability of phosphorus. Supplying organic matter is the key to providing the essential nutrients a crop needs from biological sources. As is the case with pest management, the nutrient supply from natural sources can be supplemented with carefully applied inorganic fertilizers, as required, to put together an integrated nutrient management program.
The above concept of nutrient management differs greatly from the concept commonly followed—that the main useful source of crop nutrients is inorganic fertilizers. The practices involved in an integrated nutrient management program maximize biological inputs to crop production and minimize the use of inorganic amendments so as to create a much more sustainable
pattern of crop production, not only ecologically and environmentally, but also economically and socially.
DEVELOPMENT OF AN INTEGRATED NUTRIENT MANAGEMENT PROGRAM
The steps needed for the practical integration of nutrient management are analogous to those used in integrated pest management. In essence, three steps are required:
Assess the nutrient status and needs of the system.
soil type and potential for proposed cropping
Establish an economic threshold.
availability of inorganic amendments
cost of nutrient input (chemicals, manures, and labor)
potential financial return
Develop a nutrient management strategy.
minimal inorganic chemical needs and optimal timing and placement
nutrient supply from plant inputs
nutrient supply from animal inputs
nutrient inputs from other potential sources of nutrients
Such an integrated nutrient management program is a critical component of the type of integrated farming systems that are essential for the development of sustainable agriculture and natural resource management (Edwards, 1989; Edwards et al., 1990).
Edwards, C. A. 1989. The importance of integration in sustainable agricultural systems Agriculture, Ecosystems, and Environment 21:25–35.
Edwards, C. A. R. Lal P. Madden R. H. Miller and G. House, G. eds. 1990. Sustainable Agricultural Systems. Ankeny, Iowa: Soil and Water Conservation Society.
Liebig, J. 1840. Organic Chemistry in Its Applications to Agriculture and Physiology London: Taylor and Walton.
Stern, V. M., R. F. Smith, R. Van den Bosch, and K. S. Hagen. 1959. The integrated control concept. Hilgardia 29:81–101.