The Intelligent Choices

As mentioned in the introduction, an agroecosystem has a purpose. It is designed to obtain certain goals, and the state of the system at any given point in time is a consequence of an array of intelligent choices by the farmer, complementing the border conditions set up by weather and soils, etc. The following decision matrix (Figure 2) illustrates how decisions made by a maize farmer in sub-Saharan Africa can be supported by basic science knowledge. Note that the chemical analyses are not necessary for every farmer and decision. Instead, typical values for the different organic resources are estimated, and the individual farmer uses the rule of the thumb based on these estimates.

In the upper part of the Figure 2, the general decision matrix is shown. Let us assume that we have leaves from a

N > 2.5%

YES

NO

Lignin < 15%; phenol < 4%

Lignin < 15%

YES

NO

YES

NO

Incorporate directly with annual crops

Mix with fertilizer of high quality organic matter

Mix with fertilizer or add to compost

Surface application for erosion and water control

For N on maize

For N on maize

- Need 2 t /ha as minimum to have an effect (50 kg N/ha)

- Apply more than 5 t /ha to give a reasonable response (1-2 t grain per hectare)

- Do not apply more than 10 t /ha as much of the N may be lost

■ Need 40 kg N/ha of fertilizer N for each ton of potential yield

■ Need 40 kg N/ha of fertilizer N for each ton of potential yield

- Do not apply more than 10 t /ha as much of the N may be lost

Stall-fed cattle roofed, hard floor

Figure 2 Example of farmer's decisions regarding N management for a maize crop in sub-Saharan Africa, using a decision support system for organic N management depending on resource quality, expressed as N, lignin, and soluble polyphenol content. General decision matrix (top), more detailed for N economy in a maize cropping system (bottom). Modified from Vanlauwe B, Sanginga N, Giller K, and Merckx R (2004) Management of nitrogen fertilizer in maize-based systems in subhumid areas of sub-Saharan Africa. In: Mosier AR, Syers JK, and Freney JR (eds.) Agriculture and the Nitrogen Cycle. 124p. SCOPE 65. Washington Island Press.

Stall-fed cattle roofed, hard floor

Figure 2 Example of farmer's decisions regarding N management for a maize crop in sub-Saharan Africa, using a decision support system for organic N management depending on resource quality, expressed as N, lignin, and soluble polyphenol content. General decision matrix (top), more detailed for N economy in a maize cropping system (bottom). Modified from Vanlauwe B, Sanginga N, Giller K, and Merckx R (2004) Management of nitrogen fertilizer in maize-based systems in subhumid areas of sub-Saharan Africa. In: Mosier AR, Syers JK, and Freney JR (eds.) Agriculture and the Nitrogen Cycle. 124p. SCOPE 65. Washington Island Press.

tree, which we know have a low N content and less than 15% of lignin. Then we should mix the leaves with fertilizer or add to compost. Now, in the lower part of Figure 2 we can see that if we look in more detail at the N economy of a maize system, we have other options -maybe add the low N material to the cattle corral (kraal/boma) to trap urine N or feed to livestock to produce higher quality organic inputs. Organic resources belonging to the third column from the left could be fed to livestock and the manure thus produced could belong to the first or the second organic resource class, depending on the management of that manure.

All over the world, farmers make these kinds of choices, based not only on biophysical knowledge and constraints, but also on economic and sociopolitical opportunities and constraints. An agroecosystem is not only controlled by farmers, but also by the society the farmer operates in. Subsidies can make growing products that have no market an intelligent choice for the farmer; lack of money can make fertilization impossible, even if it would be profitable in the long run, or real or imaginary environmental concerns from the society can force a farmer to, for example, abandon fertilizer use, cereal cropping, or pig farming.

Summing up, the agroecosystem, although limited by climatic constraints, is a product of decisions made by generations of farmers, supported by advice from agronomists and extension workers - all within a societal context of values, traditions, and legislation. In fact, the present and future agroecosystems are at least equally dependent on the societal context as on the climate and soil. However, the organisms involved are, as in any ecosystem, products of millions of years of evolution, and crop and animal breeding has only contributed with small, although important changes to the germplasm.

See also: Agriculture Models; Agriculture Systems; Soil Ecology; Soil Erosion by Water; Xenobiotic (Pesticides, PCB, Dioxins) Cycles.

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