Introduction to agricultural systems



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Natural Resource Accounting. People are now thinking that countries should develop methods of accounting for the costs of natural resource depletion, or the depreciation in the national economy associated with the use of resources. Currently in the national accounts, lumber, the equipment and labour used to harvest timber, and the products made from lumber are valued, but forests are not. The GNP grows as forests are converted into lumber, which has a market value, but the depreciation of the natural resource, the forest, is not reflected anywhere. The environmental services that the forest provides are also not accounted for.


Theoretically, a resource-based country could exhaust its mineral resources, cut down its forests, erode its soils, pollute its waters, and hunt its wildlife and fisheries to extinction, but measured income, on a national scale, would not be affected as those assets disappear. The problem is that it is difficult to value natural resources for which there is no market - goods obtain there value by trading in the market place.

How much is soil organic matter worth, and how does its value change as it is lost from the soil (what is its marginal value)? How much are forests worth as potential sources of lumber and jobs? How much are oil and gas in the ground worth? How much is clean air worth? These questions are difficult to answer, especially on a national scale.


Example: Soil organic matter. The value of soil organic matter may be reflected in the value of land. If farmers are willing to pay more for land with high quality soils in which the organic matter has been conserved than for land with eroded and degraded soils with low fertility, then the value of the organic matter has been recognized by farmers.

But how do governments assess the cost of soil degradation to the productive capacity of the country in the future? What is required is a method of determining the depreciation associated with various land uses and applying the depreciation against the value of crops produced from the land. This would show whether the country is drawing down, rather than building up, its soils and capacity to produce food at a reasonable cost in the future. Are current increases in farm output being achieved at the expense of future output? By ignoring the future costs of land degradation, we overestimate today=s income; we chose to pay the degradation costs in the future rather than now.

Gross domestic product (GDP) and gross national product (GNP) are short-term measures of economic activity for which exchange occurs in monetary terms. They cannot be used to measure long-term sustainable growth because natural resource depletion and degradation are being ignored under current practices. Countries are now trying to develop accounts of natural resource stocks and stock changes for determining national balance sheets. Two approaches are used: the physical approach and the monetary approach.


The Physical Approach keeps track of various Adeliveries@ of resource stocks to producing and consuming sectors (use and depreciation of natural resources) and Adeliveries@ of materials from producing and consuming sectors to receiving bodies in the environment (pollution).

Economics is generally based on production and consumption, so this method fits well with common economic models. However, it does not solve the problem of determining the value or environmental effect of a certain amount of resource use, depreciation or pollution, only the amount of it.



The Monetary Approach tries to put values on non-market resources. For example, agricultural output, yields, and income reflect the environmental inputs of solar energy and precipitation that increase the value of purchased inputs (fertilizer and machinery). Increased concentrations of ozone, acid rain, or salt dust from a potash mine reduce agricultural yields (or increases production costs) and thus income in the agricultural account. This method can be used to reflect their value indirectly in the national accounts. The direct value of environmental quality is not counted - the effect of salt dust on health, ruined hydraulics, or willingness to pay to live near a potash mine - are not known.

More information about natural resource accounting is available in Repetto et al., 1989.




3. AGRICULTURAL SYSTEMS
We have discussed biophysical systems and socio-economic systems. To understand agricultural systems, we need to incorporate both the biophysical and the economic view. Agricultural systems are economic and biophysical systems.

Examples of Agricultural Systems


  1. A rice paddy in Indonesia

  2. A feedlot in Colorado

  3. A potato farm in Ukraine

  4. A wheat field in Saskatchewan

These are all parts of different agricultural systems.

A wheat field in Saskatchewan? What is missing?



  1. people, tractors, trucks hauling fertilizer and pesticide in and wheat out, grain elevator, rail road, town, school, i.e., any information about the people

  2. temperate climate, one crop per year, young soils, i.e., any information about the biophysical environment.

How is a rice paddy in Indonesia different?

  1. lots of people, bullocks and water buffalo, but not many tractors, local consumption and marketing;

  2. tropical climate, two or three crops per year, not as much use of fossil fuels, old soils

How is a feedlot in Colorado different?

  1. a few people, a lot of equipment, grain and hay go in, meat and manure go out

  2. temperate climate, continuous production

  3. problem: what to do with manure

How is a potato farm in Ukraine different?

  1. people, machinery and equipment, but it may be difficult to get repairs, transportation system, but may not function well, fossil fuel based,

  2. problem: contamination by radionuclide fallout

  3. temperate climate, one crop per year

Agricultural systems are land, energy and people. Agricultural systems are complex (Figure 3-1). The complexity of agricultural systems arises from the interaction between social, economic and ecological processes which shapes agricultural systems. Agricultural is a cultural behaviour, involved in the management of plants, animals and ecosystems. It is subject to the same biological laws as natural systems (Vasey, 1992, p. 4).


Agricultural systems are diverse (Figure 3-2). Cereal production systems in Saskatchewan are very different than banana production on plantations in Guangzhun, China. Why? A major reason is that agricultural systems are a product of their environment, which is everything surrounding the system that interacts with it. Since physical, social, and economic environments throughout the world are very diverse, agricultural systems are diverse.

Farming systems are adaptations to the environmental conditions of the ecoregions of the world. They have evolved as methods of survival, tested over thousands of years. People use farming practices that provide them with enough food and fibre, given the amount of energy and labour they have available, to satisfy their needs and the needs of the whole community. The amount of energy available is determined by climate (solar energy) and by technology, which includes fossil fuels, machinery, and fertilizers.

Agricultural systems can be defined in terms of basic trophic chains (Figure 3-3, Loomis and Conner, 1992, p. 17). All of the trophic pathways in agricultural systems end with human consumers. Humans derive most of their food from plants, but they also eat considerable amounts of herbivores (game, domesticated animals and their products).

The goal of most agricultural systems is to increase the net primary productivity of the ecosystem, i.e., the amount of food produced per unit area and unit of input. The land or ecosystem is the biophysical resource base of the system, and determines its net primary productivity or potential productivity. The potential productivity can be enhanced by the use of subsidies, such as fossil fuels and fertilizers, which increase the energy of the system. The agricultural infrastructure, which includes the marketing, processing and distribution of food, is a mainly human component of the system, which governs the transfer of food from the people who produce it to consumers (Figure 3-4, Smith, 1974, p. 76).

Agriculture is both a biophysical and a human system, with the agricultural infrastructure is overlayed on the biophysical system. The basic ecological processes, such as photosynthesis and nutrient cycling remain, but they are overlaid and regulated by agricultural processes of cultivation, energy and capital subsidy, control, harvesting and marketing and (Figure 3-4, Smith, 1974, p. 76). Producers attempt to increase potential productivity by pushing the limits of the natural system, but ultimately, agricultural production is subject to the same environmental limitations as natural systems.

Agricultural systems are ecological systems modified by human beings to produce food, fibre, and other agricultural products. Inputs, such as seed, fertilizer, machinery, fossil fuel, precipitation, and solar energy are transformed into outputs, such as food (Figure 3-4). Natural systems also convert inputs (solar energy, CO2, and nutrients) into output (plants and animals). The difference is that in agricultural systems the conversion is managed and the quality and quantity of output is predictable. In other words, they are purposeful systems.

In purposeful systems, the goal or output can be changed even though the biophysical systems remains constant. They can also pursue the same goal in different environments by using different strategies. Natural systems cannot do this. A given environment at a given stage of succsession produces what is Acorrect@ or what has evolved and adapted to that system.

Agriculture involve several kinds of activity: crop production, conversion of crops to livestock, processing to transform crops or livestock products into a factory product, procurement activities including investments and farm maintenance works (drainage, fencing), and marketing. Each location, with its particular climate, soils and economy (price relations) is best suited to only a few crops. Thus, while natural systems are very diverse, farming areas are differentiated into areas that specialize in a few crops within each large natural biome. Only 15 species provide most of the world's food.

Being purposeful, agricultural systems exist because someone wants them to. They are the product of the knowledge, skills, attitudes and values of people, and represent the interaction between people and the physical and biological resources available to them.


The purpose of agroecosystems is increased social value. There is a connection between the goals of an agricultural system, the properties of the system and the human values involved, and those connections may include conflict among humans (i.e. producers versus consumers).

Succession. One way to demonstrate the purposeful nature of agriculture is to compare levels of succession in natural and agricultural systems. Agricultural systems deliberately mimic natural systems in early stages of succession (Figures 1-7 and 1-8). The two major reasons for this, reflect the major goals of agriculture:

i. to keep production high, which is achieved through constant disturbance (tillage, harvest), and

ii. to keep species diversity low (monoculture cropping and pesticides to control weeds).

In agricultural systems, the production of seeds is high, but overall production and biomass are low, and nutrient cycles are poorly developed. Soil organic matter content is reduced by constant disturbance and very high rates of microbial decomposition. As a result, nutrient cycling systems are inefficient and leaky, with leaching (N) and atmospheric (C) losses from the system.

Succession is a very slow process and humans are not good a perceiving very slow change. Because of our ignorance of slow, subtle, but very powerful and, in human-time-scales, irreversible processes, we are vulnerable to making catastrophic errors on a gigantic scale. Many of the environmental problems associated with our crop and forestry practises, such as loss of species diversity, instability and leaky nutrients cycles occur because these systems are maintained in early stages of ecological succession.

3.1 Land, Energy and People in Agricultural Systems (Vasey, 1992, p.12)

3.1.1 The land resource base.

Climate. Climate determines the amount of solar energy, seasons, precipitation, precipitation distribution, and the rate of evapo-transpiration of a region. All of these factors are important determinants of the potential productivity of the region.

Soil. Soil is not uniform, it is heterogeneous. Soil is part of the land, or ecosystem, which is a product of the whole environment. There are five soil-forming factors: heat, moisture, plants, time, and people. Heat, moisture, and plants create soil by accelerating the rate of weathering of parent material, but chemical weathering is a process that takes time. High heat, or high rainfall climates result in higher rates of weathering than cool dry climates. Soils are therefore soils are more highly weathered and more strongly developed in the Tropics, and less weathered and more fertile in the Temperate zones.

There are many different types of soils throughout the world, or even within Saskatchewan. Figure 3-5 shows the soil zones in Saskatchewan, ranging from dry Brown grassland soils that are low in organic matter, and have thin A horizons to moist Black grassland soils with high organic matter, and thick A horizons, to forest soils with very low organic matter, and more highly weathered profiles.

Soil is the unique combination of amount and type of minerals, organic matter, water and air that provides a rooting medium, a source of plant available nutrients, a habitat for microorganisms, water filtering and storage.

Plants and crops. What is the difference between plants and crops? Crops are groupings of domesticated plants, cultivated and harvested for a specific purpose, such as food, fibre, feed, medicinal, or ornamental uses.


3.1.2 What to plants need?
The Law of the Minimum. The growth and productivity of plant or crops is controlled by requirement that is most limiting. On the Prairies, either water or heat are usually most limiting, but if the area is irrigated, another factor, possibly a nutrient such a N, will become the limiting factor.

Heat. Plants are adapted to specific temperature regimes. Plants adapted to cool temperatures will not do well in hot climates and warm climate plants will not thrive in cold regions. The threshold temperature is the minimum temperature at which a plant commences growth. The rate of growth increases above that temperature until an optimum is reached and then declines at temperatures above that range. Crops such as oats, barley, rye and peas commence growth at ~5o, but most temperate crops have a threshold of about 10o. To mature, a plant requires a minimum amount of accumulated heat above its threshold and a sufficiently long growing season to set seed. Early crops require less accumulated heat to mature than mid- or late season crops. Many crops that are grown in tropical regions with long, warm growing seasons would not accumulate sufficient heat to mature on the Prairies.

Heat interacts with precipitation to determine the effectiveness of the precipitation, and the evapo-transpiration rate. As temperature increases, a given amount of rainfall is less effective as a source of plant available water. Why? At higher temperatures more water is evaporated and less is available for transpiration by the plant. For example, 35 cm of precipitation per year in hot Mexico results in dessert conditions, whereas in temperate Saskatchewan the result is a mixed grassland.


Light. Photosynthesis increases in response to increasing light up to the point of light-saturation, after which more intense light will not increase photosynthesis. Light can be limiting where a plant is shaded by other plants. Generally, both natural plant communities and crops under-utilize incoming solar radiation, so it is generally not a growth rate limiting factor.

Water and Air. The water and air content of the soil environment are usually considered together because they both occupy the pore (or void) space of the soil. A soil completely filled with air cannot accommodate water and a completely saturated soil has no air. Plant roots require both oxygen and water, so they growth best in a soil that is near field capacity, with roughly half of the pore space occupied by air and half by water.

Water is a growth limiting factor in arid and semiarid ecosystems, and occasionally in humid areas. Sandy and gravelly soils have a low water holding capacity and can rapidly become water limiting.



Nutrients and Nutrient Cycling. Carbon is the most abundant nutrient in plant tissue, but many other nutrients are necessary for plant growth, including the macronutrients, N, P, K, Ca, Mg, and S, and the micronutrients, Fe, Cu, Zn, B, Mb, Co, and Cl. Carbon and N originate from the atmosphere, whereas the other nutrients are derived from the soil parent material. Carbon is fixed in plant tissue through the process of photosynthesis (Section 2.1.4), and atmospheric N enters the soil system through N fixation by Rhizobia microorganisms in the roots of leguminous plants.

Nutrients that enter the soil in plant residues are in organic form, as compared to nutrients that weather out of mineral material, which are in mineral form. Since most C and N enter the soil as plant residues, they are most commonly found in the soil in organic form.

Whether they are in organic form and part of the soil organic matter, or in mineral form and part of the soil parent material, nutrients are steadily transformed in the soil into available forms that plants can use. After they are taken up by the plant, they are returned to the soil as plant residue when the plant dies, and become incorporated into the soil organic matter. Microorganisms Aeat@ soil organic matter because they use the fixed C it contains as a source of energy, a process which is called decomposition. In the process of decomposition, microorganisms consume C and the other nutrients they need for growth. If the nutrient content of the organic matter is greater than they require, they excess nutrients are released into the soil and can once again be taken up by plant roots. Thus, soil organic matter and soil fertility are closely linked.

The soil nutrient status of a soil depends on its parent material. The amount of nutrients in the minerals of which it is composed, its age (soils lose nutrients over time by leaching and erosion), clay content and climate, which determines the rate of leaching and erosion, together determine the nutrient-richness of a soil. In agricultural systems where crops are harvested and removed from the field year after year without replacement of the harvested nutrients, available nutrient levels will steadily decline (Figure 3-6 from Tivy, 1990, p. 66). Nutrients cannot be exported from the soil forever.


  1. Carbon. Humus, organic matter that has been decomposed by soil microorganisms and is no longer recognizable has plant tissue or cells, is important for the storage of nutrients, for soil structure, cation exchange capacity, and moisture holding capacity. The amount of humus that accumulates in the soil depends on how much plant material (organic carbon) is added to the soil, how rapidly it is decomposed to form humus, and how rapidly the humus is mineralized to release nutrients. These processes all depend on the activity of soil microorganisms - the decomposers of the food web.

Organic C levels in the soil are usually lower in agricultural systems than in natural systems. Major reasons for C losses from agricultural soils are disturbance due to cultivation which result in increased rates of organic matter decomposition, increased rates of soil erosion, and a reduction in the amount of plant residues returned to the soil.

The loss of stored C from agricultural soils is a contributing factor to the increased levels of CO2 in the atmosphere. Atmospheric CO2 was 350 ppm in the 1950s and is about 380 ppm now (Figure 3-7 - C loss from soil). This is important because CO2 is a greenhouse gas which means it has a role in the potential for global warming, or global climate change. Increased levels of C in the atmosphere could increase plant production through increased rates of photosynthesis, if moisture and other nutrients are available to support increased growth. Some scientists speculate that as CO2 increases, plant production will increase, tying up more C in plant biomass, keeping the system in balance. However, large areas of forest and other native systems are being harvested and replaced with agricultural or agroforestry systems that store less C than natural systems. The net effect may be increased plant growth rates, but fewer plants in agricultural crops rather than dense and diverse native systems, with less C fixed in plant tissue and a net increase in the atmosphere.


Increased C fixation in plants in a high C atmosphere may also change the C:N ratio of plant tissue. Alteration of C:N ratios could affect the decomposition rate of plant residues. The consequences of such a shift are still unknown.

  1. Nitrogen. Nitrogen is the most commonly growth-limiting nutrient (Figure 8 Nitrogen cycle from Cox and Atkins, 1979, p. 47). There are two mechanisms of N fixation:

i. chemically in the production of N fertilizer, a highly fossil fuel intensive process; and

ii. microbiologically by cyanobacteria and free-living bacteria, mainly Rhizobium species associated with legumes.



  1. Phosphorus. Phosphorus is the next most limiting nutrient If water and N limitations are removed, crops will often respond to additions of P fertilizer.

Agricultural systems that relied on specific crops, livestock and methods of cultivation evolved. As people moved, so did crops and the knowledge of how to grow them (i.e., European settlers brought their agricultural systems to North America). The limitations to the establishment of old crops in new environments were quickly realized; as crops were introduced into an area, selection began for those plants which did best in that environment and new genotypes evolved. The cropping systems that we have today are a reflection of this development or adaptation to "environment".

Agricultural systems have less species diversity, and less genetic diversity within each species than natural systems (Cox, 1984, p. 188.). All plants are of one crop and tend to be the same height, size, age and nutritional state. Cropped systems rely less on ecological organization for maintenance of soil fertility, they involve less cycling of nutrients, and they have higher losses due to leaching, erosion and export. Mono-cropping practices and lack of genetic diversity mean that they are less biologically stable than natural systems.



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