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Chapter 5. Are Agroecosystems Sustainable in Semiarid Regions?

Chapter 5. Are Agroecosystems Sustainable in Semiarid Regions?

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billion in 1950,4.7 billion in 1983 (a 1.9% average annual increase), and is projected to reach 6.1 billion in 2000 (a 1.6% annual increase) (World Bank, 1986).

Agricultural production, however, increased even faster, doubling between 1950

and 1980. This increased production resulted in about a 5% increase in per capita

daily calorie consumption. People in developed countries consume approximately 3315 calories per capita compared to only 2180 in developing countries. Approximately 2300 is generally considered the minimum requirement (FAO, 198 1).

From the beginning of agriculture until approximately 1950, increased food

production resulted almost entirely from an expanded cropland base. Since 1950,

however, the yield per unit of land area for major crops has increased dramatically. Much of the increase in yields is due to increased inputs of energy. Between

1950 and 1985, the farm tractor fleet quadrupled, world irrigated area tripled, and

fertilizer use increased ninefold. Between 1950 and 1985, total energy used in

world agriculture increased 6.9 times (a 5.7% average annual increase) (Brown

and Postel, 1987). Some of the agroecosystems that propelled food and fiber production during this period are not being sustained. In addition, there are limited areas, particularly under favored environments, where further development can occur. Consequently, agroecosystems will become increasingly present on marginal

lands in semiarid regions. Semiarid regions are characterized by insufficient precipitation, low soil fertility, and a rapid loss of arable land to soil degradation. An

imbalance between natural resources, population, and basic human needs exists in

these regions. The warm seasonally dry tropics occurring primarily in subSaharan Africa, Southwest and Southeast Asia, Central and South America, and

Northern Australia contain more than 1.5 billion people (Technical Advisory

Committee, 1990). Several of the countries in these regions have population

growth rates among the highest in the world. Kanemasu et al. (1990) estimated

that the semiarid tropics contained 13% of the world’s land and were inhabited by

15% of the world’s people but produced only 11% of the world’s food.

There are growing concerns about the ability of the agricultural community to

continue production of food and fiber at a rate sufficient to meet the demands of a

growing population; this is particularly so if income growth results in dietary

changes that require more grains for meat, eggs, and milk.

Brown (1995) summarized the strategy developed at the 1994 Conference on

Population and Development in Cairo. He concluded that their strategy reflects a

sense of urgency-a feeling that unless population growth can be slowed quickly,

it will push human demands beyond the carrying capacity of the land in many

countries, leading to environmental degradation, economic decline, and social disintegration. Some important trends that Brown emphasized include a significant

decline in the rate of irrigation development, a leveling off of worldwide fertilizer usage, and a slowing of cropland productivity.The world’s irrigated land in 1950

totaled 94 million hectares but increased to 140 million by 1960 (a 4.5% average

annual increase), to 198 million hectares by 1970 (a 3.5% average annual in-



crease), and to 271 million hectares in 1985 (a 2.1% average annual increase)

(Rangely, 1985). Since 1985, however, the rate of expansion has slowed to less

than 1% per year (Council for Agricultural Science and Technology, 1988). Worldwide, fertilizer use increased 10-fold between 1950 and 1989, when it peaked and

then began to decline. During the following 4 years it fell 15%, with the decline

concentrated in the former Soviet Union following the withdrawal of subsidies

(Brown, 1995). The rise in rice yield per hectare, which halted in Japan a decade

ago, is now slowing nearly everywhere, edging up only 2% from 1990 to 1994

(Brown, 1995). Another major concern in some countries, particularly China and

India, is the loss of cropland to industrialization.

All these factors are causing the agricultural community to question all agricultural ecosystems regarding their sustainability. The question of sustainability is

especially relevant to agroecosystems in semiarid regions. Are such systems sustainable over the long term? The short answer is yes because there are many examples in which systems have been sustained for centuries. However, there are

also many examples in which the development of agricultural ecosystems has led

to severe soil degradation and a collapse of the agroecosystem. The objective of

this treatise is to examine some of the more important soil-plant-water relationships in semiarid regions so that the steps required to sustain agroecosystems in

these areas will be better understood.


Agroecosystems are ecological systems modified by human beings to produce

food, fiber, or other agricultural products (Squires, 1991). In agricultural development, natural ecosystems are transformed for the purpose of food and fiber production. An agroecosystem is a complex of air, water, soil, plants, animals, microorganisms, and everything else in an area that people have modified for agricultural

production. Crops and livestock are the major components of agroecosystems.

An agricultural ecosystem such as a wheat field may be more productive than

the native grass that originally occupied the same land. A greater percentage of the

wheat produced is fit for human consumption than is the grass, but wheat is not

self-sustaining and may be relatively vulnerable to external disturbances. Regular

inputs of energy and nutrients are required to maintain production levels. For example, the soil may be too infertile for continuous crop growth unless it is fortified with fertilizers. Agroecosystems therefore tend to be less sustainable than natural ecosystems unless sustainability is very carefully designed into the system.

Squires (1991) emphasizes that there are three qualities of agroecosystemsproductivity, stability, and sustainability. The productivity of an agroecosystem is

not determined simply by the yield potential of the particular crop or livestock that



is used. The yield that actually occurs depends on the climatic and nutritional (soil)

environment, which is, in part, a consequence of how farmers manage. Productivity is therefore a consequence of the processes in the total interactive agricultural-environmental-social system.

Stability is the reliability or consistency of farm production. It is important because people depend on a certain amount of production year after year. Fluctuation is a normal part of all ecosystems and agroecosystems are no exception. Agricultural production often fluctuates from year to year, and this is particularly true

for agroecosystems in semiarid regions. Cropland in semiarid regions is often

more marginal and precipitation is both lacking and highly sporadic. Although the

risks of partial or complete crop failure are an unavoidable part of farming in semiarid regions, farmers place a high priority on minimizing risk.

Sustainability concerns whether a given level of productivity can be maintained

over the long term. Sustainability is defined as the ability of an agroecosystem to

maintain productivity when subject to a major disturbing force (Squires, 1991).

Salinity, toxicity, erosion, indebtedness, or declining market demand are examples

of such forces. Sustainability thus determines the persistence or durability of an

agroecosystem’sproductivity under known or possible conditions. Sustainability has

a variety of measures associated with various measures of productivity. Some measures of sustainability can be high while others are low for the same agroecosystem.

Although it is desirable for an agroecosystem to be high in all three qualitiesproductivity, stability, and sustainability-the three may conflict. An agroecosystem can often be managed in such a way that the short-term gains are very beneficial, but the long-term consequencesare devastating. Identifying and understanding

the key relationships among these qualities are essential and require that appropriate questions be developed, asked, and answered. The key questions for semiarid

regions are: (i) How can semiarid agriculture be intensified? (ii) Is this possible or

desirable considering possible land degradation and other environmental consequences that threaten its long-term success? and, perhaps even more important, (iii)

How far can agroecosystems be extended into a region of increasing marginality?


Semiarid is a comparative term implying a moisture state intermediate between

truly arid conditions and others that are more humid. Semiarid regions have four

unique characteristics. The first is that no growing season will have nearly the same

amount, kind, or range of precipitation as the previous season; and the temperature average, range, and extremes will also be vastly different. Second, crops cannot be planned or managed the same from season to season. Third, and perhaps

most important from a sustainability standpoint, the soil resource base and water



holding capacity does not remain the same for any long period once an agroecosystem is introduced into a semiarid region. Fourth, abundant sunshine and

cloud-free days induce rapid growth when moisture conditions are favorable, but

these conditions cannot be sustained through the season, thereby demanding careful soil water management.

Semiarid regions typically receive substantial precipitation for at least a few

months of the year-enough to replenish soil water content to levels sufficient to

produce amounts of biomass that far exceed those produced in arid regions. Dryland farming systems are common in semiarid regions. Dryland and rain-fed are

often used synonymously, but they are considered vastly different by some workers. They both exclude irrigation, but beyond that they can differ significantly.Dryland agroecosystems in this paper are based on the concept presented by Stewart

and Burnett (1987) that elucidates the differences between dryland and rain-fed

systems. Stewart and Burnett stated that dryland agricultural systems emphasize

water conservation, sustainable crop yields, limited inputs for soil fertility, and

wind and water erosion constraints. Rain-fed systems, although they include dryland systems, can also include systems that emphasize disposal of excess water,

maximum crop yields, and high inputs of fertilizer. Stewart and Burnett (1987)

stressed that dryland farming in semiarid regions has been and will continue to be

a high-risk undertaking and that the key to success is the use of systems and practices that take advantage of the favorable years.


The United Nations Conference on Desertification (UNESCO, 1977) defined

bioclimatic zones based on the climatic aridity index: PIETP, where P is precipitation and ETP is potential evapotranspiration calculated by the method of Penman

(Doorenbos and Pruitt, 1977),taking into account atmospheric humidity, wind, and

solar radiation. The zones established by the conference were as follows:

The hyperarid zone (PIETP = <0.03)-consisting of areas largely void of

vegetation except for ephemerals and shrubs in riverbeds and that are virtually


The arid zone (0.03 < PETP < 0.20)&comprising dryland areas with sparse

perennial and annual vegetation utilized mainly in pastoral systems.

The semiarid zone (0.20 < PIETP < OSO)-including steppe or tropical

shrubland with a discontinuous herbaceous layer and increased frequency of

perennials where dry land farming is widely practiced.

The subhumid zone (0.50 < P E T P < 0,75)--characterized by more dense

vegetation, where rain-fed farming is widely practiced with crops adapted to

seasonal drought.




The Food and Agriculture Organization of the United Nations (FAO, 1978) used

the growing period as the basis for assessing climatic resources in developing

countries. The growing period is the number of days during a year when precipitation exceeds half the potential evapotranspiration, plus a period required to use

an assumed 100 mm of water from excess precipitation (or less, if not available)

stored in the soil profile. Any time interval during the period when water is available is excluded if the temperature is too low for crop growth (mean temperature

below 6.5OC). Areas where precipitation never exceeds half the potential evapotranspiration are classified as dry with no growing period. An area having a growing period between l and 74 days is classified as arid, and areas with growing

periods between 75 and 119 days are considered semiarid.



The average monthly precipitation, potential evapotranspiration, and half POtential evapotranspiration for three locations are presented in Fig. 1. All three locations are classified as semiarid by the aridity index (0.20 < P E T P < 0.50).

However, by the FA0 growing period classification, only the Rajkot, India, location is classified as semiarid. The Amman, Jordan, location has a growing period

in excess of 119 days, so this would be considered subhumid. Bushland, Texas, is

classified as dry, with a 0-day growing period, because the average monthly precipitation never exceeds 0.5 ETP.

These examples illustrate that there is still a lack of precision in defining and classifying climatic zones. Each classification scheme presented, as well as others in

the literature, has advantages for specific purposes and locations; however, subjective judgment is required for their interpretation. The most important point, however, is to recognize and understand that there are very major differences in specific climates among semiarid regions and that each one must be addressed differently.

Average precipitation values in semiarid regions can also be very misleading.

For example, the average annual precipitation for Bushland, Texas, is 470 mm, but

rainfall for any given year has been as low as 240 mm and as high as 830 mm. For

a month, the variation can be even much greater. The average monthly precipitation for June is 76 mm, but the range has been from 1 to 248 mm, and less than 62

mm has been received in one-half of the years. In semiarid regions, there are more

years below the average value than above because the high-rainfall years raise the

average more than the low-rainfall years lower the average; thus, there is a degree

of skewness inversely related to the amount of rainfall. The degree of skewness

generally increases as the amount of annual precipitation for an area decreases.

Mageed (1986) stated that in areas where annual precipitation amounts are

200-300 mm, the amount received in a given year ranges from 40% of the aver-




















Figure 1 Agroclimatic characteristicsof three representativesemiarid locations. (From Stewart et

al.. 1989).

age to 200%; for areas where the average is 100 mm per year, the range is from

30% of the average to 350%. This makes it imperative that more emphasis be

placed on probabilistic analysis, with less attention given to average values when

designing agroecosystems in semiarid regions.

Kanemasu et al. ( 1990) have extensively reviewed the agroecological features



of the semiarid tropics and presented strategies for identifying agroclimatic constraints for increased agricultural production. They concluded that the rainfall variability in the semiarid regions of the world is on the increase. They further concluded that the productivity of traditional agriculture in these areas is declining,

and that the threat of serious food shortages is rising. At the same time, they proposed that by properly evaluating the agroclimatic resources and transferring and

adapting improved technologies, the yet underexploited potential of semiarid tropical regions can be realized for sustained and improved agriculture.


Until the past few years, sustainability was seldom mentioned in agricultural

literature. Now, it is one of the most widely used words. High costs of irrigation

development, escalating energy costs during the 1970%public concern over potential negative impacts of fertilizers and pesticides on water supplies, soil erosion, soil compaction, and salinity, as well as other concerns, have caused many

people to question whether many of the current agricultural systems can be sustained.

There are many different concepts of sustainability, but none is generally accepted. Sustainability, to many, conveys the idea of a balance between human

needs and environmental concerns. The United States Congress defined sustainable agriculture in the 1990 farm bill as

an integrated system of plant and animal practices having a site-specific application that will over the long term: satisfy human food and fiber needs; enhance

environmental quality and the natural resource base upon which the agriculture

economy depends; make the most efficient use of nonrenewable resources and

on-farm resources and integrate, where appropriate, natural biological cycles

and controls; sustain the economic viability of farm operations; and enhance the

quality of life for farmers and society as a whole.

Ruttan (1989) proposed, as a guide to research, that the definition of sustainability should include (i) the development of technology and practices that maintain and/or enhance the quality of land and water resources, and (ii) the improvement in plants and animals and the advances in production practices that

will facilitate the substitution of biological technology for chemical technology.

The first part of the Ruttan definition is similar to that of the United States Congress, but the key to the second part of the Ruttan definition is the emphasis on

more reliance on biological versus chemical technology. Sustainability should be

considered dynamic because, ultimately, it will reflect the changing needs of an

increasing global population. The common thread among all definitions of sustainability, however, is that quality of the resource base should be enhanced.



Francis and Youngberg (1990) stated that sustainable agriculture is a philosophy based on human goals and on understanding the long-term impact of human

activities on the environment and on other species. Use of this philosophy guides

our application of prior experience and the latest scientific advances to create integrated, resource-conserving, equitable farming systems. These systems reduce

environmental degradation, maintain agricultural productivity, promote economic viability in both the short and long term, and maintain stable rural communities

and quality of life. Despite the appeal of the philosophy for sustainability, there is

often controversy about how best to achieve it. It is not known what methods and

systems in diverse locations will really lead to sustainability, and this lack of

knowledge often leads to conflict.

Rodale ( 1988) listed seven reasons that sustainability has risen to importance

on the farm policy agenda. The first is that there is currently enough food in the

world. Surpluses and their timely distribution at an affordable price are more of

a problem than shortages per se. Although many people are going hungry, it is

not because of the inability to produce. Some policymakers are, however, wondering if this level of production can be sustained. The second reason is that nonrenewable resources are vitally important to the operation of the conventional

system of agriculture in the United States and most other developed countries.

Without adequate and low-cost supplies of oil, gas, phosphate rock, and other

similar resources, U.S. agriculture would not be able to turn out large amounts

of low-cost food and fiber. Third, high levels of production are having negative

impacts on some systems. Soil erosion, degradation, and deforestation are widespread. This is particularly true in some developing countries where millions of

people are literally trading soil for food. Even in the U.S. corn belt, there are approximately 2 tons of soil lost for every ton of corn produced and the loss ratio

is even greater in wheat-producing areas (Larson et al., 1981). The fourth reason is that pollution problems can be traced to agriculture. Increasingly, residues

of fertilizers and pesticides are being found in surface and underground water

supplies. Fifth, world population continues to grow-in some places very rapidly. Although there is enough food in total today, there will be a much larger population to feed and clothe in future years. Sixth, society will possibly need agricultural production for energy and chemical feedstocks as well as for food. As

fossil fuel reserves become more expensive, agriculture may have to meet new

needs. Lastly, Rodale listed the concern for the family farm. He questioned

whether the good life in rural areas can be sustained without a vibrant family

farm sector.

Sustainability clearly involves technical, economic, and social conditions. All

three conditions are highly important and interrelated. It may be entirely possible

to have an agroecosystem that is technically sustainable that is neither economically feasible nor socially acceptable. Herein, however, we will deal primarily with

sustainability in regard to how agricultural practices and systems affect the sustainability of the soil resource base.

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Chapter 5. Are Agroecosystems Sustainable in Semiarid Regions?

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