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IV. The Issue of Sustainability
AGROECOSYSTEMS SUSTAINABLE IN SEMIARID REGIONS?
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
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.
B. A. STEWART AND C. A. ROBINSON
Whenever a natural ecosystem is transformed into an agroecosystem for the purpose of food and fiber production, there are several soil degradative processes set
into motion. Hornick and Parr (1987) reported that most agroecosystems have
degradation processes and conservation practices occurring simultaneously. The
relation of soil productivity to soil degradation processes and soil conservation
practices is illustrated in Fig. 2. As soil degradation processes proceed and intensify, there is a concomitant decrease in soil productivity and sustainability.Conversely, soil conservation and reclamation practices tend to increase soil productivity and sustainability. Therefore, the productivity level of an agroecosystem at
any time is a result of the interaction of degradation processes and conservationheclamation practices, some of which are shown in Fig. 2. In natural ecosystems, sustainability is achieved through the efficient but delicate balance between
all necessary inputs and outputs. Failure of agroecosystems to maintain this balance will ultimately lead to systems that cannot be sustained.
The relation shown in Fig. 2 applies to all agroecosystems, although the importance of specific processes will vary substantially between agroecosystems.Also,
the processes included in Fig. 2 are not all inclusive but are some of the most important and will be used for discussion. It is also important to recognize that some
of the processes that are generally considered positive can, in some cases, result in
soil degradation. For example, the use of organic wastes increases soil organic matter, improves soil structure, enhances soil water storage, and reduces erosion. In
some circumstances, however, use of organic wastes results in accumulations of
toxicants or nutrient depletion caused by increased leaching. The important point
illustrated by Fig. 2 is that degradative processes and soil improvement processes
always occur simultaneously, and the net result can be positive or negative.
The negative impacts of soil degradation processes can become dominant. In
such situations,agroecosystemscannot be sustained over the long term, and in ex-
Or anic Matter Loss
Nutrient Depletion by Leaching
Improved Nutrient Cycling
Improved System to Match Soil,
Climate and Cultivars
Figure 2 Relation of soil productivity to soil degradation processes and soil conservation practices (modified from Hornick and Pam,1987).
AGROECOSYSTEMS SUSTAINABLE IN SEMIARID REGIONS? 201
treme cases soil productivity can be reduced to zero. On the other hand, soil conservation practices rarely improve the productivity of cropland beyond the initial
levels experienced when virgin land is cultivated. However, soil restoration practices, to be economically feasible and fonvard-looking, must, of necessity, be focused on improving the soil productivity beyond the initial level experienced after cultivation. Examples of such practices include adding fertilizers in areas where
essential plant nutrients are deficient and installing water conservation practices to
significantly reduce runoff. The most notable increases in dryland productivity
generally occur when irrigation practices are established or when salt-affected
areas are reclaimed.
The concepts illustrated in Fig. 2 apply to all climates and all levels of inputs.
In every case, a truly sustainable agroecosystem results when the effects of conservation practices equal or exceed the effects of the soil degradation processes.
Climate is often the most critical factor determining the sustainability of an agroecosystem. Stewart er al. (1991) presented a generalized view, shown in Fig. 3, of
the effect of varying temperature and moisture regimes on the difficulty of achieving sustainability in an agroecosystem. As temperatures increase and the amounts
of precipitation decrease, the development of sustainable cropping systems becomes more difficult. The reasons for these effects are readily apparent when the
processes and practices presented in Fig. 2 are analyzed.
The most serious degradation processes in semiarid regions are erosion and loss
of soil organic matter. As temperatures increase, organic matter decline, particularly in frequently tilled soils, is greatly accelerated. The potential for wind erosion especially and also water erosion generally increases in warmer areas. These
same degradation processes are also accelerated as the moisture regimes become
more arid because, initially, there is a lower inherent organic matter level and less
natural vegetation to prevent erosion.
Not only do the soil degradation processes accelerate under hot and arid climatic
regimes, but the benefits that can be derived from soil conservation practices in
these regimes are fewer than those in the cooler and wetter areas. For example, the
most important soil conservation practices to alleviate organic matter loss and control soil erosion usually involve crop residues, but the availability of crop residues
decreases sharply in semiarid regions. Consequently, the negative processes
shown in Fig. 2 can easily become much greater than the positive benefits and
make the agroecosystem unsustainable.
The relations presented in Fig. 3 do not apply to all climatic regimes. Soil degradation processes also accelerate under very high precipitation regimes due primarily to water erosion, nutrient depletion by leaching, and acidification. Cold
conditions severely limit the choice of cropping systems and can also result in
B. A. STEWARTAND C. A. ROBINSON
Figure 3 Generalized representation of the effects of temperature and precipitation on the difficulty of developing sustainable agroecosystems in semiarid regions (from Stewart e r a / . , 1991).
waterlogging. The relations represented in Fig. 3 should be restricted to moving
from the more favored environments toward semiarid and arid regions.
The soil is also an important factor to consider in the development of sustainable agroecosystems.The generalized representation shown in Fig. 3 holds for all
soils, but the potential rate and extent of soil quality decline depend on soil type.
Pierce et al. (1983) developed a productivity index (PI) for assessing the effect of
erosion on soil productivity. Briefly, the PI assumes that soil is a major determinant of crop yield because of the environment it provides for root growth. They
evaluated the relative productive potential of soil by calculating a PI based on the
available water capacity, resistance to root growth and development (bulk density), and adequacy of pH to a depth of 100 cm.
A study by the National Research Council (1993) stated that the quality of a soil
depends on attributes such as the soil’s texture, depth, permeability, biological activity, capacity to store water and nutrients, and the amount of soil organic matter.
The report stressed that soils are living, dynamic systems that are the interface be-
AGROECOSYSTEMS SUSTAINABLE IN SEMIARID REGIONS? 203
tween agriculture and the environment. High-quality soils promote the growth of
crops and make farming systems more productive. High-quality soils also prevent
water pollution by resisting erosion, absorbing and partitioning rainfall, and degrading or immobilizing agricultural chemicals, wastes, or other potential pollutants.
Soil degradation is an outcome of human activities that deplete soil and the interaction of these activities with natural environments. La1 and Stewart (1990a)
stated that the three principal types of soil degradation are physical, chemical, and
biological. Each type is made up of different processes, as illustrated in Fig. 4.
Physical degradation leads to a deterioration of soil properties that can have a serious impact on water infiltration and plant growth. Wind and water erosion are
generally the dominant physical degradation processes, but compaction is also a
widespread concern in places where heavy machinery is commonly used. Hardsetting of a cultivated soil is also a process of compaction, but it results from wetting structurally weak or unstable soil rather than from the application of an external load. Laterization is the desiccation and hardening of exposed plinthitic
material (material consisting of clay and quartz with other diluents; it is rich in
sesquioxides, poor in humus, and highly weathered).
Chemical degradation processes can lead to a rapid decline in soil quality. Nutrient depletion, acidification, and salinization are common soil degradation
processes that have serious impacts on crop production. Chemical degradation is
also caused by the buildup of toxic chemicals resulting from human activities.
Biological degradation includes reductions in organic matter content, declines
in the amount of carbon from biomass, and decreases in the activity and diversity
of soil fauna. Biological degradation is perhaps the most serious form of soil degradation because it affects the life of the soil and because organic matter significantly
affects the physical and chemical properties of soils. Biological degradation can
also be caused by indiscriminate and excessive use of chemicals and soil pollutants. Biological degradation is also generally more serious in many semiarid regions, particularly in the tropics and subtropics, because of the prevailing high soil
and air temperatures. Tillage also stimulates biological degradation because it increases the exposure of organic matter to decomposition processes.
Soil degradation is a complex phenomenon driven by strong interactions among
socioeconomic and biophysical factors. These interactions, as perceived by La1
and Stewart (1990b), are illustrated in Fig. 5. Soil degradation is fueled worldwide
by increasing human populations, fragile economies, and misguided farm policies.
There is also often a conflict between short-term benefits and long-term consequences. This conflict is perhaps the most serious problem that must be addressed
if truly sustainable agroecosystems are to be achieved.
B. A. STEWART AND C. A. ROBINSON
17, 71 Ti
Compaction Laterization Erosion and
Acidifi- Sodication Toxic
Figure 4 Qpes and processes of soil degradation (from La1 and Stewart, 1990a).
Soil degradation in semiarid regions can lead to desertification. Desertification,
until recently, was generally considered to be primarily the result of prolonged
drought. However, it has become apparent in recent years that human misuse of land
is perhaps the basic factor, and drought is only an exacerbating factor (Dregne,
1989). In a paper prepared for the World Bank, Newcombe (1984) described the
stages of land degradation that occur when natural forests are cleared and plowed
as people seek new agricultural land. Nutrient cycling is drastically altered, and
soil fertility declines following clearing. In the first stage, wood supplies remain
plentiful, and gradual erosion is largely unnoticed. As the population increases,
demand for wood increases for both construction and fuel, initiating a self-feeding cycle of degradation involving cutting wood from remnant forests to generate
income, burning crop residues and dung for household fuel, reduced soil fertility,
degraded soil structure due to residue and dung removal, and increased vulnerability to wind and water erosion. Eventually, dung and crop residues turn up in
markets where only wood was previously sold. As a result of declining organic
matter, the cropland becomes less productive, and crop yields prove barely sufficient even for subsistence. Eventually, dung becomes the main fuel sources in villages, and rural families use crop residues for cooking and for feeding livestock,
which can no longer be supported by grazing land. In the final stage of degradation, crop failures become common even in normal seasons because topsoil and
organic matter depletion have lowered the soil water-holding capacity. As a result,
both food and fuel prices rise rapidly. Newcombe (1984) believes a critical point
occurs in subsistence economies when more trees are cut for fuel than to make way
for farmland. This is an excellent example of how short-term benefits can lead to
AGROECOSYSTEMS SUSTAINABLE IN SEMIARID REGIONS? 205
long-term disasters. This results in people moving to even more marginal areas. In
essence, droughts seldom move to the people; rather, the people move to the
V. TECHNOLOGIES FOR INCREASING
Stewart and Burnett (1987) listed three components necessary for successful
agroecosystems in dryland areas. These are (i) retaining the precipitation on the
land, (ii) reducing evaporation, and (iii) utilizing crops that have drought tolerance
and that fit the rainfall patterns. Although these components have been known for
SOIL DEGRADATION 4-b
. Population density
. Land: people ratio
Land tenure systems
Figure 5 Interdependence of soil degradation on biological and socioeconomic factors (from La1
and Stewart, 1990b).
B. A. STEWART AND C. A. ROBINSON
centuries, new technologies and strategies have been developed that increase crop
production in water-deficient areas. Some of these technologies and the principles
on which they are based are presented in the following sections.
One of the oldest, and most controversial, technologies for increasing plantavailable water is lengthening the fallow period. This is generally called summer
fallow, defined as a practice wherein no crop is grown and all plant growth is controlled by cultivation or chemicals during a season when a crop might normally be
grown. Proponents have emphasized the water-conserving, weed-controlling, and
crop yield-stabilizing virtues, whereas critics have emphasized the inefficiency in
soil water storage and the wind and water erosion and declining organic matter
problems associated with fallow. Fallow efficiency is defined as the percentage of
precipitation occurring during the fallow period that is stored in the soil profile at
the end of the fallow period. Historically, fallow efficiencies have been in the range
of 15-20%. The remainder of the precipitation is lost as runoff and evaporation
and, on some soils, as drainage below the root zone. Stewart et al. (1 994) summarized long-term data from different cropping systems at Bushland, Texas, to illustrate the effect that lengthening the fallow period has on increasing soil water
storage (Table I). The cropping systems were (i) continuous wheat, where winter
wheat is seeded each year in October and harvested in late June or early July;
(ii) wheat-sorghum-fallow (two crops in 3 years), where the land is fallowed for
approximately 11 months following wheat harvest and the land is then seeded to
grain sorghum in June and harvested in November and then fallowed for 1 1 months
until the following October when the land is again seeded to winter wheat; and
(iii) wheat-fallow (one crop in 2 years), where the land is fallowed for approximately 16 months from wheat harvest in July to October of the following year
when wheat is seeded again. The amount of precipitation lost during the rotation
as evaporation during the nongrowing (fallow) season ranges from 36% for the
continuous wheat system to 61% for the wheat-fallow system. In the wheatfallow system, where the fallow period is 15 or 16 months, only approximately
15% of the precipitation that occurs during the fallow period is stored in the soil
for later use by a growing crop. Even under the continuous wheat system, where
the fallow period is only 3 or 4 months, the fallow efficiency is less than 20%.
However, despite the very low efficiency of water storage during fallow periods,
the wheat-fallow system is practiced widely because of the importance of sustaining crop yields under dry farming conditions. As illustrated in Fig. 1, there is
no period during the year at Bushland, Texas, when average precipitation is as
much as 50% of the potential evapotranspiration. Therefore, without a substantial
amount of stored soil water present at time of seeding, the chance of a crop fail-