Agriculture and the Environment

Primitive societies obtained food through hunting and gathering. Although some food is obtained from oceans and fresh waters, 95% of the human population’s protein and most of its calories are obtained from traditional land-based agriculture of crops and livestock. The major agricultural challenge facing us today is to achieve sustainable production of crops.       

                 

Crops

Most of the world’s food is provided by only 14 crop species. Of these 14, six species provide more than 80% of the total calories consumed by human beings either directly or indirectly. Other crops, called forage, are important food for domestic animals.

It is useful to group crops into cash crops and subsistence crops:

Cash crops are grown to be sold or traded in a large market. e.g., tea, tobacco, jute, etc.

Subsistence crops are used directly for food by the farmer or sold locally where the food is used directly. e. g., rice, wheat, etc.  Some cash crops may provide nonfood products (latex from rubber trees).

Seasonal Crop Species in Bangladesh

Cropping seasons in Bangladesh can be broadly divided into two: Rabi (dry period; October to February) and Kharif (wet; March to September). Although some of the crops are sown in one season they are harvested in another season, i.e., there is overlapping of seasons. Again crops are also divided into different groups as cereals, pulses, fibre crops, oilseeds, root crops, vegetables, spice crops, fruit crops, etc.  

Soil and Soil forming factors

Soils are earth materials modified over time by physical, chemical, and biological processes that support rooted plant life.  It can be defined as : “Soil is a collection of natural bodies occupying a portion of the Earth’s crust that support plant growth which have acquired properties due to the integrated action of climate and vegetation upon parent material as conditioned by relief over a period of time.”  

The four major components of soil are air (25%), water (25%), mineral matter (45% {sand, silt, and clay}), and organic matter (5%).

Soil forming factors

The type of a soil at a particular site depends on five factors such as

• Parent material (geological or organic precursors to the soil),
• Climate (primarily precipitation and temperature),
• Topography (relief) (slope, aspect, and landscape position),
• Biological activity (living organisms, especially native vegetation, microbes, soil animals, and human beings), and
• Time (the period of time since the parent materials became exposed to soil formation).

Soils are extremely important in many environmental considerations. As a result, the study of soils continues to be an important part of environmental sciences.

             Soil material  S = f(p+cl+r+v+t......)

Soil Fertility

Soil fertility refers to the capacity of a soil to supply the nutrients and physical properties necessary for plant growth. Ironically, agriculture depends heavily on soil quality, but agriculture can lead to a decline in that quality. A high-quality agricultural soil has all the chemical elements required for plant growth and a good physical structure that lets both air and water move freely through the soil, yet retains water well. Such a soil has high organic matter (Soil organic matter is the plant and animal residues, leaves, forest litter, etc. at various stages of decomposition, considered as the storehouse of nutrients) content. Organic matter in soil is rich in chemical nutrients and provides a physical structure conducive to plant growth.

Plowing (shattering of soil uniformly with partial to complete inversion) the soil and planting crops has been a way of life for several thousand years and continues today.                            

                       

                        Fig: Tillage operation in soil to receive the crop.

Loss of soil fertility: Erosion

When land is cleared of its natural vegetation, such as forest or grassland, the soil begins to lose its fertility. Some of this occurs by physical erosion. Erosion is the wearing away and transportation of land surface by running water, wind, ice, or other natural agents. Once the protection of the vegetative cover is lost, the soil is exposed directly to water and wind, which remove the loosened soil upper layers, where the most fertile organic matter is found. The less organic matter present in the soil, the more vulnerable the soil is to further erosion. Once erosion starts, the process can easily accelerate. The loss of soil fertility is much faster in warmer and wetter climates, such as tropical rain forests, than it is in colder or drier climates.   

Population pressures have led to overgrazing rangelands, deforestation, and destructive crop practices like clearing and burning steep, forested slopes and plowing grasslands. All these activities degrade or remove natural vegetation, causing the underlying soil to become much more susceptible to the destructive action of erosion. The result is a vicious downward cycle of deterioration—land degradation. Such land degradation results in a reduced productive potential and a diminished capacity to provide benefits to humanity.

              

                         

All forms of agriculture lead to soil loss, but the rate of loss varies with the crop and the methods of agriculture. Land used for row crops and small grains without soil conservation practices result in greater erosion loss. Worldwide, erosion removes about 25.4 billion tonnes of soil each year. Erosion is estimated to be worse now everywhere.                               

Consequences of erosion: Sediment Damage

Much of the eroded soil ends up in waterways causing downstream sedimentation which is a serious environmental effect of modern agriculture. Sediments fill in otherwise productive waters, destroying some fisheries. Nitrate, ammonia, phosphates, and other fertilizers carried by sediments can cause eutrophication in downstream waters; the resulting buildup of algae reduces fish production. Polluted sediments also can transport toxins. Sediment damage costs the US about $500 million/year in dredging expenses.

          

Making Soils Sustainable

Of a large number of factors determining sustainability of agriculture in a region, population pressures and the availability of arable land are the most important. Whether the land is plentiful or in short supply, maintenance and management of soil fertility is central to the development of sustainable food production systems. The principles that regulate soil fertility are fundamental to the philosophy of sustainability.

Reducing soil erosion through various measures help make soils sustainable. Proper use of such conservation practices as contour farming, strip farming, mixed cropping, rotation, terracing, waterways, windbreaks, and conservation tillage can reduce soil erosion.                                                                                               

Contour Plowing

Contour plowing, which is tilling at right angles to the slope of the land, is one of the simplest methods for preventing soil erosion. Contour farming reduces soil erosion by as much as 50% and, in drier regions, increases crop yields by conserving water. In the recent past, contour plowing has been the single most effective method for reducing soil erosion.

No-Till (Conservation) Agriculture

An even more efficient technique to slow erosion is No-till agriculture, also referred to as conservation tillage, a recent form of combination of farming practices that includes not plowing the land, using herbicides to keep down the weeds. In no-till agriculture the land is left unplowed most years. Plant residues or other materials are left to cover the surface (30% of the soil surface) and allowed to decay in place (mulch tillage). These practices can greatly reduce soil and water loss, reduces traffic operations over the field which decreases soil compaction, reduces the use of tractor fuel, and increases the profit.

 The wisest approach to sustainable agriculture involves a combination of different kinds of land use:

• Using the best agricultural lands for crops
• Poorer lands for pastures and rangelands, and
• Avoid using of the best lands for grain production for animal feed. 

Effects on the Environment

Agriculture is the world’s oldest and largest industry; more than one-half of all the people   in the world still live on farms. Because the production, processing, and distribution of food all alter the environment, and because of the size of the industry, large effects on the environment are unavoidable.

Agriculture has both primary and secondary environmental effects. A primary effect, also called an on-site effect, is an effect on the area where the agriculture takes place. A secondary effect, or off-site effect, is an effect on environment away from the agricultural site, typically downstream and downwind.

Major environmental problems that result from agriculture include deforestation, desertification, soil erosion, overgrazing, degradation of water resources, salinization, accumulation of toxic metals, accumulation of toxic organic compounds, and water pollution, including eutrophication.

Global Effects of Agriculture  

 Modern agriculture increases carbon dioxide in two ways. As a major user of fossil fuels, it contributes to the increased concentration of carbon dioxide in the atmosphere, adding to the buildup of greenhouse gases. Also, clearing land for agriculture increases the decomposition of organic matter in the soil, transferring the carbon stored in organic matter into carbon dioxide, increasing its concentration in the atmosphere.

Agriculture can also affect climate through fire. Fires associated with clearing land for agriculture may have significant effects on the climate because they add small particulates to the atmosphere.

Another global effect of agriculture results from the production of nitrogen fertilizer, which may be leading to significant changes in global biogeochemical cycles.

Agriculture affects species diversity. The loss of competing ecosystems (because of agricultural land use) reduces biodiversity and increases the number of endangered species. 

Pest Control and Agricultural Chemicals

All agriculture suffers from pests. From an ecological point of view, pests are undesirable competitors, parasites, or predators.

Worldwide only about 100 species of weeds, insects, fungi and microbes cause about 90% damage to the crops we grow

Pre harvest losses are due to competition from weeds, diseases, and herbivores; post harvest losses are largely due to herbivores.

Pests

A pest is any species that competes with us for food, invades lawns and gardens, destroys wood in houses, spreads disease, or is simply a nuisance.

The major agricultural pests are insects (feeding mainly on the live parts of plants, especially leaves and stems); nematodes (small worms that live mainly in the soil and feed on roots and other plant issues); bacterial and viral diseases; weeds (flowering plants that compete with the crops); and vertebrates (mainly rodents and birds that feed on grain or fruit).

Weeds

Although we tend to think that the major pests are insects, in fact, weeds are the major problem in terms of potential crop loss. Farming produces special environmental and ecological conditions that tend to promote pests.

There are about 30,000 species of weeds, and in any year a typical farm field is infested with between 10 and 50 weed species. Weeds compete with crops for all resources: light, water, and nutrients. The more weeds, the less crop. Some weeds can have a devastating effect on crops. Agricultural losses in the US as a result of weeds exceed $ 16 billion/year.

Pest Control—Pesticides

Worldwide, only about 100 species of weeds, insects, fungi, and microbes cause about 90% of the damage to the crops we grow.

To help control pest organisms, a variety of pesticides are developed. Pesticides (or biocides) are chemicals used to kill or control organisms we consider undesirable. Pesticides are classified according to the target organisms they are designed to control. Common types of pesticides include insecticides (insect killers), herbicides (weed killers), fungicides (fungus killers), nematocides (roundworm killers), and rodenticides (rat and mouse killers). 

Many of the earlier chemical pesticides were broad spectrum, and remained active for long periods of time. These are called persistent pesticides. One of the earliest pesticides used was arsenic, a chemical element toxic to all life, including people.

Development of more sophisticated pesticides began in 1939 when Paul Muller discovered DDT. DDT soon became the world’s most used pesticide and Paul Muller received the Nobel Prize in 1948 for his discovery.

Since then, over 60,000 different compounds that have potential as pesticides have been synthesized. However, most of these have never been put into production because of cost, human health effects, or other drawbacks.

At first, DDT was thought to be the long-sought magic bullet.  It appeared to have no short-term effect on people and seemed to kill only insects.

Eventually, three facts about DDT were discovered: (1) It has long-term effects on other, desirable organisms because of its long persistence; (2) It is stored in oils and fats and is concentrated as it is passed up food chains, so that the higher an organism is on a food chain, the higher the concentration of DDT it contains, a process known as biomagnification; and (3) The storage of DDT in oils and fats allows the chemical to be transferred biologically. As a result of these problems, DDT was considered as the most notorious chemical of the last century, and as a result was banned in most developed nations.

Fig.: Different ways of pesticide application

Alternatives to DDT include organophosphates and carbamates. These chemicals are more specific and decay rapidly in the soil. They too, are toxic to people and must be handled extremely carefully by those who apply them.

A perfect pesticide would have the following characteristics:  

1. It would be inexpensive.
2. It would affect only the target organism.
3. It would have a short half-life.
4. It would breakdown into harmless materials.

However, the perfect pesticide has not been invented.

Problems with Pesticide Use—Impact on Other Organisms

Many of the pesticides are toxic to organisms other than those especially targeted. One of the greatest concerns with the development and use of these substances is their slow breakdown and their ability to accumulate in organisms.

A major problem of pesticide use is that their widespread use accelerates the development of genetic resistance to pesticides. Because of genetic resistance, many insecticides (such as DDT) no longer protect people from insect-transmitted diseases (malaria, for example) in some parts of the world.

Another problem is that broad-spectrum insecticides kill natural predators and parasites that help control the populations of pest species.

Also, pesticides do not stay put. According to USDA, no more than 2% of the insecticide applied to crops by aerial or ground spraying reaches the target pests. Also, less than 5% of herbicides applied to crops reach the target weeds.

Pesticides that miss their target pests can end up in the air, surface water, groundwater, bottom sediments, food, and nontarget organisms, including humans and wildlife: at high enough levels, most pesticides can be toxic to humans, cause nervous system disorders (especially behavioral disorders), affects immune system, can cause cancer in animals, have adverse reproductive and developmental effects in wildlife, signs of growth irregularity, loss in biomass, or death to plants, in an extreme case of contamination, the number and activity of soil microorganisms may be reduced to essentially zero.  

Integrated Pest Management

Modern approaches to pest control involve integrated pest management (IPM), an ecosystem approach to pest management that integrates a variety of techniques that include:

• the use of natural enemies of pests, including parasites, diseases, and predators (biological control);
• the planting of a greater diversity of crops to reduce the chance that pests will find a host plant;
• no-till or low-till agriculture, which helps natural enemies of some pests to build up in the soil; and
• the application of a set of highly specific chemicals,
• the development of genetically resistant stock.

Biological Control

Biological control (biocontrol) is an alternative to pesticide use that relies upon natural enemies to suppress pest populations. Biocontrol is a set of methods to control pest organisms by using natural ecological interactions including predation, parasitism, and competition. It includes the intentional introduction of predators, diseases, or other parasites of a pest. For example, ladybugs are common predators of many plant-eating pests. It is possible to buy quantities of ladybugs for release in home gardens or farms. The hope is that these ladybugs will feed on pests and reduce their abundance.

There are many specialized and effective biological controls. One of the most effective is a bacterial disease, Bacillus thuringiensis, which kills larval forms of many insect pests, including many caterpillars. It is used widely.

Another technique to control insects involves the use of sex pheromones, a chemical usually releases by the female insect, acts as an attractant to members of the opposite sex. These chemicals have been identified and synthesized and used in insect control as bait in traps. In some species it has been shown to be effective up to 4.3 km away. Since many moths are pests, synthetic odors can be used to control them. Spraying an area with the pheromone confuses the males and prevents them from finding females, which results in a reduced moth population the following year.

Biocontrol has also been successfully used to control certain terrestrial and aquatic weeds.           

Genetically modified crops

Genetically modified crops (GM crops, or biotech crops) are plants, the DNA of which has been modified using genetic engineering techniques, to resist pests and agents causing harm to plants and to improve the growth of these plants to assist in farmers efficiency.

Genetically modified crops produce more food, but some worry it could upset delicate ecosystems.

More Food

Genetically modified foods produce higher crop yields, and so some have proposed it as a solution to solving hunger in developing countries. Also, although the seeds cost more up front, the yield is so great that such crops are cheaper to produce in the long run.

Reduced Need for Pesticides

Farmers do not need to use as much pesticides and herbicides on the crops, thereby reducing harm to the environment. They also decrease the amount of manpower needed to grow the crops.

Better Food Quality

The food quality is better. Genetic engineers can cause a fruit or vegetable to stay fresher for longer, extending its shelf life. They can also be engineered to withstand years of drought or other weather extremes while still providing a good crop yield. Engineers can also add essential vitamins to the food that are lacking.

Disadvantages of GM crops

Destabilization of Ecosystem

Genetically modifying foods could harm other organisms and upset the balance in the ecosystem. If engineers eliminate a pest from the crop, it could remove a food source for an animal. The GM crops could also be toxic for organisms.

Creation of New Diseases 

Some modification involves bacteria and viruses, so some people worry that this engineering could create new diseases. Also, pests could absorb the gene in the genetically modified crop and become resistant to sprays and other means of eliminating them.