It is clear that current practices need to be changed, since most arable soils in Asia are tending to deteriorate rather than improve over time.
Heavy metals are a major concern, since the presence of these in the food chain in more than the minimum permitted levels means that crops may cause health problems, and may be almost unmarketable. Monitoring soil quality is also important, since it is much cheaper to prevent soil contamination than it is to apply remedial measures to contaminated soils.
Agricultural soils in many parts of Asia are being contaminated by industrial wastes. This is especially common in fields which lie near industrial zones or are irrigated by water from rivers or canals used for discharged industrial wastewater.
Many factories in Asia are located in rural areas and surrounded by paddy fields. Sometimes this is because of lax zoning laws, but more often it is the result of a deliberate government policy to provide jobs for rural people.
Since industrial and agricultural activity are taking place side-by-side, it is not surprising that many farmers are finding that their soils are being contaminated by industrial wastes, sometimes to toxic levels.
Of particular concern are the heavy metals. Factory wastewater is a major source of heavy metals in field soils, as is sewage sludge applied as an organic fertilizer. Another common source of heavy metals in soil is livestock wastes, whether applied intentionally as fertilizer or unintentionally as a contaminant in irrigation water. Copper and zinc are often used as feed additives, and high levels may be present in livestock manure. Water which has leached through landfill may also be contaminated with heavy metals, especially around large cities. Car exhaust fumes may be a cause of lead pollution. This is common in fields which lie alongside busy roads.
Sometimes heavy metals are present in soils naturally, rather than as the result of human behavior. The parent materials beneath the topsoil may contain high levels of arsenic or lead. For the same reason, phosphate fertilizer may be a source of cadmium, although the levels in fertilizer are generally below toxic levels.
There is a need for monitoring and assessment. Each country should establish a database to record the relationship between heavy metals in soil, agricultural land use and other human activities. Agricultural production on soils contaminated by heavy metals derived from industrial waste must be carefully supervised, so that the levels in harvested food are within permissible levels. Special care should be taken with the application rate and quality of sewage sludge. Regulations and guidelines for optimum loading capacity must be established, if this resource is to be used safely.
Where levels of heavy metals in soils are high, intensive monitoring is needed, and possibly remediation. The easiest form of remediation is to convert contaminated land to non-food crops such as ornamentals. However, this may not be acceptable to farmers. For soils with only slight contamination by heavy metals, lime applications may help. Lime increases the soil pH, and thus reduces the uptake of heavy metals by the crop.
Another approach is phytoremediation, planting selected species to take up the heavy metals. (
Fig. 1 and
Fig. 2). Suitable plant species should have a high uptake, and the ability to grow in contaminated soils. Ideally, they should also translocate the contaminants from the root zone to the leaf, so that the leaves can be cut, burned and treated as solid phases, and the contaminants thus removed in the cuttings.
Vetiver grass combined with applications of zeolite is being used to remove boron, cadmium and lead from Indonesian soils contaminated by sludge from the textile industry. The water plant Eichhornia crassipes (water hyacinth) is also able to absorb lead and cadmium which has accumulated in flooded soil. Another promising plant species used for phytoremediation in Thailand is silverback fern (Pityrogramma calomelanos).
Agriculture itself causes a great deal of pollution. On a global level, paddy rice cultivation is a major source of methane gas (CH4), which contributes to global warming. It also produces other important trace gases such as nitric oxide (NO) and nitrous oxide (N2O). On a local level, agricultural pesticides and fertilizers are a major contaminant of groundwater and rivers.
Only 15% of applied pesticides hit the target. Most of the pesticides applied to crops are deposited, not on the pest, but on the surrounding soil, particularly the top 2-5 cm of topsoil. They do not stay in this upper soil layer for long. They soon leach downwards into the groundwater.
Studies have shown that many wells used by Asian farm families for drinking water are polluted by pesticides. This is common when farm families are living near their paddy fields and using wells fed by shallow aquifers.
In some countries, the main pollutants are organochloride insecticides such as dieldrin, which are very dangerous to human health. Even in countries where these pesticides are now banned, residues persist for years in the groundwater, and therefore in wells which supply drinking water.
Wells are also contaminated by fertilizer residues. In some Asian countries, more than a quarter of drinking wells contain higher than acceptable levels of nitrates. Some also contain high levels of phosphorus.
Preventing contamination by pesticides needs more efficient application methods. It also needs a new approach to pesticides. Before pesticides are recommended for use by farmers, we should consider the solubility, mobility and degradation of the different pesticides available.
With regard to fertilizers, pollution is caused, not so much by the use of fertilizers, as by their overuse. In countries where the cost of fertilizers is low compared to market prices, farmers tend to apply far too much fertilizer, especially nitrogen.
Studies have been made which compare the amount of nitrogen applied to crops in Asia with the amount taken up by the crop. The surplus can be alarmingly high, especially with horticultural crops. One study found that more than 700 kg/ha of the nitrogen applied to celery was not being absorbed by the crop. This excess nitrogen became a pollutant of both the soil and the groundwater around the crop.
The way to overcome this problem is by site-specific nutrient management, with balanced applications of nitrogen, potassium and phosphorus, and also micronutrients. These should be based on soil testing and plant analysis, to diagnose the nutrient status of soil and crop.
A promising low-cost technology is the MSL (multi-soil-layering) method of treating wastewater. It is based on a combination of physical filtering and biological decomposition. Pilot projects have been operating in Japan for some years, and have now been established in Thailand. The system works well under both temperate and tropical conditions.
Soil mixture blocks (soil mixed with organic matter and iron particles) are alternated with zeolite. Aerobic conditions occur in the zeolite, and anaerobic conditions in the saturated soil blocks. This mixture of aerobic and anaerobic conditions allows for the efficient purification of wastewater.
Nitrogen is removed by both (aerobic) nitrification, and reduction under anaerobic conditions. Phosphate is removed by forming insoluble compounds. The rate and duration of aeration are key factors in increasing the efficiency of the MSL system.
To maintain soil quality, we need to be able to measure it by scientific methods. If it remains high or increases, we can then be sure that soil management is following a sustainable path.
In the case of agricultural soil to which chemical fertilizers are applied, soil enzyme activity is a good biological indicator of soil quality. Some soil biochemical properties are significantly related to crop yield. They play an important role in the decomposition of soil organic matter. This makes them a good indicator of soil quality.
Erosion is a major cause of poor soil quality. It affects not only the eroded areas which lose their topsoil, but lowland soils and rivers downstream. It is not easy to protect lowland areas from non-point source pollution. The best approach is to intercept soil erosion, by improved land manage-ment and land use. The use of soil conservation practices can reduce soil erosion, water loss and nutrient loss.
Some successful practices found in Asia today include the use of grass strips on the lower parts of cultivated slopes, and the use of forest buffer strips as vegetative barriers around lakes and reservoirs. These buffer strips intercept nutrients and pesticides dissolved in run-off from cultivated fields on higher slopes. There is also widespread use in Asia today of mulches and cover crops to protect the soil.
Agriculture and industry are commonly treated as two separate concepts and economic sectors. In practice, they have a strong and continuous interaction. Agriculture in Asia is greatly influenced by industrial practices, particularly wastewater disposal and treatment.
However, it is agriculture itself that is the main cause of deterioration in the quality of agricultural soils. Best management practices can prevent soil pollution and soil erosion.
Pollution of groundwater by fertilizers and pesticides should be a major health concern, particularly for farm families who take their drinking water from shallow wells near agricultural fields.
Promising new technologies and approaches include low-cost wastewater treatment by MSL, and improved indicators for assessing soil quality, such as physicochemical properties and soil enzyme activity.
To prevent heavy metal contamination, information is a vital asset. Each country needs to establish monitoring and regulation levels of heavy metals in soils and crops, particularly in vulnerable areas which are already contaminated. For this to be meaningful, each country also needs to establish background levels of heavy metals in various parts of the country where there are different types of parent materials.
With regard to further information needs, these include further testing of MSL, and models to evaluate soil quality. We also need more data about the impact of pesticide pollution on human health, both at a national and regional level. We particularly need data from high-risk areas where farm families are drinking contaminated water.
From the policy aspect, more needs to be known about the economic losses of farmers as the result of non-point source pollution, and possible compensation and remediation programs which may help them.
Held at Kasetsart University, Bangkok, Thailand on May 6-11
No. of countries participating: 7 (Indonesia, Japan, Korea, Malaysia, Philippines, Taiwan ROC, Thailand)
No. of papers presented: 14
No. of participants: 21 plus observers
Co-sponsor: Kasetsart University, Thailand
Figure 1 Vetiver Grass
Figure 2 Stone Cress