The Quality of Agricultural Soils
In May, FFTC held an international seminar to discuss the widespread deterioration in the quality of agricultural soils, and what can be done about it.
The other is pollution to agriculture. Fields are often affected by off-site pollution, especially those which lie near industrial zones.
Soil Pollution from Industry
Wastewater from factories is becoming a problem for many Asian farmers. Even after treatment, it often contains high levels of heavy metals or toxic compounds. From the factory, the wastewater flows into rivers and irrigation canals, and from there into farmers' fields as irrigation water.
In some parts of Asia, farmers have seen their rice yields fall by 80% because of the contamination of irrigation water by polluted sludge from textile factories and other industries. Industrial pollution is particularly dangerous, because it may contaminate both soils and crops with heavy metals.
The Problem of Heavy Metals
Heavy metals in agricultural soils are a major concern. Taken up by crops, they may enter the food chain and contaminate human food. Not much is known yet about the pathways whereby toxic heavy metals enter human food. Studies of crops growing in contaminated soils show that levels of heavy metals are generally higher in the roots than in the edible grain or leaves. However, even as little as 2 ppm (parts per million) of elements such as cadmium and mercury, not to mention arsenic, can be harmful to the health.
As a first step to coping with the problem, every country needs to establish baseline levels of heavy metals in local soils. Heavy metals are present naturally in some rocks such as granite, and therefore in the soils formed from these rocks.
Other sources of heavy metals in agricultural soils, beside the parent material, are contaminated irrigation water, landfill leachate and atmospheric fallout, especially around large cities. Car exhaust fumes may be a cause of lead pollution in fields close to busy roads. Surprisingly, phosphate fertilizer is a source of cadmium, although the amount in fertilizer is generally below toxicity levels.
Care must be taken when sewage sludge is being applied to land as a fertilizer. It often contains high levels of heavy metals. They are less of a problem with composted livestock manure. However, livestock manure may contain high levels of copper and zinc, which are used as additives in livestock feed. Where levels of heavy metals in soils are high, monitoring is needed, and possibly remediation.
Soil Pollution from Agriculture
Agriculture itself is a major cause of soil contamination and loss of soil productivity. The main culprits are agricultural chemicals and erosion. Of the agricultural chemicals, both pesticides and fertilizers may cause a deterioration in soil quality.
Pesticides
Eighty-five percent of applied pesticides are deposited, not on the target pest, but in the soil, water or air. It is the soil which receives the largest part. Soil-bound residues then become biologically available to the crop, and enter the human food chain.
Most pesticides in soil are in the uppermost 2-5 cm of soil. They soon leach downwards into the groundwater, where they may become a potential health threat.
Fertilizers
Pollution of soil by fertilizers is caused, not so much by the use of fertilizers, as by their overuse. Solutions to this problem are based on more accurate assessment of the nutrient levels of soils and crops by soil testing and plant analysis, and balanced fertilizer use. By such means, fertilizer applications can often be reduced with no loss of yield.
Soil Erosion
On both a global and a regional level, the most important cause of loss of soil quality is erosion. Soil erosion affects, not only the denuded agricultural areas which lose their topsoil, but the rivers, lakes and lowland fields which receive the sediments.
Contour farming has long been recommended as a way of controlling erosion. However in practice, farming along contours is difficult on steep slopes, especially if farmers are using machinery. A study in Korea compared the cost effectiveness of various conservation practices. Terraces were effective but expensive. Management practices such as applying mulch to the soil surface and leaving grass strips on lower parts of slopes were effective and did not cost too much.
In Taiwan, vegetative barriers are being planted as buffer strips along the shores of lakes and water reservoirs. These riparian buffer strips can intercept runoff of nutrients and pesticides from cultivated areas on higher slopes.
Index of Images
Figure 1 Factory Located in Rural Area. This Is Often a Deliberate Policy, to Create Jobs for Rural People. However, Wastewater from Factories May Contaminate Paddy Rice.
Figure 2 Rice Irrigated with Wastewater Containing Cadmium. the Harvested Rice Grain Also Contained High Levels of Cadmium, and Could Not Be Marketed.
