No. 3: DRAIN TILE WATER QUALITY
The Science Council of Canada study on sustainable agriculture pointed out that farmers must now contend with public concerns over the effect of pesticide residues, chemical preservatives, irradiation and animal hormones on food safety as well as a host of other environmental, technological and management issues. Environmental problems include the contamination of ground and surface waters by fertilizers, pesticides and animal wastes.
Water management includes the control of the water available to crops. A supply of both water and nitrogen is essential for commercial agricultural production. Water is generally available in most soils of Ontario, although irrigation is often needed in the coarse sands usually used for the production of tobacco and horticultural crops. Too much water can also bring the attendant problems of crop losses from drowning and a loss of soil through erosion.
Nitrogen comprises 80 percent of the air we breathe. When nitrogen is combined with hydrogen, oxygen or carbon it may be converted to a nitrate by nitrification in which bacteria changes ammonia (nitrogen) into nitrate in the presence of oxygen and moisture. Ammonia is produced in soil when organic matter decomposes, it is also a component of mixed fertilizer, and can be added as anhydrous ammonia. When nitrate is produced it moves in the soil water as it is very soluble. When nitrate nitrogen is formed it may be taken up by plants to be used in building organic matter. Septic tank effluent, sewage lagoons, industrial waste sites, soil, rainfall, legume crops and animal manure are other important sources of nitrogen.
Nitrogen contamination of our water supplies is now a matter of record and concern. Many research reports point at agriculture as a source of much of the nitrate found in both ground and surface waters. Besides being a source of water pollution, such loss of nitrogen also represents an economic loss in fertility to the farmer.
High nitrate levels in the ground water is the result of activities some years previous when the use of fertilizer to increase agricultural production was being actively encouraged. Many recommendations have been made to reduce the impact of agriculture on nitrate concentrations in water supplies. It is a very difficult problem and realistic economic solutions are few.
Nitrogen, particularly in the form of nitrate, is highly soluble in water, and consequently highly mobile. Application of nitrogen at rates over and above the ability of the crop to use it results in losses of that nitrogen, normally by leaching.
Denitrification is the conversion of nitrate into nitrogen gas. This process occurs through the action of bacteria in water-logged soils. Commercial crop production is not possible on water-logged soils so the process may only hold promise in deeper soil profiles.
Septic tank systems and animal manure are common sources of nitrate and bacterial pollution of ground water. Bacterial tests of water are free in Ontario, but such tests do not detect other than bacterial contaminants of water. Chemical analysis of water are provided by commercial laboratories and water conditioning companies. A nitrate test costs about $20, so large scale testing for nitrates in water is expensive.
If growth is not restricted by water deficit, then nearly all the applied nitrogen is recovered at moderate rates of application. When there is a soil moisture deficit, growth is restricted and applied nitrogen is not taken up, but is available for leaching. Irrigation and controlled drainage, because they control the soil moisture deficit and stimulate growth to take up nitrogen, may in fact reduce the rate of nitrate leaching.
When a pasture is grazed, it changes the nitrogen cycle. Any nitrogen applied is still taken up by the crop, but is used in the field by the animals, and is then returned in a highly mobile state in the form of manure and urine. One researcher showed that an animal removes 30 kg/ha of nitrogen in the form of live weight gain, whereas cut forage would have removed 300 kg/ha. The remaining nitrogen must either accumulate in the soil or be lost by leaching deeper into the soil, or denitrify near the surface of waterlogged soils.
The Role of Tile Drainage
Hay and forage crops
Good fertilizer practices, including split applications of nitrogen top-dressing, and delaying applications when soils are wet, or if heavy rain is forecast, can prevent excessive leaching of nitrogen fertilizer. The economic loss to the farmer and the consequent effect on the environment can be predicted.
For more information ask your LICO drainage contractor.