LICO Factsheet No. 3 - Drain Tile Water Quality

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
Commercial fertilizers provide the major portion of the common plant nutrients - nitrogen, phosphorus, and potassium, used in crop production. The remainder is supplied through animal manure and natural sources such as soil, legumes, and precipitation.

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.

Nitrogen Cycle
The nitrogen cycle is quite simple for cash crops, grain, cut hay and forage crops. The two dominant processes are the addition of nitrogen fertilizer, which is taken up by the plant, and the removal of nitrogen in the crop by repeated cutting and removal from the field. Only when there is an excess supply of nitrogen over the demand is there any great potential for losses from cut hay and forages; either gaseously by denitrification, or by direct leaching.

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
Water table management has an important effect on the nitrogen economy. Because of the high potential for loss of nitrogen, and the important role of soil water in both controlling the rate of plant growth, and in providing one of the routes for nitrogen loss, soil water management - both drainage and irrigation, can be expected to have large effects on the nitrogen economy. These are summarized:-

Hay and forage crops

Nitrogen inputs are roughly balanced by the outputs.
As nitrogen input increases, so does the relative importance of leaching.
Leaching losses are higher from tile drained soil than from undrained soil, when the pool of available nitrogen is high.
Gaseous nitrogen losses are higher on undrained soils due to waterlogged anaerobic conditions.
Drainage leads to better aeration of the soil in which nitrogen mineralisation processes are encouraged.
Undrained soils tend to greater gaseous losses through denitrification, and greater accumulation of nitrogen in the soil organic matter.

Field Crops

Concentration of nitrogen in the tile drain discharge is lower with moderate rates of fertilization.
Research in the UK showed the annual loss of nitrogen from drained plowed land was 40 kg N/ha, equivalent to 23% of the applied fertilizer. Most of the nitrate leached was lost through the tile drains.
Loss of nitrate was greater in the period between the harvest of one crop and the spring application of fertilizer for the next crop.
There is a linear relationship between the rainfall after top-dressing with nitrogen and the loss of fertilizer through leaching.
If tile drains did not intercept the nitrogen leachate the leachate would continue to percolate down to the ground water table where its removal is much more difficult.
Manure nitrogen is much less available to crops than is fertilizer nitrogen.

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.