No. 7: WATER TABLE MANAGEMENT INCREASES YIELD AND SAVES FERTILIZER
Water table management is recognized as an essential best management agricultural practice throughout North America. Due to its agronomic, environmental and economic benefits, water table management is a successful technique adopted by crop producers in eastern Canada. Data from field studies demonstrate the many advantages of water table management to producers.
There are three forms of water table management which farmers will recognize. These are represented by (1) common tile drainage where the water level in the field is reduced to permit earlier seeding, efficient use of fertilizer, and a substantial increase in a crop yield. Tile drainage pays for itself!
A variation on the above practice is termed (2) controlled drainage. With controlled drainage the existing, or modified, field tile drainage system is designed such that the water table can be lowered to permit planting operations. During the growing season, drain discharge is restricted from the tile outlet, resulting in a higher water table. The water table drops with time due to evaporation and deep seepage, and is only raised if there is rain.
The third system of water table management is termed (3) subirrigation. Here, water is pumped slowly, and continually, into the drainage system, to maintain a near constant water table during the growing season. When large rainfalls occur, and the water table rises above the desired level, the irrigation pump is stopped, and the excess water drained through an overflow pipe connected to the outlet.
These three water table management systems are depicted in Figure 1.
In short, a water table management system allows a crop producer greater flexibility with the management of the soil-water regime--lowering the water table during periods of high precipitation to remove excess water for better root aeration and optimum crop growth, and raising the water table with controlled drainage or subsurface irrigation when the crops need water during the summer months. This ability to adjust the water table level helps stabilize crop yield which is the biggest economic benefit of these practices. Yield increases come primarily from the increased moisture content of the soil. This available water is critical during the summer period when water-stress adversely affects grain yields. Water from the raised water table moves up to the plant roots by capillary rise.
A site near Bainsville, Eastern Ontario, was used to assess the effects of different water table levels on strip-cropped corn and soybeans. The farmer practiced a no-till ridge system and 140 kg per hectare of nitrogen fertilizer (28% UAN) were applied to the corn rows. No fertilizer was applied to the soybean rows. In 1995, corn yielded 12.6 tonnes per hectare with a 50-cm controlled water table.
For soybeans, a 75-cm controlled water table gave the maximum yield of 3.6 tonnes per hectare. In 1996, the highest corn yields, 7.3 tonnes per hectare, were found in both the 50 and 75-cm water table depths. For soybeans, the highest yield, 3.2 tonnes per hectare, was obtained from the 50-cm depth. Yields in 1996 were lower due to a wet spring and late planting.
A silt loam soil at St. Emmanuel, Quebec, produced the highest corn yields in 1993 and 1994 with a 50 cm controlled water table depth. Overall, corn yield increases by adopting subirrigation were in the range of 1.25 to 13.5% higher compared to conventional drainage. For soybeans, the yield increases were much higher, ranging from 13.5 to 32.7% over conventional drainage. Precipitation during the growing seasons of 1993-1996 were higher than normal, which reduced the beneficial effect of subirrigation. In drier years, however, much higher yield increases can be expected.
(To convert tonnes/hectare to bushels/acre -- multiply by 14.9 for soybeans and 15.9 for corn.)
Other economic benefits from water table management include savings in production costs. Subsurface irrigation reduces energy and maintenance costs since it is a very efficient irrigation method compared to other methods of irrigation. Water table management is very affordable. It can be easily integrated with existing subsurface drainage systems and be fully automated. Furthermore, grain and cereal crops which do not lend themselves easily to sprinkler and surface irrigation systems do very well under subirrigation. Since the nitrogen use efficiency of the plant is improved, less nitrogen inputs are required. These savings also translate positively for the environment in that nitrate (NO3-N) pollution of streams, rivers, lakes and ground water are significantly reduced.
In the Bainsville and St. Emmanuel studies, the plots which gave the highest yields also gave the least water pollution. Because water table management increases crop yields while improving drainage water quality, this technique will play a key role in the sustainability of subsurface drainage systems in the future.
Agricultural non-point source pollution is a serious problem. Land needs to be tile drained; however, when the land is tile drained the potential for leaching of agrochemicals is increased because the drains may serve as conduits for the transport of nitrates into ditches and surface waters.
Excessive use of N-fertilizer also increases nitrate leaching into these waters. In particular, nitrate-nitrogen (NO3-N) pollution may create serious environmental and health concerns which threaten the health of the receiving water bodies as well as the safety of human drinking water supplies, and accelerate eutrophication of lakes and rivers. Efficient methods of nutrient and water management must be adopted to preserve the quality of our water.
Water table management is effective in reducing (NO3-N) loading to drainage water. It keeps water and nitrates in the soil profile for plant use rather than being drained away. The higher water table enhances denitrification which reduces the amount of leachable nitrates. Water pollution can be significantly reduced by as much as 30 to 50% when compared to conventional drainage.
In the United States, it is estimated that about 8 million kg of nitrogen are no longer getting into surface waters because of the adoption of this practice. With full implementation of controlled drainage to areas which are physically suited to the practice, nitrogen loading to surface water could potentially be reduced by nearly 100 million kg annually. This environmental benefit can also lead to yield increases and significant N-fertilizer savings for producers.
Saving on N-fertilizer costs is the other economic benefit offered by water table management. Significant N-fertilizer savings were achieved through this practice in experimental fields in Quebec and Ontario.
Figure 1: Not Available
Leaching predominantly occurs during the non-growing period and most of the data from Bainsville was collected during the growing season. NO3-N losses were thus lower in Bainsville than in St. Emmanuel where the losses were measured on an annual basis. The equivalent amounts of N-fertilizer saved (kg/ha) with water table management as compared to conventional drainage was calculated. Using current N-fertilizer prices for three fertilizer types (Ammonium nitrate, Urea and Anhydrous ammonia), savings in input costs in terms of dollars per hectare were calculated.
Overall, the research findings show that farmers can save significant amounts of fertilizer (16 to 42 kg per hectare, or, $10 to $16 per hectare) with water table management. Economic benefits not only include reduced fertilizer losses, but also increased crop yields. There is also an enormous positive environmental benefit, in that nitrate pollution of streams, rivers, lakes and groundwater is significantly reduced.
Where topography and soil conditions are suitable, and a water supply is available, subirrigation is preferable to controlled drainage. This is because subirrigation will have a greater effect on reducing drought stress, thereby providing an additional economic benefit of increased crop yields. Your LICO drainage contractor is capable of designing subirrigation and water control systems. Ask him for details about your project.
These variations in the amount of NO3-N lost demonstrate that leaching is highly dependent on rainfall, fertilizer application rates, cropping systems, soil type and other management practices. However, it is clear that water table management invariably reduced NO3-N leaching at both sites when compared to conventional free drainage. This efficient use of N and water translates into improved drainage water quality and considerable savings in N-fertilizer cost for the farmer, as well as higher crop yields.