Top Crop Manager

Features Fertilizer Seed & Chemical
Controlled drainage has big benefits

Managed tile outflow systems can reduce losses and improve yields.

November 12, 2007
By Top Crop Manager


14aThe Gulf of Mexico is suffering from the impacts of agriculture hundreds of
miles upstream from the mouth of the great Mississippi River. A zone of hypoxia,
in which oxygen levels are extremely low, has left a vast area of water unable
to support marine life. The cause of the problem has been identified as excessive
nutrients delivered by the Mississippi from its huge watershed. Now a focus
is on eight mid-west states: Minnesota, Wisconsin, Iowa, Missouri, Illinois,
Indiana, Ohio and Michigan to discover the most practical and efficient way
to reduce nutrient flow into the basin and in the case of the northern-most
regions, the Great Lakes and north-flowing rivers.

Conceptual drainage outlet control structure for subsurface drainage
main line.

“The source of the problem is somewhat complex, but is becoming clearer,”
says professor Larry Brown of the Department of Agriculture and Biological Engineering
at Ohio State University. “However, how to reduce the nitrate load is the
real question. Our research demonstrates a strong linkage between agricultural
subsurface drainage and nitrate-N losses to surface waters.” Scientists
and engineers have formulated a number of potential solutions to help address
the problem. One obvious, but least economical, method to reduce nitrate-N losses
is to abandon subsurface drainage systems throughout the mid-west. This approach
is practically impossible, however, as crop production would be reduced substantially
on millions of acres of productive poorly-drained cropland soils in the mid-west.
In addition, sediment and phosphorus concentrations in surface waters would

Brown says there are far more practical methods, including:

1. Implementation of wetland restoration areas, denitrifying ponds or managed
riparian zones where drainage water could be 'treated' to remove excess nitrate-N,
and other potential pollutants, before discharge into drainage ditches or streams.

2. Use of alternative cropping systems that contain perennial crops to reduce
nitrate-N losses.

3. Increasing the use of improved soil N testing methods to determine the availability
of mineralizable N and N carried over from the previous crop, especially following
dry years, legumes, or past manure applications.

4. Fine-tuning fertilizer N management; applying the correct rate of N at the
optimum time might substantially reduce nitrate-N losses.

5. Improving the management of animal manures to lower nitrate-N losses in
livestock producing areas. This would include better information on manure nutrient
and water contents, and improving application rate uniformity and incorporating
manures in a timely manner.

6. Designing new subsurface drainage systems, or retrofitting existing drainage
systems, to manage soil water and water table levels through controlled drainage,
and/or subirrigation, thus lowering concentrations of nitrate-N in shallow groundwater
and reducing the discharge of water and nitrate-N to surface waters. This is
called agricultural drainage water management (ADWM).

The idea of installing controlled outlet drainage is not new. Systems have
been in practical use in eastern Ontario and in southwestern Ontario to retain
subsurface water and to provide subirrigation. Research by Agriculture and Agri-Food
Canada's Chin Tan at the Harrow Research Station and by Dr. Chandra Madramootoo
at the Bruce Centre of Water Resources Management, Macdonald Campus of McGill
University in Quebec, have shown that nitrate N and herbicide losses to surface
waters can indeed be significantly reduced and crop yields can be increased.

Madramootoo says that by maintaining the water table at 24 to 30 inches (60cm
to 75cm) below the soil surface during the growing season, the crops are able
to extract moisture from the water table through a process known as upward flux
or capillary rise. Water can be pumped from a ditch or reservoir, or from a
groundwater well to the tile drain outlet to raise the water table. In order
to achieve subirrigation, several types of inexpensive water control structures
are available for installation on the tile outlet. The number and size of these
would depend on the topography, soil type and area being managed. “Should
rainfall occur, the gates or valves on the water control structures can be opened
so that drainage occurs and in this way, the water table is kept out of the
crop root zone,” he adds.

Table 1. Grain corn yield (t/ha).

Free drainage
Percent increase in grain yield

The goal is to use the soil as a water storage reservoir and only remove water
from the soil in order to ensure field machine trafficability and prevent waterlogging
in the crop root zone. Studies conducted by drainage researchers at Macdonald
College show that grain corn yields can be increased by 33 percent with subirrigation
(see Table 1). This is financially advantageous, in terms of the extra revenue
to the crop producer.

An added advantage of subirrigation is that keeping the water table at about
24 to 30 inches (60cm to 75cm) below the soil surface will reduce the amount
of nitrates leaving the tile outlets. In the same studies conducted by McGill
researchers, it was found that nitrate-nitrogen concentrations from subirrigation
systems could be up to 80 percent lower than from conventional drain tile outlets
(see Table 2). And total nitrate-nitrogen losses, in terms of loads (kg/ha),
could be 60 percent to 88 percent less with subirrigation. There are two principal
reasons for this reduction in nitrate loads with subirrigation: denitrification,
and the dilution of the nitrates in the saturated zone between the water table
and the tile lines due to the introduction of the subirrigation water.

Table 2. NO3-N concentrations (mg/l).

Free drainage
Free drainage

Tan's work in the early 1990s found similar results: nitrate N loading in tile
outflow was dramatically reduced using water table control during the winter
season. From November to April, the loss of nitrate N with free drainage was
reduced by 37 percent and the total drainage water outflow was reduced by 33

Tan notes that an integrated approach that includes water table control, reduced
tillage, intercropping with cover crops and managed water tables in the crop
season to nutrient uptake can both enhance soil conditions and further reduce
losses. Recently he has worked with Essex County farmers to design systems that
store drainage water from tile outflow in reservoirs and recycle the water through
subirrigation to the crop.

In Tan's most recent research, in 1999, a wetland reservoir was constructed,
approximately 45 by 30 metres and 2.75 metres deep (140ft x 95ft x 8ft). Subsurface
tile discharge and surface runoff from plots in an area measuring 131 by 54
metres (430ft x 174ft) was stored in the reservoir, then used to subirrigate
the crop in 2001 and 2002. “The system increased grain corn yield by 91
percent and soybean yield by 49 percent relative to non-irrigated plots,”
says Tan. As well, nitrate concentration in tile water was reduced by 14 percent
and total nitrate loss compared to free drainage was reduced by 27 percent.
The equivalent dissolved organic and inorganic phosphorus losses were reduced
by 47 percent and 54 percent respectively.

These benefits far outweigh any losses in yield by taking the reservoir area
out of crop production. “Most importantly, for growers, is that they will
achieve an increase in yield while benefitting the environment,” adds Tan.

A research task force has been established in the US to provide data to support
the idea of controlled drainage. As well, an industry coalition is busy with
developing practical control stations for installation into existing or new
subsurface drainage systems. There is very real pressure on US agriculture to
reduce its impact on the environment and these two groups are working towards
practical solutions that will achieve this while also maintaining or even improving
agricultural productivity. There is also a high expectation that US farmers
will be able to apply for government assistance to incorporate drainage management

How big is the US problem?

The Mississippi River basin covers 41 percent of the contiguous US. The basin
contributes 55 percent of US agricultural outflow, and 36 percent of the runoff
to the gulf. Scientists estimate that agricultural production contributes 2.6lb
of nitrate-nitrogen per acre. Considering there are 1.2 billion acres involved,
it is cause for concern.

Many of the sources are traceable, at least to some of the smaller watersheds
that make up the Mississippi watershed. About 90 percent of the nitrate-N discharged
via the Mississippi is most likely from agricultural non-point sources, and
as much as 56 percent of this nitrogen may be traced to the portion of the Mississippi
basin above the Ohio River, and thus about 34 percent comes from the Ohio basin.
Also, nitrate contributions are not evenly distributed between watersheds. For
instance, the upper Mississippi River basin accounts for about 15 percent of
the total Mississippi basin area and 22 percent of the water discharge, but
half of the nitrate discharge.

Since 1980, about 1.6 million tons of nitrogen discharges into the Gulf of
Mexico annually. A number of scientists suggest that a 40 percent reduction
in nitrogen loading to the gulf would be necessary in order to return levels
to what they were between 1950 and 1970. Without continued efforts to reduce
nitrogen loading, the problem promises not to go away and more pressure from
development and intensive agriculture are sure to continue.

The Bottom Line

The idea of controlled drainage fascinates me. The ability
to water your crop at a critical time would be very beneficial. We are
very careful with the amount of nitrogen applied to our crops. Grahame Hardy, Inkerman, Ontario.

As an industry, we have come a long way to reduce contamination in our
waterways. Conservation farming, controlled runoff, reducing erosion,
variable rate application of fertilizer are a few of the methods we have
developed. Managing tile outflow seems to be the next logical step. The
question is, how affordable is it to the cash crop farmer? 
Leo Guilbeault, Belle River, Ontario.