What to Know Before You Drain

March 5, 2016 02:15 AM

Improved drainage, the key to higher yield, hinges on sound planning  

Often the shortest route to higher yield is improved soil drainage. But planning the perfect system for each field is no simple task. On-farm experience and a long-term drainage study have supplied Farm Journal Field Agronomist Ken Ferrie with tips that can help make drainage effective and cost-efficient.  Among them:

  • Be creative with landlords.
  • Know the capacity of your outlet.
  • Consider the drainage ability of each soil.
  • Don’t place tile too deep.

As you make drainage plans, remember its fundamental purpose. “What you really are doing is controlling the depth of the water table,” Ferrie explains. “Regulating the water table makes conditions more conducive for tillage, planting and harvesting, and increases crop yield.

“While the crop is growing, your drainage system must prevent soil from being saturated with water for more than 24 hours to avoid crop damage. The ideal condition for roots is 60% water and 40% oxygen in the soil macropores, where roots and beneficial soil organisms thrive. After 48 hours of saturation, the root system starts to suffer stress and begins to die,” he adds.

In the Farm Journal Test Plots, moisture sensors revealed exactly when soil was dry enough to plant. “We found installing tile in undrained soil expanded the planting window from 12 to 30 days in wet seasons,” Ferrie says. “Waiting for soil to dry is one of the greatest obstacles to timely planting.”

            Your drainage contractor will               determine the size of tile, depth                    and spacing to meet your                  objective—as long as you have an                   outlet that can handle the                    volume of water. 

Good drainage makes the difference between easy farming and a struggle. “Each farmer must decide what is the most cost-effective way to install it in each of his fields,” Ferrie says. 

Yield maps—the more years, the better—will reveal if you are losing yield in certain areas of a field. If you are, and the cause is poor drainage, your first decision is between pattern-tiling—often considered the ideal system but also the most expensive—and what you can actually accomplish. That usually depends on funding.

“We often find situations in which a pocket of poorly drained soil is reducing crop yield almost every year,” Ferrie says. “The ideal solution would be to tile the entire field. But if the field is owned by an absentee landowner who doesn’t want to pay for tile, frequently nothing gets done.”

Don’t give up. “For relatively little cost, you may be able to run one tile line, and maybe a few laterals, to the wet spot,” Ferrie says. “That may improve yield in the wet area enough to pay for the tile, with no cost to the landowner. The increased yield may even help persuade the landowner to drain the rest of the field.”

If it’s a large farm, you might offer to drain the wet spots in return for a reasonable lease long enough to recoup your investment.  “Look for a creative solution that lets you do something to improve drainage, rather than continuing to waste fertilizer on areas that don’t produce a full crop,” Ferrie suggests. “I’ve seen $2,500 pay for enough tile to drain a 10-acre wet area, increasing the corn yield by almost 50 bu. per acre. The tile paid for itself in two years.”  

Deciding to drain is easier if you own the land. There, drainage improves the economic value as well as crop yield and soil health. 

         Gated control structures must be           included in the 
          design of a new drainage project      before tile is laid.

“The first thing to check is whether you have access to an outlet that can carry the water off your field,” Ferrie says. “Sometimes water has to travel a mile from a field to reach an outlet. Occasionally, the water has to be pumped up a grade. If you tap into an outlet used by others, the outlet must have enough capacity to accept the additional water. Frequently, the outlet influences the design of a tiling system. 

“The faster you move water away from your field, the bigger outlet you require,” Ferrie continues. “If you install a system to drop the water table 6" in 24 hours and then discover the outlet can only take 2" in 24 hours, you could waste a lot of money.”

A drainage contractor or engineer determines the size of tile, depth and spacing to meet your drainage objective. “The basic principles are the closer together the tile lines, the faster you move water and lower the water table,” Ferrie explains. “The deeper the tile lines, the more water you will remove.”

You might need to make some compromises. “The formula for draining your soil may say 30' is the ideal tile line spacing,” Ferrie says. “If you have a poor outlet and can’t move water quickly, you may have to place lines wider and deeper. That will pull the water table lower, providing a place to store water when you get a saturating rain.”  

A good outlet lets you design a system based on your soil’s ability to drain, which is influenced by hydraulic conductivity (how easily water flows through the macropores), permeability and the seasonal water table depth. 

For example, a 68 Sable silty clay loam has moderately high hydraulic conductivity, moderate permeability and a high water table. In contrast, a 91 Swigart silty clay loam has moderate hydraulic conductivity, moderate to slow permeability and might have a parched water table. These two soils must be drained differently.

“With higher conductivity, you can move water farther, so you can use wider tile line spacings, maybe 60' or 100',” Ferrie says. “With low conductivity, tile lines must be laid closer together, or it will take too long to unsaturate the area between the lines.”

Tile design is more challenging if you have multiple soil types in the same field. “If a field is split between high- and low-conductivity soils, spacing lines at 90' or 100' might correctly drain only half the field,” Ferrie says. “If cost is no object, you can space the lines to drain the toughest part of the field. But it’s more cost-effective to use different spacings suited to each soil.”

Tile depth is influenced by spacing. “The farther apart the tile lines, the deeper they must be to drain the centers within 24 hours after a saturating rain,” Ferrie says. “The challenge is whatever depth we put the tile is the depth where the water table ends up.” 

That can create a problem if you have to tile through a rise to reach a low, wet area behind it. Because the tile is farther below the surface in the high ground, the water table might fall too low to meet the crop’s water requirements during dry periods. 

“We encountered this in our study, when we pattern-tiled an area of varying elevation,” Ferrie says. “We had to place tile 6' deep to drain an area behind a rise. We found that, while yield increased in the wet areas of the field, it declined on high ground, for soybeans as well as corn, especially if we had two or three weeks of dry weather during the growing season.”

The study also showed it’s possible to have too much drainage and how to avoid the problem. “One way is to run non-perforated tile through the rises,” Ferrie explains. “Another is to run a submain through the hill and connect laterals behind it. A third is to use gated control structures to maintain the water table in the higher part of the field. After we installed gates in 2015, the high area showed a yield increase for the first time since we tiled the field.”

Placing tile too deep could affect an entire field. “It might sound cost-efficient to tile a field at 120' intervals, planning to use the increased revenue to add more lines later and splitting the intervals to 60',” Ferrie says. “But to space lines 120' apart, and not have wet areas between them, you have to go deep. If it turns dry, the water table will fall to that depth.” 

A better solution is to tile just part of the field at 60' intervals, and then tile more of the field later, as crop revenue increases. Or you can place lines deep on 120' spacings and install control gates. “Later, if you split the spacing to 60', you can lay the new tile lines shallower and use the gates to hold the water table in the 120' lines at the same depth as the shallower tile.”

Gated control structures must be included in the design of a new drainage project. “You can’t just stick them in anywhere, after the tile has been laid,” Ferrie cautions.

Just as in every other aspect of crop production, sound planning of your drainage system will make you a better water manager.  

As a limited yet vital input, water demands a high level of diligence. The Water Management series details how farmers can manage earth’s most valuable resource to boost yields and profit.

www.FarmJournal.com/ water_management

Drainage Ratings 

Drainage ability ratings for most soils can be found on the USDA–Natural Resources Conservation Service (NRCS) Soil Survey website at http://websoilsurvey.sc.egov.usda.gov. But if you’re not familiar with the site, finding the information you want can be challenging, so you might want to ask for help from your NRCS district conservationist or drainage engineer. 


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Spell Check

Lillian Moore
Heriman , UT
7/6/2016 12:37 PM

  I thought it was interesting that often the shortest route to higher yield is improved soil drainage. Would draining the soil allow for the water to drain only if it is excess or will any water put on the crop be drained? What do you suggest as the best method of draining water into other parts of your crop? http://www.hazelbros.com/about.html


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