Manage Your Water Table

Manage Your Water Table

Drainage boosts yield and impacts field operations, soil characteristics and soil health

What single step can you take that will improve yield the most, and the fastest? After examining thousands of yield maps in various locations, Farm Journal Field Agronomist Ken Ferrie believes the answer is to improve drainage in fields that need it.

When soil is waterlogged, crops stop growing because of the lack of oxygen. Beneficial microorganisms die off, and nitrogen is lost to volatilization.

An ongoing study, now in its 14th year, that Ferrie is conducting on a 240-acre field in central Illinois confirms his belief. Besides the effects on yield, the study is documenting drainage’s impact on the efficiency of field operations, soil characteristics and soil health. 

“Especially in a wet year, or in an average year with a month of wet weather, we can see the detrimental effects of excess water on yield maps, whether the crop is wheat, soybeans or corn,” Ferrie says. “When farmers ask if they should spend money on fertilizer or tile, we answer tile. You can’t fix the effects of poor drainage by applying fertilizer.”

Think of drainage as controlling seasonal high water tables, Ferrie says. “Before we installed drainage, the seasonal water table was 6" above the soil surface in portions of the Illinois field. When soil gets saturated during the growing season, it is very hard on a crop. Roots need oxygen to drive cell respiration. In saturated soils, without oxygen, plant growth ceases.

“If the soil stays saturated long enough, plants die,” Ferrie says. “If saturation occurs intermittently, perhaps two days at a time, throughout the growing season, it lowers the crop’s yield potential, and there’s no way to recover it. If soil is only saturated for even one week, at the wrong time, it triggers many problems.”

Besides root growth, oxygen is important for soil health (and healthy soil produces higher yielding crops). “Most of the soil microbes that play a big role in soil health are aerobic, or oxygen-breathing,” Ferrie says. “They can’t survive very long in saturated soil. Anaerobic bacteria, which generally are detrimental to crop production, will increase. That causes nutrients that have a gaseous state, such as nitrogen and sulfur, to volatilize into the air and be lost to crops.”

In summary, when you lower seasonally high water tables, oxygen enters the soil, roots are able to flourish and soil is healthier. Beneficial microbes thrive, mineralizing nutrients locked up in the soil and making them available to plants. “You produce more yield, the soil releases more tied-up nutrients, fewer nutrients are lost to volatilization and you can apply less fertilizer,” Ferrie says. “As the cycle continues, healthier crops produce more residue for the good microorganisms to recycle into the soil, releasing the nutrients later for future crops.” 

The study has documented the benefits of drainage on field operations and fertilizer efficiency. “In 2008, during a 48-day period of wet weather, the tiled area of the field with 30' spacing had 38 days when the top 6" of soil was dry enough to plant without compacting soil,” Ferrie says. “The area without tile had only four days, and they were not consecutive. Tile lines spaced at wider intervals fell in between.” 

Consider the impact of wet soil not just on planting efficiency, but also on crop growth through the entire season. “When a field is too wet to plant and you have to pull out and move to another area, efficiency suffers,” Ferrie says. “You have to choose between planting late or working wet fields to dry them out. Usually, you’ll choose to work the soil in order to plant on time. So poor drainage is responsible for compaction that you’ll have to deal with all season long.”  

But that’s only one way poor drainage leads to inefficient planting, Ferrie continues. “Wet areas in a field usually are not conveniently located,” he says. “They may stretch all the way across the field. That makes it impossible to cross the field to plant the drier areas. If 80% of the field is ready to plant, but 20% is wet, most farmers will delay planting until they can plant the entire field at one time. So they sacrifice yield on 80% of the field by planting after the optimum window.” 

Improved drainage facilitates less intensive tillage, which saves time and fuel and improves soil health. “Before our study, the operator of that field would not have considered no-tilling because the soil took too long to dry out,” Ferrie says. “But since we drained the field, he has implemented vertical tillage and even some no-till, especially on soybeans.” 

Increased machinery efficiency does not end with planting. “During one 20-day sidedressing window, installing drainage provided 12 more days in which we were able to sidedress fertilizer without creating wheel-track compaction,” Ferrie says. 

At harvest, while neighboring fields are experiencing delays and combine ruts, tiled fields can be harvested on schedule. “Timely harvesting, in turn, lets the operator be timely with fall fertilizer and lime application—and tillage if he needs to do any,” Ferrie says. 

In the swampy area that’s outlined, corn yield increased by about 60 bu. per acre thanks to drainage. The average yield of the entire field increased by almost 20 bu. per acre. Drainage not only eliminated standing water but also facilitated timelier planting.

How much can drainage really increase yield? Before installing tile, Ferrie collected baseline yield data on the field. After drainage was installed, he tracked the yield for 14 years.

“We’ve seen corn yield in some areas of the field respond to drainage with a 60-bu. yield increase,” Ferrie says. “The field average increased by nearly 20 bu. per acre. With soybeans, we’ve seen from a slight yield decrease to an increase of 3 bu. to 4 bu. per acre.”

Installing drainage also paid off in more efficient fertilizer use. “Because the drained soil is better aerated, micro-organisms release more nutrients and less nitrogen is lost through denitrification,” Ferrie says. “So the operator is producing 250 bu. to 270 bu. of corn per acre with the same amount of nitrogen fertilizer he used to apply to produce 210 bu. to 215 bu. The amount of applied nitrogen per bushel of corn produced has fallen from 1.1 lb. to 0.7 lb.” 

In drought years, the study showed tile can have a negative impact on yield. “In 2003, a very dry season, soybeans planted over the deeper tile lines suffered,” Ferrie says. “But even with a little yield loss in dry years, the return on investment for tile is strong.”

It might be possible to overcome the negative effect of tile in dry seasons. “In one area of the field, the ground rises, so we had to place the tile deeper than normal in order to drain the land behind it,” Ferrie says. “Over the deeper tile, the yield went down every year. But when we installed gates at the outlet and held some of the water in the tile lines, the yield went back up.

“There seems to be a strong correlation between the depth of tile and how the crop reacts in a dry year,” Ferrie says. The continuing study will examine the effect of depth, use of gated structures to regulate the water table and how fast water leaves the field. 

Thanks to tile, the days available for planting without causing soil compaction increased almost 10-fold when wet soil was drained. (Planting days were calculated by measuring the amount of water in the top 6" of soil.) 
Source: Farm Journal Test Plots

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. water_management


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