Tile Drainage Brings Benefits and Challenges
Written by Ev Thomas and Dan Wiersma*
|There Are Clear Benefits||Less Runoff|
We last discussed tile drainage in our Field to Feedbunk in the May 10, 2012, issue on page 343. At that time, public concerns about the potentially negative impact of tile drainage on water quality were being expressed. These concerns certainly haven’t abated and in fact are intensifying as new data becomes available.
We refer to these systems as “tile drainage” even though that’s become a misnomer since a high percentage of subsurface drain systems installed in recent years use PVC tubing, not the old clay tiles our grandparents used in yesteryear.
The history of tile drainage in the U.S. goes back a lot further than our grandparents: The idea of tile drainage was brought to the U.S. from Scotland in 1835, and tile drainage installations rose considerably in the decades following the Civil War.
There Are Clear Benefits
The results of university research and farmer experience arrive at the same conclusion: Tile drainage is a huge benefit where soil drainage is less than ideal. Some tile drainage systems aim at eliminating wet areas in fields while parallel, whole-field systems are more common where seasonally high water tables or poorly drained soils limit productivity.
The spacing in parallel systems depends to some extent on soil type, with normal spacing ranging from 30 to 120 feet. The trend in the past 10 to 20 years has been to space tile lines closer together, and in some cases farmers have had fields redrained, with the contractor running new tile lines midway between the old ones.
By allowing farmers to get on their land earlier in the spring and by permitting deeper rooting of forage crops, tile drainage can in some cases double yields and also result in better forage quality. The importance of deeper roots is especially noticeable during a dry summer. In fact, one of the best times to visually locate tile drains in an alfalfa field is during drought stress, since the alfalfa growing over the tile line is often noticeably more vigorous.
Tile drainage reduces runoff and surface erosion, but there are concerns about the nitrates and phosphorus discharged from tile outlets. The initial concerns were mostly about nitrates since they’re water soluble and there’s a good correlation between the amount of nitrate in the soil and what’s in tile discharges.
The soil will usually do a good job of tying up phosphorus, especially in alkaline soils — those with a pH over 7.0. It’s not only the topsoil pH that’s important but the pH of the subsoil as well.
More recently small amounts of dissolved phosphorus have been found in tile system discharges. However, research at the William H. Miner Agricultural Research Institute in Chazy, N.Y., measured nitrogen and phosphorus losses in two tile drained fields from October 2015 through March 2016. About 80 percent of the total water loss from these fields was through tile drainage, yet 90 percent of the bioavailable phosphorus was in the comparatively small volume of surface water runoff. The situation was much different for nitrogen: Over 90 percent of the nitrate nitrogen loss was from tile drainage.
In new tile drainage installations, there’s recent interest in the use of in-line structures to regulate the amount of water — and, therefore, nutrients — in tile outlet discharges. The structures are opened early in the spring to allow excess water to exit through the tiles, allowing for timely fieldwork and planting. After planting, the structure is closed to maintain the water table and provide water to the growing crop. Following harvest the structures remain closed to prevent nutrient losses.
Based on preliminary data, an in-line structure system would cost between $500 and $2,000 for fields with 0.5 percent or less slope, for new tile drainage installations. That’s quite a range in price, but field conditions (including tile line spacing) vary greatly which influence the cost.
Existing systems can potentially be retrofitted as long as the slope is very small — probably 0.2 percent or less, but there’s little experience with this. USDA research suggests that in-line structures may reduce nitrate losses by almost half. Yields would be improved by the use of inline structures during a dry summer but probably not if precipitation was adequate.
Experience with these structures is limited to corn-soybean rotations, so it may take time to learn how they might work on dairy farms with a forage rotation. In the future, less expensive types of in-season tile line management may become available, and they appear to hold promise, so stay tuned.
At the end of the day, tile drainage, like many other things in life, has a good side and a challenging side. Learning to manage these systems to reduce nitrate and phosphorus runoff will allow farmers to use these systems for improving yield and reducing risk in their operations well into the future.
*Ev Thomas is retired from the William H. Miner Agricultural Research Institute and president of Oak Point Agronomics Ltd. Dan Wiersma is the alfalfa business manager with DuPont Pioneer.
Used by permission from the January 10, 2017, issue of Hoard’s Dairyman. Copyright 2016 by W.D. Hoard & Sons Company, Fort Atkinson, Wisconsin.