Deciding whether it’s economically viable to tile drain is not simply a matter of looking at a muddy field and saying, “I sure as heck want to fix that.”
Crop value is an obvious factor that goes into the equation, but another factor that may not be as well understood is soil permeability.
Soil that allows water to quickly travel laterally to the tiles is a good bet. Producers can get by with a lower investment per acre because tiles can be more widely spaced.
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On the other hand, soil that wants to hold onto the water and not let it flow freely to the tiles will require closer row spacing, driving up the cost. If that higher investment is not justified by potential profitability once the field has been tiled, it’s a no go.
In southern Ontario, where agricultural land sells for $25,000 per acre and a standing grape crop might be worth $1 million per acre, it’s not unusual to see tile drainage installed on eight foot or 10 foot centres to match the rows of grapes. After all, with that kind of money on the line, tile drainage is cheap insurance.
Those big numbers don’t make sense on the Prairies, but the basic math to arrive at the cost justification point is similar.
Bruce Shewfelt of the Prairie Farm Rehabilitation Administration’s office in Morden, Man., works with producers who are trying to determine the economic viability of tiling. Once a project receives the go ahead, he works with the producer to come up with the best possible design.
One instrument in Shewfelt’s toolbox is a device called the Guelph Permeameter, which measures hydraulic conductivity.
“Hydraulic conductivity determines how fast water will move toward the drain tile. This determines what the tile spacings must be. From there it’s easy to determine whether or not a project is economically viable.”
While soil maps and soil classification can provide a general idea of permeability, the data is often not specific enough and may vary 10-fold for a particular soil series.
Shewfelt said the concept behind the Guelph Permeameter is simple. The instrument drives a shaft into the soil profile, to a maximum depth of 315 centimetres. Once the desired depths have been reached, 2.5 litres of water are poured into the top and the time it takes to infiltrate the soil is measured. Depending on the soil, it takes from half an hour to two hours for the water to dissipate into the soil.
Shewfelt said large continuous pores have high hydraulic conductivity. Small interrupted pores have low hydraulic conductivity, which means sandy soil has better hydraulic conductivity than clay. Compacted soil always has a low hydraulic conductivity rating.
