The debate over how close to lay tile lines would seem to be one of economics. How close can you afford to go to best drain the field and expect a payoff from the investment? Experience has shown that 100 feet is too wide in almost any soil. Some have settled on 50 to 60 feet in typical silt loam soils that need drainage. Others have tried narrower spacing with three-inch tile instead of four-inch tile.
The one variable you may not have considered is what happens to nitrate that could be carried out with tile water. Eileen Kladivko, Purdue University Extension researcher in agronomy, has studied this in different soils and at different spacing.
Most of her long-term data has been collected at the Southeast Purdue Ag Center near North Vernon. The soil is Primarily Cobb's Fork, once classified as Clermont soil. The change was primarily made because Cobb's Fork typically does not have a fragipan above three to four feet, and best describes the soil instead of Clermont.
Whatever you name it, the soil is still what many refer to as the 'buttermilk flats'' which people once thought couldn't be drained. Leveling the surface helps on these soils. Kladivko and others have shown they can be drained.
She has tested three tile spacings. While her work is in meters, it translates roughly into 66, 33 and 15 feet spacing between tile lines. Obviously the 15-foot spacing would provide the best drainage potential in the field.
However, it also provides the most challenge for managing nitrogen that could get into tile water, she says. The narrower the tile line, the more nitrate found in the water coming out of the tile when collected and sampled in a lab.
She expects nitrate levels in tile water to become an even bigger environmental issue in the future. It's worth considering when determining how you will tile a field.