A recent tour began at the National Soil Tilth Laboratory at Iowa State University. It is the second largest research facility of the entire U.S. Department of Agriculture. The Walnut Creek project has been going on for seven years and has focused specifically on how nitrates, a primary agricultural nutrient, and the four most commonly used herbicides move within the watershed both on the surface and through ground water.

      Dramatic shifts in climate patterns in recent years have allowed researchers to get a much better picture of how weather drives movement of these chemicals. It was interesting to note that no two moved alike.

      The process begins with a system of remote testing stations scattered throughout the watershed. Water samples and flow rates are automatically taken from both surface and tile drainage flows. Hundreds of samples are taken back to the lab where they are evaluated by a one-of-a-kind array of computer-driven, robotics-testing equipment. It operates pretty much around the clock collecting and recording a range of data on each sample. Chemical contaminants are extracted from the samples and sent to another lab where their various components are identified and measured to a level of parts per billion.

      It's been discovered that little, if any, contaminants reach deep ground water at the upper end of the watershed where glacial clays dramatically slow downward water movement. Flowing water and all that it carries mixes directly with deeper ground water once the stream reaches its lower end in the Skunk River's broad sand and gravel aquifer. In fact, the stream often seeps entirely into the sand before it even reaches the river when flow rates decline.

      Nitrates dissolve move readily in water and easily can be lost before growing crops use them. It's been calculated that fields in the watershed lost an average of $7 worth of nitrate fertilizer per acre per year over the past seven years. That loss rate jump to about $18 per acre in 1993, more nitrate than was actually put on as fertilizer.

      Herbicides, thankfully, tend to bind closely to soil particles and come into the water primarily with surface runoff and the silt loads it carries. Only very tiny percentages of applied herbicide tended to be lost to erosion most years of the study.

      Why do we care? For one thing, nitrogen fertilizer costs are rising and farmers can't afford to have it washed out of the soil and wasted. For another, it tends to fertilize aquatic plants causing algae blooms that reduce water quality as far away as the Gulf of Mexico.

      It's fairly certain, now, that a "dead zone" that forms seasonally in the Gulf of Mexico west of the Mississippi's mouth is a direct result of lost nutrients flowing from the Corn Belt states. No life other than anaerobic bacteria survives there. Those nutrients did no good for farmers who spread them to feed crops, either.

      The Tilth Lab's mission doesn't stop with identifying problems but continues with attempting to find solutions that will insure long-term enhancement of the nation's soil and water resources. I'll share some of their ideas in another column.