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Rain, runoff, and underground water.

When farmers aren't praying for rain they're praying for a way to save it.

Just an inch of rain on a 1-acre field amounts to 27,152 gallons. After a month's accumulation of 4 inches of rain, a farmer might find 1 or more of those inches have run off the field.

And with that water goes precious topsoil and costly pesticides and fertilizers. The topsoil and chemicals that reach streams, rivers, and lakes become contaminants.

If farmers wish to keep surface flow and erosion to a minimum they might switch to no-till, an energy-saving method of planting without prior plowing. This leaves previous crop residue on the surface to protect the soil. The residue slows the movement of water across the field, giving it time to soak into the soil.

Farmers planted crops with no-till on about 17 million acres of U.S. cropland last year and used reduced tillage on another 56 million acres. That's a quarter of the cropland with conservation tillage.

But critics point out that no-till farmers have more water going down toward groundwater, shifting the pollution threat from streams to aquifers. What's more, they say, no-till usually requires increased herbicide use to kill weeds that would have been uprooted by plowing.

Are these problems real and, if so, how do we deal with them? A long-term experiment on 1-acre corn plots in Beltsville, Maryland, is delivering some answers.

Agricultural Research Service scientists are already in their sixth year of building what plant physiologist Allan R. Isensee considers "one of the most complete data sets in the country."

Isensee, with the ARS Pesticide Degradation Laboratory, says he and soil scientist Ali Sadeghi are measuring just about everything involved in the movement of water carrying atrazine, alachlor, and cyanazine herbicides across and under cornfields.

The rain that falls on the fields is measured. The water evaporating from the fields is accounted for. The soil is sampled at 4-inch increments, down to 20 inches, to see how far the chemicals are moving down with the water. The groundwater is sampled from 128 wells drilled to depths ranging from 5 to 36 feet.

And rainwater that runs off the field is channeled through stainless steel flumes where ultrasonic sensors measure its level. Flowmeters connected to the ultrasonic sensors electronically convert the readings into flow rates and volumes. The meters also trigger automatic water sampling for every 75 or 100 gallons of flow. Back at the lab, the samples are analyzed for herbicide content.

As Isensee shows a visitor the fields, he notes some welcome news for farmers. With both no-till and conventional tillage, he hasn't found pesticides in the deepest wells, where groundwater might be used for drinking water.

As for the soil, with no-till there is less chemical residue in the first 4 inches than with conventional tillage.

But in the next 8 inches, the situation reverses--higher residues of herbicides with no-till. The good news is that there isn't much getting past the 1-foot mark, says Isensee.

And the chemicals that are in the wells are usually well below health advisory levels issued by the Environmental Protection Agency for drinking water. When levels approach or exceed these amounts, within 2 or 3 days they drop back as the aquifer dilutes them.

"No-till is here to stay," Isensee says, "so we have to learn how to take advantage of it and minimize any harmful side-effects. It does a great job of stopping soil erosion and the movement of chemicals attached to soil. We have very little slope here but we still see a substantial amount of soil eroding on the conventional plots and almost none on the no-till plots. It's saving farmers soil as well as fuel and time."

He says that on the Beltsville plots, no-till cut runoff from low-intensity storms (about an inch of rain) by at least half. However, on more intense storms, preliminary observations indicate that soil wetness before a storm affects runoff and pesticide loss more than tillage practice.

Isensee says that the conditions at Beltsville are generally similar to those in the midwestern Corn Belt. "The herbicides, the amounts, the silt-loam soil, and the rainfall pattern are all similar," Isensee says. "If anything, the conditions in the Midwest, with slightly less rainfall, are even less conducive to leaching than in the East." Leaching is the carrying downward of chemicals by irrigation or rainwater.

"We couldn't afford this intensive instrumentation in every part of the country," he says, "but if we can refine existing leaching computer models to account for regional differences, we could make accurate predictions of chemical movement for each major crop growing area in the country.

"Our long-term field database will provide modelers the opportunity for testing, validation, and perhaps improvement of existing models."

A comprehensive comparison of field observations and model predictions is the final step in verifying any model's worth, Isensee explains.

Of course, he notes, the real test of any model will be whether it helps farmers find conditions under which no-till needs fine-tuning to protect groundwater quality. And will it help them choose solutions such as applying encapsulated pesticides or applying pesticides only in narrow bands to restrict pesticide-soil contact as much as possible?

As a preliminary accuracy check, Sadeghi recently fed 2 years worth of data into an Environmental Protection Agency computer model. He points out that the predictions of pesticide residue amounts in the first 4 inches of topsoil came very close to those observed in the cornfields. However, because of model limitations, the predictions of pesticide residue below 4 inches were not comparable with field observations.

"We need models that can account for localized shortcuts for the transport of pesticides dissolved in rainwater or snowmelt," Isensee says. "These shortcuts might be root channels, soil cracks, and worm holes. They can cause a great increase in pesticide residues at greater depths in one small part of a field. This |hot spot' condition is missed by models that compute averages for entire fields."

PHOTO : Soil scientist Ali Sadeghi (right) and plant physiologist Allan Isensee take samples from a shallow well for pesticide analysis. (K-4516-14)

PHOTO : Plant physiologist Allan Isensee and technicians Valerie McPhatter (center) and Juanita Yates (right) collect soil samples from a cornfield for further analysis. (K-4518-6)

Allan R. Isensee and Ali M. Sadeghi are at the USDA-ARS Pesticide Degradation Laboratory, Beltsville Agricultural Research Center, 10300 Baltimore Ave., Beltsville, MD 20705-2350. Phone (301) 504-5533.
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Title Annotation:critics of no-till farming fear pollution threat to groundwater
Author:Comis, Don
Publication:Agricultural Research
Date:Feb 1, 1992
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