According to researcher Lynn Forster from Ohio State University agricultural contaminants such as nitrogen and phosphorus in two major rivers flowing through rural northwestern Ohio have dropped by up to 50% over the past 10 years, mainly due to farmers’ rapid adoption of a technique called conservation tillage. This technique involves minimal ploughing, which leaves plant stubble intact on at least 30% of a field after harvest. These crop residues anchor the soil, making it more resistant to water or wind erosion.

The proportion of farmland in Ohio’s Maumee and Sandusky River watersheds under conservation tillage rose from 5 to 50% between 1985 and 1995. At the same time, nitrogen levels in these rivers dropped by around 15% while phosphorus fell by about 48%. Forster used a computer simulation developed by the US Department of Agriculture , the Erosion Productivity Impact Calculator, to confirm that the decreases in river pollution observed were linked to changes in agricultural practices.

Government incentive programme to curb water pollution have encouraged farmers in this predominantly corn- and soybean-farming area of Ohio to switch to conservation tillage, according to the university. Ohio farms in the two river watersheds occupy about half of all American and Canadian cropland draining into Lake Erie, one of the world’s largest freshwater lakes and one that has historically suffered from rural and urban pollution.

However, conservation tillage should be combined with other practices that minimise the use of chemicals, warns Rattan Lal, a soil scientist also from Ohio State University. Because ploughing controls weeds, farmers who plough less rely more heavily on herbicides for weed control.

If herbicides and other water-soluble chemicals aren’t applied in the proper amounts, using the appropriate methods or at the right time, Lal says they could still flush into waterways even if soil is minimally tilled. This might explain the 10% increase in water-soluble nitrates in the two Ohio rivers.

Although erosion is only a moderate problem in Ohio, it is the major cause of agricultural land degradation around the world, especially in Asia, says Eric Craswell, director of the International Board for Soil Research and Management in Bangkok, Thailand. Brazilian farmers have also cut back on erosion by adopting minimum tillage, Craswell explains, and researchers are working with other tropical farmers to implement this technique and to plant hedgerows and grass strips in order to try to reduce erosion.

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