A fifty percent reduction in nitrogen and phosphorus concentrations in eroded sediment is possible under conservation tillage, according to latest findings* from the ongoing SOWAP (Soil and WAter Protection) project.

But while there is potential for conservation tillage to reduce nutrient loss from fields into watercourses, the project has yet to see this realised at a catchment level, said SOWAP’s Ceris Jones.

This could be due to the fact that the N & P reduction was seen in sediment in a relatively small area of a field, whereas catchment results are for levels in water relating to a number of fields, she explained.

Differences in location and when samples were taken (e.g. after rainfall) could also be factors. “As we get more results from the trials we will try to better understand the effects of all these factors.

“The SOWAP results to date show that conservation tillage has the potential to reduce nutrient (and sediment) loss from fields into watercourses, but we need to better understand why we haven’t seen this potential realised at the catchment scale.”

Other findings from the trials include:

• Yields were similar for ploughed and conservation-tilled plots (around 8t/ha for winter wheat and 4-5t/ha for winter oilseed rape)
• But, the risk of getting lower yields increases as tillage intensity decreases
• Ploughing results in greater variation in the microbial community structure than in soils under conservation tillage
• Conservation tillage has increased herbicide costs where blackgrass is present, but there has been little change where the weed is not a problem
• Conservation tillage could favour earlier skylark nesting and may increase the length of the effective breeding season (but this may be due to wide row spacing and large amounts of residue on this particular farm)
• Ploughed catchments saw a slight decrease in invertebrate species compared to conservation tilled and semi-natural catchments

For more information, see www.sowap.org

*The trial was carried out in Leicestershire from March 2004 to December 2005