Getting a precise fix on N

28 March 1997

Getting a precise fix on N

MORE precise nitrogen management is the goal of a new series of Velcourt trials. The role of soil structure and the over-winter fate of N residues are already proving vitally important.

"Against the background of Velcourts objective to produce crops at the lowest unit cost, we need to further develop and implement technology which allows us to optimise N usage for yield in a way that is in harmony with the environment," says Ken Shipley, Velcourts northern farms director.

He views current models which consider soil N reserves in the spring and then calculate how much bagged N is needed to get the expected yield as a starting point at best. "Its pretty crude."

A series of replicated trials is now comparing IACRs Sundial system, Hydros Extran Plan, Velcourts own balance sheet and the use of Solomon Tubes (Arable, Feb 28) which assess available N levels.

The need for such a study is demonstrated by work at Epton (Farms) Ltd, Croft, near Skegness. As part of an exhibit at the Association of Drainage Authorities event on Jun 25/26, Velcourt has monitored the fate of residual N in wheat plots given an appropriate dose of N, plus and minus 50kg/ha and none.

The work is being conducted in association with Silsoe College and uses a standard N rate of 200kg N/ha as predicted by Velcourts model for the silty clay site.

Key to that model forecast were soil cores taken in spring 1996, showing 63kg/ha residual N from the preceding wheat crop. Using the Solomon Tubes to track the fate of N over the winter proved very revealing.

"It seems that the 75kg/ha applied in late April last year was not well utilised, because of the dry conditions that followed. You cant forecast that, but you do need to take account of it."

Sampling has shown the excess nitrogen moved to the sub-soil over the winter, confirming the importance of sampling the whole profile. "We take cores from 0-30cm, 30-60cm and 60-90cm," explains farm manager Andrew Spoor.

Sample analysis by NRM confirmed the loss of N to the lower soil profile, which was greatest where the N rate was highest. Testing also shows the topsoil contains less N than expected. "We think that is because once it rained it quickly became saturated, and anaerobic denitrification converted nitrate to nitrogen gas," says Mr Spoor.

"So soil structure is very important. You need good structure at the surface to avoid denitrification and good structure lower down to help root growth."

Indeed vigorous early root growth from the early-drilled crop has helped mop up much of the N that was not used last summer. Soil testing shows just 47kg N/ha is left, suggesting a higher rate of bag N would be needed this year than in 1996.

"The whole issue is clearly very complex. Measuring how much N is left in the soil in spring is a good indicator. But its not that meaningful if viewed alone. What we need is to understand what is happening over winter better so we can take account of it when setting our N rates in the spring," concludes Mr Shipley.

MORE precise nitrogen management is the goal of a new series of Velcourt trials. The role of soil structure and the over-winter fate of N residues are already proving vitally important.

"Against the background of Velcourts objective to produce crops at the lowest unit cost, we need to further develop and implement technology which allows us to optimise N usage for yield in a way that is in harmony with the environment," says Ken Shipley, Velcourts northern farms director.

He views current models which consider soil N reserves in the spring and then calculate how much bagged N is needed to get the expected yield as a starting point at best. "Its pretty crude."

A series of replicated trials is now comparing IACRs Sundial system, Hydros Extran Plan, Velcourts own balance sheet and the use of Solomon Tubes (Arable, Feb 28) which assess available N levels.

The need for such a study is demonstrated by work at Epton (Farms) Ltd, Croft, near Skegness. As part of an exhibit at the Association of Drainage Authorities event on Jun 25/26, Velcourt has monitored the fate of residual N in wheat plots given an appropriate dose of N, plus and minus 50kg/ha and none.

The work is being conducted in association with Silsoe College and uses a standard N rate of 200kg N/ha as predicted by Velcourts model for the silty clay site.

Key to that model forecast were soil cores taken in spring 1996, showing 63kg/ha residual N from the preceding wheat crop. Using the Solomon Tubes to track the fate of N over the winter proved very revealing.

"It seems that the 75kg/ha applied in late April last year was not well utilised, because of the dry conditions that followed. You cant forecast that, but you do need to take account of it."

Sampling has shown the excess nitrogen moved to the sub-soil over the winter, confirming the importance of sampling the whole profile. "We take cores from 0-30cm, 30-60cm and 60-90cm," explains farm manager Andrew Spoor.

Sample analysis by NRM confirmed the loss of N to the lower soil profile, which was greatest where the N rate was highest. Testing also shows the topsoil contains less N than expected. "We think that is because once it rained it quickly became saturated, and anaerobic denitrification converted nitrate to nitrogen gas," says Mr Spoor.

"So soil structure is very important. You need good structure at the surface to avoid denitrification and good structure lower down to help root growth."

Indeed vigorous early root growth from the early-drilled crop has helped mop up much of the N that was not used last summer. Soil testing shows just 47kg N/ha is left, suggesting a higher rate of bag N would be needed this year than in 1996.

"The whole issue is clearly very complex. Measuring how much N is left in the soil in spring is a good indicator. But its not that meaningful if viewed alone. What we need is to understand what is happening over winter better so we can take account of it when setting our N rates in the spring," concludes Mr Shipley.

Getting to grips with the fate of N applications are Velcourts Ken Shipley (left) and Andrew Spoor.

&#8226 Over-winter fate of residual N critical.

&#8226 Sample carefully.

&#8226 Precision needed to cut cost and benefit environment.

&#8226 Soil structure has a role.