Nitrate losses still weather dependent

18 May 2001



Nitrate losses still weather dependent

By Andrew Blake

COVER crops, minimal cultivations and early establishment do help reduce nitrate leaching into drainage water, a 10-year research project concludes.

But nothing is foolproof when it comes to reducing the risk, admits ADAS scientist Mark Shepherd who was involved in the 500,000 project.

If it starts raining in the autumn and then, like last year, doesnt stop there is nothing much you can do to avoid leaching, said Dr Shepherd.

Along with colleague Paddy Johnson, he monitored levels at Peter Smiths farm at Oasby, Lincolnshire for the Ministry of Agriculture funded research.

The experiment, within the Aswarby Nitrate Sensitive Area, was set up to explore ways of controlling nitrate losses when NSAs first came in.

Three management systems — standard, protective and intermediate — were tested over two complete five-course all arable rotations.

These included peas, winter milling wheat, winter barley, winter oilseed rape and winter feed wheat on shallow soil over limestone.

The standard approach involved ploughing before all crops.

The protective policy introduced minimal cultivations and cover crops were used to mop up residual nitrogen before spring sowings.

Bridging the gap between these two schemes was the intermediate system explained Dr Shepherd.

In both five-year periods we showed we could reduce nitrate losses by over 40%, but there are pitfalls, he said.

The average annual nitrogen loss in drainage water under the standard approach was about 56kg/ha (45 units/acre).

Intermediate measures trimmed that to about 40kg/ha (32 units/acre), and the protective policy cut the figure to about 30kg/ha (24 units/acre).

A key message is that whichever management system was used it had no effect on the yields of peas or of milling wheat after peas.

It also looks as though minimal cultivations are useful in reducing N leaching, said Dr Shepherd.

But where we used them we eventually got a build up of grass weeds and ran into problems.

• Rain fails to wash out soil nitrogen supplies, FWi, 16 February 2001
• More N leaching than from grass, FWi, 03 March 2000
• Nitrogen losses need watching after wash-out, FWi, 24 April 1998
  

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