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Reducing the risk of nitrate loss after harvest

Last updated: 27 Jul 2015

With all the emphasis on NVZs, it’s sometimes forgotten that leaching of nitrate is a natural process that will happen whatever we do and on most types of land. The key point is the amount. We can’t stop nitrate leaching but we can influence the amount leached.

Nitrate is lost by leaching when water is moving down the soil profile, typically during late autumn and winter. Nitrate also can be lost by conversion to nitrogen gas and nitrous oxide especially when the soil is very wet. As a rule of thumb, if drains are not running, little nitrate is being lost. Usually, very little of the nitrogen applied in spring is lost by leaching or as gas until the following winter. So, the amount of unused nitrate in the soil after harvest is an indication of the extent of nitrate loss during winter and the less there is the better.

There are three ways to minimize the loss of nitrogen from the soil over winter:

  • Make sure there is as little nitrate as possible in the soil after harvest
  • Make sure soil nitrate after harvest is not replenished by any increase in ammonium-N supply (ammonium-N can convert to nitrate during autumn)
  • Make sure as little as possible of this nitrate gets lost to water or air

Soil nitrate after harvest

The most effective practical way to minimize soil nitrate after harvest (‘residual soil nitrate’) is to ensure the supply of available nitrogen is no greater than crop requirement. If more nitrogen is applied than the crop needs, the amount of residual soil nitrate is likely to increase. However, applying less than the crop needs will not necessarily reduce the amount of residual nitrate. It has been shown in many field experiments throughout Europe that, for winter cereals at least, the amount of residual soil nitrate does not increase significantly as nitrogen supply increases up to the economic optimum rate. If the optimum nitrogen rate is exceeded, residual soil nitrate increases sharply. So the best way to minimize residual soil nitrate is to identify and apply the optimum rate of nitrogen in spring.

cereal yield diagram_443_281

Ammonium-N supply

Ammonium-N is added to soil after harvest whenever manures are applied or soil is cultivated (mixing soil with air promotes breakdown of organic matter and release of ammonium-N). Applying in autumn organic manures which contain a large proportion of readily available nitrogen (mostly ammonium-N) can increase the amount of nitrate formed and the loss of nitrogen. Changing from autumn to spring application (encouraged by NVZ rules) will prevent much of this loss and help get best value from the manure. You can check this using MANNER-NPK (free from www.planet4farmers.co.uk) by entering dates for an application just before and after the closed period.

Keeping soil cultivations in autumn to the minimum practically possible will help to retain nitrogen and minimize formation of loss-prone nitrate.

Loss of nitrate over winter

If land is left bare over winter, all of the nitrate in the soil after harvest will be at risk of loss. If the next crop is autumn sown or a cover crop is established some of the nitrate will be mopped up, held in plant tissue and protected from loss. Cover crops can be effective at this provided they are established early, preferably by the end of August and latest mid-September. There are rules on species and date of sowing and retention if cover crops are used as an EFA option. The nitrogen taken up by the cover crop, sometimes as much as 80 kg N/ha, might not be released after incorporation in time for the next crop but at least it is retained in the soil and not lost down the drain or into the air.

Under oxygen-free soil conditions, nitrate converts to nitrogen gas which is lost to the air. These conditions occur when the soil is waterlogged or compacted. Improving drainage in known wet patches and keeping autumn traffic to a minimum will help.

There isn’t a magic bullet to prevent nitrogen loss over winter but a great deal can be done by management to ensure as little nitrate is left or formed after harvest and as little as possible of this is lost.