The environmental benefits of analysing soils part 1: the benefits and issues of nitrogen

Author: Alli Grundy

1st December 2021

NRM

Agriculture

Environment

Two of the most important crop nutrients, nitrogen and phosphate, are known to have major environmental implications if applied in excess of crop requirement. It is valuable to assess the soil supply of nitrogen annually in spring and to monitor the phosphate status of fields through soil analysis every 3 to 5 years. Planning appropriate nutrient use will ensure optimal yield and crop margins are achieved. An appropriate balance of all crop nutrients is vital to ensure optimal yield, but the oversupply of nitrogen and phosphate can have a significant negative impact on the wider environment. Read on to learn about how to use analysis to avoid pollution from excessive nitrogen applications.

The importance of measuring soil nitrogen levels

Nitrogen, a significant driver of yield and productivity, is preferred and used by plants mainly in its nitrate form. Nitrate-nitrogen is, however, highly mobile. If present in excessive quantities, it can rapidly leach into ground and surface waters. Ammonium nitrate fertiliser is a 50:50 combination of nitrate and ammonium ions, the nitrate portion being immediately available to plants. When soils are moist and above 5^oC, nitrification occurs, whereas ammonium ions, which are generally held in the soil, are converted initially to nitrite and then to nitrate ions by soil bacteria. Once the ammonia ions have been converted to nitrate-nitrogen it becomes freely available for crops to access. However, if there is no crop requirement nitrate-nitrogen is at risk of loss to ground and surface waters.

To understand the potential of soils and organic matter to supply crop-available (soil mineral nitrogen) and organic nitrogen, samples should be taken in spring before any fertiliser or organic inputs have been made. The release of organic nitrogen from the soil is temperature dependent and can be considerable, and therefore needs to be accounted for. It can be estimated by measuring the total amount of organic matter a soil contains.

By using the measured soil mineral nitrogen results to calibrate the soil nitrogen supply (SNS) tables in RB209, the quantity of additional fertiliser nitrogen needed to support optimal crop production can be determined.

In season, monitoring crop nitrogen content with tissue testing can help fine-tune or plan later nitrogen applications for improved quality. Currently many winter cereal crops have an average nitrogen use efficiency of between 50 to 70%, while spring cereals are, more often, much less than 50%. Measuring the soil and realistically estimating yield potential will help to increase the recovery of soil and applied nitrogen. If possible, we should aim for the crop to recover 85% of the nitrogen supplied.

The environmental impact of excessive nitrogen applications

The impact of losing nitrogen to watercourses can be very damaging. The drinking water regulations set a maximum permissible level of 50 mg/l.

The main concerns with high concentrations of nitrate in water are:

• The risks to human health from drinking water after abstraction from ground or surface waters;
• Eutrophication (the nutrient enrichment of water) of lowland surface waters;
• Acidification and eutrophication of upland waters;
• Nutrient enrichment in other sensitive habitats like Groundwater Dependent Terrestrial Ecosystems.

Agriculture is the dominant source of nitrate in England, coming to about 70% of total inputs, while sewage effluent comes in second place at 25-30%. In general, nitrate concentrations are greatest in the drier, arable-dominated southern and eastern areas of England. This is also where we are most dependent on groundwater for public water supply and base flow to rivers.

When rivers and streams have high levels of nitrate and/or phosphate, this causes eutrophication, leading to the development of algal blooms. These are especially prevalent in warm weather and can destroy the biodiversity living in the water environment.

Agriculture has room to become more efficient and use nitrogen much more effectively, for both environmental and economic reasons. Better planning ahead of the cropping season, including measuring soil mineral nitrogen for fields within the rotation, will provide a good benchmark of the soil nitrogen levels across the farm. This is a highly cost-effective way to begin to improve nitrogen use efficiency.

Keep an eye out for the second half of my blog, where I talk about the benefits of replacing nitrogen with phosphorus and ensuring responsible applications.

NRM offers services which measure not only standard nutrients, but also soil mineral nitrogen and organic matter with N-Check and N-Check Plus. For a broader picture of the benefits of organic matter, NRM also offers CarbonCheck and CarbonCheck Plus, enabling a better understanding of the quality and functionality of soil.

Contact your agronomist or NRM directly to arrange for samples to be tested today.