NITRATES - Reducing Water Pollution from Agriculture
High Soil Nitrogen Reserves (Spring 2005)
In many parts of England, recommended rates of fertiliser nitrogen should be lower in 2005 than previous years. This is due to the dry winter causing only low levels of N leaching, and the high price of nitrogen fertilisers.
Nitrogen use on each field should be carefully assessed, taking full account of any above average soil N supply (SNS) and the supply of crop available N from organic manure applications.
This years Met Office map of excess winter rainfall (EWR) up to 16 March 2005 (See Figure B), confirms that winter 2004/05 has been much drier than average. A dry winter means that less N is leached from soils. This will result in fields having more soil N for crop use in 2005, and thus a lower requirement for fertiliser N.
 |
 |
 |
| Figure A: Excess winter rainfall (long term average) |
Figure B: Excess winter rainfall (at 16 March 2005) |
Figure C: Excess winter rainfall (at 24 March 2004) |
These maps are based on data from the Met Office MORECS model. They can be found at www.adas.co.uk/home/springn.html. This information is provided by ADAS on behalf of Defra.
Fertiliser Recommendations for Agricultural and Horticultural Crops (RB209)
Farmers and advisers should consider carefully which RB209 table to use for determining the SNS Index for 2005 crops. Table A (up to 150mm EWR) should be used in most arable areas of England, Table B (150-250mm EWR) in the North East and parts of the West Midlands and South coast, and Table C (over 400mm EWR) in the far West.
Low winter rainfall will have the greatest effect in the following situations:
- On medium and heavy soils following crops that leave significant soil N residues (e.g. following oilseed rape, peas and beans, potatoes, brassicae, grassland).
- Where frequent applications of organic manures have been applied in recent years.
- Where organic manures have been applied since autumn 2004.
|
Example The above crop available N values are calculated using the PLANET software. Register at www.planet4farmers.co.uk for your FREE copy. |
Reductions in N use will not be appropriate for all crops. Where the SNS Index is 0 or 1, the effects of a dry versus wet winter are minimal. Also, on easily leached sandy or shallow soils, soil N is lost quickly and most will be gone even after a dry winter.
Further information supports advice that SNS levels in 2005 are, on average, likely to be higher than normal.
- Many soils analysed for SMN (soil mineral N) are showing higher levels than normal.
- ADAS data from 80 field sites as part of the NVZ monitoring programme can be used to compare N leaching this winter with the long term average for each site. Leaching data is based on regular samplings of 10 porous cups at 90cm soil depth at each site and is currently available up to mid Feb. A consistent pattern is emerging. By mid Feb, between 10-30kg/ha less N had been leached than in a normal year. Examples of data from some individual fields are shown below.
N leaching from selected NVZ monitoring sites (mean of 10 porous cups at 90cm at each site)
Coping with Higher Fertiliser Prices
Although nitrogen remains a usually very cost-effective input, fertiliser prices have increased significantly and recommended rates are therefore slightly lower. Adjustments to the RB209 recommendation depend on knowing the breakeven ratio which is calculated from the cost of N and the value of the crop.
Winter cereals are based on a breakeven ratio of 3:1 (i.e. 3kg grain needed to pay for each 1 kg of N), and a ratio of 2.5:1 for oilseed rape. The look-up table below shows the breakeven ratios for various cereal/rapeseed crops at different ammonium nitrate prices. In 2005, many cereal farmers will be judging N use where the breakeven ratio is between 5 (AN bought at £120/t) and 6 (AN bought at £145/t).
|
Cereal/OSR product
sale price (£/tonne) |
Ammonium nitrate (34.5% N) (£/tonne)
|
||||
|
90
|
110
|
130
|
150
|
170
|
|
|
Breakeven ratio
|
|||||
|
50
|
5.2
|
6.4
|
7.5
|
8.7
|
9.9
|
|
60
|
4.3
|
5.3
|
6.3
|
7.2
|
8.2
|
|
70
|
3.7
|
4.6
|
5.4
|
6.2
|
7.0
|
|
90
|
2.9
|
3.5
|
4.2
|
4.8
|
5.5
|
|
110
|
2.4
|
2.9
|
3.4
|
4.0
|
4.5
|
|
130
|
2.0
|
2.5
|
2.9
|
3.3
|
3.8
|
|
150
|
1.7
|
2.1
|
2.5
|
2.9
|
2.9
|
Once the farm breakeven ratio is known, the following adjustments to the RB209 recommended rates should be made with the larger adjustments on clay and medium textured soils. Reductions in N use may not be appropriate for milling wheats where achieving a grain protein content of 13% is important.
| Crop and Breakeven ratio | Change in recommended N rate |
| Feed winter wheat, feed winter barley | |
| <4.0 | No change |
| 4.0 | Reduce by 10-15kg/ha N |
| 5.0 | Reduce by 15-20kg/ha N |
| 6.0 | Reduce by 20-30kg/ha N |
| 7.0 | Reduce by 30-40kg/ha N |
| Winter oilseed rape | |
| <2.0 | Increase by up to 20kg/ha N |
| 2.0 to 3.0 | No change |
| 3.0 to 3.5 | Reduce by up to 30kg/ha N |
For further information, contact your FACTS adviser.
Page last modified:
6 April 2005
Page published: 29 April 2004