Adjustments to main season nitrogen use in spring 2011?
The information below is provided by ADAS and has been agreed with Defra.
The advice here supplements rather than repeats the information given previously in the technical note
‘Cost-effective fertiliser use in spring 2011’ issued in February 2011.
Summary
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Correct nitrogen decisions need the soil N supply (SNS) to be assessed as accurately as possible.
The effect of past cropping and organic manure use will usually have a much more significant impact
on SNS than seasonal factors such as rainfall or temperature. Fields with a history of manure use
usually have higher soil N residues than those with no manure use history. Residues in fields where
organic manures were applied were consistently higher (average 18 kg N/ha) than where no manures
were applied.
Winter rainfall to 30 March has been below average in most areas so N leaching losses this winter are
likely to be lower than normal – many farmers may need to use a different N Index table in RB209
than usual (low, moderate or high rainfall). For instance, some farms in Central and West Midlands
should consider using Table A rather than Table B, and Table B rather than Table C in parts of the
South and South West. In parts of Cambridgeshire and Lincolnshire, there has been very little excess
winter rainfall so N leaching losses will be low.
However, some data indicate that soil mineral N levels in autumn 2010 (before winter leaching) were
also lower than normal. This, combined with the low rainfall, means that SNS levels this spring are
likely to be close to average.
It is important to assess and allow for any overwinter uptake of nitrogen by the crop, especially
advanced winter oilseed rape crops which usually contain a large quantity of N - this is nitrogen that
might otherwise have leached. The amount of nitrogen in the crop at the start of spring will influence
the amount of nitrogen fertiliser that needs to be applied.
Make use of the new Fertiliser Manual (RB209) recommendations and comply with all relevant NVZ
rules. Defra has recently issued some errata to the Manual including a modified breakeven ratio (BER)
adjustment table for winter oilseed rape, and modified N recommendations for grassland used for beef
production.
PLANET version 3.1 will be released soon and will generate nutrient recommendations for grassland
fields according to Defra’s Fertiliser Manual (RB209). Registered users will be informed when v3.1 is
available.
NVZ reminder – you must have a record of your Nitrogen Plan before any N is applied this spring.
NVZ reminder – make sure you comply with the N max limits. Defra’s Fertiliser Manual (RB209)
provides recommendations only which are NOT N application limits for crops grown in NVZs.
NVZ reminder - following amendment, the N max limit for winter oilseed rape is now 250 kg N/ha but
including the quantity of N applied in autumn 2010 (previously the limit was 220 kg N/ha irrespective of
autumn N). This can be increased by 30 kg N/ha per 0.5 t/ha for crops yielding over 3.5 t/ha. The limit
of 30 kg N/ha in the autumn is unchanged.
NVZ reminder – the minimum slurry and poultry manure storage must be in place by the end of 2011.
Assessing Soil N Supply (SNS) using Field assessment method (FAM) – seasonal adjustments
Figure 1 shows average soil mineral N (SMN) levels found in autumn 2010 in over 175 fields used to
monitor soil nitrogen changes in different soil and cropping situations, compared to 2009 and the long term
average since 2004 (source ADAS, NIT18 project funded by Defra). These data measure the SMN residues
following harvest in 2010 before any leaching of N. Levels in autumn 2010 were mostly lower than autumn
2009 and always lower than the long term average, though the differences were mostly less than 20 kg
N/ha. Autumn soil N residues following winter wheat and oilseed rape were typically higher than following
spring barley and sugar beet which reflects the RB209 soil N Index tables. Residues in fields where organic
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manures were applied were consistently higher (average 18 kg N/ha) than where no manures were applied,
confirming the important impact of previous manure applications on the SNS of following crops.
Autumn 2009 no manure
Autumn 2010 no manure
Long-term (2004-2010) no manure
Long term (2004-2010) with manure
Autumn SMN 0-90cm depth (kg/ha)
140
120
100
80
60
40
20
0
Wheat
(winter)
Barley
(winter)
OSR (winter)
Barley
(spring)
Sugar beet
Grass
(permanent)
Figure 1. Average soil mineral N (kg/ha to 90 cms) in autumn 2010 compared to autumn 2009
and the long term average following different previous crop types
Figure 2a shows Excess Winter Rainfall (EWR) this winter from October to 30 March 2011 (source Met
Office) – this is the rainfall that drains through medium textured soils causing leaching of nitrate-N. February
and March have been relatively dry in most areas and most drains have now stopped running. So this map
provides a good basis for estimating how much soil N has been lost by leaching during the winter compared
to the ‘average’ year. Comparison of this years map (Figure 2a) with the long term average (Figure 2b)
confirms that it has been a drier than average winter.
The Fertiliser Manual (RB209) uses EWR to assess the SNS Index of fields (Table A <150mm; Table B 150250mm; Table C >250mm). This year, many farms should therefore consider using a lower rainfall table
than normal for assessing the SNS Index of crops. Since this will tend to reduce the RB209 recommended
N rate compared to an ‘average’ rainfall year, make sure that other factors support this approach before
making any decision to reduce the total amount of N to apply to the crop. For instance, some farms in
Central and West Midlands should consider using Table A rather than Table B, and Table B rather than
Table C in parts of the South and South West. In parts of Cambridgeshire and Lincolnshire, there has been
very little excess winter rainfall so N leaching losses will be low.
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Figure 2a. Excess Winter Rainfall
to 30 March 2011
Figure 2b. Long term ‘Average’ Excess
Winter Rainfall (whole winter to 30 March)
Commentary and Conclusions
Data confirms the importance of previous cropping and organic manure use on residual soil N levels.
Previous crops such as oilseed rape and grass leave higher residues than sugar beet. Residues are usually
higher where there has been a history of organic manure use.
Seasonal factors of rainfall and temperature usually have less influence on SNS than the effect of previous
cropping, manure use and soil type. However, some evidence indicates that residual soil mineral N levels
(SMN) at the end of 2010 might be slightly lower than the long term average, but also that leaching of this
nitrogen has been lower than normal due to the low over-winter rainfall. Also, the cold weather in
November/December is likely to have delayed the mineralisation process and thus reduced the potential
leaching of this source of soil N. The contribution of mineralised N might therefore be greater this year. The
net effect of these factors is probably that average SNS is about average this year.
The other important factor to consider is how much nitrogen is contained in autumn sown crops at the start
of spring growth. Early autumn sown crops, especially oilseed rape, that established quickly with significant
autumn growth, will have taken up much of the residual autumn soil mineral N which might otherwise have
leached. This crop nitrogen content should be counted as part of how much N fertiliser needs to be applied.
Crop inspections are essential therefore before final N application decisions are made as the crop is the
best indicator of the SNS – if the crop is more lush and green than normal, then the SNS is also likely to be
above normal, if the crop is pale and backward then the SNS may be low unless there is something else
causing these symptoms. Winter oilseed rape crops can take up large quantities of nitrogen in the
autumn/winter period (up to 100 kg N/ha) but this is very variable this year depending on sowing date and
growth. High yields of oilseed rape come from avoiding excess leaf production, thus allowing light into the
canopy. Nitrogen use should be based on assessing the Green Area Index (GAI) and for crops over 30 cm
tall, this can only be by weighing a 1x1 m area of whole crop (kg) - multiply this weight by 0.8 to get GAI,
then multiply GAI by 50 to get the crop N content (kg/ha).
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Overall, the effect of soil type, past cropping and (especially) organic manure use will usually
have a more significant impact on SNS than seasonal factors such as rainfall or temperature.
The available evidence and opinion suggests that SNS levels for crops other than winter
oilseed rape are probably about average this year but, as always, are likely to be variable from
field to field.
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Crop inspections are essential to help final N decisions, especially oilseed rape as crops are so
variable. Adjustments to the long term strategy of N use should take account of how previous
crops have performed and the appearance of this years crop relative to previous crops.
The below-average winter rainfall will mean that some farmers may need to use a different
RB209 soil index table when assessing the SNS – i.e. a table appropriate for a below-average
winter rainfall. But also take account of crop appearance and the factors discussed above.
The new Fertiliser Manual (RB209) 8th edition, and PLANET
The latest RB209 publication (published June 2010) can be found at www.defra.gov.uk/RB209. Hard copies
(priced) are available from The Stationery Office (www.tsoshop.co.uk), 0870 2403701, ISBN 978 0 11
243286 9.
Note that the Fertiliser Manual (RB209) provides nutrient recommendations only which are NOT mandatory
limits for crops grown in NVZs. There are mandatory nitrogen application limits for crops grown in NVZs but
these are set as the crop specific N max limits (see NVZ Guidance documents 3 and 7 for full details).
Defra has recently issued some errata to the Fertiliser Manual (RB209).
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Adjustments to N recommendations for winter oilseed rape according to the breakeven ratio (BER)
have been modified.
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Nitrogen recommendations for grassland (beef production) have been modified.
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The recommendation that ‘No adjustment for yield should be made where the soil (PK) index is higher
than target’ should be deleted. This was given for several arable/forage crops.
All of the Fertiliser Manual (RB209) recommendations for arable crops and organic manures are included in
the PLANET version 3 software available free from www.planet4farmers.co.uk.
The new grassland recommendations will be provided in the next version of PLANET that will be released in
spring 2011 – registered PLANET users will be notified when this is available and how to obtain a copy.
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The Nitrate Vulnerable Zones (NVZ) regulations (in force from January 2009)
Approximately 62% of land in England is designated as an NVZ, and the relevant NVZ rules must be
complied with on this land. Non-compliance with the rules will risk a deduction from the Single Farm
Payment.
Key points of the NVZ regulations that need to be thought about this spring are as follows.
For individual fields this spring
• A Nitrogen Plan must be prepared before any manure or N fertiliser is applied to the crop. The soil N
supply, crop N requirement, crop available N from manure applications and the need for
manufactured fertiliser N must all be assessed or calculated. Records must be kept of the Nitrogen
plan for 2010 crops before applying any N this spring, and of each fertiliser and manure application
after application. These records must be available for inspection and kept for at least 5 years
(Guidance leaflet 6).
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Defra’s ‘Fertiliser Manual (RB209)’ (8th edition, published June 2010) provides best practice advice,
but it is not mandatory to follow or keep within the RB209 recommendations.
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DON’T apply (Guidance leaflets 8 and 9):
o manufactured N fertiliser or organic manure if the soil is waterlogged, flooded, frozen or snow
covered (under extreme conditions, farmers should seek advice from the EA),
o organic manure within 10 metres of a surface water, or 50 metres or a spring, well or
borehole,
o N fertiliser within 2 metres of surface water.
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All other required field Records must be kept, including details of the crop grown, the use of N
fertiliser and manure following application, and any written advice from a FACTS qualified adviser. All
records must be available for inspection and kept for at least 5 years (Guidance leaflet 6).
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The quantity of N applied (from manufactured fertiliser plus livestock manure N) must not exceed the
N max limit for the whole area of each crop type grown on the farm (Guidance leaflet 7). Note:Following amendment, the N max limit for winter oilseed rape is now 250 kg N/ha but including the
quantity of autumn N applied (previously 220 kg N/ha irrespective of autumn N). This can be
increased by 30 kg N/ha per 0.5 t/ha for crops yielding over 3.5 t/ha. The limit of 30 kg N/ha in the
autumn is unchanged.
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The amount of total N in organic manures applied to land must not exceed 250 kg N/ha in any rolling
12 month period (Guidance leaflet 8).
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A Risk Map of the farm must be prepared to identify suitable field areas for manure applications, and
for locating temporary field heaps of solid manures (Guidance leaflet 8).
For whole farms that carry farm livestock
• By end 2011 (less than 12 months away!), there must be at least 6 months storage capacity for pig
slurry and poultry manures, and at least 5 months storage capacity for cattle and other types of
slurry. The minimum storage requirement must be calculated using the specified calculation method
(Guidance leaflet 4). Livestock farms should investigate ways of minimising their slurry storage
requirements such as:
o re-routing clean water away from the slurry store
o using a slurry separator
o exporting manure off the farm
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Show compliance with the Livestock Manure N Farm Limit – 170 kg N/ha produced by livestock in a
calendar year, or 250 kg N/ha for farms that have applied for and received a derogation for the
calendar year.
Remember! Non compliance is likely to result in a reduction in the Single Farm Payment and
further inspection.
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Further reading
Fertiliser Manual (RB209), 8th edition. Defra (June 2010), available from the Stationery Office,
www.tsoshop.co.uk or www.defra.gov.uk/rb209.
Nitrogen for winter wheat – management guidelines. HGCA (autumn 2009), www.hgca.com.
Crop nutrition for potatoes. Potato Council (November 2009), www,potato.org.uk.
PLANET. www.planet4farmers.co.uk.
Full details of the NVZ rules can be found in Defra’s NVZ Guidance leaflets (go to
http://www.defra.gov.uk/environment/quality/water/waterquality/diffuse/nitrate/help-for-farmers.htm, or ring
0845 955 6000 for a copy).
Further important clarification of the rules by the EA can be found in the NVZ Q&A at
http://www.environment-agency.gov.uk/business/sectors/54714.aspx
April 2011
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