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Guidelines for farmers in nitrate vulnerable zones

Table 3: Site Classes- Applicable to grassland only

Grassland production is limited by growing conditions, in particular the quantity of rainfall between April and September and soil type. The combined effect of these factors defines the site class. From the soil type and the average summer rainfall for your area (Table 2), find the most appropriate site class for each field within the NVZ, using Table 3 below:

*Add 1 to the site class for farms above 300m (1000 ft)

Site Class 1 is limited by nitrogen not by water supply since nitrogen is utilised very efficiently and rainfall is above 500mm between April and September.

Site Class 5 is severely limited by water supply since rainfall is below 350mm between April and September. The potential yield of grass growing within a Site Class 5 is about half of that growing in a Class 1 site.

N residues from previous crops or grass/clover swards

• For fields that were in an arable or non-grass fodder crop, a residual N group is allocated according to the previous crop (see Table 4 on the next page).

• The groups are numbered 1 to 5, in ascending order of residual N available in the soil, following the harvest of the previous crop.

• Residual levels vary according to the management and performance of the previous crop.

• Residues may be lower following a high yield, or where N application has been less than normal.

• Residues may be higher if the crop has performed badly due to problems such as drought and disease.

• The tables of N requirements in this guidance assume that previous crops have been well managed and that N fertiliser use was close to the recommended rate, taking account of any organic manures used.

• For fields that were in grass/clover swards, a residual N group is allocated according to fertiliser and manure use in the last 2 years and the grazing management immediately prior to ploughing out (see Table 4).

• The groups for grass/clover swards are numbered 1 to 6 in ascending order of residual available N in the soil going into the winter.

• Analysis of the sward for total N and C content prior to ploughing out is recommended, in order to help predict the release of available N for the next crop.

• Available N is also influenced by the time of ploughing, with less residue being available from autumn ploughing, compared to spring ploughing.

• The higher N recommendations for groups 5 and 6 spring sown/planted crops are based on ploughing out grassland in November/December or March/April.

• Where grassland is ploughed out in January or February use the lower N recommendation, in the absence of sward analysis.

• Where grassland is to remain in grass and is not being ploughed out, the N residue group is still determined from the fertiliser and manure use in the last 2 years, and the grazing management immediately prior to ploughing out.

** N residues can be very variable in groups 5 and 6 and analysis of the crop debris or grass sward for N and C content, prior to ploughing down, is recommended to help predict release of available N for the next crop.

Soil mineral N testing is not recommended in established grassland, nor in the first year after ploughing out. It can however be used one year after ploughing out, in order to help predict the amount of N that will be available to the second crop after grass.

Winter cereals

• Autumn nitrogen is NOT generally recommended, as there is not normally a profitable response and such applications will also increase N leaching.

• Responses to autumn N have been recorded on sandy soils with a high pH and low manganese content. There is also a possible N requirement in some winter barley crops, established by direct drilling/minimal cultivation.

• Spring N is best applied as a split dressing. The general recommendations are:

• A 33%/67% (one-two thirds) split between the start of spring growth and growth stage 30-31.

• A 20/80 split will improve bread-making quality and help to prevent lodging.

• A 50/50 split will help to reduce grain N% for malting or distilling.

Spring cereals

• If crops are sown up to the beginning of April, apply half of the fertiliser N to the seedbed, then:

• For low N malting, apply the balance at leaf stage 2-3.

• For high N malting, apply the balance at the start of tillering.

• From the beginning of April onwards, all of the N may be applied to the seedbed.

• Combine drilling is recommended for early sown crops and crops grown in high soil pH (>6.4).

• Combine drilling of urea is not recommended as close contact can damage the germinating seed.

• Combine drilled fertiliser should be limited to 150kg/ha N + K ₂O on sands and sandy loams.

* avoid growing malting barley after crops in groups 4 - 6 and humose/peaty soils that leave high N residues

Oilseed Rape

Winter Oilseed Rape

• Winter barley is the most suitable crop for early entry as it leaves low N residues.

• Rotational set-aside is expected to leave 10-20kg/ha more N residues, depending on cover (select the appropriate Previous Crop Group for your set-aside).

• Where crops are sown into cold silty and heavy clay soils, higher N is recommended in the seedbed, compared with the lighter soils.

• Autumn sown rape can produce about 20-25 t/ha fresh material by December. N top dressing in spring is best split, applying half at the start of spring growth and half prior to stem elongation.

Spring Oilseed Rape

• Generally utilises soil N more efficiently than winter crops as its requirement for N coincides with the normal period of soil N release in May and June.

• Trials have confirmed that optimum N rates can be substantially reduced for 'swede' types and further reduced by 20kg/ha for turnip rapes, compared to N rates for winter oilseed rape.

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