Recommendations for fertiliser use

To ensure crop yields with economical fertiliser applications, Defra offers a guidance framework for growers. Dr Clive Rahn and Nigel MacDonald report on the latest revision to recommendations for field vegetables.

It became clear in 2006 that the seventh edition of the national Fertiliser Recommendations (RB209), published by MAFF in 2000, was in need of revision. Defra has funded the revision, with Professor Keith Goulding of Rothamsted in overall charge. Several sections are being revised and it is hoped that the new fertiliser manual will be available in spring 2009.

The field vegetables section has been revised by the authors of this article, supported by an expert group consisting of representatives from the main commodity organisations, crop associations and HDC. Changes were needed to reflect increasing yield levels and to provide a transparent system for calculating fertiliser demand. The new recommendations are based on the best data available. The organisation of the recommendation tables remains the same, but there are some differences.

The largest changes have occurred with the nitrogen recommendations, which are now based on a transparent framework linked to crop demands. This enables some adjustment of recommendations to take account of local conditions and crop type. The lack of recent field trial research testing nitrogen response of the great diversity of crops was problematic but overcome by asking a series of questions. These included making some assessment of commercial yields in practice, how much crop had to be grown in order to achieve such yields, what was the associated nitrogen content and where that came from. These questions formed the basis for the framework for revised nitrogen recommendations.

The answers to these questions came from a variety of sources, with contributions from growers and the expert group. Experimental data some going back to the 1970s provided some fundamental information about the relationship between the size of the crop and its nitrogen off-take when optimally fertilised. It provided information on the relationship between marketable yield and whole crop yield, again at optimum nitrogen levels. Where data was not available from the UK it was sourced from overseas, with data from a German recommendation system filling some gaps.

Some of the information used was already present in decision support models such as WELL_N and EU-Rotate_N. In the new fertiliser manual the release of nitrogen from soil organic matter is explicitly taken into account. These models have provided estimates of the amounts supplied. Large amounts of nitrogen, around 120kg/ha, can be supplied from soil organic matter to long-season crops such as Brussels sprouts. This is in contrast with short-season crops such as lettuce, where only 20kg/ha nitrogen is supplied.

The amounts of nitrogen supplied by the incorporation of previously incorporated crop residues are taken into account in the SNS supply index as in the previous version of RB209. The importance of the assessment of soil mineral nitrogen, especially where brassica residues have been incorporated, cannot be underestimated. Residues can contain over 200kg/ha nitrogen and, if well-managed, can provide most of the needs of following crops. In dry winters on silt soils, SNS 4 or 5 can be achieved. Measures of soil mineral nitrogen status made at appropriate times can provide information on the ability of the soil to supply nitrogen. This is important in a wide range of conditions: in dry winters to confirm the amounts available and after wet winters to check how much nitrogen is available in the rootzone.

Vegetables have a range of rooting depths; some crops, like brassicas, can be well in excess of 90cm but others, like salad onions, might only root to 30cm. Rooting depth makes a large difference to the ability of a crop to extract nitrogen, particularly from previously incorporated crop residues. The framework used in the new fertiliser manual was estimated from yields and by comments from the expert panel, along with data from experiments carried out in the UK and other parts of Europe.

In the new recommendations, greater allowance has been made for rooting depth and as a consequence the recommendations for shallow-rooted crops at high nitrogen-supply indices have been increased. Some crops have increased recommendations due to larger expected yields. High yields do not always mean more nitrogen fertiliser being required as increased rooting depth might make a larger supply of nitrogen available from the soil. Calabrese and cauliflower are dealt with separately as it was felt that the amount of nitrogen given to calabrese should be lower to reflect the high risk of spear rot.

As a result of HDC and Defra-funded work on over-winter brassica crops, more specific recommendations are provided in the new edition to support raising of the maximum proposed limits on nitrogen supply in the nitrate vulnerable zone close periods. Another group has extended the manures section to include nutrient supply from a wider range of materials, including composts.

The framework in the field vegetables section of new fertiliser manual provides a set of recommendation tables that can be adjusted according to yield, crop rooting depth, previous crop residues and organic-matter status. Additionally, the framework does allow flexibility for adding new crops or for more easily revising further editions of the recommendations (an example of the data in the framework is shown in the panel, right).

The new fertiliser manual can only represent a few of the myriad possible cropping scenarios, and computer decision support systems such as WELL_N and EU-Rotate_N still have an important place in providing recommendations in those situations.

Phosphorus and potash

Recommendations for phosphorus (P) and potash (K) have remained largely the same. However, in using the recommendations more account should be taken of yield - while at index 3 there is unlikely to be a response it would be unwise to neglect the balance between what is put on as fertiliser and the amount removed in marketed crop. Some crops, for instance high-yielding carrot crops, can remove over 300kg/ha K20. Some of this can be offset by straw but it is important that the net balance is calculated.

Yield levels of other crops have generally increased since the previous revision so it is important to take account of the amounts of nutrients removed and check soil P and K status every four or five years. Where P levels are high, there is no need for broadcast fertiliser - small, targeted doses of starter fertiliser have been shown to be very beneficial for lettuce and onion crops. Where P and K levels in soil are already low it is important to apply more than is taken off to allow a steady build up.

Other nutrients

No new information is available on the needs of other nutrients, so the recommendations remain as in the previous version. Some indications are provided on the requirements of sulphur for brassica crops.

Future research requirements

Work funded by the Home Grown Cereals Authority (HGCA) has demonstrated that about 30 per cent of all wheat crops grown in the UK could benefit from sulphur fertiliser applications.

No recent experiments are available to test the requirements of brassica crops in the UK. It is known that brassica crops have a large requirement for sulphur; there are instances of sulphur deficiency but no data on which recom-mendations can be based.

There are some crops where we need more information, such as leeks, which have a high nitrogen requirement but are equipped with a poor root system - making scheduling of nitrogen for quality crops a challenge.

Dr Clive Rahn is a senior research scientist at Warwick HRI and Nigel MacDonald is a consultant at ADAS

Example of the recommendation framework - lettuce

This is the framework used behind the recommendation tables in the new version of the fertiliser manual (RB209) for field vegetable crops.

Crop needs

Expected marketable yield (tonnes/ha) - 46
Dry matter of marketable crop (per cent) - 5.3
Proportion of crop grown marketable (per cent) - 50
Total dry matter yield whole crop (tonnes/ha) - 4.8
Associated nitrogen content (kg/ha) - 165
Soil nitrogen supply index 2 (kg/ha to 90cm) - 90
Rooting depth - 45
Release of nitrogen from soil organic matter - 22
Total supply from soil to rooting depth - 67

Fertiliser requirement

Crop needs minus supply from soil (adjusted for rooting depth) equals fertiliser requirement. A further adjustment has to be made for fertiliser recovery, which is assumed to be 60 per cent. Therefore, the fertiliser requirement for this crop is:

(165 - 67)/0.6 = 163kg/N fertiliser required.

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