ARCHIVE: Fertiliser Manual (RB209)

Fertiliser Use for Vegetables

The principles governing the use of fertilisers are given in Section 1.

Crop nitrogen requirement

Where sufficient data are available the nitrogen recommendations are based on a three-step process:

  • Size of the crop – the size, frame, or weight of the crop needed to produce optimal economic yields.
  • Nitrogen uptake – the optimum nitrogen uptake associated with a crop of that size.
  • Supply of nitrogen – based on the nitrogen supply from the soil within rooting depth including any nitrogen mineralised from organic matter during the growing season.

Recommendations are given for typical crops produced in the main part of the growing season. As vegetable crops are planted at many different times of the year and have a range of expected yields, the table in Appendix 10 can be used to customise individual field recommendations. Earlier planted crops may need extra nitrogen because the supply of nitrogen from mineralisation is less than later in the growing season. Some vegetable such as beetroot can have wide ranging yield potential depending on the market. The baby beet crop will have a smaller nitrogen demand but is shallow rooted compared with larger yielding processing crops so will require similar amounts of nitrogen.

The recommendations assume effective pest and disease control. Where crops are grown with minimal control measures or the crop is intended for storage, smaller amounts of nitrogen fertiliser should be considered. In all cases too much nitrogen fertiliser can give rise to poor quality crops especially when growing conditions are difficult. Where large amounts of nitrogen residues from previous crops are expected measurement of soil mineral nitrogen can be helpful.

The use of a decision support system, such as WELL-N to help interpretation of soil nitrogen measurements can be advantageous. Decision support systems can also assist in assessing the need to vary nitrogen recommendations particularly for high or low yielding crops or in cases of extreme weather where nitrogen may have been leached out of the immediate rooting depth of a crop.

Nitrogen residues following vegetable crops

On deep silty or deep clayey soils, nitrogen residues in predominantly vegetable rotations can persist for several years especially in the drier areas. These residues are likely to be evident following ‘High or medium N vegetables’. The SNS tables make some allowance for this long persistency of nitrogen residues. However, the Index may need to be adjusted upwards particularly where winter rainfall is low or where the history of vegetable cropping is longer than one year, and in circumstances where larger than average amounts of crop residue or
unused fertiliser remain in the soil to rooting depth (see Tables A-C in Section 3). In rotations where vegetable crops are grown infrequently in essentially arable rotations, the Index may need to be adjusted downwards.

Nitrogen rich leafy trash from many brassica crops is ploughed into the land at various times of the year. The nitrogen in these materials can become available for use by the next crop very rapidly in summer but more slowly in the winter when the soil temperature is lower. In this situation where double cropping is practised in the summer season, the SNS Index can be increased by 1 Index if following ‘Medium N vegetables’ and by up to 2 following ‘High N vegetables’ as indicated in Section 2. It is important that the growing conditions of the first
crop are fully taken into account, especially where nitrogen may be leached from light sand soils in wet seasons or where excess irrigation has been applied. Sampling and analysis for SMN before the second crop is worthwhile.

Care needs to be taken where residues are ploughed in after late December; the nitrogen may not become available for uptake by the next crop until after that crop requires the bulk of its nitrogen supply.

Where there is uncertainty about the amount of nitrogen in the soil, soil sampling for Soil Mineral Nitrogen (SMN) may be appropriate.

Assessing Soil Nitrogen Supply (SNS)

Many vegetable crops are shallow rooted and cannot take full advantage of mineral nitrogen to 90 cm depth. The SNS Index tables may indicate relatively high levels of SNS from previous crops, but not all of this nitrogen will be in the rooting zone of shallow rooted crops. This will be particularly so following wet winters on medium soils where nitrogen is leached to lower depths. In this situation, even deep-rooted crops may require small dressings of nitrogen (up to 50 kg N/ha) to support establishment. Soil mineral nitrogen sampling is recommended to identify the distribution of mineral nitrogen with depth.

Soil sampling should be carried out to the rooting depth specified in Appendix 2. The amount of nitrogen to rooting depth can be used to estimate soil mineral nitrogen to 90 cm assuming that the distribution of mineral nitrogen is uniform. i.e. 0-90 = 0-60 cm value x 1.5. The recommendation tables will take account only of nitrogen available to the depth sampled.

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