ARCHIVE: Fertiliser Manual (RB209)

Finding the nitrogen recommendation

Fields vary widely in the amount of nitrogen available to a crop before any fertiliser or manure is applied. This variation must be taken into account to avoid inadequate or excessive applications of nitrogen. The Soil Nitrogen Supply (SNS) system has been developed to assign an Index of 0 to 6 that indicates the likely extent of this background nitrogen supply. Once the Index is identified, it can be used in the recommendation tables to indicate the amount of nitrogen, as fertiliser, manure or a combination of both that typically would need to be applied to ensure optimum yield.

The SNS Index for each field can be arrived at either by the ‘Field Assessment Method’ using records of soil type, previous cropping and winter rainfall or by the ‘Measurement Method’ using measurements of soil mineral nitrogen (SMN) plus estimates of nitrogen already in the crop (at the time of soil sampling) plus an estimate of available nitrogen from the mineralisation of soil organic matter and crop debris during the period of active crop growth.

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