ARCHIVE: Fertiliser Manual (RB209)

Field Assessment Method

(for the Measurement Method, go to page 95)

The Field Assessment Method does NOT take account of the nitrogen that will become available to a crop from any organic manures applied since harvest of the previous crop. The available nitrogen from manures applied since harvest of the previous crop, or those that will be applied to the current crop, should be calculated separately using the information in Section 2, and deducted from the fertiliser nitrogen application rates shown in the recommendation tables.

There are five essential steps to follow to identify the appropriate SNS Index:

Step 1. Identify the soil type for the field.

Step 2. Identify the previous crop.

Step 3. Select the rainfall range for the field.

Step 4. Identify the provisional SNS Index using the appropriate table.

Step 5. Make any necessary adjustments to the SNS Index.

In detail these steps are:

Step 1. Identify soil type for the field

Careful identification of the soil type in each field is very important. The whole soil profile should be assessed to 1 metre depth for arable crops and to rooting depth for shallow-rooting vegetables. Where the soil varies, and nitrogen is to be applied uniformly, select the soil type that occupies the largest part of the field.

The soil type can be identified using the following flow chart that categorises soils on their ability to supply and retain mineral nitrogen. The initial selection can then be checked using the soil category table below. Carefully assess the soil organic matter content when deciding if the soil is organic (10% to 20% organic matter for the purposes of this Manual) or peaty (more than 20% organic matter). If necessary, seek professional advice on soil type assessments, remembering this will need to be done only once.

Soil category assessment – flow diagram


 Soil category assessment - table

Soil Category Description of Soil Types Within Category Properties
Light sand soils Soils which are sand, loamy sand or sandy loam to 40 cm depth and are sand or loamy sand between 40 and 80 cm, or over sandstone rock. Soils in this category have poor water holding capacity and retain little nitrogen.
Shallow soils Soils over impermeable subsoils and those where the parent rock (chalk, limestone or other rock) is within 40 cm of the soil surface. Sandy soils developed over sandstone rock should be regarded as light sand soils. Soils in this category are less able to retain or supply nitrogen at depth.
Medium soils Mostly medium-textured mineral soils that do not fall into any other soil category. This includes sandy loams over clay, deep loams, and silty or clayey topsoils that have sandy or loamy subsoils. Soils in this category have moderate ability to retain nitrogen and allow average rooting depth.
Deep clayey soils Soils with predominantly sandy clay loam, silty clay loam, clay loam, sandy clay, silty clay or clay topsoil overlying clay subsoil to more than 40cm depth. Deep clayey soils normally need artificial field drainage. Soils in this category are able to retain more nitrogen than lighter soils.
Deep silty soils Soils of sandy silt loam, silt loam or silty clay loam textures to 100 cm depth or more. Silt soils formed on marine alluvium, warp soils (river alluvium) and brickearth soils are in this category. Silty clays of low fertility should be regarded as other mineral soils. Soils in this category are able to retain more nitrogen than lighter soils and allow rooting to greater depth.
Organic soils Soils that are predominantly mineral but with between 10 and 20% organic matter to depth. These can be distinguished by darker colouring that stains the fingers black or grey. Soils in this category are able to retain more nitrogen than lighter soils and have higher nitrogen mineralisation potential.
Peat soils Soils that contain more than 20% organic matter derived from sedge or similar peat material. Soils in this category have very high nitrogen mineralisation potential.

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