ARCHIVE: Fertiliser Manual (RB209)
C:\Documents and Settings\Andy\Desktop\RB209\RB209_2010_sec8_find.htm

Finding the Nitrogen Fertiliser Recommendation


Procedure

  • Using the table on page 188, identify the Soil Nitrogen Supply status of the field using information for:
    • typical nitrogen use in the last 2-3 years
    • grass management last year
  • Using the table on page 188, identify the Grass Growth Class of the field using information for:
    • the predominant soil type in the field (see page 17 and Appendix 1)
    • average summer rainfall
  • Using the factors on page 189, calculate stocking rate in livestock units/ha (LU/ha).
  • Decide on the intended grass management and select one of the recommendation tables on pages 191-213:
    • Grass Establishment
    • Dairy production
    • Beef production
    • Sheep production
    • Grass/clover swards
    • Hay
  • During the season adjust nitrogen use according to the weather and actual grass growth performance (see page 189).

Assessing the Soil Nitrogen Supply status

The nitrogen recommendations are based on the requirement of the crop to be grown, making allowance for soil nitrogen residues. In grassland systems, these residues are assessed in a different way to that used for arable or vegetable cropping systems. Three levels of soil nitrogen status are recognised and used in the recommendation tables. Fields with a low soil nitrogen status need more nitrogen compared with fields with a moderate or high status.

Nitrogen fertiliser, organic manure use and the grass management history in the last 1-3 years is of most importance for determining the soil nitrogen status, but longer histories can be relevant. Applications of organic manures over the last few years must also be taken into account.

Soil Nitrogen Supply (SNS) Status in Grassland Systems According to Previous Grass Management

 

Previous grass management

Previous nitrogen use
(kg/ha)a
High Long term grass, high input. Includes:
  • grass reseeded after grass or after 1 year arable
  • grass ley in second or later year

over 250

Moderateb First year ley after 2 or more years arable (last crop potatoes, oilseed rape, peas or beans, NOT on light sand soil)

Long term grass, moderate input. Includes:

  • grass reseeded after grass or after 1 year arable
  • grass ley in second or later year
All

100 – 250
or
Substantial clover content

Low First year ley after 2 or more years arable (last crop cereal, sugar beet, linseed or any crop on a light sand soil)

Long term grass, low input. Includes:

  • grass reseeded after grass or after 1 year arable
  • grass ley in second or later year
All

Up to 100

  1. Refers to typical fertiliser and available manure nitrogen used per year in the last 2-3 years.
  2. The nitrogen values in the recommendation tables assume a moderate Soil Nitrogen Supply status and so adjustments need to be made only for high or low Soil Nitrogen Supply: increase total fertiliser nitrogen input by 30 kg/ha in a low SNS situation; decrease total fertiliser nitrogen input by 30 kg/ha in a high SNS situation.
  • Increase the soil nitrogen status by one class if more than 150 kg/ha of total nitrogen has been regularly applied as organic manure for several years. Reduce the soil nitrogen status by one class if grass was cut for silage and less than 150 kg/ha of total nitrogen as organic manure has been applied on average in previous years.

Assessing Grass Growth Class

Grass growth will be restricted where summer rainfall plus the moisture stored in the soil (the soil available water) are inadequate to meet the grass demand for water. Although there can be wide variations in summer rainfall between years, the table below gives an indication of the grass growth potential in an average season, based on the risk of summer drought. For simplicity within the recommendation tables, we combine the 'very poor' and 'poor' grass growth classes. We also combine the 'very good' and 'good' grass growth classes.

Grass Growth Classes

Soil Available Water Soil types a Rainfall b
(April to September inclusive)
    up to 300 mm 300 – 400 mm over 400 mm
Low Light sand soils and shallow soils (not over chalk) Very poor Poor Average
Medium Medium soils, deep clay soils, shallow soils over chalk, and organic soils Poor Average Good
High Deep silty soils, peaty soils and soils with groundwater (e.g. river meadows) Average Good Very good
  1. See Appendix 1 for soil descriptions.
  2. Mean summer rainfall (April to September) is usually about half of annual rainfall.

For sites above 300 m altitude, reduce the growth class by one. This is because lower temperatures will restrict growth.

The recommendation tables indicate how it is possible to decrease total nitrogen requirements the better the Grass Growth Class in order to produce the target amount of home grown forage, as nitrogen is used more efficiently. In other words, the better the Grass Growth Class, the greater the efficiency of nitrogen use and a reduced risk of nitrogen losses to the environment. Conversely, the poorer the Grass Growth Class the higher the nitrogen requirement (assuming no other limiting factors, such as shortage of phosphate, potash and
sulphur supply). This reflects the need to support a target economic level of animal production rather than maximum yield of forage.

Good nutrient management is important to all farming systems and at all levels of nutrient application. In some situations where higher nitrogen requirements are advised, carefully planned use of fertiliser will maximise the nutrients taken up by the crop rather than being lost to the environment. Where good growth is expected lower nitrogen requirements are possible and may be advised, and in this situation good nutrient management is important to ensure nutrients are used efficiently in order to maintain yield.

Calculating stocking rate

Stocking rate is one important factor in determining requirement for herbage. It is usually expressed in livestock units/ha (LU/ha). In estimating the LU for N recommendations: 1 (650 kg) cow = 1LU (other stock can be expressed as LU by pro rata on liveweight, e.g. calves), 1 average beef animal = 0.6 LU and 1 ewe with lambs = 0.17 LU. Calculate the average number of livestock as LU and divide by the total area of grass and forage crops in the enterprise to give stocking rate.

Adjusting Nitrogen Use for Weather during the Growing Season

If grass yields are restricted due to drought, reduce the use of nitrogen once growth restarts following rain. As a guide, if grass does not grow for about 2 weeks in June or July, the yield will be reduced by about 1 t/ha of dry matter and there will be about 40 kg/ha of unused nitrogen remaining in the soil. This residual nitrogen must be allowed for when deciding on nitrogen use once grass growth starts again following rain.

ADLib logo Content provided by the Agricultural Document Library
© University of Hertfordshire, 2011