Nutrient management: Getting nutrients exactly right on grassland

Soil analysis – what and when

• The Farming Rules for Water state that soils must be tested for phosphate, potassium, magnesium and nitrogen every five years.
• However, SNS calculations from the RB209 Fertiliser Manual can be used to establish N levels. Taking a soil sample correctly is crucial because it will impact results.
  
  

How to take samples:

• Sample between October and November
• Do not sample within three months of applying organic manures or mineral P and K fertiliser
• If lime is applied, it is best to wait 12 months for a stable soil pH reading
• Sample at least every five years, and preferably every three years
• Take one sample per four hectares (9.8 acres). Divide fields greater than 4ha (9.8 acres)
• Take separate samples from areas that are different in soil type and cropping history
• Use a soil corer to sample to a depth of 7.5cm on long-term grassland fields
• Sample the area, walking in a W pattern
• Avoid headlands, gate entrances, or where manure or lime was heaped
• Take 20-25 soil cores from the sampling area
• Place soil cores in the sample bag and label clearly with the farm and field name
  
  

Soil testing is the first part of a four-part process for formulating a farm nutrient management plan. The RB209 Fertiliser Manuals form the basis for all calculations and should be referred to for the subsequent steps:

Calculating crop requirements

Use RB209 Section 3 Grass and forage crops to establish the nutrient requirements of specific crops.

Phosphate (P) and potash (K)

There are two tables in RB209 (page 10) which provide guidance on silage ground P and K requirements depending on whether you know your expected grass silage yields or not. If you are unsure, use the standard values for P and K recommendations after each silage cut. Grazing ground figures are available separately.

Nitrogen (N)

Use the Soil Nitrogen Supply (SNS) calculations in the RB209 Nutrient Management Guide to calculate nitrogen levels.

• The SNS will depend on previous management and nitrogen use. On dairy farms which are likely to be using >250kg N/ha/ year, it is likely the SNS will be ‘high’ or ‘moderate’.
• Grass growth class (GGC) of the ground should then be established – this describes the ability of the site to respond to nitrogen, which depends on soil type and rainfall. For example, good or very good GGC sites are swards dominated by productive grass species which respond well to increased nitrogen supply as soil drainage, temperature and water supply are conducive to growth.
• Table 3.8 can then be used to establish nitrogen application rates, depending on target dry matter yields. These recommendations are applicable to grass swards with low clover content in a very good/ good GGC and moderate SNS situation.
• Poor GGC sites are likely to achieve DM yields towards the lower end of the range in most years.
• Application rates should be adjusted depending on SNS sites. For example, on low SNS sites, 10kg N/ha more should be applied for first cut and 20kg N/ ha more for second cut.

Sulphur

Sulphur is considered an essential nutrient in maximising dry matter yield and protein percentage in both grazed and conserved grass (see Use sulphur as standard panel).

Sulphur should be applied on all grass grown on:

• Sandy and shallow soils.
• Loamy and course silty soils in areas with >200mm rainfall between November and February. JClay, fine silty or peat soils in areas with >400mm rainfall between November and February. On soils at risk of sulphur deficiency apply:
• Silage: 40kg SO3/ha before each cut.
• Grazing: 20-30kg SO3/ha when up to 100kg N/ha is applied and an additional 20- 30kg SO3/ha for each additional 100kg N/ha applied.

Knowing the value of farm organic matter

The next step is understanding what nutrients you have on-farm in the form of slurry and farmyard manure (FYM).

The RB209 Fertiliser Manual Section 2 Organic Materials, provides guide values for the nitrogen levels of a typical dry matter slurry. However, Mr Cosgrave says it is well worth testing your farm’s supplies as in practice, NPK levels, can vary considerably.

He explains: “The standard book value for cattle slurry is 6% dry matter, but on two grass trials last year in Derbyshire and Devon, where we tested each farm’s slurry, one had a dry matter of 3.3%, while the other was 1.56%. That shows how you can be way off the mark by assuming a standard book value.” (see Table 1).

He advises testing slurry and FYM at least two to three times a year and using this to balance mineral fertilisers.