Phosphorus (P)

Phosphorus fertiliser advice for mineral soils is based on increasing the soil P reserves to a level at which the amount of available soil P supports the production of herbage of optimum quantity and quality.

This is referred to as “soil P build-up”. Once this soil P level has been achieved, the objective is to maintain the soil P level by replacing the P removed in animal products or in hay or silage. This is called “soil P maintenance”. Where the soil P level is greater than that required, there is an increased risk of P loss to the environment. In such cases, P application should be avoided so that the soil P level will decline to the optimum level, so that this risk is reduced. It is advised that wherever possible, animal manures should be recycled to minimise chemical fertiliser needs.

Phosphorus fertiliser advice for grazing is based on the following 6 steps:

  1. Determine P required for soil P build up
  2. Determine P required for soil P maintenance
  3. Take account of P in slurry and farmyard manure
  4. Take account of P in concentrate feed used
  5. Calculate chemical fertiliser P requirements

Determine maximum P allowance, and confirm that the P fertiliser requirement of the holding is compliant with SI 31 of 2014.

Step 1: determine P required for soil P build up

P fertiliser rates advised for build-up to soil P index 3 are presented below. Note that application of build-up rates may be required over a number of years for P-deficient soils to move from Index 1 to 2 and from index 2 to 3. At very extensive stocking rates, or in areas governed by specific legislation such as national heritage areas (NHAs) or special areas of conservation (SACs), it may be appropriate to fertilise soils to a level such that index 2 is maintained.

Available P rates (kg/ha) for build-up on mineral soils

Soil P Index Mineral soils
1 20
2 10
3 0
4 Do not apply chemical P

Step 2: Determine P required for soil maintenance

Each year, P is removed in either animal produce (meat, milk) or as silage or hay. This P must be replaced by applying “maintenance rates” of P as recycled P from concentrate feeds used, recycled animal manures or chemical fertiliser P. Soil P maintenance rates advised for grazed pasture should be equal to the amount of P that leaves the holding as product. In general, 1 kg of P is required to replace the P removed in either 1000 litres of milk or 100kg of liveweight gain.

Example maintenance rates of available soil P to replace grazing offtakes (kg/ha) (taken from Teagasc Green Book)/h3>

Grassland stocking rate
(kg/ha Org N)
System
Dairy Drystock
≤100 6 4
130 10 7
170 14 10
210 19 13
≥210 23 16

Rates of P fertiliser based on concentrate feed usage level of zero. P maintenance requirements for silage and hay are listed in the silage section.

Step 3: take account of P in slurry and farmyard manure

To calculate the available P contained in slurry and farmyard manure, multiply the quantity of each material that is applied to grassland by the P content of that material.

Typical P levels in organic fertilisers¹´²

Organic fertiliser type Total P contained in 1 tonne³ (kg/t)
Cattle slurry 0.8
Pig slurry 0.8
Sheep slurry 1.5
Dungstead manure (cattle) 0.9
Farmyard manure 1.2
Poultry
Slurry (layers 30% dry matter) 2.9
Broilers/deep litter 6.0
Layers (55%) dry matter 5.5
Turkeys 13.8
  1. Values for P are those that must be used for compliance with SI 31 of 2014
  2. Dry matter and nutrient contents can vary widely between farms
  3. 1 tonne of slurry=1m³. 1000 gallons=4.5tonnes. 1000gallons/acre=11tonnes/ha

Step 4: Take account of P in concentrate feeds

Concentrate feed must be considered as a source of P for grassland since most of the P consumed will be recycled in animal excreta to the soil. A deduction of 5kg P should be made from the total P advice for each tonne of concentrate feed that is used on the farm. A deduction for concentrate feed used can be made across the entire grassland area as follows:

Step 5: Calculate chemical fertiliser P requirements

Annual maximum fertilisation rates of phosphorus for grassland (grazing plus silage)

Grassland Phosphorus Index stocking rate (kg/ha/year) Phosphorus Index
1 2 3 4
Available Phosphorus (kg/ha)
≤130 35 25 15 0
131-170 39 29 19 0
Grassland stocking rate greater than 170 kg/ha/year
171-210 44 34 24 0
211-250 49 39 29 0
>250 49 39 29 0
  1. Total annual nitrogen (kg) excreted by grazing livestock averaged over the net grassland area (grazing and silage area). Stocking rate refers to grassland area only.
  2. The fertilisation rates for soils which have more than 20% organic matter shall not exceed the amounts permitted for Index 3 soils.
  3. Manure produced by grazing livestock on a holding may be applied to Index 4 soils on that holding in a situation where there is a surplus of such manure remaining after the phosphorus fertilisation needs of all crops on soils at phosphorus indices 1, 2 or 3 on the holding have been met by the use only of such manure produced on the holding.
  4. The maximum phosphorus fertilisation of grassland shall not exceed that specified for stocking rates less than or equal to 170 kg/ha/year unless a minimum of 5% of the net area of the holding is used to grow crops other than grass or a derogation applies in respect of the holding.
  5. This table does not imply any departure from Article 20(1) which prohibits the application to land on a holding of livestock manure in amounts which exceed 170kg Nitrogen per hectare per year, including that deposited by the animals themselves (or 250kg in the case of a derogated holding).
  6. An additional 15 kg of phosphorus per hectare may be applied on soils at phosphorus indices 1, 2, or 3 for each hectare of pasture establishment undertaken.