How to use the Fertilizer Calculator:
  • Click on the link to ‘Fertilizer Calculator’ above
  • Select crop category from the list of options (e.g. Dairy Grazing, Cereals, etc)
  • Enter details on soil P and K index, stocking rates, concentrate feed usage, yields, and organic manures by selecting from the lists provided
  • Click submit to see the breakdown of the P and K requirements for maintenance, soil buildup, P and K applied in organic manure, and chemical P and K required.
  • Note that P fertiliser rates may need to be adjusted based on Nitrates limits on your farm.
  • Repeat steps 2-5 to calculate and P and K requirement for a different crop or grassland scenario.

Note 1 – To calculate stocking rate as kg/ha Org N, allow 85 kg of Org N per for every 1 LU/ha

Note 2 – To calculate concentrate feed used in t/ha, divide the total concentrate feed used on the farm (t) by the total grassland area (ha)

Note 3 – For crop yield, use DM yield for silage crops, and fresh yield for cereals, oilseed, legume and root crops

Note 4 – 1000 gallons per acre of slurry applied is equal to 11 t/ha. 1 t/acre of solid manure is equal to 2.5 t/ha

Note 5 – Multiply kg/ha * 0.8 to convert to units/acre

The P and K Calculator was developed by the Fertilizer Association of Ireland in Association with Teagasc and K+S UK & Eire Ltd.

K + S Teagasc


This P and K calculator tool provides general guidelines for crop offtake and/or general agronomic advice for P and K application to grassland and arable crops in Ireland.

For more detailed information, see Teagasc Nutrient Advice.

The guidelines included for crop requirement and organic fertilizer value may result in recommendations that exceed the maximum N and P allowances on the farm.

Fertilizer usage must be in compliance with Nitrates Regulations and other nutrient management legislation. This may constrain the rates of N and P fertilizer that are permissible relative to the guidelines shown by this tool.