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

The P & K calculator is an online calculator for estimating P and K fertilizer requirements for grassland and crops.

It is available online, or via the iOS and Android Smart Phone Apps.

It requires details of crop yield, grassland stocking rate, concentrate feed usage, soil test results, and slurry application to estimate chemical fertilizer P and K requirements.

All calculations are consistent with Best Practice Recommendations from Teagasc.


Soil pH & Lime

Apply lime to increase soil pH. Optimum soil pH is critical for efficient uptake of fertilizers.

Optimum soil pH for crops

Cereals and Maize 6.5
Grassland 6.2-6.3
Potatoes 6.0

Sulphur (S)

A significant proportion of Irish soils will show a response to applied sulphur (S) There is no reliable soil test for S, but responses are most likely on light textured soils with low organic matter content.

The following rates of S are advised for crops grown on soils responsive to S:

Grass Silage 20 kg/ha per cut
Cereals 15 kg/ha per year
Oilseed Rape 25 kg/ha per year

Magnesium (Mg)

Mg application to tillage crops is only advised on soils with < 50 mg/L of soil test Mg (Index 1 and 2).

On such soils, using magnesium limestone when applying lime will supply sufficient magnesium to the crop.

Where there is no lime requirement, Mg should be supplied with Mg-containing fertilizers once every 3–4 years as per table below.

Foliar Mg products can also be used to help to prevent and alleviate Mg deficiency in growing crops.

Mg recommendations (kg/ha) – Tillage crops

Cereals, Maize, Oilseed Rape 60-75 30
Beet 100 50
Potatoes 90 45

Soil Index System

Index 3 is the target Index for Soil test P, K and Mg

Soil Index System for P, K and Mg for Grassland and Tillage Crops (Morgan’s soil test)

Index Phosphorus (P)
Grassland Tillage All Crops All Crops
1 0-3.0 0-3.0 0-50 0-25
2 3.1-5.0 3.1-6.0 51-100 26-50
3 5.1-8.0 6.1-10.0 101-150 50-100
4 >8.0 >10.0 >150 >100

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


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.

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