Phosphate and Potash Nutrient Calculator 2024

This phosphate and potash nutrient calculator is for farmers and crop nutrition advisors in England, Wales and N Ireland*. It does two calculations, nutrient offtake and nutrient deficiency correction.

You can download the app on Apple and Android devices using the links below:

• Apple: iPhone or iPad
• Android: phone or tablet

Nutrient offtake calculation

It enables a quick calculation of the phosphate (P₂O₅) and potash (K₂O) offtakes by most UK crops at harvest. Enter the crop details and yield in the top section, then click the ‘Calculate’ button to show the nutrient offtakes on the bottom lines. These quantities are often termed ‘maintenance dressings’, and usually provide the recommendation for fertiliser use when the soil Indices are at target level (with some exceptions, such as potatoes). If the soil Index is above the target level a fertiliser dressing may be reduced or considered unnecessary. The cost of such maintenance dressings should of course be charged to the crop being grown.

Nutrient deficiency correction

If soil Indices* are below the target level the second section of the Calculator provides a guide to the quantity of nutrient which is likely to be required to correct the deficiency. This will vary according to the level of deficiency and the soil type. The quantity will usually be more than can reasonably be applied in one season, and the Calculator asks by how much and how quickly you want to correct the deficiency, i.e. over how many years. The desired improvement can be input by Index or more precisely by mg/litre as determined by soil analysis*. Once all relevant information has been input (for one or both nutrients) clicking on the ‘Calculate’ button will provide a guide to the total quantity of nutrient(s) which will be required. The annual dressing is also shown, according to the timescale selected. The bottom line shows this annual deficiency-correcting application, together with the maintenance dressing. The cost of the nutrient(s) required to correct the deficiency should not be charged to the crop being grown but should be accounted for separately.

It is important to read the notes attached to the Calculator. Also, further information is provided in the PDA Newsletter Calculator for estimating nutrients needed to correct soil deficiency, with additional background in Standard recommendations cannot make a poor soil good overnight.

The Calculator cannot be used to indicate timescales for any reductions in Indices higher than target.

* The soil nutrient Indices are those described in the PDA Leaflet 24 ‘Soil analysis: key to nutrient management planning‘ and in the AHDB Nutrient Management Guide (RB209). The analytical values (mg/litre) are those derived by Olsen’s method for P and by exchange using ammonium nitrate for K.