The latest Teagasc crops update reviews the range of phosphorous (P) and potassium (K) requirements that will deliver optimal results for growers in 2022.

P and K have many roles in cereal crop nutrition from rooting to nitrogen (N) use efficiency.

To maximise the return from applied N, it is very important to ensure the crop has a balanced supply of major P, K and sulphur (S), plus the minor nutrients – copper, manganese and zinc (Cu, Mn, Zn). 

Requirement for phosphorous and potassium

Phosphorous is very important for rooting and tillering, especially in spring cereals, while K is very important for straw strength and plant tolerance to such diseases as powdery mildew.

Teagasc said that it will be very tempting to reduce or omit P and K in 2022 to control production costs. However, it will be prudent to tailor P and K application rates based on soil test results.

Soils at Index 1 and 2 will be most responsive to applied P and K and have a very low to low P and K supply. These soils will have higher P and K requirements as soil supply will be lower. 

Spring cereals

For spring cereals, where possible, combine drill P at sowing time to increase the efficiency of applied P fertiliser. It will be important to fertilise these soils to their expected grain yield in potential. 

Therefore, in order to control fertiliser costs, omit additional P and K application for soil fertility build-up in 2022.  

Soils at Index 3 have a good nutrient supply; aim to replace P, and K removed at harvest time to maintain soil fertility in the optimum range. 

Again, it is important to fertilise to crop yield potential to maintain soil fertility levels. 

These crops will use N most efficiently and produce the largest grain yields annually.  Omitting P and K will result in soil P and K levels declining, thus reducing grain yield potential in the years ahead. 

Winter wheat

For higher yielding crops, such as winter wheat, maintaining higher soil indexes produces higher grain yields.   

Soils at Index 4 are very fertile and have a good supply of P and K to meet crop requirements during the growing season. 

Up-to-date soil analysis will help identify these soils on the farm as they don’t have a P and K requirement. 

The Teagasc soils database shows that 31% of tillage soils are at Index 4 for P and K in 2020, offering major savings potential on P and K applications in 2022. 

According to Teagasc, soil fertility levels have improved on Irish tillage farms, resulting in higher grain yields. 

To maintain profitable grain yields and hold soil fertility levels at their current levels, it is recommended to fertilise crops to their grain yield potential.

E.g. take the average yield over the last three years to form the basis for calculating P and K requirements in 2022.