When both phosphorus (P) and potassium (K) are both low in the subsoil there’s a strategic advantage to managing these two nutrients together. Deep phosphorus (P) deficiency is more common than deep potassium (K) deficiency. It’s unusual to see low subsoil K not accompanied by low subsoil P.
Without adequate P root systems don’t develop well. In P sufficient soils, root systems tend to be more extensive and more able to take up other nutrients. Phosphorus status and root vigour affect the crop’s use of other nutrients, including nitrogen. Co-placement of deep fertiliser is a strategy being investigated in current research into managing multiple nutrient deficiencies.
Improved K uptake occurs when a subsoil with low P is remediated with deep P application, even where subsoil K remains low. An improved root structure making use of a larger contact area with soil allows the plant to access more K. This gives a boost to crop performance but further runs down the soil’s store of K.
While applying deep K with deep P may not be immediately required for a yield benefit, long term nutrient budgeting suggests it may be strategic to address low subsoil K at the same time as applying deep P.