Autumn 2023

Measuring and managing soil P and K

Ben Wainwright
Fertiliser Technical

Phosphate (P) and Potassium (K) are two of the big three macro-nutrients for plant nutrition; they are also some of the most considerable crop input requirements for development, growth, and yield. This places a high significance on managing the application and utilisation of P and K.

Throughout the last cropping year, P and K prices along with most other fertilisers, were significantly inflated, leading many growers to take a PK holiday to manage input costs. Despite this, growers should continue to take into account the needs of the soil and crop.

Soil testing via RHIZA or standard sampling is the foundation, but understanding how to achieve the best from soil nutrient inputs will help enormously. Couple these results with broad spectrum analysis of macro-nutrients, micro-nutrients, organic matter and soil texture, and you’ve got a holistic view of understanding nutrient availability. Understanding these values helps to target specific inputs.

Phosphate’s primary roles in crop nutrition is to aid root development, photosynthesis, storage and transport of other nutrients. As such, P needs to be understood and measured to avoid any shortfalls in availability. The rate of freshly applied P ranges between 5-25%, which indicates that the remainder of the P used by the growing crop must come from pre-existing soil reserves of P.

From all the samples taken by Agrii and RHIZA in 2022, 21% yielded below an Index two for phosphate, meaning applications of fresh P are essential for healthy crop production in those scenarios. Conversely, 16.1% of samples yielded an index of four or higher. In those cases, taking an extended P holiday may still be possible by utilising Agrii-Start Release. This soil phosphorus activator makes existing locked-up soil phosphorus available to the plant.

The main functions of potassium are to aid cellulose production, photosynthesis, and nutrient, starch and sugar transport. Of all the samples Agrii and RHIZA took in 2022, 26% of soils yielded below an index of two. If this is the case, fresh K should be applied to supply the crop with sufficient nutrition to establish, grow and optimally, yield.

24% of UK soils yielded a K index of three or higher, again similar to P. In these instances, it may be possible to take a K holiday. Where K levels are at an index of three or higher, take steps to measure magnesium (Mg) levels in the soil and conduct tissue testing to quantify if the plant utilises these soil-based nutrients effectively. This is because K and Mg have a unilateral antagonistic relationship, meaning high levels of K can negatively impact the capability of plant utilisation of soil Mg (Mg can still get into the roots but have a suppressed rate of translocation into the shoots) leading to K induced Mg deficiency.

It is worth considering that UK soils are typically high in Mg levels (56% at index three or higher) due to applying dolomitic lime.

For all of the reasons highlighted above, it remains essential to frequently test your soils to maintain the optimum nutrition levels to best set up your cropping for success. As mentioned, it works both ways; testing may reveal that you need to increase or decrease fertiliser application rates. Either way, testing
helps you rectify nutrition problems rather than compounding them.

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Agrii-Start Release

Agrii-Start Release is a unique and ground-breaking soil complexing agent which works in the soil to prevent both lock-up and to release phosphate and other essential nutrients for uptake by the roots. Suitable for a wide range of crops including: cereals, vegetables, root crops, maize, grass and OSR.

Contact for more information.