The Potash Development Association (PDA) is highlighting the benefits of measuring the potassium (K) concentrations of growing crops.

In a lot of cases, farmers may base their K requirements on the needs of a growing crop only.

This would assume that the field, or fields, in question had previously been confirmed as index 2 for this crop nutrient, based on a soil test result.

But according to the PDA, this one-off assessment may need monitoring to ensure this remains the case for each specific soil type and across the years between sampling and even within fields.

Plant potassium measuring

Any in-field tests for K sufficiency should meet a number of basic requirements. These are – it is easy and inexpensive to carry out, and it gives a definitive answer on sufficiency/deficiency.

In addition, the timing of measurement and turnaround time gives growers the opportunity to respond. And, finally, the testing should allow management practices that give a high level of success.

The most basic method for monitoring potassium levels and identifying deficiency would be through a visual assessment.

For nutrients in general, waiting for deficiency symptoms to become visible may be too late as crops can appear to be perfectly well fed yet still be deficient.

Potassium deficiency

When K is deficient, several processes are impaired. Low K inhibits enzyme activation, making plants more susceptible to fungal attack.

Impaired stomatal activity results in poor control over gas exchange, impacting on photosynthesis and water control, making plants more susceptible to stresses from drought, frost, water uptake, and soil salinity.

Low K also impairs proton exchange across membranes in chloroplasts, resulting in worsening symptoms under higher light intensity, according to the PDA.

Transport of photosynthates can also be impaired, resulting in a build-up of sugars (potentially worsening aphid attacks) and a reduction in protein and starch synthesis, lowering the plant’s dry weight.

Potassium deficiency may first appear as deep green plants with shorter and fewer internodes and smaller leaves, followed by the rapid development of necrotic spots along the margins and across leaf blades of recently matured leaves.

Potassium deficiency can cause changes in both individual plant organs and, collectively, the crop canopy. When leaf tissue becomes chlorotic or necrotic, it no longer reflects light the same way it did when it was healthy.

Visibly, leaves change from green to yellow or brown, but changes also occur outside the visible spectrum.

These changes can be identified by measuring the reflectance of the crop using specific wavelengths which can then be linked back to the potassium status of the plants.

Identifying deficiency

The most common method for identifying deficiencies in crops and grass is tissue analysis, measured by the laboratory on samples collected from the field.

However, although tissue analysis is the most common means of measuring nutrient levels in plants, it may not be the most appropriate for potassium, as most K is held in the ‘liquid’ fraction of the plant, not the dry matter.

Another approach to analysing the potassium status of plants is to measure the content of the plant sap.

As potassium is held in solution within plants, rather than within the dry matter, sap analysis can be a more accurate method of measuring potassium levels.

Work at Rothamsted Research has indicated that the adequacy of potash in winter wheat is represented by tissue water concentration in leaf one.