Pink straw: High potash or something else…?

August 2022

One of the defining memories of the 2018 cereal harvest must be the colour of the straw. Social media was alive with pictures of pink looking crops. Rumours soon followed regarding the reasoning behind this, with higher potash levels coming out on top. The theory behind this presumably related to the colour of many MOP products which tend to have a pink colour, despite the fact they can have a variety of colours from tiny quantities of impurities, such as clay and iron compounds, as the deposits were laid down.

This theory was further strengthened through laboratory tests which identified higher than average potash levels in the crop residues. The mystery had been solved. Or had it…

In 2018, the prolonged dry spell during the build up to harvest meant many crops senesced prematurely. In these situations, some of the natural plant processes are not fully completed. Where the grain fill period is shortened, elevated potash levels may remain in the straw at harvest. As the crop matures and senesces the cells of plants crack, allowing some of the potash they contain to be washed back into the soil. Where there is little or no rainfall prior to straw being removed from the field, it will result in elevated nutrient concentrations as less potassium is washed from the harvested straw back to the soil.

So, this may help explain the higher levels of potash found, but it does not show where the change in colour came from. The answer may lie with the plant’s response to stress. Anthocyanins are plant pigments which can give flowers, fruits, and seeds a red-purple colour. They naturally occur in late summer from the breakdown of sugars as leaves senesce and phosphate levels fall, resulting in the red and purple colours seen in autumn leaves. While they are not normally present in actively growing leaf tissue, they can be induced in response to abiotic stresses such as drought, high salinity, phosphorus deficiency, excess light and cold. The build up to the 2018 harvest saw two of these abiotic stresses in elevated levels to normal – drought and light.

Once again, the pink straw is back this harvest, and once again we have had a particularly dry spell, with high light intensity and high heat, which is likely to have stressed plants, leading to increased levels of anthocyanins, giving off the pink colour in straw.

Although the colour and the elevated potash levels are not unrelated, the high potash is not the cause, but a symptom, along with the colour, of other factors at play within plants.

A final comment on the learnings from 2018 was that analysis of cereal straw (and grain) showed some very low levels of phosphate, especially where the soil P status was Index 1, but even at Index 2. Straw and grain from wheat grown on soil with a P Index of 3 or above showed more normal levels of P in that year (see chart below). This, along with the changes in potash levels this year would suggest taking analysis for grain (and straw) could be worthwhile, to ensure more accurate offtake values are used, especially in a time of such high input costs.