Potash requirements of potatoes

This review from 2011 summarises the information that was available at the time on the response of tuber yield to potash application, the removal of potash in potato tubers, and the effect of potassium supply on potato health/quality including the effects on bruising, hollow heart, tuber size, dry matter content, mechanical damage, storage, cooking quality and cooking colour.

The research results indicated that application of potash at rates for maximum yield is sufficient to minimise the risk of bruising (blackspot). The use of higher rates to achieve further reduction gives benefits that are too small and inconsistent to be of practical value. Most reports suggest a reduction in specific gravity or percentage dry matter with increasing rates of potash fertiliser. Potassium chloride has been shown to result in a larger reduction in tuber dry matter concentration compared to use of potassium sulphate or nitrate, but the effect is inconsistent. It is unclear if applying potassium chloride well in advance of planting would have any additional effect on dry matter content.

Research evidence indicates that the effect of potassium on Enzymic Browning was not large, and was mainly observed on soils that were low or deficient in potassium. It is concluded that extra potash above that needed to replace potash removal is not required to reduce Enzymic Browning. The effect of potash on fry colour has been variable. In response experiments, the largest improvement in fry colour, usually statistically significant, came from the first increment of potash. Further Improvements in fry colour diminished with higher potash rates, and these improvements were rarely significant. Once again, it can be concluded that potash should be applied to replace offtake even where fry colour is the major concern. The effect of potash on After Cooking Darkening is variable and other climatic and management factors may be more important. To minimise acrylamide, it can be concluded that the potash rate should be such that reducing sugars are at their lowest at harvest, that is optimal for processing. 

Overall, the research results indicated that levels of potash that give optimum yield, and are sufficient to replace potash offtake, are also sufficient to reduce quality defects. There was no evidence to indicate that higher rates provide any consistent or significant improvement.