Potash for Potatoes

Potash and quality

The amount and form of potash can affect various aspects of quality but other factors are involved and often have a greater impact. Requirements for different markets will require particular attention to specific aspects of quality. As indicated above, extra potash beyond response or replacement recommendations is not normally justified by quality benefits alone.

Dry matter

Applying large amounts of potash fertiliser generally reduces the dry matter (DM) content of tubers by up to 2%. The effect of K supply on tuber DM appears to vary greatly with variety. Where DM is important and levels need to be improved when muriate of potash (MOP, potassium chloride) is being used, splitting the total requirement to half in the autumn and half in the seedbed can help. Switching from MOP to SOP (sulphate of potash) applied in the spring may have a greater effect than splitting the MOP application and may improve DM by up to 2%. DM is affected to a greater extent by factors such as N supply, irrigation and variety.

Both specific gravity and starch content tend to follow dry matter and where these characteristics are important it is necessary to ensure that the correct amount of K is applied and to consider the use of SOP rather than MOP.

Tuber size and number

% of yield by size of tubers
Fry colour
Internal blackening

Yield response to potash is normally associated with an increase in average tuber size and weight. The use of sulphate of potash instead of muriate of potash may be beneficial where larger numbers of small-medium size tubers are required such as for seed, canning, salad etc. The benefit will be more pronounced under dry or stressed growing conditions.

Fry colour

Lighter colours on frying can be very important for crisping potatoes and adequate potash supply can improve this aspect of quality. Muriate appears to be marginally superior to sulphate for this characteristic, however sulphate does appear to reduce the quantity of fat absorbed on frying and this has important cost implications for the processor.

Tuber damage

Tuber bruising and damage is caused by mishandling but will be worse where there is a limited supply of potash. Additional potash beyond the optimum required for yield or replacement will not help to prevent a damage problem.

Internal blackening / Black Spot

This is aggravated when tubers have a large DM content and is alleviated by applying larger amounts of potash. Muriate is somewhat more effective than sulphate in reducing the occurrence of this problem but an adequate amount of K is the more important factor.

% of tubers with Black Spot

Cracking on high pH soils

It is desirable to avoid growing potatoes on high pH soils but there is some evidence that where this is practised, SOP can reduce the severity of tuber cracking.

Flavour

This is a very subjective characteristic but there have been a number of taste experiments that suggest that the use of sulphate of potash results in a product with a better flavour.

Cooking quality

Many other factors especially the method of cooking and variety are more important for cooking quality than nutritional factors, but there is some evidence to suggest that potash reduces disintegration on boiling and mealiness of the cooked product.