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Oct 27, 2009

This year’s process potatoes feature high under-water weights and frequent incidence of black spots

Process potatoes currently exhibit high under-water weight, i.e. high starch content and an excessively high percentage of potatoes with black spots. Both are quality degradations with significant influence on the crop’s marketability and the prices achieved by process potatoes. Batches heavily affected by black spots may even be rejected after acceptance inspection. These quality degradations are caused both by 2009’s cultivation conditions, and by agrotechnical measures.

The potatoes´starch content

An important quality indicator for potatoes is their starch content. While high starch content is very valuable in starch potatoes, and may raise prices here, process potatoes (commonly used for French fries or chips) require medium starch contents. In table potatoes, excessive starch contents will lead to disintegration of the potato during boiling.

Influencing factors for starch content

This year, two factors have heavily influenced the potatoes’ starch content. The first influencing factor were this year’s weather conditions, with an abundance of sunshine promoting photosynthesis, as well as drought towards the end of the vegetation period. Both of these factors together have generated extremely high dry matter content, resulting in high starch content at the time of harvesting. However, another important factor in determining starch content is the potassium supply. Low to moderate potassium supply already generates highest starch contents. If this potassium supply is raised, the starch content is reduced.

Potassium decreases potatoes´susceptibility to black spots

In addition to reducing starch content, increasing the potassium supply decreases potatoes’ susceptibility to black spots. Black spots are caused by oxygen entering the spud through lesions, e.g. caused by pressure during harvesting, and oxidizing phenolic amino acids. These oxidations actually cause the discoloration of the spud. By its reducing effect ascorbic acid may chemically counteract these discolorations caused by oxidised decomposition products. Ascorbic acid content may easily be increased by providing potassium, and may effectively prevent black spots.


Important quality parameters for potatoes, in relation to potassium supply based on Patentkali, Kuras variety, Kleinau 2007

K2O-fertilisation Dry matter content (%) Starch content (%) Ascorbic acid (mg/100 g FM) Oxidative potential (EE 445 nm)
without K2O 30,5 24,3 17,4 0,457
80 K2O/ha 30,1 24,5 18,9 0,318
160 K2O/ha 28,0 22,6 20,0 0,818
240 K2O/ha 28,2 22,4 21,2 0,255


Potassium supply is an important steering instrument for influencing not only potato yields, but also quality. Field trials have shown that potassium fertilisation of 300 kg K2O per ha significantly reduced the percentage of potatoes with black spots. A more precise answer to the question of whether potassium supply was sufficient may be provided by an analysis of the harvested crop. If the potassium content is below 2.5% K, the potassium supply was not sufficient for achieving optimal quality (see graph).

Influence of potassium fertilisation on black spot index and spud potassium content, Langwedel 1995 – 2001


A balanced supply of nutrients secures both potato yield and quality. Potassium does not only influence starch and ascorbic acid contents, but also influences contents of other components, such as reducing sugars, thereby affecting the potatoes processing qualities. Depending on soil condition and yield level, a supply of 300 kg K2O may be sufficient for achieving spud-content of 2.5% K, which is required for good quality.


In addition to potassium, magnesium and boron may also positively influence potato quality, and particularly black spots. Both magnesiumand boron may be applied as leaf fertilisers, as required, during the vegetation period. Balanced plant nutrition, i.e. optimised supply of all nutrients relevant to yield and quality, is a decisive factor for the economic success of potato cultivation. Cutting the costs of fertilisation, as was partially attempted this year, may lead to immediate quality degradations, and may therefore decisively decrease the profitability of potato cultivation.


Dr. Hans-Peter König

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