Aug 23, 2010
Potato cultivation II: Balanced nutrient supply essential for quality
The positive influence of potassium for potato yield has long been proven in a great number of field trials worldwide. Additionally, potassium positively influences many quality characteristics, thereby heavily influencing the marketability and therefore ultimately profitability of the crop. Balanced fertilisation, with consideration of both yield and quality, is therefore crucial for the economic success of potato cultivation.
Starch content – the most important quality characteristic of potatoes
Starch content is the most important quality characteristic of potatoes. Depending on the intended use of the crop, different starch contents are required. While starch potatoes are valued and achieve premium prices for their starch levels, medium starch contents are best suited for chips and crisps.
For table potatoes, starch contents are less important than cooking properties. These table potatoes should not disintegrate or show discolouration either before or after cooking.
Factors for starch content: potassium fertilisation and form of potassium
Two factors are important for potato starch content: Moderate applications of potassium raise starch content to an optimal amount, while exceedance of this optimum slightly decreases starch content.
Another important factor is the form of the potassium. While after fertilisation with potassium chloride, the chloride negatively influences the transport of assimilates, thereby reducing starch content whereas fertilisation with potassium sulphate may raise starch content by around 2%. In addition to starch content however, the quality of table potatoes or processing potatoes is defined by several other factors.
Potassium fertilisation influences the content of reducing sugars
The quality of processing potatoes for chips or crisps is primarily determined by their content of reducing sugars (e.g. fructose or glucose). Increased content of reducing sugars impairs the processing properties of the potatoes. This will not only negatively influence deep-frying-colour leading to dark brown chips and crisps, but will also contribute to the generation of unwanted acrylamides, due to an interaction with aspartic acid.
A potassium fertiliser trial showed that application amounting to 300 kg K2O ha-1 showed an impressive initial decrease of reducing sugars. Further increases in the applied rate of potassium however, did not significantly decrease the remaining content of reducing sugars.
Potassium and magnesium help to prevent black spots
Marketability of potatoes is significantly impaired by a physiological internal bruising commonly known as ‘black spot’. Many different factors influence the crops’ susceptibility to black spot; there are however several targeted crop husbandry measures, which may be used to reduce the incidence of this condition.
Potato variety, skin stability, harvesting and post-harvesting conditions, as well as storage conditions (such as mechanical stress) directly affect black spot susceptibility. Fertilisation also influences black spot occurrence, albeit indirectly.
Potassium and magnesium improve cell tissue stability, thereby reducing pressure sensitivity of the tuber and therefore help to reduce black spot. Skin lesions occurring during the harvesting process may allow oxygen to enter, causing phenolic amino acids to oxidise and to effect discolouration. The same mechanism also causes crude pulp discolouration. Due to its inhibiting effect on the oxidised degradation products of the phenolic amino acids, the ascorbic acid contained in the tuber may counteract discolouration. Levels of tuber ascorbic acid are directly and positively influenced by a sufficient supply of potassium and magnesium.
A trial lasting several years clearly showed the connection between potassium supply and black spot occurrence. Potassium application of 300 kg K2O ha-1 resulted in the lowest number of tubers affected by black spot, as well maintaining a potassium content of approximately 2.5 % in the potatoes.
Micronutrients influence black spot occurrence
A series of trials researching foliar fertilisation showed that foliar application of EPSO Top (magnesium sulphate), applied twice, reduced the proportion of tubers showing black spots. This effect was increased further when EPSO Microtop was used (Magnesium sulphate with boron and manganese).
Potassium influences citric acid content, thereby also influencing after-cooking blackening
Contrary to black spot, after-cooking blackening is caused by the reaction of phenols (e.g. chlorogenic acid) with free metals, such as iron. Due to the complex formation between free metals and citric acid, iron, for example, is bound and can no longer react with phenols. This reduces the tendency to darken after cooking. The citric acid content can be influenced by fertilisation. As is shown in the following graph, an increased supply of potassium raises the potatoes’ citric acid content. The highest proportion of citric acid is achieved by a potassium content of 2.5%.
Potassium, magnesium and boron promote optimal quality and yield
In addition to improving yield, a balanced nutrient supply also safeguards potato quality. Potassium not only enhances the starch content, but also positively influences other attributes essential for potato quality. Depending on soil K content, an application of 300 kg K2O ha-1 may be sufficient to achieve the tuber potassium content of 2.5%, which is necessary for premium quality.
In addition to potassium, magnesium and boron may also be used to improve potato quality, specifically susceptibility to black spot. For this purpose, magnesium and boron may be applied as foliar fertiliser, during the vegetation period, and according to demand.
Dr. Hans-Peter König, Kassel, Germany