Dec 2, 2013
Potato Fertilisation – Potassium Sulphate or Potassium Chloride?
Potassium is the one nutrient with the greatest influence on the interior and exterior quality of the potato. First of all, it is important to supply a sufficient amount of potassium. Taking into consideration the German soil classification system it seems advisable to aim for the upper segment of nutrient range C for potassium in potato cultivation.
Special attention should be given to the nutrient form of potassium. For basic fertilisation potassium is available in sulphatic or in chloridic form. Potassium chloride is present in manure, for example, while mineral fertilisers are offered in sulphatic as well as in chloridic form. The chemical form of potassium has a decisive influence on quality parameters such as starch content of the tubers and on taste – both actually respond better to potassium sulphate.
It should also be noted that an excessive supply of chloride would negatively influence yields – as is also the case for a host of other crops (see chart).
Potassium sulphate increases starch content
It has long been known that application of potassium chloride as opposed to potassium sulphate will lower the starch content of potatoes by 0.5 – 2.5 %. Chloride ions interfere with the transport of carbohydrates from photosynthetic processes in leaves to tubers. This results in an over-supply in leaves, while tubers have too little starch to store. Chloride is also responsible for raising the turgor pressure, and for decreasing the dry substance content of the tubers.
Ware potatoes grown on good to excellent soils (loess-loam-soils) may exceed their variety-typical starch content, sometimes impairing their `waxyness´. If this is the case, chloridic potassium fertilisation in spring or autumn will help to lower starch contents. This approach may however also reduce yields. Its application should therefore be limited to situations in which excessive starch contents are to be expected.
Potassium sulphate improves taste
In general, potatoes are grown on lighter soils so excessive starch content is not a common problem for ware potatoes. Chloridic potassium fertilisation will quickly lower starch contents far below variety-typical values. This will however negatively influence the quality of ware potatoes, as tubers will be watery inside and will be less storable. As starch is also an important flavour carrier low-starch potatoes will not meet the consumer’s expectations on taste. Sulphatic potassium fertilisation on the other hand will increase the tubers’ dry matter content, which in combination with a desirable starch-protein-ratio (12-16:1) is responsible for a delicious, hearty taste. Specifically direct marketers need to pay attention to this quality characteristic.
Potatoes are sensitive to chloride
Starchy potatoes and processing potatoes may therefore be considered chloride sensitive, while seed potatoes and ware potatoes are to be considered partly chloride tolerant. Potassium fertilisation of ware potatoes may partially or entirely be done in chloridic form, as long as this is done in good time prior to the onset of vegetation. Under certain conditions it may also be advisable to fertilise in autumn of the previous year. The idea is that a large proportion of the potassium fertiliser’s chloride content will have shifted to lower soil levels or will have been washed out. Using this approach does however entail the risk of losing large proportions of potassium, too – especially if soils are light and there is high precipitation. On heavier soils it is possible that during summer months the chloride is transported back to the root zone by the capillary flow of water, resulting in an unexpected negative influence.
For starchy and processing potatoes potassium fertilisation should partially or entirely be done in sulphatic form. For these intended purposes higher starch contents are always desirable. The guiding principles are the requirements of the various potato processing industries and the amount of the premium paid for starch content. Exclusively chloridic potassium fertilisation is risky and may easily lead to failure.
Many factors need to be taken into consideration
With respect to the potassium form it is not easy to generalise fertilisation recommendations for potatoes. There are practically no standard recommendations. The choice of potassium form for potato fertilisation depends on quite a few factors. The following factors need to be taken into consideration:
- the intended type of utilisation,
- the potato variety,
- the fertilisation time,
- the type of soil,
- other site characteristics
- as well as the level and distribution of precipitation.
Play it safe with potassium sulphate
In view of the above-described risks potato growers are well advised to conduct at least part of their potassium fertilisation (one half or two thirds) in sulphatic form. For ware potatoes it is however advisable to apply potassium sulphate only, such as by using Patentkali®.