The Authority in Potassium and Magnesium
August 19, 2010
The potato is a high-quality food product but it can only develop full quality and taste characteristics with access to an optimum supply of nutrients. Of all the required nutrients, potassium (K) has the biggest effect on the quality of potatoes. The chemical form of potassium is of great importance too: Compared to a chloride based K source, a sulphate based K fertilisation increases the tuber’s starch content.
Recent high cost pressures in agriculture coupled with some disappointing produce prices has lead to intense discussions about the necessary amount of basic K fertilisation. Many farmers have tried to minimise their costs by going against agronomic advice and reducing their basic fertilisation to a level far below the recommended practice. This is confirmed by the fact that according to the results of recent soil analyses in Lower Saxony, more than 40 percent of the soil samples are rated nutrient ranges A and B – insufficiently fertilised.
When potassium fertiliser prices were high, applications of K were neglected, particularly in the starch and industrial potato production in order to reduce costs. It is well known that a moderate K supply can increase the dry matter content and the starch content of the tubers. In turn, this affects the amount of respectively the crop value and the specific quality bonuses: The higher fertiliser costs are therefore cost effective because of the considerable increase in revenue achieved.
Insufficient K fertilisation quickly causes storage and quality problems. In order to prove this theory, K+S KALI GmbH and its agronomic application consultants have carried out comprehensive field trials using potassium in the potato production. The results are presented in the following.
Tests over several years show that the tuber yield is closely linked to the K content in the haulm. With a K content of 4-5 percent in the dry matter (dm) of the leaf, tuber yields of up to 57 t ha-1 can be produced. Due to the strong correlation of potassium content in the leaf and tuber yield, the leaf tissue can be analysed in order to check the nutrient supply of the potato plant. Thus, the potato can be efficiently and optimally fertilised.
The potato is the crop with the highest K uptake in our cropping systems. At a yield of 50 tons per hectare, a total of 336 kg K2O ha-1 is taken up from the soil. 300 kg K2O ha-1 is taken up into the tubers and 36 kg K2O ha-1 remains in the field within the crop residues. This high K removal has to be replaced by fertilisation.
Multi-year tests on light soils have shown that the optimum fertilisation with respect to tuber yield is on average 300 kg K2O ha-1. A potassium supply of 100 kg K2O ha-1 compared to 200 or 300 kg K2O ha-1 produced significantly lower tuber yields. The highest yield effects due to a balanced K supply could be observed on soils with lower potassium content (nutrient ranges A + B). That result clearly shows how important K supply is particularly on these soils to secure high yields. On a soil with normal K content (nutrient range C), K fertilisation of at least 250 kg K2O ha-1 is necessary for the realisation of an optimal tuber yield.
But not every K fertiliser is suitable for a quality-oriented potato production. A chloride based potassium application has the disadvantage that the plant’s uptake in chloride is too high which compromises the starch formation and translocation in the tuber. Patentkali contains potassium in the sulphate form and is therefore perfectly suited for the quality-conscious K fertilisation. Additionally Patentkali contains magnesium and sulphur – two further nutrients which have a positive influence on high yields and best quality. In comparison with the Patentkali fertilisation, the application of chloridic potassium by means of Korn-Kali led to a drastic decrease in starch content of about two percentage points. For the production of industrial or starch potatoes, it is therefore preferable to use sulfatic K fertilisation in order to realise higher revenues.
A comparison of different application dates of K fertiliser showed that K supply before planting is better than after planting. The reason might be the greater proximity of the nutrients to the mother tuber as the nutrients are very close to the root formation.
The starch yields per hectare are less correlated with the starch contents of the tubers but instead with the tuber yields per hectare. Field tests prove again that a K supply of at least 250 kg K2O ha-1 is necessary to secure optimum starch yields. Thus, a normal soil fertilisation (nutrient range C) is the prerequisite.
Depending on the soil fertility, the starch content in the tubers is influenced by the K application. While a K supply of 100 kg K2O ha-1 in combination with a high soil provision already results in a starch content decrease, the tests show that at low soil provision, starch content increases can be observed up until the application of 200 kg K2O ha-1. Given a moderately fertile soil class (C), on average it can be expected that the starch content in the tuber is positively affected up until an application of 150 kg K2O ha-1. A calculation of the K supply only based on the tuber’s starch content has negative consequences on the tuber yield as well as on the starch yield per hectare. The evaluation of the 1995 trial results according to the currently valid payment modalities in the starch potato production makes this point very clear.
The gross margin and therefore the profitability of starch potato production is less linked with the starch content of the tuber and more to do with with the outright tuber yield. The decreased payment resulting from a lower starch content is more than compensated for by the increased starch yield per hectare from a heavier crop.
In order to achieve optimum results in the production of starch and industrial potatoes, K fertilisation has to be targetted to the specific requirements of the site and of the potato crop. Only that way can maximum tuber yields be secured. Furthermore the basic nutrient potassium has an important influence on the outer and inner quality of the potato.
Dr. Bernd Ditschar, Kassel, and Dr. Dietrich Lampe, Nordstemmen, Germany
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