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Needs of the plant
Normal plant growth requires large quantities of potassium (K, often referred to as potash, K2O). In fact, throughout growth most crops contain more potassium than any other nutrient including nitrogen (N). Small quantities of potassium are needed to support many of the crucial enzyme processes within the plant but very much larger amounts are used to control the water relationships in the plant. Potassium also plays a vital role in the transport of sugars and other products of photosynthesis from leaves to storage organs. Adequate quantities of potassium are thus essential for a crop to achieve its full yield potential and also for many aspects of product quality such as grain size and appearance, tuber size, oil content, dry matter and starch content, percentage sugar and fruit ripening and quality.
Many of the functions of potash in the plant are related to physiological conditions and stress. These functions are diverse and include efficient nitrogen and water use, drought tolerance, frost resistance and resistance to pests and diseases. It is therefore not surprising that lack of plant-available potash in the soil results in weaker, less vigorous crops that suffer major penalties in difficult growing seasons. In years when growing conditions and yields are good the response to potash may be modest, especially for crops like cereals, but in adverse years its contribution to optimum yields will be substantial. Adequate potassium thus provides some 'insurance' against adverse conditions in difficult growing seasons.
Fertilisers and organic manures release potassium into soil solution as the potassium ion, K+ and plants take up potassium in this same form. However the potassium content of fertilisers is usually referred to and measured as 'potash' or K2O. This is just a convention and 'potassium', 'potash', 'K' and 'K2O' are often used interchangeably when referring to fertilisers or application rates. Soil analysis results are given as mg/litre (or ppm) of K (potassium) which relates to an Index, not to an amount of fertiliser, and should not be confused. If it is necessary to convert K2O to K (for example if converting the UK recommendations in this leaflet for use in the Irish Republic) the K2O value should be multiplied by 0.83. Similarly K values can be converted to K2O by dividing by 0.83.
Potassium and nitrogen are strongly associated in plant processes and should be considered together.
Firstly in terms of uptake, both nutrients are needed in large amounts at the same time. N is mainly taken up as the negatively charged nitrate anion NO3 - and potassium as the positively charged cation K+. When equal amounts of both are taken up together, the neutrality of the system is maintained. Nitrogen application and uptake stimulates the growth of plant cells which take in water. Potassium is required to maintain the turgor (rigidity) and regulate the water content of the cell. Maintenance of cell turgor is essential to optimise the interception of solar radiation (sunlight) to provide energy for the conversion of carbon dioxide to sugars, the basic building block for the production of harvestable yield.
Within the plant the complex formation of protein from nitrate and its distribution around the plant are highly dependent upon adequate K supply. If 'normal optimum' rates of N are applied in the absence of sufficient K, full response to nitrogen will not be obtained and residues of nitrogen may remain and be leached at the end of the season (see Figure 1).
Typical patterns of K uptake for cereals and roots are shown in Figures 2 and 3. For combinable crops, uptake will peak at around flowering, whereas for root crops maximum uptake is shortly before harvest or at haulm destruction.
In most plant species the total content of potassium is larger than that of any other nutrient, even nitrogen. For optimum growth the plant-available supply of potassium in the soil must meet both the maximum total demand and the required rate of daily uptake. This will vary from season to season depending on soil conditions and weather. Whenever there is a rapid surge in growth the soil supply must be able to meet the demand of the crop. Hence the need to maintain soil reserves is a sound insurance. Typical peak rates of uptake are 5-10 kg K2O/ha/day for root crops and oilseed rape.
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