Potatoes
Potatoes can remove more than 270 kg potassium/ha in harvested tubers. Production of high yield potato crops depends on adequate potassium fertilisation. Potassium improves yield of potato crops through an increase in tuber number, but most importantly in tuber size. (See photo.)
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The tuber size and yield of potato crops are increased with potassium. |
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Potassium is also important for processing quality of tubers. Deficiency can increase bruising, black spot and will darken chip colour.
Where potassium is severely deficient, application of potash fertilisers can improve specific gravity of potatoes. But where potassium is adequately supplied for maximum yield, additional potassium can sometimes reduce specific gravity.
There is much debate about the effect of different forms of potassium fertilisers on the yield and quality of potatoes. Some research suggests that sulfate of potash (SOP) is superior to muriate of potash (MOP) in maintaining quality of tubers at high application rates. Trials in New Zealand have demonstrated that MOP is superior to SOP in increasing tuber yield, but SOP has a smaller negative impact on specific gravity at high application rates. (See table.)
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This table illustrates the effect of rate and form of potassium fertiliser on the yield and specific gravity of process grade potatoes in New Zealand. |
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Commonly growers will choose to apply up to 140 kg/ha of potassium as MOP, with the remainder as another form of potassium, however there is no research to indicate if this is economical.
In Tasmania several trials over the past 15 years have found no difference between MOP and SOP in terms of yield or quality with rates of up to 240 kg potassium/ha. Therefore forms of potassium other than MOP are recommended only at rates higher than 240-280 kg potassium/ha (depending on soil type), above which the chloride added in MOP can increase cadmium uptake to undesirable levels.