Potassium Uptake Rates During the Growing Season
Examining only the potassium content of harvested products can give a misleading indication of the potassium requirements of a crop. In most annual crops, potassium is absorbed early in the growing season, with a rapid uptake period preceding the maximum dry matter accumulation rate. (See diagram)
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The relative accumulation of potassium and dry matter over the growing season of maize grown in Queensland. |
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There are important differences in the rate at which potassium is required by various crop species during this peak uptake period. See this table for the peak daily uptake rates for several crops species.
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Maximum daily potassium uptake rates of some crop species. |
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For example, while sugarcane removes several times more total potassium than a crop of maize, during critical growth stages maize may require six times more potassium on a daily basis than sugarcane which has a lower daily requirement over a longer growth period.
In soils where the crop is relying on a slow release pool of fixed potassium, the rate at which the soil releases potassium is often insufficient to keep up with daily plant demand. Agronomically, the potassium release characteristics of a soil are just as important as the absolute size of the soil potassium store.
Furthermore, the maximum potassium content of the plant may be 40-50% more than that measured at maturity. Between flowering and maturity many crops (particularly those with determinate growth patterns like soybean, lupins and canola) lose potassium due to leaching from aging leaves, leaf fall and possibly leakage from mature roots. The potassium requirement of a crop is much greater than indicated by the quantities present in the crop at harvest.