Chronic Deficiency
Occasionally chronic potassium deficiency occurs, where small potash applications do not produce yield responses, but large applications do. In these cases the response curve is 'sigmoidal' in shape. (See graph.) This usually only occurs in severely deficient soils where yields are extremely limited. In the case of potassium, this type of response may be due to soil fixation of added potassium before any is available for plant growth.
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Typical and sigmoidal yield responses to applied nutrients. |
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An example of such a response has occurred in lucerne growing soils in the Peel Valley, New South Wales. Strip trials of potassium fertiliser (60-25 kg potassium/ha) failed to produce yield responses in irrigated lucerne crops, despite obvious visual deficiency symptoms. But applications of 500kg potassium/ha were accompanied by dramatic yield increases, increased persistence and resistance to waterlogging, and improved leaf retention - therefore providing a longer window for hay-making.