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Contents |
| Nutrient balance |
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| Functions of potash |
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| Potassium deficiency |
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| Uptake of potash |
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| Potash uptake throughout the season |
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| Potash offtakes |
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| Response to potash |
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| Potash recommendations |
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| Time of application |
Oilseed Rape & Potash |
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Potash recommendations
Soil analysis is an essential management tool and regular soil sampling should be carried out every 3 to 5 years. Both the actual soil index as well as any charges should be considered.
Soil K fertility should be maintained at least at index 2-(121-180 mg K/litre). At this target index, sufficient potash should be applied to replace that removed in the harvested crop, i.e.the crop offtake. This is termed the maintenance (M) rate of application. General values can be used as shown in the following Table, or more appropriately the maintenance application rate can be calculated from the known yield and offtake, as previously worked out for a 4 t/ha winter oilseed rape crop.
Where soil K index levels are below 2-, a 'build up' policy should be followed to raise low fertility soils back to the target index. The general recommendation is to increase the potash application rate, over and above the maintenance dressing, by 50 and 25 kg K2O/ha for soil K index 0 and 1 respectively. However it is important for yield and crop quality that a deficiency of K in the soil is corrected with as little delay as possible and larger dressings will be more effective. It will be more appropriate to consider the cost of such 'build-up' dressings as a 'capital' improvement rather than as a variable cost set against the crop. Where the soil index is well above the target index of 2- reductions in rates of application are possible.
| K Index | 0 | 1 | 2- | 2+ | 3 and above |
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| Winter oilseed rape (3.5t/ha) (kg K2O/ha) | 90 | 65 | 40(M) | 20 | 0 |
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| Spring oilseed rape (2t/ha) (kg K2O/ha) | 75 | 50 | 25(M) | 0 | 0 |
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Time of application
Where soil reserves are satisfactory, e.g. index 2 or more timing of potash application is not critical. For soils of K index 0 and 1, potash is best applied in the seedbed. Early spring top dressing with potash may have some merit where availability of K for peak spring growth may be limiting, on light soils where some leaching of K over-winter is possible, or where other management or buying advantages are obtained.