Oilseed Rape & Potash

Nutrient uptake for typical 3 t/ha oilseed rape crop

Nutrient balance

Efficient crop nutrition requires the correct balance of all nutrients, and in principle individual plant foods should not be considered on their own. Nevertheless this leaflet discusses the importance of the nutrient potassium (K) because of its critical importance to the efficient use of nitrogen (N) which is widely covered elsewhere. Each year only about two thirds of the oilseed rape crop receives a dressing of potash (K₂O) fertiliser and if sufficient K is not available to the plant from the soil, yields will be reduced.

Functions of potash

The functions of potash in the oilseed rape plant are not necessarily visible or obvious but this nutrient is nevertheless vital for vigorous, healthy crop growth and profitable yields. In fact, more potash needs to be taken up by the plant than any other nutrient including nitrogen.

Potash is responsible for the transport mechanisms in the plant whereby materials made by photosynthesis in the leaf can be moved to other parts of the plant for growth, formation of seed and deposition of oil. It is vital in the water regulation of the plant and plays an important balancing role with nitrogen to ensure healthy, vigorous growth and natural resistance to disease, pests and stress.

	Potassium deficiency causes browning of leaf margins.

Potassium deficiency

Deficiency symptoms are not commonly seen in the field but plants will suffer from potash limitation long before any visual effects on the leaf are apparent. General stress from drought, cold, water-logging, etc. will be aggravated if the plant is short of potassium and crops with better K supply will grow through such difficulties more satisfactorily. K deficiency can also be one reason for early lodging in crops.

Uptake of potash

Potassium levels in the establishing plant are quite high at 3 - 4% and although the total bulk of the crop is fairly small, there will be around 60-75 kg/ha of potash K₂O in the crop by the end of autumn. Some forward crops have been found with levels of over 100 kg/ha by November. Over winter potash content is stable or may fall with leaf loss from frost, pest and disease effects.

The rate of growth and uptake requirement of oilseed rape plants in the spring is dramatic. Potash demand maybe in excess of 12 kg/ha/day. Where conditions for growth are not ideal (poor soil structure, thin soils, very wet or dry conditions etc.) the plant may not be able to extract its full needs even though the total quantity in the soil is theoretically sufficient. Continental fertiliser recommendations tend to be more generous than in the UK in order to ensure that the peak supply is adequate for optimum potential growth and yield. As with cereals, a spring top-dressing with an NK fertiliser will be worthwhile if the K available from soil reserves is insufficient (see leaflet 22, NK Arable Top Dressing).

Maximum uptake normally corresponds with the end of flowering when a crop will contain around 250 kg K₂O/ha. Peak uptake levels of 440 kg/ha have been recorded with high yielding crops. After seed set, potash in seed and pod continues to increase whilst leaf and stem content falls.

Soil K fertility must be maintained at a level which does not restrict this large and rapid uptake. The target soil index for oilseed rape for maximum yield is 2- (121-180 mg K/litre).