According to the Potash Development Association (PDA), potassium (K) must never be overlooked when developing a fertiliser plan for oilseed rape.

It is universally recognised that potash is a vitally important nutrient for all oilseed rape crops. In many cases it will out-perform nitrogen (N), in terms of the growth response that it delivers.

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.

More potash needs to be taken up by the plant than any other nutrient, including N, with the majority of it being required to create the osmotic potential in the cell sap, enabling the leaves to maintain their turgor.

Without adequate potassium cell production and expansion is limited.

Importance of potash

K also plays a vital role in the transport of sugars and other products of photosynthesis from the leaves 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 N to ensure healthy, vigorous growth and natural resistance to disease, pests and stress.

Many of the functions of potash in the plant are related to physiological conditions and stress.

These functions are diverse and include: Efficient nitrogen and water use; drought tolerance; frost resistance; plus resistance to pests and diseases.

There are several processes by which K helps to reduce insect pest pressure.

The leaf yellowing associated with K deficiency attracts aphids to plants. High levels of K provide resistance against insect pests by enhancing secondary compound metabolism, reducing sugar accumulation and therefore plant damage.

Moreover, sufficient K levels hardens plant cell walls, stimulates lignin production leading to thicker and harder stems.

This is considered to improve mechanical resistance to feeding of insects, particularly sucking insects such as aphids.

K 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 60-75kg/ha of potash in the crop by the end of autumn.

Some forward crops have been found with levels of over 100kg/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 may be in excess of 12kg/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.

Maximum uptake normally corresponds with the end of flowering when a crop will contain over 300kg K2O/ha.

Peak uptake levels of 440kg/ha have been recorded with high yielding crops. After seed set, potash in seed and pod continues to increase while leaf and stem content falls.

Soil K fertility must be maintained at a level which does not restrict this large and rapid uptake, according to the PDA.