The Potash Development Association (PDA) has acted to update its fertiliser recommendations for oilseed rape crops.

A new advisory leaflet, published at the very end of 2021, relates the latest potassium (K), phosphate and sulphur requirements for oilseed rape (OSR).

The step has been taken in the wake of reported difficulties that growers in the UK have recently experienced in getting rape crops properly established.

According to the leaflet, rape is rapidly falling out of favour on-farm due to difficulties with establishment following the loss of neonicotinoid seed treatments in 2014. On top of this the crop has also faced increasing pest pressure throughout the growing season.

The increasing occurrences of clubroot, turnip yellows virus, erucic acid levels and weed control-related issues now constitute enhanced challanges for growers.

The new leaflet points out that the risks involved in growing the crop have increased greatly, and the greatest risk period is the autumn around establishment.

This is when the crop is most vulnerable to cabbage stem flea beetle (CSFB) and when it stands the greatest risk of failure.

Improved establishment for oilseed rape

Improved establishment is key to the success of the crop, and the primary area of focus for this should be the soil.

Soils should be well structured, at a neutral pH and soil indices should be at the target index of 2 for phosphate and 2- for potash.

The PDA points out that 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 nitrogen (N).

The bulk of the nutrient is 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.

Potassium 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.

Functions of potash and phosphate

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.

Phosphate is a component of several essential cell components and specifically helps to promote root development and early flowering and ripening.

Its role in root development is important for establishment, leading to a focus on early applications.

Phosphate plays a role in the energy storage and transfer within a plant, and is therefore important in the production of oil in oilseed rape.

A deficiency of phosphate can reduce both the above and below ground biomass, with a reduction in rooting having knock-on implications for the uptake of other nutrients from the soil.

Although the nutrient is relatively immobile in soil, it is mobile within the plant, meaning deficiency symptoms show up on older leaves first.

Symptoms of deficiency

Symptoms appear as a reddish discolouration of leaves, due to a build up of anthocyanins in leaves.

For this reason, phosphate deficiency during the colder months can be difficult to identify in the field, as anthocyanins tend to be related to an increase in stress, which may be due to a number of reasons.

Oil crops have the greatest requirement for sulphur compared with most other crops due to the requirement for glucosinolate production in the crop (although recent breeding has lowered the levels in the seeds) and the nutrient’s role in oil synthesis.

Sulphur is also required by plants for vegetative growth and is a component of plant amino acids, proteins and enzymes.

Due to being an essential component of proteins, it has a direct relationship with N, and adequate sulphur is required to achieve good N use efficiency (NUE).

Oilseed rape has a high demand for sulphur and is particularly sensitive to sulphur deficiency compared to other crops.

Visual symptoms of deficiency include yellowing of the older leaves, and later during flowering, shows up as a change in shape and colour of the petals.

Severe deficiency can result in reduced growth of the plant and a reduction in both yield and quality of up to 40%.

Sulphur applied to oilseed crops in the autumn improves the retention of a range of other nutrients, including capturing soil nitrogen reserves. It also helps to reduce senescence of lower leaves over winter.