Potash for OSR

August 2020

Oilseed rape is going through a tough period at the moment, 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 it has also faced increasing challenges with Clubroot, Turnip Yellows Virus, erucic acid levels and weed control. However, despite these challenges, OSR remains one of the more important and economic break crops on many combinable farms when it can be grown successfully.

The risks involved in growing the crop have increased significantly, and the greatest risk period is the autumn at establishment. This is when the crop is most vulnerable to Cabbage Stem Flea Beetle (CSFB) and where it stands the greatest risk of failure.

Many growers started their drilling campaign earlier this month, or even at the end of July, as improved establishment is key to the success of the crop. A lot of discussion has focussed on the role of nutrition in establishment, particularly nitrogen and phosphate. However, particular attention should also be paid to potassium as plants with a high nitrogen level and little potassium have soft tissue with poor resistance to sucking and chewing insects.

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.

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
• resistance to pests and diseases

This last point is of particular importance in the current climate, with CSFB and aphids causing such devastation to oilseed rape crops. Although there are no studies looking at the role of potassium on CSFB feeding pressure, there is work that has shown a positive correlation between potassium and aphid numbers.

Impact of potassium on population build-up of aphid and crop yield in Brassica napus

Potassium treatment (kg/ha)	Aphid population/Plant	Yield (t/ha)
Control	13.70a	2.56c
60	8.46b	3.02b
80	7.63b	3.12b
100	4.63b	3.50a
120	4.30b	3.56a
LSD	4.60	0.30
Ref: Sarwar, Muhammad & Ahmad, Nazir & Tofique, Muhammad. (2011) Journal of Zoology

The role of K in mitigating crop damage from insects

There are several processes by which potassium helps to reduce insect pest pressure:

• Leaf yellowing due to potassium deficiency attracts aphids to plants
• High levels of potassium in plants provides resistance against insect pests by enhancing secondary compound metabolism, reducing carbohydrate accumulation and plant damage.
• Sufficient potassium hardens plant cell walls and stimulates lignin production leading to thicker and harder stems. This results in improved mechanical resistance to feeding by insects, particularly sucking insects such as aphids.