Oilseed Rape & Potash

Potash uptake throughout the season (by winter rape)

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Potash offtakes

Potash removal and yield

The yield of seed is closely related to the total dry matter production and because the % K between different healthy crops does not vary greatly it follows that total potash removal (offtake) is directly related to yield. Higher yielding crops take up more potash and where more nitrogen is used to achieve higher yields potash removal is increased.

The % K content of a healthy crop declines during growth from around 3-4% in the young plant to 2% at flowering. The seed itself contains only around 1-1.2% K. The amount of potash actually removed in the seed at harvest therefore represents only a very small proportion of the total potash which was in the growing crop.

Typically 11 kg K₂O/ha are removed for every tonne of seed harvested. However the offtake rises to 17.5 kg K₂O/ha per tonne of seed harvested if the rape straw is also removed from the field.

As a guide, a 4.0 t/ha winter oilseed rape crop harvested for seed only will remove approximately 45 kg K₂O/ha (4 x 11 = 44). If the straw is also removed, the crop offtake increases to just over 70 kg K₂O/ha (4 x 17.5 = 70).

Response to potash

Uptake and removal should not be confused with response. Whilst oilseed rape takes up large quantities of potash, UK trials have not often shown yield responses. However, little yield response work has been done in recent years with new higher-yielding varieties.

Whilst potash is known to improve oil content in a range of other crops, no relationship has been found in this country with oilseed rape field trials although pot experiments have shown an increase in oil content of winter oilseed rape with increasing rates of potash fertiliser.