Offtake values for phosphate and potash in crop materials

For grass, mean P₂O₅ (1.2 kg/t fw at 24% dry matter) and K₂O (5.2 kg/t fw at 24% dry matter) offtakes from the experimental database were lower than the current reference values in RB209 for grass silage at 25% dry matter (1.7 and 6.0 kg/t fw for P₂O₅ and K₂O, respectively). In contrast, the reviewed commercial datasets had P₂O₅ and K₂O concentrations that were higher than the values in RB209. Overall, due to the range and variability of P₂O₅ and K₂O concentrations in grass dry matter from the different data sources no changes to the offtake values in RB209 values were recommended.

For wheat and barley, P₂O₅ grain offtakes were lower than the reference values in RB209 (7.8 kg/t fw); the reviewed data suggested that there may be justification in reducing the value in RB209 to 6 kg/t fw. In contrast, there was little difference between the RB209 reference value for K₂O (5.6 kg/t fw) and the reviewed data (5.0 ± 0.1 kg/t fw) and it was not recommended that the K₂O value for cereal grain was updated. For both oilseed rape and forage maize no recommendations for changes to RB209 were made due to the limited size and variability of the available dataset.

At target soil indices, phosphate (P₂O₅) and potash (K₂O) fertiliser recommendations in the AHDB Nutrient Management Guide (RB209) are intended to replace crop offtake. The amount of fertiliser to apply can be adjusted using the target yield and RB209 offtake values for P₂O₅ and K₂O per tonne of yield. Thus to accurately estimate crop offtake it is imperative that P₂O₅ or K₂O offtake values for crop materials in the RB209 reference tables are representative of ‘real world’ data. Where the reference concentrations are lower than ‘actual’ data then the calculated offtake, and hence fertiliser requirement, will be lower than required to maintain soil nutrient reserves, risking a decline in soil index and, in responsive situations, a loss of yield. In contrast, where reference values are too large then the calculated offtake, and hence fertiliser requirement, will be more than required, which is both economically and environmentally undesirable.

The project reviewed recent data on P₂O₅ and K₂O offtakes for arable, grassland and forage crops (c.2,800 data points). Data were reviewed from field experiments carried out between 2009 and 2017 in England, Wales and Scotland where P and K offtakes had been quantified for arable, grassland and forage crops. Data was sourced from ADAS field experiments, the Yield Enhancement Network, NIAB and CF Fertilisers. Additional data from NRM laboratories, SEGES and commercial organisations were also reviewed to benchmark the experimental findings.