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Offtake values for phosphate and potash in crop materials

Research Review No. 92

Offtake values for phosphate and potash in crop materials 

Alison Rollett1, John Williams2 and Paul Newell-Price1

1ADAS Gleadthorpe, Netherfield Lane, Meden Vale, Mansfield. NG20 9PD.

2 ADAS Boxworth, Battlegate Road, Boxworth, Cambridge. CB23 4NN.

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.

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.

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