A greater focus on the amount of phosphorous a crop is getting rather than just trusting soil analysis will form part of updated RB209 guidelines due out next month [January].
Speaking at the AHDB Agronomists’ conference, Professor Roger Sylvester-Bradley, head of crop performance at ADAS says the principles of the new guidance are evolving towards feeding the crop not the soil, setting target indices according to rotation rather than crop by crop, monitoring soil P status more carefully, and managing soil P build-up and run-down more closely.
Regarding sampling and analysis, Prof Sylvester-Bradley advises leaving a six-month gap after any P application, keeping to the same routine concerning season, sample points, depth, and lab, and checking results against last time, each other and crop balance calculations, repeating sampling if necessary.
“Don’t use soil analysis as the only guide to P management. Interpret results together with phosphate balances and crop (grain) P analyses. Use soil analysis to tell two things: As before, average soil P index and hence P requirements but also rate of soil P change. This can be very variable and the main differences are between farms rather than soils.”
Prof Sylvester-Bradley advocates using grain P to support soil P analysis. “It offers easy and accurate sampling compared with soil or leaf testing, can be used to measure P and K offtakes, P and K deficiency and gauge all nutrient levels. It summarises the whole crop and whole season.”
The grain P level that is critical in terms of cereal crop nutrition is 0.32 per cent of grain dry matter, he says.
When introducing routine grain P analysis, Prof Sylvester-Bradley suggests favouring fields at lower soil P levels – Index 0-2, particularly if soil P has recently built up and yield maps match soil P maps.