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Best Use of Limited Supplies: Phosphate & Potash Fertilisers

  • Fine-tuning your P fertiliser decisions can use a soil specific approach to P management assessing the relationship between differing soils capacity to regulate your PK recommendations for your area (see the regional Technical Notes listed).
  • Grain P and K analyses after harvest of winter wheat can provide more accurate estimates of crop phosphate and potash offtakes than the typical values in FAS TN 715-718, 2020. This information also complements soil analyses for P and K. AHDB-funded research on P nutrition has shown that the critical level of grain P in winter wheat is 0.32%. Grain P contents repeatedly below this level indicate crop P uptake was deficient enough to reduce grain yield. If soil conditions are satisfactory but a field repeatedly shows grain P deficiency, available soil P should be increased for future crops. Grain samples must be representative of fields or areas from where routine soil analyses are taken.
  • Care should be taken to avoid building up crop-available phosphate above the target soil P Status. Recent research has shown that phosphate transfer from soil to surface water increases as soil P Status increases. Much of this transfer is due to the loss of crop-available phosphate through land runoff.
  • For cereal-based arable rotations, the target soil P Status is lower moderate (M-) on low and moderate P-fixing soils and upper-moderate (M+) on high P-fixing soils. The target soil P Status is M+ for rotations including potatoes and P responsive vegetable crops.
  • For cereal-based arable rotations, the target soil K Status is lower moderate (M-) on all soils and upper-moderate (M+) for rotations including potatoes and vegetable crops.
  • Adjustments to P and K fertiliser recommendations can be made to build up or run down to the new target soil PK Status by following FAS TN715-718s.
    • PLANET Scotland, a free to download and use software tool designed for Scottish farmers and consultants, can be used to plan and manage nutrient use on individual fields.Planet Scotland Logo(


Dr Alex H Sinclair for the Farm Advisory Service

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