Winter wheat (Triticum aestivum L., cv Mercia) was grown at Rothamsted over three seasons (1992/94/95) with plant-available soil P (Olsen-P) ranging from very deficient to superoptimal (1–52 mg P/kg; ADAS soil P Index 0–4). Critical Olsen-P for 95% maximum grain yield changed little between years and averaged 9 mg P/kg. Total-P and inorganic-P (Pi) were measured in whole shoots several times between late tillering and anthesis to see if Pi was a better indicator than total-P of the adequacy of P supply for maximum grain yield. Inorganic-P accumulated in shoot tissue water with increasing Olsen-P to a maximum concentration of c. 15 mM. Inorganic-P was a better indicator of crop P status as there was less variation in critical values between seasons and with crop growth stage. Expressing Pi concentrations on a tissue water basis (Piw) reduced the effect of differences in soil water supply and also offers the prospect of in situ testing. Over the three seasons, critical shoot Piw for 95% maximum grain yield averaged 4.7 mM (range 4·3–5·8 mM) during stem elongation.
