PHOSPHORUS-NUTRITION OF SPRING WHEAT (TRITICUM-AESTIVUM L) .4. CALIBRATION OF PLANT PHOSPHORUS TEST CRITERIA FROM RAIN-FED HELD EXPERIMENTS

Five single-year field experiments conducted on phosphorus (P) deficie nt soils were used to calibrate plant P test criteria for rain-fed, sp ring wheat. At each site, P concentrations in whole shoots and young a nd mature leaf blades reached asymptotic values in the adequate-luxury P zone: within 6 weeks of sowing. The asymptotic P concentration vari ed between sites and declined as plants aged. The applied P level requ ired to reach the asymptote increased with advancing plant age. Two co nsistent trends in relationships between relative shoot yield and P co ncentrations in whole shoots and leaf blades were observed. Firstly, a s plants aged; the slope of the relationship in the zone of deficiency progressively increased and at later stages of growth became nearly v ertical so that severely deficient plants had P concentrations only sl ightly lower than plants of adequate P status. Secondly, there was onl y a narrow range of P concentrations in the zone of adequate to luxury P status for whole shoots and young leaf blades. Plant P test criteri a derived from field-and glasshouse-grown wheat coincided when related to stage of ontogeny as defined by the level of leaf insertion on the main culm. Using this as a basis of stage of plant growth, plant P cr iteria in shoots and young leaf blades were constant up until early ti llering, declined rapidly until late tillering, and thereafter decreas ed more slowly. Critical concentration ranges for total P are proposed for YEB and for whole shoots. Critical values for grain P were estima ted to lie between 0.19% and 0.23% P for 90% maximum grain yield and b etween 0.21% and 0.24% for near maximum grain yield. Soluble P and lab ile P fractions were highly correlated and produced similar relationsh ips with relative shoot yield. The diagnostic relationships for both f ractions had steep slopes in the zone of deficiency and criteria for s tandard leaf blades derived for either fraction sometimes declined as plants aged. The labile P:total P ratios for YEB and YEB+1 were also o f diagnostic value; where this ratio was <30% during tillering, P defi ciency was assured.