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Five single-year field experiments conducted on phosphorus (P) deficient soils were used to calibrate plant P test criteria for rain-fed, spring wheat. At each site, P concentrations in whole shoots and young and mature leaf blades reached asymptotic values in the adequate-luxury P zone, within 6 weeks of sowing. The asymptotic P concentration varied between sites and declined as plants aged. The applied P level required to reach the asymptote increased with advancing plant age. Two consistent trends in relationships between relative shoot yield and P concentrations in whole shoots and leaf blades were observed. Firstly, as plants aged, the slope of the relationship in the zone of deficiency progressively increased and at later stages of growth became nearly vertical so that severely deficient plants had P concentrations only slightly lower than plants of adequate P status. Secondly, there was only 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 criteria 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 criteria in shoots and young leaf blades were constant up until early tillering, declined rapidly until late tillering, and thereafter decreased more slowly. Critical concentration ranges for total P are proposed for YEB and for whole shoots. Critical values for grain P were estimated to lie between 0 |
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