Abstract
Phosphorus (P) deficiency is a universal constraint to crop production, including wheat (Triticum aestivum L.), and fertilizer P use in deficient situations has biological and economic benefits. Although plant analysis is a fundamental tool for determining P nutritional status of crops, foliar analysis diagnostic indices are not well defined even for major crops like wheat. The critical concentration approach to interpreting plant analysis has inherent limitations; thus, critical concentration ranges have been suggested instead. We conducted six rainfed field experiments on P-deficient soils (Ustochrept, Haplustalf) for three seasons (1997-2000) to develop plant analysis interpretative guidelines for diagnosing the P nutritional status of spring wheat. In each experiment, substantial crop yield increases as well as plant P concentration enhancements were observed with applied P. In general, fertilizer P requirements for maximizing P concentration in foliar plant parts were similar to the ones needed to attain maximum yields. Grain P concentration increases were comparable to whole-shoot P concentrations. As with crop yields, P concentrations were greater with banded than broadcast P. However, there was no consistency between plant P concentration relationship with soil type or seasonal rainfall. Estimated critical P concentrations for near-maximum crop yield (95%) were greater with banded than broadcast P, except for whole shoots and grain grown in the noncalcareous soil. Moreover, estimated critical levels for the same plant part varied between soils, years (rainfall regimens), and even fertilizer application methods. To cope with such inconsistencies, the proposed critical P concentration ranges for spring wheat in Pakistan are as follows: young whole shoots, 0.13-0.28%; recently matured leaves, 0.27-0.36%; and mature grain, 0.13-0.27%. Plant analysis remains imprecise and should be used to complement growth observations and soil analysis.