DIAGNOSING PHOSPHORUS DEFICIENCY IN IRRIGATED DURUM-WHEAT USING BASAL STEM PHOSPHATE TISSUE-ANALYSIS

The occurrence of phosphorus (P) deficiencies in small grain crops are normally predicted through the use of preplant soil analysis and verified by tissue analysis for total P often at the beginning of reproductive growth. Few studies have addressed the potential use of tissue phosphate (PO4-P) tests to characterize the P status of grain crops during early vegetative growth which could allow for correction of P deficiency in irrigated production systems during the current season. Three field experiments were conducted in southeastern Arizona from 1986-89 to examine the effects of P applications, and residual soil P on tissue phosphate (PO4-P) concentrations and yield of irrigated durum wheat (Triticum turgidum L. var durum cv. Aldura). Fertilizer P additions resulted in significant increases in basal stem and leaf tissue PO4-P concentrations at GS 2, 6 and 10. Fertilizer P applications of up to 40 kg P/ha increased extractable P levels in the soil by 82% at the beginning of the next growing season but had no significant effect on grain yields or tissue PO4-P concentrations of the succeeding wheat crop. Band applications of P resulted in higher tissue PO4-P concentrations versus broadcast applications. While both basal stem and upper leaf tissue PO4-P concentrations were responsive to differences in P availability, routine use of basal stems for nitrate testing and significantly higher PO4-P levels in basal stems at GS 2 and GS 6 favor its use in diagnosing P deficiency. Basal stem tissue PO4-P analysis seemed to accurately reflect P nutrition of durum wheat from GS 2 to 10. Preliminary critical levels of PO4-P in basal stem tissue of irrigated durum wheat were 2000, 1200 and 500 mg/kg at GS 2, 6 and 10, respectively.