Nitrate‐N and phosphate‐P concentration in winter wheat at varying growth stages(1)

Winter wheat triticum aestivum L.) was sampled at selected stages of growth from N and P fertility experiments at one location in 1981 and three locations in 1982. Specific growth stages were determined using the Feekes scale. Plants were separated into roots, crowns, and leaves at each growth stage, dried, ground, and analyzed for NO3‐N and PO4‐P. Crown and leaf NO3‐N at growth stages 4 and 5 were significantly correlated with yield in 1981 at the Stillwater site. Crown and leaf NO3‐N and PO4‐P were also positively correlated with N and P applied at growth stages 4 and 5, respectively. In contrast to 1981, no moisture stress was present during 1982 and as such contributed to lower tissue concentration of NO3‐N and PO4‐P at all stages of growth. Limited positive correlation between independent crown and leaf PO4‐P and NO3‐N concentration at all growth stages with yield was found in 1982. However, surface response modeling using NO3‐N and PO4‐P in the leaf samples at or before stage 5 with yield provided similar models for both years and locations sampled. Combined location critical levels for NO3‐N and PO4‐P in the leaves at Feekes stage 5 were 2220 and 1843 mg kg‐1, respectively. These combined location critical levels (restrictions defined) entered back into site‐specific models accounted for 81 to 94% of predicted maximum yields by location. Root NO3‐N and PO4‐P were not affected by treatment and did not correlate with yield or any other independent tissue variable at any stage of growth, location, or year sampled. Data from these experiments indicate that sampling for yield prediction models should take place at or before Feekes stage 5. Crown and leaf samples for NO3‐N were both highly correlated with each other at Feekes stage 4 and 5 and were equally effective in predicting yield. This suggests that the entire above‐ground portion of the plant can be used for sampling, thus eliminating the need to separate the sample into different plant parts.