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Plant Tissue Tests for Predicting Nitrogen Fertilizer Requirements of Winter Wheat

Dep. of Agronomy, The Pennsylvania State Univ., University Park, PA, 16802

^* Corresponding author.

Field experiments were conducted to evaluate three plant tissue tests for estimating the need for supplemental nitrogen (N) fertilization of ‘Tyler’ winter wheat (Triticum aestivum L. em Thell) in Pennsylvania. The tests were used between Feekes growth stages (GS) 3 and 6, and measured stem nitrate concentration, whole-plant Kjeldhal-N concentration, and crop N uptake. The accuracy of a test for predicting N deficiency was evaluated in four to eight experiments conducted on either Hagerstown (fine, mixed, mesic, Typic Hapludalfs) or Murrill (fine-loamy, mixed, mesic Typic Hapludults) soils. Spatial variability of the test procedures was evaluated at GS 4 in a 0.25 ha area of a production wheat field. In a third study the temporal variability of each test procedure was monitored during 37 and 25 d periods in 1985 and 1986, respectively. The whole-plant N concentration accounted for the most variation in relative yields, had the lowest spatial variability, and appeared to be best suited for the current wheat management system in Pennsylvania. The wholeplant N concentrations necessary to produce 90% of the maximum yields or critical levels, were 39.0, 35.0, and 26.5 g N kg^–1 for GS 4, 5, and 6, respectively. The stem nitrate tissue test was most sensitive to short-term changes in the soil N supply. The critical level for the stem nitrate test did not fluctuate more than 10% between growth stages and averaged 2.15 g kg^–1 NO₃–N. The N uptake test was the weakest predictor of N deficiency, had the largest spatial variability, and was affected by factors that limited crop dry matter production.

Received for publication April 22, 1988.

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