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Long-term N-P Fertilizer and Climate Influences on Morphology and Yield Components of Spring Wheat¹

The long-term availability of one-time, high-rate, broadcast application of P fertilizer has not been adequately evaluated in relation to variable water supplies and N fertilization for optimum production of spring wheat (Triticum aestivum L.) on dryland. Additional knowledge of the interrelationship of plant morphological characteristics, plant growth, and components of grain yield to soil fertility will be useful in the development and validation of wheat yield models.

Spring wheat was grown for 12 years (winter wheat in 1 year) in two separate alternate crop-fallow plot series on a Williams loam (fine-loamy, mixed, Typic Argiborolls). Phosphorus fertilizer as treble superphosphate was broadcast at 0, 22, 45, 90, and 180 kg P/ha at the beginning of the study in 1967 or 1968. Ammonium nitrate was broadcast each crop year at 0, 45, and 90 kg N/ha. With relatively shallow sweep-tillage, soluble P increased proportionate to rate of P applied and remained relatively high over the 12-year period in the 0 to 15.2 cm soil depth. Significant grain yield responses to initial and residual P fertilization occurred up to a soluble P level of about 15 ppm independent of available water supplies. In contrast, yield responses to N fertilization increased as total available water supplies increased from 22.6 to 52.8 cm.

Number of heads/m² could be predicted more reliably from number of adventitious roots/plant than from number of tillers. Number of head/m² accounted for 86% of the yield variance associated with years (climate) and N-P fertilizer treatments excluding one disease-year when number of kernels/head was significantly decreased. Combining number of heads/m² and kernels/head accounted for 91% of the yield variance associated with fertilizer treatments over all years combined. Predicting grain yields from dry matter production at tillering or heading was unsatisfactory compared to yield predictions based on total dry matter production or straw production at maturity.

Key Words: Adventitious roots • Tillers • NaHC0₃-soluble P • Residual P availability • Water-use efficiency • Triticum aestivum L.

² Supervisory soil scientist, USDA, ARS, Northern Great Plains Research Center, P. O. Box 459, Mandan, ND.

Received for publication August 13, 1981.

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