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Friday, July 20, 2018

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4022. Mahler, R.L.. 1990. Nitrogen database project - final report.. unpublished report for Dryland Cereal/Legume LISA project.
This project had two components: 1) development of a comprehensive database on winter wheat response to nitrogen fertilizer rates; 2) evaluation of the potential of peas, alfalfa, and wheat straw as nitrogen sources for a following wheat crop in rotation. The database study examined winter wheat yield response to 41 nitrogen rates. When soil test N + mineralizable N + fertilizer N ranged from 101 to 175 kg/ha, a requirement of 2.75 lb N per bushel of wheat was calculated. This agrees with the figure calculated by Leggett in the 1950's, indicating that modern varieties have not changed in their basic nitrogen requirement, when nitrogen fertilizer efficiency is assumed to be 50%. At total available N rates greater than 175 kg/ha, the N requirement per bushel of wheat increased dramatically. Low rates did not show a large increase in efficiency on a per bushel basis. At Moscow, N fertilizer application rates less than 95 kg/ha resulted in greater than 50% N use efficiency. Efficiency declined rapidly at rates above this. The green manure study compared alfalfa, pea, and green wheat straw residues applied at 1, 2, and 3 mt/ha. In general, higher rates of pea and alfalfa resulted in higher wheat yields. The highest yields were with the high rate of pea residue. It was more effective than alfalfa residue, probably due to faster decomposition. Alfalfa provided more N per ton of residue (31 kg/mt) than the peas (29 kg/mt), while straw added 19 kg/mt.

9903. Smith, C.J., D.M. Whitfield and O.A. Gyles. 1989. Estimation of available N status of soil by wheat and barley: A-values.. Soil Biol. Biochem. 22(1):169-172..
The A-value is a measure of soil nutrient availability and is expressed as the ratio of nutrient in the plant derived from labelled fertilizer to that derived from soil. The assumptions of this model are that the A-value is constant, independent of rate of fertilizer-N addition, and that the fertilizer use efficiency of the fixing and non-fixing plant is the same. Using 15N-labelled fertilizer, an experiment was run to examine the effect of nitrogen form, rate of addition and the timing of the nitrogen fertilizer application on the A-value. The results showed a decline in A-values with increasing rate of fertilizer nitrogen. From their results, the authors concluded that considerable doubt is cast on the assumptions that the A-value is independent of nitrogen rate for all plants.

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