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Phosphorus and Nitrate-Nitrogen Immobilization by Wheat Straw¹

With stubble-mulch tillage systems, relatively large quantities of wheat straw with low N and P concentrations are being incorporated near seeding time in inherently P-deficient soils of the northern Great Plains. This study was conducted to determine what additional adjustments in fertilizer applications might be necessary to compensate for the amount of N and P immobilized by known quantities of incorporated wheat straw under defined soil fertility levels.

Wheat straw was incubated with four soils—Tally loamy sand, Dooley sandy loam, Williams clay loam, and Cherry silty clay. Straw was added at rates up to 10,000 ppm; and P, up to 260 ppm. The experiment was a randomized complete factorial design with three replications for soils, P rates, and straw rates. NaHCO₃-soluble P and nitrates were determined after seven alternate wet and dry incubation periods during a 60-day period.

Soluble P increased progressively as rate of P applied increased. Soluble P decreased progressively (P=.05) as straw rates increased. In all four soils, straw incorporation caused a much larger proportional decrease of soluble P at low than at high levels of applied P. The soil by straw treatment interaction was not significant (P=.05), indicating that incorporated straw rates had the same effect on soluble P in all soils. Soil nitrate mineralization was not significantly influenced (P=.05) by P rates even though the soils were P deficient. Nitrate levels decreased significantly as straw increased.

Key Words: Crop residue • Fertilizer recommendations

² Soil Scientist and Hydrologic Technician, respectively, USDA, Sidney, Montana 59270.

Received for publication March 4, 1972.