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Predicting Nitrogen Availability to Rice: I. Comparison of Methods for Determining Available Nitrogen to Rice from Field and Reservoir Soils¹

ABSTRACT

Organic matter or available N in soil before or after incubation under waterlogged conditions each was used to predict soil N availability to ‘Nato’ rice (Oriza sativa L.) plants in a greenhouse experiment. The 42 silt loams had much lower organic matter and available N contents than the 19 clays (or reservoir soils) used in the study. Soluble + extractable NH₄⁺-N after 6 days incubation accounted for 91% of the variation in yield on the 19 clay (or reservoir) soils, but accounted for only 18% on the 42 silt loams.

Initial NH₄⁺-N, initial NO₃^–-N, NH₄⁺-N production during 6 days incubation, and NH₄⁺-N production during 6 to 12 days incubation, as independent variables in a multiple linear regression, were used to predict grain yield. Little was gained by including NO₃^–-N or 6 to 12 day NH₄⁺-N production. Of the independent variables included, NH₄⁺-N production during 0 to 6 days incubation alone accounted for the greatest percentage of yield variation but including initial NH₄⁺-N together with 6 day NH₄⁺-N production (also initial NH₄⁺-N + that produced during 6 days) improved the prediction.

Organic matter or methods which measure total N in soil alone were poor predictors of N availability to rice. Organic matter accounted for 41 % of the variation in yield on clay soils and < 1% on silt loams, whereas total N accounted for 56 and < 1% of the yield variation on the clays and silt loams, respectively.

Key Words: ammonium • soil organic matter

¹ Contribution from the Ark. Agr. Exp. Sta.

² Former Assistant Professor, Ark. Agr. Exp. Sta., Stuttgart; former Assistant Agronomist, Arkansas Agr. Exp. Sta., Fayetteville; and former research assistant, Ark. Agr. Exp. Sta., Stuttgart, respectively. The authors presently are Assistant Professor of Agronomy, University of Kentucky, Lexington; Agronomist, American Rice Growers Coop. Assoc., Lake Charles, La.; and Chemist, USDI Bureau of Sports Fisheries and Wildlife, Fish Farming Experimental Station, Stuttgart, Ark.

Received for publication October 27, 1966. Accepted for publication June 30, 1967.