Date of Award

1993

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Plant, Enviromental and Soil Sciences

First Advisor

Donald L. Robinson

Abstract

Annual ryegrass (Lolium multiflorum Lam.) is frequently grown on acid soils with potentially toxic levels of aluminum (Al). Interaction effects of nitrogen (N) form and Al on growth and nutrient relationships in ryegrass require further definition for environmental, biological, and economic reasons. Nutrient solution and soil experiments were conducted in growth chambers to determine interactive effects of ammonium: nitrate (NH$\sb4$:NO$\sb3$) ratios and Al on growth, N absorption, nutrient composition, cation influx rate, and N use efficiency. In nutrient solution studies, highest dry weights of shoots were obtained when the two N forms were present in equal proportions, while highest dry weights of roots were obtained with 100% NO$\sb3$-N. Weights of both roots and shoots were lowered by Al. Aluminum increased the ratio of K/(Ca + Mg) in the forage. The grass tetany potential of the forage was high when NH$\sb4$-N was 50% or more of the available N. Aluminum stimulated the absorption of NH$\sb4$-N and depressed the absorption of NO$\sb3$-N. Influx rates of Ca, Mg, and K decreased with increasing plant age and with increasing levels of NH$\sb4$-N in solution. Significant positive correlations were found between influx rates and adsorption of Ca and Mg but not of K in the Donnan Free Space. In soil experiments, maximum shoot growth was found at the highest lime rate when the two N forms were present in equal proportions, while maximum root growth was obtained at the highest lime rate and 100% NO$\sb3$-N. Yields decreased as NH$\sb4$-N exceeded 50% of the available N, and concentrations of Ca and Mg in the ryegrass decreased as the NH$\sb4$-N level in solution increased, with both effects being greater at lower lime rates. The ratio of K/(Ca + Mg) decreased with increasing lime rates and with increasing plant age. Total N uptake decreased when NH$\sb4$-N was more than 50% of the available N, particularly at higher Al levels. Like yields, total N uptake increased with lime rate and decreased as NH$\sb4$-N exceeded 50% of the N. The studies indicate that increasing exchangeable Al and NH$\sb4$-N levels in soils significantly interact to depress yields and increase the grass tetany potential of ryegrass.

Pages

130

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