EFFECT OF APPLICATIONS OF MANGANESE AND ALUMINUM, SOIL-PH AND MOISTURE REGIMES ON ALFALFA YIELDS AND THE AVAILABILITY OF MANGANESE AND ALUMINUM

Alfalfa (Medicago sativa L., ''Saranac'') was grown on Ste. Rosalie clay, Greensboro loam and St. Jude sand adjusted to .apprx. pH 5.0, 6.5 and 7.5 in a greenhouse experiment, to determine the changes in response of alfalfa to Al and Mn resulting from variations in soil pH and soil moisture. Rates of Mn were equivalent to 0 and 200 kg .cntdot. ha-1, and rates of Al were 0, and 100 kg .cntdot. ha-1. Three soil moisture regimes were used: optimum, with soil moisture between field capacity (FC) and 70% of this value; wet, with soil moisture between saturation point (SP)and FC; very wet, with soil moisture between saturation point and a value half way between SP and FC. Mn applied on acid soils (pH 5.2) under optimum soil moisture regimes decreased alfalfa yields by 3% only, compared to a 62% decrease in alfalfa yields by Mn applied on acid soils of the 2 high soil moisture regimes. This was due to a high level of Mn in alfalfa on the wet acid soils. A large quantity of Al was also found in alfalfa grown in acid soils, along with a high concentration of extractable aluminum. This resulted in a 54% reduction of alfalfa yields. Content of Al and Mn in alfalfa top and in soils was decreased sharply by liming soils at pH of 6.5 or 7.5. On soils limed to a pH of .apprx. 7.0, alfalfa survived at high levels of Mn and Al, such as frequently encountered in some acid and very wet soils.