2019 ASHS Annual Conference

Manganese is an essential plant nutrient that can become toxic to plants at high concentrations. Most of the production areas in Central Oahu have Oxisol soils (Wahiawa Series) that have been in pineapple production for many years. Due to the management practices associated with pineapple production, the average soil pH in these areas are near 4.5. These highly weathered soils are known to be naturally high in manganese which becomes soluble and toxic to plants under low pH conditions. To better understand how management practices affect the plants ability to uptake manganese in these low pH soils, lettuce was planted in one gallon pots containing oxisol soils and amended with common liming materials. Treatments included an unamended check, commercially produced compost at a rate of 20 tons per acre and calcium carbonate as Microna at a rate of 2.5 tons per acre. The starting unamended soil had an average pH of 4.5 and had a manganese concentration range of 648-934mg/dm³. Soil amended with calcium carbonate and compost showed to increase pH to 6.3 and 4.8, respectively. Above ground biomass from both calcium carbonate and compost amended soils were significantly higher than that of unamended soils. Above ground biomass from calcium carbonate amended soils were numerically higher but not significantly different than biomass from soils amended with compost. Post-harvest tissue analysis showed manganese was significantly higher in plants grown in unamended soil than plants grown in soils amended with both compost and calcium carbonate. Although there were no significant differences in tissue manganese between soils amended with compost and calcium carbonate there were significant visual toxicity symptoms on plants amended with compost. This study provides valuable information that can be used to make management decisions in areas that have low pH and high soil concentrations of manganese.