Effects of Soil pH and Eh on Growth and Nutrient Uptake by Rice in a Flooded

Rice grown on a flooded Sumatra Oxisol soils frequently exhibits poor growth and low yield. A study has been made of the effects of pH and redox conditions on growth and nutrient uptake by rice, and the relationship between nutrient release in the flooded soil and nutrient uptake by the rice plant. At pH 4.5 rice plants produced low plant height, low root dry weight, low shoot dry weight, slow relative growth rate, and short root length under both reducing and oxidizing conditions. Under both soil conditions these growth parameters tended to increase from pH 4.5 to 6.5 and then decrease at pH 7.5. The rice plants under reducing conditions showed a significantly higher score of orange leaf discoloration at pH 4.5 and 5.5 than at pH 6.5, and 7.5. At pH 4.5 the rice plants absorbed significantly high concentrations of Zn, Fe, Mn, Ca, Mg, and Al under reducing conditions. However, only the Fe content observed at pH 4.5 and 5.5 under reducing conditions was above the critical level of toxicity of 300 mg kg1, indicating that the rice plants may have suffered from Fe toxicity at these two pH levels. Under oxidizing conditions, rice plants showed a significantly higher score of yellow leaf discoloration at pH 4.5 and 5.5 than at 6.5 and 7.5. Under oxidizing conditions, the Al content in the rice plants at all pH levels was above the critical level of Al toxicity of 300 mg kg1, indicating that the rice plants may have suffered from Al toxicity. The contents of Cu, Zn, Mn, Ca, Mg, P and K in the rice plants at all pH and redox conditions were in the normal range except for Si which was found to be below the threshold deficiency value of 5%. A significant, high positive correlation was measured for the water-soluble Fe in soil and uptake by the rice plants under reducing conditions (r = +0.962**). A significant, positive correlation was also measured for the water-soluble Al in the soil and uptake by the rice plants under oxidizing conditions (r= +0.658*). A significant correlation was not measured for water-soluble Si in soil and uptake by the rice plants under both soil redox conditions.