Iron Availability and Uptake by Rice in Acid Sulfate Soils

Abstract

AbstractIron toxicity is one of the main constrains to rice (Oryza sativa L.) growth on acid sulfate soils. The objectives of this study were to determine the solubility of Fe under natural conditions in these soils, to determine what governs Fe solubility, and to investigate Fe uptake by rice growing on these soils. Iron availability and uptake by rice was evaluated in 134 flooded acid sulfate soils in the Central Plains region of Thailand and in a growth chamber experiment utilizing 50 of the same soils. Soil and plant metal analyses were conducted at the panicle differentiation stage of growth in both studies and in the soil prior to transplanting in the growth chamber experiment. The ‘R.D. 23’ rice cultivar was used in the growth chamber study, while 88 different cultivars (native and improved) were sampled in the field study. Metal activities in soil solutions were determined from free metal concentrations using GEOCHEM and activity coefficients using the Davies equation. Exchangeable metals were determined from sodium acetate extracts buffered to the soil pH. The results indicated that Fe2+ activities in flooded acid sulfate soils are seldom in equilibrium with pure Fe solid phases under natural conditions. This is believed to be due to (i) transient redox conditions, (ii) the presence of ill‐defined ferric oxides or hydroxides, and/or (iii) cation exchange reactions. A better relationship existed between leaf Fe and the divalent charge fraction in the soil solution attributable to Fe2+ (E′‐Fe) than to Fe2+ activity indicating that ion competition may be important in determining Fe uptake in rice. If the reported critical concentration for Fe toxicity in rice of 300 mg kg−1 is correct, then the critical E′‐Fe observed for Fe toxicity conditions in this study was approximately 0.75. These findings clarify the role of other nutrients in Fe toxicity in rice.