Characteristics of selected soils of Madagascar and influence of water regime on the chemical changes and growth of rice on a flooded iron toxic soil

The effects of water regimes (standing water, surface waterflow, upwelling water) on soil solution kinetics, nutrient uptake and growth of IR26 and IR46 grown on an iron toxic soil were likewise investigated. Upwelling waterflow distinctly lowered soil solution pH, EC, HCO3 ion, total Fe, Fe (II), water-soluble P, Ca++, and Mg++ compared to standing water and surface water regimes. The symptoms of nutritional disorders observed at the maximum tillering stage were more severe for plants grown under upwelling water, although no pronounced difference between the water regimes was detected in their plant growth parameters (plant height and tiller number), element content and uptake. The symptoms cannot be attributed solely to an excessive uptake of iron since there was no significant differences in plant Fe concentration. An imbalance of nutrients assimilated by plant may explain the growth differences. The most severely affected plants were characterized at the maximum tillering stage by low N/Zn and P/Zn ratios in the shoots, and high Fe/Cu ratio. Better plant growth observed in standing water and surface waterflow was associated with a low Fe/Mn ratio. Applications of Mn and Cu fertilizers may alleviate Fe toxicity by counterbalancing high Fe concentration and reducing sterility. The difference in the behavior of the IR26 and IR46 with regard to iron toxicity would be in their ability to maintain a certain range in the element ratios. Lower K/Fe ratios for IR46 throughout the growing season would reveal a lower K status in this plant compared to IR26. The increase of this ratio from maximum tillering stage to maturity would confirm that K deficiency in iron toxic soil occurs, partly because of a delay in K uptake due to the release of toxic substances in soil solution. Upwelling water resulted in a substantial increase of available Zn in the soil. Moreover, the decrease in active iron was least in flowing water regimes (surface waterflow and upwelling water).