Effects of micronutrients on Khao Dawk Mali 105 rice in Thai paddy soils and assessment of their availability indices

Abstract

Background: Micronutrient deficiency is currently the major cause of declining productivity in rice-growing countries, including Thailand. A pot experiment was conducted to determine limiting micronutrients (Fe, Mn, Zn, and Cu) for Khao Dawk Mali 105 (KDML 105) rice, in four soil types. Soil micronutrient speciation and in-soil extracts were examined to identify the species controlling the availability to the plant and the efficiency of soil extract to assess the availability indices of multi-micronutrients for KDML 105 rice.Results: Micronutrients in the studied soils were in the order Fe > Mn > Cu > Zn, mostly in stable forms accounting for 96.8%-99.7%, 81.3%-96.5%, 95.6%-97.6%, and 83.3%-98.0%, respectively, and in available fractions of only 0.32%-3.2%, 3.55%-18.66%, 2.44%-4.42%, and 2.03%-16.74% respectively. The omitted Fe had the greatest effect on limiting the rice grain yield with a significant reduction of 24.12%-46.46%, whereas the omitted Zn had the least effect. To assess the availability for KDML 105 rice, DTPA pH 7.3 was the most suitable multipurpose extractant for Fe, Mn, and Cu, while HCl was superior to other extractants for Zn. All oxide fractions were responsible for supplying available Fe, Mn, and Cu, while Zn was generated from the organic and exchangeable fractions. Conclusions:The yield of KDML 105 rice can be increased by adding each of these micronutrients but yield magnitude was different depending upon their deficient levels in soils. The oxide-bonded micronutrients were the species controlling its availability and DTPA pH 7.3 was suggested as the multipurpose extractant for micronutrient availability indices indicators.