Influence of potassium chloride and polyphenols on the dynamics of the release of manganese from soils

Assess the effect of potassium chloride fertilization, and presence of polyphenols on the release of managanese from soils.


30 Aug 2001


    The deficiency of toxicity of manganese in soils depends on the nature of Mn-bearing minerals in soils and their physico-chemical behavior in the dynamic soil environment caused by the agricultural practices in vogue. Research emphasis regarding Mn availability has, so far, been to vary the soil pH thereby affecting the Mn availability. Some neutral chloride salts were found to increase the soluble Mn2+ and also to decrease the soil pH in the soil layer surrounding the fertilizer placement. It was also reported that fertiliser carriers containing Ca, K, Na or OH reduce Mn toxicity whereas carriers containing Cl, SO4, Br or H tend to increase it. However, the increase in Mn solubility associated with Cl- could not be attributed to the salt effect as the other K salts such as K2SO4 or K2CO3 had no effect on the Mn uptake by plants. The possibility of the reduction of MnO2 by Cl- under the experimental conditions of the soils studied was suggested.

    The influence of KCl on the release of Mn in soil most likely would vary with the forms, structure and chemical composition of Mn-bearing minerals, as well as with the soil associated factors such as pH, Eh, and soil temperature. Data on the influence of KCl solutions on the release of Mn from selected MnO2 minerals is reported in the present study.

    Pyrolusite, birnessite and cryptomelane, the common MnO2 minerals found in soils, were selected for the study. The minerals were prepared by the procedures outlines by Mackenzie. The release of Mn by 0.01 M, 0.1 M and M KCl was studied at 23+ 10C for various periods. Results of Mn release from other metal chloride solutions Viz., NaCl, NH4Cl and CaCl2 were compared with the data obtained using KCl. A solid-solution ratio of 2:1 (50 mg of minerals in 25 ml of salt solution) was used for the Mn release experiments.

    After 20 hrs of equilibration in water, the water soluble Mn was found to be 3, 1 and 390 mg/100g from pyrolusite, birnessite and cryptomelane, respectively. The release of Mn from these minerals consistently increased with increasing concentration of KCl (1, 0.01, 0.1, 1M). At the 1M KCl concentration, the release of Mn from birnessite, pyrolusite, and cryptomelane increased by 21, 8.7 and 2.2 times, respectively. The enhancement of the release of Mn from these minerals by KCl was accompanied by the decrease in pH and the increase in Eh. The variable increase in Mn release from different metal chlorides was observed. The Mn release was in the order: CaCl2 > KCl > NH4Cl=NaCl.

    Further study on the kinetics and mechanisms of Mn release from Mn-bearing minerals commonly found in soils is in progress to explain the K-fertiliser-associated changes in Mn availability to the plants.

    Research is also in progress to investigate the role of Mn-bearing minerals in the polymerization of polyphenols and the subsequent formation of humic substances and release of Mn in the redox reactions.