Impact of manganese oxides on plant growth

Abstract

Manganese oxides have a strong sorption capacity for metals, effectively binding and reducing bioavailability of essential trace elements to plants. This study was initiated to evaluate the impact varying proportions of manganese oxides in different "soil" media had on plant growth. Manganese oxide-coated sand, described e l sewhere(Eley and Nicholson , 1993), was employed as this was known to have a uniform c o m p o s i t i o n which had ear l ier been well-characterised. Varying proportions of the oxides (0%, 0.5%, 2%, 4%, 6%, 8%, 10% and 50% oxide by volume) were mixed with two growth media: multipurpose compost and vermiculite. Mung beans were grown from seed in each media and their growth rate monitored over 16 weeks. Mung beans were selected for study because of their rapid germination and growth rate. The metal content of the plants and manganese oxides was d e t e r m i n e d by f lame a tomic abso rp t ion spectroscopy. The deve lopment of the plants visually deteriorated with increasing concentrations of manganese oxides in the growth media. Analysis of the p lan ts g rown in the c o m p o s t showed concentrations of Fe, Zn, Mg and Ca were less than in the control samples. Potassium showed a variable b e h a v i o u r but was gene ra l ly at h igher concentrations than in the control, while Mn levels did not display any consistent trend. Plants grown in the vermiculite showed Mn and Fe concentrations greater than the control, but lower concentrations of Zn and Mg. The low levels of magnesium, an order of magnitude less than the control samples, was particularly striking. Potassium and Ca showed a variable behaviour but were generally at levels greater than in the control samples. The variations in element behaviour, with respect to the control, between growth media is in part due to the properties of the medium itself Analysis of the manganese oxides retrieved from the "soils" shows, for example, than the striking loss of Mg from the plants is probably due to take-up of Mg on the vermiculite rather than on the oxides.