Soil manganese and some factors affecting its availability

Studies were carried out to investigate the effects of seedbed cultivation practices on the availability of soil manganese to barley. Generally, overall seedbed consolidation had little effect on manganese uptake, but the availability of the element was found, in many cases, to be considerably enhanced where soil had been compacted by repeated passes of tractor wheels during cultivation operations. The effect of this wheeling phenomenon on manganese uptake was governed by the method of fertiliser application. Often, the soil beneath the wheel tracks was found to be more acidic and to have higher concentrations of CaCl^- extractable manganese. At a number of sites the soil pH was highly correlated with the logarithm of the extractable soil manganese. Possible causes of enhanced manganese availability in the more compacted soil were (1) soil acidification ^rought about by nitrification of ammoniacal fertiliser and/or by+H +ion exudation as a result of enhanced availability of fertiliser (NH, ,K ); (2) a greater exudation of compounds able to dissolve insoluble manganese; (3) contact reduction mechanisms.The principles of radioisotopic exchange and isotopic dilution analysis were applied to the study of soil manganese using radioactive 5i+Mn. The addition of the tracer to a wide range of soil types showed that the rate of disappearance of 5t+Mn from soil solution, and its distribution in various fractions, differed greatly between the soils. Generally, the radioisotope labelled all the soil fractions determined, with the majority of 51+Mn associated with the water-soluble + exchangeable, organically bound and easily reducible oxide fractions. I'ftien the soils were subjected to air and oven-drying both the native and radioactive manganese behaved in similar fashion. However, the changes in native manganese were proportionately greater than 5t+Mn in the former two fractions while in the easily reducible fraction the reverse was true. Net gains of 5t+Mn were observed in the resistant and residual manganese fractions as the moist soils were dried, probably because of occlusion or oxidation of the radioisotope in these fractions.An assessment of the manganese labile pool using S1+Mn was also carried out. The two techniques exployed - chemical extraction and plant uptake - were found to be of limited value, which was solely or partly due to sorption of ^Mn. Also, measurements of the labile pool by plant uptake were found to be markedly affected by different levels of soil consolidation.The chemical and microbiological release of manganese under waterlogged conditions was investigated using sterilised and unsterilised soils. During the 28 day submergence period, a direct microbial contribution to the release of manganese appeared to be small in three of the four soils investigated. Chemical reduction was thought to be attributable to the reaction of manganese oxides to microbially synthesised organic compounds and/or to enzymatic systems that remain operative following sterilisation by gamma irradiation