THE DYNAMIC NATURE OF PLANT-AVAILABLE MANGANESE DURING STORAGE OF A CALCAREOUS SOIL

The effect of length of dry storage period and subsequent moist incubation on the availability of Mn was examined in a calcareous soil. Increasing the time of dry storage (for up to 4 years) generally increased the availability of Mn as determined by plant growth and Mn concentration in wheat and barley. Moist incubation of stored soil had variable effects on Mn availability depending on how long the soil had been stored before use and on the method used to assess Mn availability. When assessed by Mn concentration in plant tissues, increasing the moist incubation time (from 0 to 30 days) of soil stored dry for 4 years increased Mn availability in soil initially and thereafter decreased it. However, incubation time had little effect on Mn availability in soil stored for only 1 year or soil used fresh from the field. When Mn availability was assessed using a chemical extractant (DTPA; diethylenetriaminepentaacetic acid), both soils showed an initial increase in Mn availability immediately on wetting. However, Mn availability in the soil stored for only 1 year decreased rapidly and remained consistently below that of soil stored for 4 years. In the latter soil, Mn availability also decreased but only after a few days. Microbial studies indicated that there was a decrease in the ratio of Mn oxidising to Mn reducing microorganisms with increasing storage time. Inoculation of one soil with another suggested that the factor responsible for the low Mn availability in soils stored for a short period could be transferred to soils stored for a longer period. These results suggest that the change in Mn availability in a calcareous soil with dry storage is a result of changes in microbial populations that cannot be fully restored by moist incubation.