Ferric iron transformation in soils with rotation of irrigated rice uplandcrops and effect on soil tillage properties

We examined the relationship between the form of iron and the tillability ( defined as the degree of ease of pulverizing a soil into small clods) of so ils in upland fields that had been converted from paddy fields. The amount of iron (Fe,,) extractable with acetate buffer (pH 3.0) decreased from 0.95 9 g kg(-1) in a field that has been continuously used as a paddy field to 0 .104 g kg(-1) in a field that had been converted into an upland field for a period of 5 y. There was no significant change in the free iron oxide cont ent under upland conditions. These results indicate that ferric iron oxides are gradually crystallized to less reactive forms after the conversion of a paddy field into upland conditions. Both soil tillability (represented by the mean clod diameter after tillage) and the stability of the soil micros tructure (represented by the sediment volume) also increased during the 3-y period after conversion and then remained constant for the last 2-y period of the study. On the basis of these results, two mechanisms for the improv ement of soil tillability can be proposed as follows: crystallization of fe rric iron oxides increased their resistance to microbiological reduction an d due to this stabilization the iron oxides as a cementing reagent that con tributed to the soil microstructure, which in turn affected the soil tillab ility. During the first year after drainage, however, there was no signific ant correlation between the soil tillability and amount of Fe-ac, presumabl y because the soil was not sufficiently dry in the first year after convers ion, and the iron oxides did not affect appreciably the soil structure.