THE EFFECTS OF CROPPING CORN ON THE EXTRACTABILITY OF ZINC ADDED TO ACALCAREOUS SOIL

The effects of cropping corn on the decrease in the extractability of Zn added to a calcareous soil were studied by a pot experiment and che mical extractions. The results show that the concentrations of Zn exch angeable with MgCl2 (EXC-Zn) and extractable with DTPA (DTPA-Zn) in th e soils with added Zn decreased with time. The processes associated wi th the decrease in extractability in DTPA of Zn added to soil can be d escribed aptly by a diffusion equation which gives the proportion of a dded Zn in the non-DTPA fraction as a function of the square root of i ncubation time. This result suggests that the diffusion of Zn cations into microporous solids is a rate-limiting reaction. The relative diff usion rate coefficients (D/r(2)) were found to be 1.95x10(-10) and 3.3 4x10(-10) sec(-1) in the soils with added 217 of 20 and 60 mg kg(-1), respectively. Compared with uncropped soil, the concentrations of DTPA -Zn in the soils with added Zn were decreased by cropping. The decreas e of DTPA-Zn in the soils in the presence of corn can be attributed to both its acquisition by corn and other processes associated with the growing of corn. The activity of plant roots would appear to enhance t he process of decrease in the extractability in DTPA of Zn added to th e soil. The source of Zn uptake by corn was affected by the lending or activity of Zn in soil. In the soil with low available Zn, the DTPA n on-extractable Zn (non-DTPA-Zn) was mobilized and taken up by corn. In the soils with high available Zn, e.g. the recently added Zn, only EX C-Zn and a part of the DTPA-Zn were taken up by corn.