| Variable | Regression equation | R2 |
| Cu rate vs extractable soil Zn | Y = 0.51 + 0.019X | 0.92** |
| Cu rate vs extractable soil Mn | Y = 10.96 + 0.369X – 0.011X2 | 0.68** |
| Cu rate vs extractable soil Fe | Y = 29.39 + 0.255X – 0.019X2 | 0.92** |