The impedance of soil seals and the forces of emerging seedlings

Abstract

Surface seals were produced on field plots of Tippera sandy clay by simulated rainfall and natural drying. The impedance was determined by recording the force required to bring about the emergence of mechanical probes buried prior to seal formation. In comparison with the impedance of loose dry soil, wetting and draining to field capacity caused a two- to threefold increase in impedance. On drying to 6% water content the impedance increased to a maximum of four to nine times the initial dry soil value. On further drying to 2% water content the impedance decreased to about two-thirds of the maximum value. Axial seedling forces calculated for a range of stem diameters and assumed turgor pressures were matched with the seal impedance data to produce a model which provides a quantitative assessment of the mechanical impedance met by seedlings.