Vertical wind shear and concentration gradients of viable  airborne bacteria were used to calculate the upward flux of viable cells above bare soil and canopies of several crops Concentrations at soil or canopy height varied from 46 colony-forming units per m3 over young corn and wet soil to 663 colony-forming units per m3 over dry soil and 6 500 colony-forming units per m3 over a closed wheat canopy