Abrasive grits (sieved to less than 0.85 mm when necessary)
were applied via compressed air at approximately 700 kPa through
a hand-held siphon-fed sand-blasting unit (Powermate, Inc.;
Aurora, Illinois, USA; Model 0090367CT). The air compressor
(BelAire Compressors; Rock Hill, South Carolina, USA; Model
3G3HH) was transported in a tractor cart (Ravenna Officine Metalmeccaniche;
Italy; Model MT 500 V) towed by a walk-behind
utility tractor (BCS America; Portland, Oregon, USA; Model 853)
and grits were siphoned from different buckets in the cart (Fig. 1).
The siphon line was removed from each bucket and cleared between
different abrasive grit treatments. One person drove the
tractor approximately 1.6 km h1 and a second person walked
behind the cart and aimed the blasting nozzle at weeds growing
within each planting hole of the plastic mulch. This speed allowed a
maximum application time of 4 s per planting hole. The tip of the
nozzlewas maintained between 30 and 60 cm from the soil surface.
Grits were applied continuously within each plot and the application
rate varied by abrasive grit type (Table 1).