Are penetrometer measurements useful in predicting emergence of sugar beet ( Beta vulgaris L. ) seedlings through a crust

Abstract

Soil crusting strongly affects seedling emergence. Laboratory experiments were carried out to analyse the emergence of sugar beet (Beta vulgarisL.) seedlings from beneath a crust, and to determine whether mechanical models using penetrometer measurements could predict the final emergence rates. Wet and dry crusts were created under a rainfall simulator. An ascending micropenetrometer was used to measure the crust strength in conditions as close as possible to those encountered by seedlings. The mode of the emergence was observed for both micropenetrometer probe and seedlings. While 94% of the emerged seedlings penetrated the wet crusts, only 6% broke it. The probe penetrated the wet crust in only 46% of cases, and broke it in the other 54%. In contrast, 39% of the seedlings emerged directly through a crack in the dry crust and 55% emerged by breaking (5% lifted a detached fragment and 1% penetrated the crust). In the same conditions, the probe emerged directly through a crack in only 4% of cases; 67% by breaking and 29% by lifting a crust fragment. The force recorded by the penetrometer was 0.05 – 0.80 N, and it varied with the mode of emergence. The seedling growth force distribution was 0 – 0.30 N (mean = 0.09 N). Mechanical models comparing the force exerted by the micropenetrometer probe to the seedling force distribution according to the mode of the emergence were used to predict final emergence rates. Penetrometer measurements appeared to overestimate the mechanical resistance encountered by the seedlings, leading to an underestimation of emergence rates under all emergence conditions. This overestimation of crust strength was attributed to the rigidity of the probe as opposed to the flexibility of hypocotyls.