Mechanical factors influencing cotton emergence through soil crusts

Abstract

Soil crusting is a worldwide problem for obtaining good stands after planting. This study was conducted to evaluate effects of environmental physical conditions upon mechanical factors influencing seedling emergence from soil crusts. The factors include seedling emergence force and critical time, hypocotyl elongation and swelling, crust impedance, and critical penetration distance. Critical penetration distance is the distance of penetration by the seedling when maxim um crust impedance is encountered. Seedling emergence force was measured when the seedling was supported laterally. Maximum emergence force varied quadratically with increases in soil temperature and moisture. The highest force, 11.4 N was measured at 28.5 °C and -0.1 bar. Critical time decreased linearly with increasing temperature. Maximum emergence force increased linearly with the hypocotyl cross-sectional area, while calculated turgor pressures were 768, 756, and 518 kPa at -0.1, -0.3, and -3.0 bar, respectively. Emergence force was much less when lateral support was not provided. Compaction of the top soil increased emergence force. Hypocotyl elongation decreased with increasing downward load and increased with increasing soil temperature and moisture. Swelling index increased quadratically with increasing downward load. Total fresh weight decreased with increasing downward load and decreasing soil moisture. Hypocotyl swelling generated potential emergence forces exceeding downward loads in the tested range, 0 to 1.8 N.A lower soil moisture reduced potential emergence force and caused a faster slowing of force increase. Crust impedance was measured using a mechanical probe penetrating from below. Up to four-fold increases in probe size and twenty-fold increases in probe speed did not increase measured impedance of dried crust. Crust impedance increased exponentially with time as the crust dried. An increase in initial soil moisture and compaction increased crust impedance. The increase in crust impedance by compaction was smaller when initial soil moisture was low. Crust impedance measurements by a pocket penetrometer correlated well with that by the probe but overestimated the impedance. Critical penetration distance increased with drying and increasing initial soil moisture. Drying rate of the crust did not significantly influence crust impedance.