Evaporation and crust impedance role in seedling emergence

Seedling emergence is affected by soil crusting through two mechanisms-crus t impedance (CI) and crust effect on evaporation rate that determine the mo isture content in the seed bed. The relative importance of these two mechan isms was evaluated in this study, The effects of soil sodicity, phosphogyps um (PG), polyacrylamide (PAM), and raindrop impact energies on evaporation and CI of two soils, hamra (mixed, loamy, typic Rhodoxeralf) and loess (mix ed, silty loam, calcic Haploxeralf), and their consequential effects on see dling emergence, were studied under high and low evaporation conditions (8. 5 and 5.6 mm day(-1), respectively). Increasing the exchangeable sodium per centage (ESP) from 3.0 to 7.0 decreased the rate of evaporation from the lo ess by 10% and doubled the soil CI, Under mist (low raindrop energy), crust formation did not occur, and evaporation from both soils was higher than t hat from crusted soils. Rain energy and PG had a more significant effect on the evaporation rate (E R) and CI of the hamra than that of the loess, The effect of PG on ER in th e loess was negligible, Cotton seedling emergence was affected by evaporati on rate and the moisture content in the seed bed, but it was not affected b y crust impedence, Conversely, onion seeds were affected by both, moisture content in the seed bed and by crust strength. Increasing the crust ESP fro m 3 to 7 and maintaining adequate moisture reduced the onion seedling emerg ence rate by 50%, Application of PAM plus PG to the soil surface doubled th e emergence rate of onions in loess with an ESP of 3.0 but was not effectiv e in loess with an ESP of 7.