Rapp, I., Inst. Soil, Water and Environ. Sci., Volcani Center, Agricultural Research Organization, PO Box 6, Bet Dagan, Israel
Shainberg, I., Inst. Soil, Water and Environ. Sci., Volcani Center, Agricultural Research Organization, PO Box 6, Bet Dagan, Israel, Dept. of Environmental Science, University of California, Riverside, CA 92521-0424, United States
Banin, A., Dept. of Soil and Water Sciences, Fac. Agric., Food and Environ. Sci., Hebrew University of Jerusalem, PO Box 12, Rehovot, Israel

Seedling emergence is affected by soil crusting through two mechanisms - crust impedance (CI) and crust effect on evaporation rate that determine the moisture content in the seed bed. The relative importance of these two mechanisms was evaluated in this study. The effects of soil sodicity, phosphogypsum (PG), polyacrylamide (PAM), and raindrop impact energies on evaporation and CI of two soils, hamra (mixed, loamy, typic Rhodoxeralf) and loess (mixed, silty loam, calcic Haploxeralf), and their consequential effects on seedling emergence, were studied under high and low evaporation conditions (8.5 and 5.6 mm day-1, respectively). Increasing the exchangeable sodium percentage (ESP) from 3.0 to 7.0 decreased the rate of evaporation from the loess 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 that from crusted soils. Rain energy and PG had a more significant effect on the evaporation rate (ER) and CI of the hamra than that of the loess. The effect of PG on ER in the loess was negligible. Cotton seedling emergence was affected by evaporation rate and the moisture content in the seed bed, but it was not affected by crust impedence. Conversely, onion seeds were affected by both, moisture content in the seed bed and by crust strength. Increasing the crust ESP from 3 to 7 and maintaining adequate moisture reduced the onion seedling emergence rate by 50%. Application of PAM plus PG to the soil surface doubled the emergence rate of onions in loess with an ESP of 3.0 but was not effective in loess with an ESP of 7.