Soil Crusting and Emergence of Wheat Seedlings

Soil crusting is usually referred to be the cause of impaired wheat (Triticum aestivum L.) stands in the Negev region of Israel. The soils in this region vary from mixed hyperthermic, typic Ustipsamment soils to fine loamy, mixed hyperthermic typic Ustorthent soils. The major objective of the work carried out was to estimate and analyze the soil crusting phenomena under field conditions and as a secondary objective to study and analyze wheat seedlings behavior under field and laboratory conditions. Resistance to penetration of natural soil crusts, measured by penetration from underneath the crust, and its dependence on water content and seed bed preparation methods (disking vs. plowing) were determined. These measurements were collected in a field survey carried out during early winter following the first rain that initiated both wheat germination and a sequence of soil slacking and subsequently, as the soil dried out, crusting. It was found that natural crust resistance to penetration was greater for those formed on disked plots than on plowed plots. In laboratory trials, using a sandy silt loam (typic ustipsamment) soil, crusts were formed by simulated rainfall (intensity of 10 mm/hour, rainfall applications of 5, 10, and 30 mm in one or two simulated rainstorms). These crusts were found to be less penetrable than the natural ones for the same soil under natural field conditions at the same water contents. Wheat seedling penetration of soil crusts was evaluated under laboratory conditions, whereas projected penetrability of crusts under field conditions was based on an assumed water potential and subsequent coleoptile thrust in the region of the seedlings. These projected values suggested that crusting in the field may not be the cause for a bad stand, except under extremely adverse conditions. These projections illustrate the urgency of including dynamic considerations of seedling performance in the future studies of small grains germination and emergence under and through crusting soils.