MECHANISM OF CRUST FORMATION ON A SOIL IN CENTRAL IRAN

Crust formation on soils around Isfahan reduces infiltration and seedl ing emergence in cultivated lands. Mechanism of crust formation on loc al soils was investigated under field condition. Soil and crust sample s from a field under furrow irrigation were taken for physical, chemic al and micromorphological analysis. Seedling emergence of sugar beets (Beta vulgaris) reduced by 50% due to crust forming after the first ir rigation. Rise of water table, increase of exchangeable sodium percent age to 6.4, and higher silt content resulted in higher susceptibility of soil to crust formation. Crust forming inside furrows was thicker a nd usually consisted of 3 layers as compared to a thinner crust formed on beds with only 2 layers. Layers found in crusts of Lavark soil gen erally had less sand and more silt and fine clay compared to the overa ll Ap horizon. Silt and fine clay in the middle M2 layer of crust form ing on furrows were increased from 51.6 and 5.6 to 59.8 and 21.1 perce nt respectively. Percolation of finer particles through and inside the pores created layers that were denser and less porous. Chemical dispe rsion of particles together with physical deterioration of surface str ucture by long period of mechanized agriculture in the area are probab ly the main reasons for crust formation.