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Cone Index of Loamy Sands as Influenced by Pore Size Distribution and Effective Stress¹

ABSTRACT

Tillage pans form in coarse textured soil horizons of the Atlantic Coastal Plain and develop high cone index values (soil strengths) which slow root growth and may decrease crop yields. Identification of soil properties that influence cone index may improve field identification of tillage pans and lead to the development of techniques to reduce their detrimental effects. This study examined the influence of pore size distribution, and effective stress on the cone index of loamy sand and loamy fine sand soil materials. Effective stress values were calculated from soil water characteristic curves and compared to cone index values for both natural cores and remolded materials. The remodeled samples of a loamy sand and a loamy fine sand developed different cone index values for relative saturations between 1.00 and 0.10, and these cone index differences were partially related to pore size distribution differences between materials. Pore size distribution influenced cone index by controlling effective stress. In both remolded samples and undisturbed cores of loamy sand, effective stress values ranged from 0 to 33 kPa for soil water potentials between 0.0 and –1500 kPa, respectively. For relative saturation values between 1.00 and 0.10, changes in effective stress closely conformed to changes in cone index for nondispersed soil materials. At a given effective stress, cone index was influenced by bulk density and soil particle characteristics that influence interparticle friction.

¹ Contribution from the Dep. of Soil Science, North Carolina State University. Paper no. 9414 of the Journal Series of the North Carolina Agricultural Research Service, Raleigh, NC 27695-7601.

² Assistant Professor of Soil Science, North Carolina State Univ., Raleigh, NC 27695-7619.

Received for publication October 13, 1983. Accepted for publication June 25, 1984.