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Cohesion Development in Disrupted Soils as Affected by Clay and Organic Matter Content and Temperature¹

ABSTRACT

Soils were dispersed and separated into sand, silt, and clay fractions that were reconstituted to give mixtures of each soil with 5 to 40% clay. In the range from 0 to 35% clay, higher clay contents resulted in greater stability. Rate of cohesion recovery was over 10 times as fast at 90°C as it was at 23°C, showing that the processes involved are physical-chemical rather than biological. Maximum rates of cohesion recovery occurred at moderate soil water tensions, probably because some tension is needed to pull the particles into direct contact, but a continuous water phase is also essential to allow diffusion of bonding agents to the contact points. Since diffusion rates in water increase 300%, while rate of cohesion recovery increased 1000% when temperature was raised from 23 to 90°C, other factors, such as higher solubilities at higher temperatures of compounds contributing bonding ions to the solution, probably play a role in the rate of cohesion recovery. Recovery of cohesion was more rapid in the soil with organic C contents of 0.004 kg/kg than in the soil with 0.012 kg/kg. When the organic matter was removed with H₂O₂ from the soil with 0.012 kg C/kg, its rate of cohesion recovery increased. Rate of cohesion recovery of this high organic matter soil was also increased by aging it at 0.1 kg H₂O/kg soil compared to 0.2 kg/kg. A possible explanation is that organic coatings, tending to prevent direct contact and bonding of adjacent projections of mineral surfaces, are forced away from contact points by extremely strong forces that pull the adjacent minerals together when soil water tensions are high. When the higher organic matter soil had been consolidated by air-drying and rehydrated, its rate of cohesion recovery was just as rapid as that of the soil with low organic matter.

¹ Contribution from USDA-ARS, Snake River Conservation Research Center, Kimberly, ID 83341.

² Soil Scientist, Agricultural Research Technician, and Consultant (at SRCRC in July to August 1985, from the Waite Inst., Univ. of Adelaide), respectively.

Received for publication May 12, 1986.

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