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A Century of Soil Development in Spoil Derived from Loess in Iowa¹

ABSTRACT

During the past century well-drained soils have formed in leached loess spoil materials under prairie vegetation along railroad cuts in Iowa. The spoil material was deposited on an existing Tama Variant soil which reveals the effects of 100 years of burial.

In 100 years' time a 31-cm A horizon has developed in the spoil, available phosphorus recycling has been measured, and the translocation and accumulation of illuvial clay and fine silt has occurred. The organic carbon content reached a maximum of 2.6% and exceeds 0.58% to depths of 40 to 46 cm. This 100-year-old A horizon, however, does not meet the color criterion for a mollic epipedon. Organic carbon content seems to build up rapidly in the first 30 to 50 years of soil development. After this time a slower more steady-state rate of increase seems to take place.

In 100 years' time the buried soil shows a 1% decrease (from 2.6 to 1.6%) in organic carbon in the upper 20 cm. Most of this difference resulted from the degradation of fibrous organic matter after burial. Accompanying this loss of organic matter is an increase in bulk density, which has impeded soil water movement. This density contrast has enhanced the accumulation of illuvial clay and fine silts in both the lower portion of the 100-year-old soil and in the buried A horizon.

¹ Journal Paper no. J-8872 of the Iowa Agric. & Home Econ. Exp. Stn., Ames, Iowa 50011 Project no. 2100. Contribution from the Dep. of Agronomy and the Iowa Cooperative Soil Survey.

² Chief, Res. Div., Iowa Geological Survey, Iowa City; formerly Research Associate, Dep. of Agronomy, Iowa State Univ., and now Research Geologist, Iowa Geological Survey, Iowa City; and Associate Professor, Dep. of Agronomy, Iowa State Univ., Ames, Iowa; respectively.

Received for publication July 5, 1977. Accepted for publication November 2, 1977.