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...addresses this issue by comparing properties in compacted to non-compacted soils in an abandoned farmyard and along preserved stretch of the Mormon Trail. Properties compared include soil morphology, bulk density, carbon, C/N ratio, and apparent electrical conductivity ([EC.sub.a]). Bulk density, organic carbon, and [EC.sub.a] values were consistently different in the compacted versus non-compacted soils. Darker soil colors were consistently found at greater depths and roots were more abundant in the non-compacted soils. Some changes in C/N ratios were observed and a zone of platy structure was found in the abandoned farmyard.
Keywords: soil compaction, Iowa, North Dakota
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INTRODUCTION
The frequent crossing of soil by heavy equipment in modern agricultural and forestry operations has made soil compaction one of the most important issues these industries face today (1, 2). Soil compaction has many negative influences on the soil system, including: reduced agricultural yields (3, 4, 5, 6), forage production (7, 8, 9), and tree growth (10); reduced numbers and diversity of soil fauna (11, 12, 13); restricted root development (5, 14); restricted air and water movement through the soil (1, 15); and reduced additions of organic matter to compacted areas (9). At one time it was widely believed within the soil science community that natural forces such as annual freeze-thaw cycles and wetting-drying cycles of shrink-swell clays would alleviate compaction problems in soils that were located in the appropriate climates and/or contained shrink-swell clays (16) and that tillage would address the problem in tilled soils (17). However, research over the last 20 years has disproved this belief (9, 17, 18, 19, 20, 21, 22, 23).
Most of the studies that address soil compaction issues address time periods of less than 20 years (i.e., 17, 19, 22, 24) and/or address soil compaction in forested ecosystems (i.e., 10, 18, 20, 21, 25). Longer-term studies on grassland soils are largely absent, with the notable exceptions of 9 and 23. This is probably because much of the land in grassland regions has been tilled for agricultural purposes, leaving little in the way of relatively undisturbed compacted study sites. By contrast, it is much easier to find areas in forests undisturbed for 20 plus years due to the long growth period of the crop and lack of tilling operations. There are, however, sites in grassland areas of the United States that have undergone compaction and then been undisturbed by tillage for long periods of time, including the abandoned farmyards found throughout the American Midwest and the occasional preserved portions of wagon trails. While these sites usually have not been used for production agriculture, they often include soils used for such purposes in adjoining fields. Two such sites were utilized for this study.
Study Sites
The sites chosen for this study are an abandoned farmyard near the town of Linton in south-central North Dakota and a preserved portion of the Mormon Trail near Murray in south-central Iowa. Compaction at the North Dakota site was by farm traffic such as tractors and heavy trucks, while compaction at the Iowa site was caused by covered wagon traffic. The North Dakota farmstead was established in the 1930s (A. Amundson, verbal communication, 5/14/2000) and abandoned in 1973 (P. Wagner, verbal communication, 9/1998). P. Wagner, now retired in Linton, is the wife of the farmer who worked this site until 1973, and A. Amundson is a long-time neighbor who saw the farmstead constructed when he was a young man. The Iowa site was compacted when the portion of the Mormon Trail that ran through southern Iowa was used from 1846 until approximately 1853 (26). Sampling at the two sites was done between 1998 and 2000; therefore soils had been compacted for 60-70 years at the North Dakota site and about 150 years at the Iowa site at the time of sampling. As well as can be determined, neither site has been tilled since the compacting events took place (P. Wagner, verbal communication, 9/1998, and R. Jackson, verbal communication, 7/1998). R. Jackson is a rancher who's family has owned the Iowa site for several generations.
The soil at the North Dakota site is mapped as the Bryant series (Fine-silty, mixed, superactive, frigid Typic Haplustolls), which are deep, well-drained, moderately permeable soils on loess covered uplands (27). The average summer temperature in Emmons County is 21[degrees]C (69[degrees]F) and the average winter temperature is -11[degrees]C (13[degrees]F) (27). The average annual...
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