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Influence of Slope on Size and Distribution of Fields in Iowa¹

ABSTRACT

The objective of this investigation was to determine the percentage of the land in Iowa and in the major soil associations within Iowa that would fall into various field sizes when slope criteria were the only factors determining field size. A method was developed to determine the percentage of land which would fall into various field sizes (10–160 acres or ca. 4–65 hectares). Slope and acreage of individual soil mapping delineations for selected fields within each 1/4-section (ca. 65 hectares) tract was the input in this study. The soil map data by 1/4-sections represents a 2% sample of Iowa and consists of approximately 4,500 individual 160-acre tracts. For this study, 100 1/4-section tracts were selected in each of the four replicates. The number of 1/4-sections included in each replicate for each soil association was the same as the percentage of that soil association in Iowa.

The frequency of occurrence of different size fields meeting six sets of slope criteria are given with 5 and 10% confidence intervals for Iowa, and without confidence intervals for soil associations within Iowa. As field size increased and as more level fields were evaluated, the percentage of land in Iowa meeting the slope criteria decreased. Most of the large nearly-level fields are in soil associations in northwest and north-central Iowa, and in the Missouri and Mississippi river bottomlands. The within 1/4-section variance was less than that between 1/4-sections.

Key Words: soil association areas • farm field size • soil mapping

¹ Journal Paper no. J-5402 of the Iowa Agr. & Home Econ. Exp. Sta., Ames. Project no. 1191. Contribution from the Department of Agronomy.

² Research Associate and Professor of Soils, Iowa State University. The senior author is now Assistant Professor of Soils, Department of Agronomy, University of Illinois, Urbana.

Received for publication May 27, 1966. Accepted for publication April 13, 1967.