DeKalb County is a parallelogram of rich soil, about 36 miles long and 18 miles wide (58 km x 29 km). It covers an area of 648 square miles comprising 407,040 acres (164,725 hectares).
Slope, Drainage, and Soils
Most of DeKalb County ranges in elevation between 700 and 950 feet (213-290 m) above sea level. 58 percent of the land area has a slope of 0-2 percent. 41 percent is in the 2-7 percent slope range. The soil is permeable and in most cases is readily drained by tile and ditches. Drummer silty clay loam is an important black soil type with the surface 12 to 20 inches (30-51 cm) thick. The subsoil is 18 to 24 inches thick (45-61 cm). About 98 percent of DeKalb County is considered prime farmland by Natural Resources Conservation Service criteria.
The average date of the last killing frost is May 4 and the average date of the first killing frost in the fall is October 7. The growing season averages approximately 157 days. Rainfall averages 35 inches (90 cm). Prevailing winds are from west and southwest. Snow averages 36 inches (91 cm) annually.
DeKalb County is predominately agricultural and land is used as follows: Farmland, 88%; Cultural Development 12%. Over 97 percent of the land in the county is in capability classes I and II.
Limestone, dolomite, sand, and gravel are the chief minerals in the county.