Soil survey involves the mapping, classification, correlation, and interpretation of soils. The first soil survey in Iowa was in the Dubuque County area but did not include the entire county. The field work was done in 1902 and the report was published in 1903. Since that time, most Iowa counties have had at least two soil surveys completed and some have had three. The basic factors of soil formation have not changed but the use of the soils for intensive agriculture has resulted in changes in some soil properties, especially of the surface horizons. However, generally factors other than soil differences have been responsible for multiple soil surveys over one area. Over time, our concept of soil has changed. Early soil scientists with a background in geology considered the soil to be primarily that part of the earth's surface that had been darkened by the addition of organic matter. Our concept of soil has evolved so that soil now is considered a natural body made up of several horizons or layers that are genetically related to the soil forming factors under which the soil has developed. Total analyses of soils for phosphorus and potassium was a common practice during the early 1900's. Later, it was learned that it was not the total amount of a nutrient that was important for plant growth but the amount that was available to the plant. Other major factors in resurveys were the scale and the base map used.