What is an "acre furrow slice" of soil?
Acre furrow slice is a common means of estimating the volume or weight of the surface 6.7 inches of soil in an acre of land. 6.7 inches is the approximate depth a farm plow blade penetrates the soil. Acre furrow slice volumes and weights are often used by soil test labs as a means of expressing nutrient content per acre of soil.
The typical soil volume used in acre furrow slice calculations is 24,394 cubic feet, and is determined by multiplying the number of square feet in an acre (43,560) by a depth of 0.56 feet (6.7 inches ÷ 12 inches = 0.56 feet).
Determining the weight of soil in an acre furrow slice involves knowing the dry bulk density of the soil in question. Often, a soil bulk density of 1.33 grams/cubic centimeter is used because this approximates the bulk density of a silt loam soil. Keep in mind that bulk densities of soils vary, and are usually higher than 1.33 grams/cubic centimeter for sandy soils and lower for organic soils. By multiplying 1.33 grams/cubic centimeter by a conversion factor of 62.4, you can determine there are 82.99 pounds of dry soil in one cubic foot. To determine the weight of dry soil in an acre furrow slice, multiply 82.99 pounds by 24,394 cubic feet for a total weight of 2,024,458 pounds. Typically, this number is rounded off to 2,000,000 pounds of soil per acre furrow slice.
When concentrations of nutrients are reported in pounds per acre (or pounds per acre furrow slice), think of this value as an index from which to compare future reports from the same area, or reports from other areas. In reality, many labs determine the concentration of nutrients using a certain volume of soil, and then convert the concentration to parts per million (ppm) using a standard bulk density for all soils (usually 1.33 g/cubic centimeter). Some labs multiply ppm values by 2 to come up with pounds per acre, whereas others simply report results as ppm. Therefore, if your soil test report indicates 60 pounds of plant-available phosphorus/acre, think of this as an estimated value. In fact, it is unlikely that exactly 60 pounds of phosphorus is evenly distributed throughout the acre furrow slice. However, as long as your lab is reporting results and recommendations based on field calibration tests that show consistent plant responses at certain nutrient concentrations, you can be confident that your results and recommendations are sound.