All extractable soil sample results are expressed as lbs/acre. Calibrated soil scoops are used to measure the soil rather than weighing. We use a 5 g soil scoop with a capacity of 4 mL and a 25 g scoop with a capacity of 20 mL. This system assumes a soil bulk density of 1.25 g/mL. The assumed weight is used for the calculation of lbs/acre.

An acre slice of soil 7.1 inches deep, with a bulk density of 1.25 g/mL weighs 2 million pounds. We simply multiply the parts per million found in the soil times 2 in order to convert the result to parts per 2 million, which is equivalent to lbs/acre. It is important to note that the term lbs/acre actually means the lbs contained in an acre slice of soil 7.1 inches deep.

In our system when we use a 5 g soil scoop (4 mL) and 20 mL extracting solution the calculation is as follows:

ppm in extract × (20 ÷ 5) × 2 = lbs/acre

This equation is correct for a 7.1 inch acre slice of soil having a bulk density of 1.25 g/mL. For soils having a bulk density other the 1.25 g/mL, the equation is nevertheless correct because of the following considerations: The error involved in the 5 g assumed weight may be corrected by multiplying 5 × (soil bulk density ÷ 1.25). Since the bulk density is not 1.25 g/mL, the 7.1 inch acre slice of soil will not weigh 2 million lbs. This error may be corrected by multiplying 2 × (soil bulk density ÷ 1.25). When the two error correcting terms are inserted into the equation, the bulk density terms cancel each other. Therefore, regardless of soil bulk density, this system gives accurate values for the pounds of element in an acre slice of soil 7.1 inches deep.

It is important to note that dividing our lbs/acre value by 2 will yield accurate ppm values only when the soil bulk density is 1.25 g/mL. For clients requiring their results in ppm or mg/kg, the samples should be weighed and not scooped.