What Is PPM in Soil?

Definition

• The abbreviation "ppm" stands for "parts per million." Parts per million is a standard unit of measure for laboratories to use when analyzing soil samples for certain nutrients or contaminants. Laboratory reports will often list constituent concentrations as milligrams per kilogram, or mg/kg. Milligrams per kilogram is equal to parts per million in soil. Therefore, a result of 15 mg/kg is the same as 15 ppm.

Evaluation

• When laboratories conduct analyses on soil samples, they choose the concentration reporting methods based on several factors, including the maximum contaminant limits or common concentration limits that people use to evaluate the data. If a contaminant limit or a nutrient level goal is 15 ppm, then it is logical to report the results of the test in ppm so that the users of the report do not have to convert the data. However, if the concentrations of the constituents are extremely low in the sample, the laboratory may revert to a smaller unit of measure such as parts per billion to avoid excessive use of scientific notation in the results.

Conversion

• When a laboratory uses a measurement other than parts per million, you can convert the data to ppm. Other common measurements that you will see are parts per billion, or ug/kg, or a percentage. To convert from parts per billion to parts per million, divide the number by 1,000. For example, if your soil result is 3,500 parts per billion or ug/kg of a constituent, you can convert the value to mg/kg or ppm by dividing by 1,000, resulting in a value of 3.5 ppm. To convert a ppm result to a percentage, divide the ppm value by 10,000. In the example of 3.5 ppm, the constituent percentage would be 0.00035.

Differentiation

• Parts per million is a common unit that is used for both soil and water. While the notations are similar, soil is a weight measurement and water is a volume measurement. Generally, laboratory reports list water results as milligrams per liter, or mg/L. Milligrams per liter is also ppm in water just as mg/kg is in soil. The conversion factors for water are the same; however, the unit remains a volume, typically liters.