Understanding PPM in Soil
The over-used expression, ppm, is short-hand for parts per million, and it’s one of the most common items that shows up on soil test reports. But, knowing what ppm stands for is only one part of the ppm equation.
The numbers on the soil test reports may be expressed in different units. Many times, even the same lab expresses different elements in different units. Ppm is one of the most common units on soil test reports. What does ppm stand for and how can we estimate the actual amount of the tested element in the soil?
What Does PPM Mean?
The acronym ppm stands for parts per million, or, in other words, the ratio 1:1,000,000. It is used to measure low concentrations in aqueous solutions or in soil. In solutions, ppm refers to milligrams/liter or grams/cubic meter (m3). Note that it represents a ratio of mass to volume. This can be done because the mass of water is the same as its volume (in metric units). In soil test reports, ppm usually stands for milligrams/kilogram, so that means 1 ppm = 1 mg/kg. However, many labs use soil extracts, like saturated paste, 1:2 extract, etc. In such cases, the ppm level stands for mg/l in the soil extract. To convert the ppm of a nutrient (mass/mass as in mg/kg) to an actual amount, in kilograms or pounds, we must know three other parameters:
- The area for which we want to calculate the nutrient amount.
- The bulk density of the soil. Bulk density is defined as the dry weight of soil per unit volume of soil. Most soils have a bulk density of 1.1-1.6 ton/m3.
- The layer depth for which we want to calculate.
Let’s look at an example. How many kilograms of available potassium are there in a 30 cm (12-in.) layer of a one-hectare (2.47-acre) plot, if the soil test report shows a level of 10 ppm of potassium? The bulk density of the soil is 1.2 ton/m2.
What is the amount of available potassium in this soil layer, if the soil test level is 10 ppm?
If you are used to working with US or imperial units, it is advised to convert some measurements to metric units, for ease of calculation.
The Math of PPM
US units = Metric Units:
1 pound = 0.453 kg
1 acre = 4,046.86 square meters (m2)
1 inch = 0.0254 meters
Since 10 ppm are 10 mg/kg, we should first know the mass of this layer of soil.
1 ha = 10,000 m2
The layer depth is 0.3 m, therefore the volume of the soil layer is:
10,000 X 0.3 = 3,000 m3 (cubic meters)
Once we know the volume, we can calculate the mass of this soil layer, multiplying the mass by the bulk density of the soil:
3,000 m3 X 1.2 ton/m3 = 3,600 tons (3,968 short tons)
Now, the amount of potassium in the soil can be calculated, using the definition for ppm.
1 ppm = 1 mg/kg
3,600 tons = 3,600,000 kg
We have 10 mg of potassium per kilogram of soil and therefore:
3,600,000 X 10 = 36,000,000 mg = 36 kg (79.3 lbs.)
Therefore, we have 36 kg of potassium in our 1 ha plot and layer depth of 30 cm.
Theoretically, a crop that grows on this plot and has its active root system at the top 30 cm layer will have 36 kg of available potassium.