Understanding PPM in soil
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?
PPM is used to measure low concentrations in aqueous solutions or in soil. In solutions, ppm refers to mg/liter or grams/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).
However, many labs use soil extracts, like saturated paste, 1:2 extract etc. In such cases, the ppm level stands for mg/liter in the soil extract.
To convert ppm of a nutrient (mass/mass as in mg/kg) to an actual amount, in kilograms or pounds, we must have three other parameters known:
- 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;
- And the layer depth for which we want to calculate.
How many kilograms of available potassium are there in a 30 cm (12 inch) layer of 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 work with US or imperial units, it is advised to convert some measure to metric units, for ease of calculation.
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 0.3m, therefore the volume of the soil layer is:
10,000 X 0.3 = 3000 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:
3000 m3 X 1.2 ton/m3 = 3600 tons (3968 short tons)
Now, the amount of potassium in the soil can be calculated, using the definition for ppm.
1ppm = 1 mg/kg.
3600 tons = 3600000 kg
We have 10 mg of potassium per kilogram of soil and therefore:
3600000 X 10 = 36000000 mg = 36 kg (79.3 lbs)
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.
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