To understand what a soilless mix consists of and how it differs from native mineral soils, please consider visiting the HGIC blog, Standard Soil Testing vs. Soilless Media Analysis: What You Need to Know.
A handful of bark-based soilless mix with some perlite incorporated into it.
Julia Nichols, ©2025, Clemson University
Checking pH Level in Soilless Mixes
Once you have received your soilless mix test results from Clemson’s Agricultural Service Laboratory, check your pH level reading first. The pH is the most important factor in soil and soilless mixes. It is a measure of the alkalinity or acidity of the soilless mix. It directly influences nutrient availability to plants.
Keeping the pH of your growing space within the proper range for your specific crop(s) ensures optimum plant growth. Most vegetable and herb plants thrive in a range of 6.0 to 7.0 pH. A pH reading that is too high or too low can lead to toxicity or deficiency of certain essential plant nutrients.
Recommended pH Ranges
- Bark-based soilless mix: 5.0 to 6.5
- Peat-based soilless mix: 5.3 to 6.5
If the pH is not within an ideal range, it can be raised or lowered by adding acidifying agents (to lower pH) or lime (to raise pH).
Measuring Electrical Conductivity (EC) for Nutrient Availability
Electrical Conductivity (EC) is checked next. In a soilless mix, the EC measures the dissolved salts present in their total concentration, which can indicate nutrient availability and salinity of the mix.
Recommended EC Ranges
- Bark-based soilless mixes: 0.7 to 3.0 mmhos/cm
- Peat-based soilless mixes: 0.8 to 5.0 mmhos/cm
An EC that is too high can cause nutrient toxicity and a saline environment, which leads to reduced plant growth. An EC that is too low can cause nutrient deficiency, leading to slow growth in plants.
If EC levels are too high, the excessive salts can be flushed out with more water.
If EC levels are too low, it is time to add fertilizer or organic matter to the soilless mix.
Interpreting Nutrient Levels in a Soilless Media
Next, check your individual nutrient level results. These readings will measure the concentration levels of each nutrient needed for plant growth within the soilless mix:
- Nitrogen (N)
- Phosphorus (P)
- Potassium (K)
- Calcium (Ca)
These nutrients are essential for plant growth and reproduction. Some plants are heavier feeders than others, which is something to consider when measuring or adjusting nutrient levels.
The range for each nutrient is listed in the table below. Each nutrient is measured in parts per million (ppm).
Adjusting Nutrient Levels
- Insufficient (Low) nutrient levels: Can be gently raised with the addition of fertilizers and amendments, both organic and inorganic.
- Supraoptimal (Excessive) nutrient levels: Soil leaching can gently flush some excessive nutrients out of the soil.
- Raised beds- Use a sprinkler or soaker hose to saturate the soil deeply a few times in the off-season.
- Containers- Flush the potting mix with at least twice the container’s volume of water. Allow the water to drain freely from the drainage holes of the container.
Regularly testing your soilless mixes can help you maintain the optimal healthy growing environment for your plants. Plants’ nutrient uptake and heavy irrigation/rainfall can deplete nutrient levels and raise pH levels over time.
Reference Chart for Soilless Mix Test Results
For specific ranges for each nutrient, refer to the official Soilless Mix Test Result Interpretation Chart from Clemson’s Agricultural Service Laboratory.
Need Help Interpreting Your Results?
Contact the Home and Garden Information Center:
Phone: 1-888-656-9988, Monday – Friday, 8:00 AM to 4:30 PM.
Email: hgic@clemson.edu.
Additional Resources
Clemson Agricultural Service Laboratory Results
Clemson Agricultural Service Laboratory Guidelines for Collecting Samples
