With the heightening popularity of raised bed gardening and building soil health with organic amendments, products like organic composts, potting/raised bed soils, peat moss, etc., are now being used in garden spaces more than ever. There is actually quite a large difference between native mineral soil and soilless media when it comes to uses and laboratory testing. According to the USDA’s Natural Resources Conservation Service (NRCS), “a mineral soil is one that consists predominantly of, and has its properties determined predominantly by, inorganic matter. Essentially, it’s a soil where the major components are minerals derived from weathered rocks, rather than primarily organic matter like in organic soils.”
Red clay heavy mineral soil is typically found in South Carolina’s upstate region.
Barbara Smith ©2025 Clemson Extension
What differentiates mineral soils from soilless media?
Mineral soils do contain some organic matter (OM). The OM is typically less than 5% of a soil’s volume. The inorganic matter that composes mineral soils consists of differing percentages of particles of sand, silt, and/or clay. Over time, mineral soils develop distinct horizons, or layers with differing thickness and nutrient content.
Soilless media can have varying amounts of OM depending on the material used to create the mix. Certain bagged peat-based soilless mixes contain up to 66% OM in the mixture. Soilless mixes were developed for use in containers (raised beds, pots, seedling trays) or to be added as an amendment to your native soil to improve soil health and drainage (garden soil mix). Many growers have found making their own soilless mixes more cost-efficient and effective. For more information, see The Role of Organic Matter in Healthy Soils.
A soilless mix is made up of mostly organic materials, which can improve native soil health or be used as a layer in raised beds.
Bob Polomski, ©2023, Clemson Extension
When to Choose a Soilless Mix Test
Composts, potting soils, peat moss, leaf litter, vermiculite, perlite, manures, etc., are NOT considered soil for traditional soil testing purposes, as they are deemed soilless media. As a rule of thumb, if your potting or raised bed mix contains close to 50% or more amended materials, you should select a Soilless Mix Test. Clemson’s Agricultural Service Laboratory needs at least one quart of soilless media for this test; two quarts are ideal. A saturated water extract will determine soluble salts, pH, phosphorus, potassium, calcium, magnesium, and nitrates.
Standard Soil Test
A Standard Soil Test is used for testing your native mineral soil. The Ag. Service Laboratory will test for pH, buffer pH, extractable phosphorus, potassium, calcium, magnesium, zinc, manganese, copper, boron, and sodium. Lime recommendations, if needed, along with fertilizer application rates, will be provided in the report based on the soil test results. Calculations for a CEC (cation exchange capacity), acidity, and % base saturation will also be included. Two cups of mineral soil are needed for this test.
Soil Testing Information and Pricing
Choosing the appropriate test for your growing space is crucial. Putting soilless media through a standard soil test will give skewed and inaccurate results. The same goes for testing mineral soils in a soilless media format. For all South Carolina residents, the cost of a standard soil test is $6 if the results are emailed to you or $9 if a paper copy of your results is mailed to your address. A soilless media test is $10 if results are emailed or $13 if a paper copy of results is mailed to your address. For more information on Clemson’s soil testing, visit Agricultural Service Lab’s Soil Testing.
