Should I incorporate compost into the substrate?

Adding compost can potentially suppress diseases and be a desired component for retail or organic customers. However, uncontrolled nutrient release and loss of structure can occur if the compost is not sufficiently stable and mature. Test the chemical, physical, and biological characteristics of the compost you are using.

Source and reliability

  • Find out about the materials used to make the compost, whether any components may cause issues (such as herbicide residues, trash, or human pathogens), and whether the composting source has adequate quality control procedures.
  • Professional compost manufacturers follow guidelines such as the Seal of Testing Assurance (STA) developed by the USDA and U.S. Composting Council, or the Solvita Quality Seal of Approval. Specialized laboratories can test compost batches.
  • If using biosolids, make sure the product complies with EPA rules on pathogen reduction and heavy metals.
  • Some states have additional regulations.

Compost in container

Volume of compost

Incorporate less than 10% compost by volume into the growing mix so that nutrient release or physical loss of structure does not cause major issues.

Biological quality

  • Weed seeds: the finished compost should be weed free, which can be tested by placing moist compost in a propagation tray in the greenhouse.
  • Maturity: A Solvita® test kit can be used to measure CO2 and ammonia emission, and low levels indicate a mature and stable product.
  • Plant growing response: Test germination and growth, for example with bean seeds, as a bioassay to check for plant pathogens or chemical residues

Physical structure

  • Finished compost should ideally have a similar initial moisture level as other components (around 50 %), but compost is often too wet when delivered.
  • Fine humus-like compost materials can hold a lot of water.
  • Ideal composts for container substrates should therefore have a similar particle size distribution to other components used in the mix.
  • Compost dry bulk density can range from 190 up to 300 g/L (higher than peat)
  • Less mature compost will continue decompose over time and cause shrinkage of the mix

Ammonia test on compost

Chemical structure

  • Phytotoxicity /herbicide: Immature composts could be detrimental to plant growth (phytotoxins), also can act as mild herbicides. Residual pesticides could be still in the finished composts. Plant trials are needed.
  • Composts made from biosolids normally have high contents of heavy metals, such as copper (Cu) or zinc (Zn), which could potentially lead to toxicity to the plants.
  • EC values in composts can range widely from 1 mS/cm to more than 10 mS/cm. The soluble salts in composts normally are N, P, K, Ca, Mg S and Na. High salinity may be toxic to plants. Ideal soluble salt levels will depend on the end use of the compost. Final compost blends with soil or container media/potting mixes should be tested for soluble salts.  Ammonium levels especially in immature composts can be high (above 20 ppm), leading to potential ammonium toxicity.
  • Most finished composts will have pH values in the range of 5.0 to 8.5.  Because the pH values are high in composts, less lime is usually required for growing mixes containing compost.

For more information: Contact authors Jinsheng Huang and Paul Fisher of University of Florida IFAS Extension, and Dr. Bill Argo of Blackmore Co. Thanks to our Floriculture Research Alliance at University of Florida sponsors including A.M.A. Plastics, Blackmore Co., Everris, Fine Americas, Greencare Fertilizers, Klasmann-Deilmann, Pindstrup, Premier Tech Horticulture, Quality Analytical Laboratories, Sun Gro Horticulture, and leading young plant growers. The University of Florida does not endorse any product, and our research focuses on quality testing on these and competing products to assist grower success. June 30 2015.