Potting Soil Poison

Gardeners often struggle to grow plants in containers. You may feel that you have a really black thumb at times when newly planted seedlings fall over dead or fail to thrive. The problem may not be disease or poor gardening acumen but rather your container media otherwise sold as “Potting Soil”. A trip to one of the big box stores or a larger retail nursery will offer gardeners many choices of bagged potting soils. They are marketed to give you the impression they will grow anything and everything. But do they?
Over the last couple of decades I have done comparative potting media trials where I plant small plugs (usually impatiens) three per six inch container. I go out and find every retail brand of potting mix I can find and plant them all up and then follow them for about two months. I’ve been thinking of revisiting the studies and seeing if anything has changed. I also want to test the assumption that you can’t predict the grow ability of a potting soil by reading the ingredient label as some research suggests. While there can always be a surprise with any given product, I think that from my many trials I can make some suggestions to improve the outcome of your gardening adventures in containers.

Soil on the left has no nutrients same soil on the right with 2 grams of ammonium sulfate added on the surface of the medium one time.

Growing media is not the same as soil. Since media are placed in containers, often plastic ones, they need to be very porous. Porosity of up to 50% is not uncommon in container media. The bulk of the media needs to hold water and minerals for plant growth. Usually an organic material that has a high cation exchange capacity is used. The darling of potting mixes has been Peat Moss. Since peat moss harvesting is damaging to the environment (see previous blog by Linda CS), many gardeners may want to avoid media with peat moss. Bulking agents that do not hold much water or nutrients are also added to “lighten” or aerate the medium. Horticultural perlite (expanded volcanic glass) is the most common. Sand is also sometimes used but it adds weight to the bag and is not preferred by manufacturers. Some media use bark or other wood products to provide greater porosity.
There are usually about 18 to 20 different media on the market at any given time and the results of growing plants in them is predictable. About 10 of the media will not grow anything very well, 5 give ok results and about 5 of the products will grow a nice plant. A lot of the reason for success or lack thereof is about nitrogen chemistry. If no fertilizer is added, the medium will likely not grow well. You can add your own fertilizer and make about  ½ of these poor growing media work. One quarter to one half a teaspoon (approximately 2gm) of ammonium sulfate usually peps up most media that are ok but lack nutrients.  This is an amount used in a standard height 6inch (15cm) diameter plastic container.  Larger containers and plants will require incrementally more fertilizer to achieve growth goals. 
Some media will not grow even when fertilized. This is because they may contain manures, or composts and manures that have added too much salt to the medium. Adding fertilizers to these products only makes them less growable. Sometimes these potting soils will improve with leaching but then fertilizer will need to be added back later to make up for what was leached away. Generally a salty potting mix is worth avoiding.
So how can you tell if you are getting a good or bad mix. You can start by reading the ingredient list. And you will need to decode that list to help you make some decisions. What manufacturers call things can be very misleading. Look for a medium that has fertilizer added and lists what kind of fertilizer was used. These media usually grow without help. Avoid media that use manures, they are not suitable container media ingredients.

Some potting soils claim they can grow plants bigger than others, some claim to be all organic and some claim to be friendly to the earth. This is all marketing. Look for a simple ingredient list that is fortified with a nutrient charge (fertilizer). Begin there. You may want to sieve the medium to remove large particles if you are growing seeds, add more bulking agent (bark, sand, perlite, pumice) for plants that need increased porosity such as orchids, bromeliads and cactus. Don’t be afraid to modify potting mixes to suit the needs you might have. If plants don’t grow, consider adding more nutrients. After growing for some time (months to years), many media will breakdown, and the plant will need to be repotted in a new medium.

Published by

Jim Downer

Dr. Downer has 34 years of experience as a horticulture and plant pathology Advisor with the University of California Cooperative Extension in Ventura County. Dr. Downer’s academic training is from California Polytechnic Univ., Pomona, (BSc. horticulture & botany, 1981; MSc. Biology, 1983;. In 1998 he earned a Ph.D. in plant pathology, from University of California, Riverside. Dr. Downer’s research is focused on mulch, soil microbiology and disease suppression in mulched soils, diseases of shade trees and cultural practices to maintain landscape plants. Dr. Downer is a member of the American Society of Horticultural Science, the American Phytopathological Society, The International Soc. of Arboriculture, and the Western Chapter of the ISA, and the International Society for Horticultural Science. Dr. Downer is an Adjunct professor at California Polytechnic University in Pomona. Dr. Downer serves on the Board of the John Britton Fund for tree Research as the chair of the research advisory committee, and currently chairs the regional conference committee for WCISA. Dr. Downer has a love of shade trees, Shinrin roku (forest bathing/walking) tree work, wood working, horses, gardening, horticulture and the study of plants and their biology.

10 thoughts on “Potting Soil Poison”

  1. This is very nice but discouraging. In reviewing the choices, few sources seem to be completely satisfactory.

    I have found that media using spaghnum are most satisfactory. I avoid “sedge peat” (used in some Michigan products) but I still have to bear some guilt for damage to peat bogs.

    I bought some medium from Whole Foods a couple of years ago that must have been composted lumber yard waste. Horrible. You can’t trust anyone.

    1. It is discouraging. Most container media are not that great. The ones that list fertilizer and avoid manure are usually quite a bit better. Compost and forest products are not what they seem. Coir, peatmoss, if they are used early on in the label list would indicate a possibly better soil.

  2. Yet another helpful and timely blog post from the Garden Professors.
    I just had to return several bags of potting soil. It was full of chunks of wood, BIG pieces of bark, lumps of those little green fertilizer balls, and it stunk. I did think to take the one bag I had opened with me as proof when I returned the rest. Thankfully I had no problem getting a refund for the unopened bags.

  3. Our local producers of bagged mixes use kelp and other seaweed as a significant part. I have long wondered about the salt content and it’s effects. Is there any information on this?

  4. Thank you for this post. I was just thinking it would be great to see more articles relating to houseplants and growing in containers from the Garden Professors and then I came here and saw this! I have been wondering about fertilizing my houseplants/container plants (usually annuals). I know from Linda Chalker-Scott’s work that the ground in my yard likely doesn’t need much fertilizer, if any, but are containers different? It seems like the plants would eventually use up what is in the container. Are there any studies/resources on this? Thank you.

    1. Yes you are correct plants eventually use up container minerals. Proof of essentiality of mineral elements wad developed this way long ago by Leibig. (See Leibig’s law of the minimum). Add to this the fact that media for containers have greater porosity and leaching. Thus needed elements like nitrogen run out and plants will slow their growth. Other elements and micronutrients can also run out so a balanced slow release fertilizer will work. If you want organic sources of nutrients there are many available and all work ok. I did do some research on organics for container media and Gro Power was quite good. But any work if used according to directions..

  5. Is there a better medium than potting mixes that you can buy for container plants? Can you DIY a potting mix to ensure it will have all the nutrients it needs and not have any undesirable ingredients or too much salt? What would be the “recipe” for such a DIY potting mix?

    1. Yes you can DIY. The ingredients you chose will depend on your ethic and experience you generate over time. Generally 50%-75% solids that hold nutrients and water such as peat or coir and the 25-50% bulking agent that develop porosity in the media will work. These would be bark, perlite or pumice. The nutrient charge can be from organic or inorganic fertilizer sources. I prefer timed release/slow release urea based coated fertilizers. How you make the blend depends on what you are growing, for cacti and other plants that want a well drained soil 50% bulking agent. For flowering perennials (ex. ferns and begonia) you can use more organic component. the best way to approach this is to dive in and try it.

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