Is Mold in Your Soil Good or Bad? Lessons in soil microbiology.

by Ashley Mariani March 17, 2017

 Mold. Even the word does not sound pleasant. However, do not let it's nasty reputation fool you; when it comes to gardening mold is a sign of life.

 At PittMoss our mission is to make the best soil amendments and blends on the market. For us, a sustainable and organic approach goes hand in hand with superior results for plants. So when some consumers discover mold in our products, our dedication to this mission becomes tested. Should we deliver an aesthetically familiar product to consumers even if it sacrifices it's benefits, or risk our first year on the market to stay true to our mission? Might sound like a tough decision, but when you are on a mission to disrupt dirt, the choice is clear. We believe consumers care about what they plant into, and that progress in a sustainable natural world requires innovation.

To start dispelling the old mold myths of living soil and all the awesome biologic activity that comes with it, we are going to break down what can be seen in our own PittMoss products.  


Explaining the Wonderful World of Fungi: 

The spores that produce mold, or fungi, are an underappreciated partner in the garden. For example, they are present, to some degree, in every common organic gardening mix, from peat moss to bark. However, you do not see them until spores produce fruiting bodies (like mold)- only when certain conditions are met. Many products are treated to stop this growth and improve the aesthetics of their products. PittMoss is purposefully  produced to encourage natural growth, since we are in the business of growing plants sustainably. So if the product is left in a warm, humid environment or lacks access to air, biological activity will occur. This activity is perfectly normal and natural. Mold comes from an ancient group of simple plants, called "hyphomycetes" (say that five times fast). It is these fungi that scientists consider foundational to our natural world. Don't believe us? Check out this Ted Talk. In fact, organic gardeners can attest that gardening with "living soil" that represents the natural world is the ideal environment for plants. We could spend a few blog posts on the subject of the benefits of living soil and microbes, but for now it is broken down nicely here. 

“Soil organisms show their greatest diversity of species and usually their largest populations in productive soils. The size of the microbial biomass usually shows direct correlation with the amount of plant growth…” - Soil Microbiology and Biochemistry by E.A. Paul and F.E. Clark, Academic Press, Inc. 1989, page 12 

A Breakdown of Microbes 

A quality growing mix should support a full array of beneficial soil microbes. These “Friends in the Soil” provide for the availability and absorption of essential plant nutrients. They also help to fight root diseases and break down toxins. In general terms these microbial workers are classified as:   

BACTERIA - non-visible are very small single-celled microorganisms found in growing media, native soils and compost. They are the most numerous and are only visible with a microscope. They respond quickly in favorable environments where populations multiply rapidly under favorable conditions. They produce enzymes that dissolve and transform minerals making them more available to plants. The nitrogen converting bacteria nitrosomonas and nitrobacter are the most noteworthy. Bacillus and azotobacter and the thiobascillus are very important. Bacteria do not form fruiting bodies above the surface of a growing substrate.

FUNGI- primitive (simple) plants that typically form multi-celled filaments in the growing mix. These filaments form a network called mycelium that grow in the media. Many fungi grow in association with the roots and are called micorrhizae fungi. Like most fungi they serve to decompose organic material and make nutrients available to plants but the micorrhizae also help plants absorb nutrients. Some common families of fungi include ascomycetes, basidiomycetes, trichodermas, and zygomycetes. When conditions are optimal they develop fruiting bodies which produce white or tan growth on the surface of a mix. The spores and surface mycelium are commonly referred to as mold.

ACTINOMYCETES are in between bacteria and fungi in size and complexity. They form long chains of cells within the growing media. They typically give soil the familiar earthy or musty smell. They often have anti-bacterial and disease fighting properties. Some common families of actinomycetes include micomonospora, thermoactinomycetes and streptomycetes which is a source of the familiar antibiotic streptomiacin. The fruiting bodies are like tiny mushrooms or puff balls.

ALGAE- simple plants that grow within and on the surface of nutrient rich soil and growing media. They are the most widely distributed of all green plants. Sometimes called cyanobacteria they are primarily water plants and develop where high levels of water or very high humidity are present. Primarily blue-green algae are the type that commonly grows on the surface of a constantly moist growing mix. Algae causes no harm but is an indicator of conditions that are often excessively humid and wet. These conditions can foster other undesirable infestations of insects and pests.

Biological Activity Seen in PittMoss
The Fungal Matt (mycelium) of zygomycete spp. in a blend of 60% PittMoss Prime Soil Amendment, 30% sphagnum, and 10% perlite.  This fungi work to make components in the growing mix more soluble.  That allows for easier plant absorption.
fungal matt- microbial development seen in PittMoss

Bacidomycete spp. fruiting (sporulating) bodies on the surface of a blend containing 50% PittMoss  Prime Soil Amendment.  They are also often considered “Higher Fungi”.  They are often associated with root systems and act as an ectomycorrhiza.  They are known to enhance the availability of nutrients and nutrient absorption. 

Composting is enhanced by exceptional microbiological activity of PittMoss.  That is demonstrated in the photo of a pile as seen below.  Below a depth of about 12” in the pile the composting fungi can be seen (bottom half of photo) as a white fungal mycelial growth.  It was clearly present after about 14 days in a composting pile.  Microorganisms such as Trichoderma spp., Ascomycete spp., Basidiomycete spp., and Zygomycetes spp. release enzymes that help to breakdown the substrate materials making them much more available to plants. 

composting improves with PittMoss

Ascomycetes spp. These fruiting (sporulating) bodies are growing on PittMoss – They are often considered “Higher Fungi” and  grow within a soil and on or near root surfaces producing mycorhizal associations that increase nutrient solubility and absorption.



So if all this biological activity is good, is it bad if I do not see any mold?  

Whether you can see the activity or not depends on a variety of environmental conditions. Nonetheless, these microorganisms will always be found in gardening blends even if they do not become visible.   

This all sounds good, but I really want to avoid seeing any microbial growth.  

Managing the environment will greatly reduce the occurrence of fruiting bodies. PittMoss should be stored in a cool dry space with available air circulation (for example, leaving the bag slightly open) mold is unlikely to occur.  

Should I Expect Mold in my Product? 

More often then not, you will not see any visible growth. The microbes produce fruiting bodies when not properly stored (see the previous question). Also, mold will not remain. In fact, after the first bloom any subsequent growth will be minimal.  

If I see mold in my mix what do I do?  

At PittMoss we recommend simply mixing it up with the rest of the product.

Ashley Mariani
Ashley Mariani


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