All About Inoculants – Part I

By Navona Gallegos. This is the first article in a series of two.

^{Image by Isabelle Jenniches CC BY 2.0}

Healthy soil is a robust ecological web of mostly microscopic organisms (see the Meet the Soil Food Web post) working in synchrony with plants to cycle nutrients and create more and more biomass and biodiversity. As human participants in this web of life, there are many ways that we can help support a more abundant soil organism community. Simply following the six healthy soil principles is a great start and sometimes all that is needed. With the right conditions, soil organisms will land from wind-born spores, come from animal manure, or wake up out of dormancy as soil conditions become more hospitable. However, there can be a number of reasons that we might want to jump start the soil ecosystem by inoculation, which is to say introducing more organisms from elsewhere. This series will discuss the why and how of using liquid compost amendments to inculcate soil.

Why inoculate?

1. Inoculation is a way to dramatically speed up the soil building process. Yes, dormant organisms are usually in the soil waiting to come to life when more living roots bring them the sugars they need, but sometimes a soil is so denuded from tillage, overgrazing, or chemical inputs that there are simply not very many organisms present to start cycling nutrients and feeding plants. This is exacerbated in arid climates where due to the lack of water things are moving even more slowly. If left to its own devices, soil will inevitably accumulate organic matter and cycle nutrients eventually—but this can take anywhere from years to decades to centuries depending on climate and precipitation. 
   By inoculating, we can speed this process up so that the soil building that would have taken years might be accomplished in as little as a season if we also provide the conditions in which these organisms can thrive. I want to emphasize that simply inoculating won’t do much if those organisms don’t at the very least have mulch to provide food, habitat, and moisture retention. Ideally, there are some living roots in the ground as well as mulch or plant litter. If so, these newly introduced organisms can get to work building soil structure and cycling nutrients right away and the results can be dramatic. 
2. Inoculation can treat plant disease. A healthy soil ecosystem is full of predators that cruise around and eat disease causing organisms like parasitic fungi and root feeding nematodes. Indeed, in a thriving ecosystem, every inch of a plant’s surface is covered by its microbial allies. So if a parasitic organism lands on a plant, it will quickly be eaten before it ever gets to attack the plant itself. What is more, creating a healthier soil can have repercussions that even change disease-causing organism’s behavior. For example, there are some nematodes that can switch between eating fungi and eating plant roots. If there aren’t fungi to eat, they can be very detrimental. If there are plenty of healthy fungi in the soil, then these nematodes are a boon because as they munch on the fungi, they are cycling nitrogen that plants can then use. I’ve successfully used inoculation to treat some aggressive fungal disease on tomatoes. After just one inoculation with compost extract in the springtime, the late blight that had been recurring for years disappeared and still has not been an issue two years later. It only took about half an hour to make and apply the extract.
3. Inoculation can also be a way to shift the soil ecology to favor different sorts of plants. Bacteria and fungi cycle different forms of nitrogen (nitrate and ammonium). Because of this, every plant has an ideal soil fungal to bacteria ratio it thrives in. “Early successional” plants–those that show up in degraded soil, like dandelion, goat heads, bindweed, etc.–can thrive with little to no fungi and just some bacteria in the soil. “Later successional” plants like redwood trees that grow in the abundant soil of old growth ecosystems require lots of fungi and very little bacteria (a ratio of around 1,000:1). How does this relate to inoculation? If we want to grow strawberries, a plant that naturally occurs on the forest floor, but we are starting with a compacted dirt lot in a city, then mulching and inoculating with lots of fungi can create the conditions for strawberries to thrive. Conversely, if we are making a veggie patch in an area that was recently forest, then we may inoculate with lots of bacteria to give our broccoli and kale a leg up. 

^{Image by Isabelle Jenniches CC BY 2.0}

What is a good inoculant?

Ok, so where do we get these bacteria or fungi or soil predators to inoculate with? It is possible to use a small amount of really good soil from, say, a forest floor. An area in the wild where there is an abundance of the plants we are trying to grow or a spot with lots of fruiting mushrooms can be good places to grab a handful of soil. However, for the best results, it is wise to use a compost that we know for sure contains a high density of all the necessary soil organisms. Think of this compost as the sourdough starter that gets the rest of the dough going once it’s mixed in.   

Of course, what people call “compost” ranges from backyard piles to municipal sewage slurry to bagged compost at the nursery. Making a compost that’s a good inoculant is both an art and a science. Much of what is available on the market may be more accurately called “decomposed organic matter” or “mulch” rather than “compost” because it usually lacks living organisms by the time it gets through the composting process, is bagged, and sits on the shelf in a garden store.

Dr. Elaine Ingham (of the Soil Food Web) has created a standard called BioComplete™ that is a useful guideline for identifying a good inoculant. The table below shows the levels of organisms per gram of compost necessary to ensure that a compost has enough biology to be an effective inoculant. Notice at the bottom of the chart is a maximum level for ciliates; these are potentially disease causing organisms.

These levels are ascertained by looking at compost with a microscope. It’s actually not too difficult to do if you are interested in getting some training and learning to do it yourself. And if that’s appealing to you, I highly suggest taking an online course with the Soil Food Web School. If you are just wanting to get the benefits of inoculating your soil without taking up a new hobby, then the easiest way is to find someone in your area selling compost that meets the biocomplete standard. If you ask your compost provider for a biological assessment of their product and they don’t know what you are talking about, then it is likely their material is not inoculant grade.

Many different composting styles (whether done commercially or at home) can achieve a product that meets these standards, so long as it’s an aerobic composting method. Vermicompost and thermophilic composts are a couple common approaches. Another composting method that I’d like to give an extra shout-out to is the Johnson-Su bioreactor (affectionately referred to as the JSB). This is a static, aerobic compost that takes one year to mature and, if done correctly, usually far exceeds the BioComplete™ levels, especially in those all-important fungi. That means even less of it goes a long way. In order for JSB to be successful it has to be built with the correct surface area to volume ratio to allow it to stay aerobic. Also make sure that it stays watered and doesn’t freeze over the winter. Because it isn’t turned, it gives fungal mycelium lots of space to proliferate in the compost and nematodes lots of habitat (when compost gets turned, nematodes literally run away from the disturbance). There are many compost providers using the JSB these days. It is also absolutely possible to make one for yourself as one of the easier ways to go about making inoculant-quality compost because you don’t have to turn it or do much but water it after the initial build. If you’re interested in learning about the Johnson-Su method, keep an eye out for upcoming field days and/or check out this blog post on the Johnson-Su Bioreactor.

Usually a compost that has high levels of beneficial soil organisms may cost more than the bulk compost one might buy by the yard. Luckily, we only need a few pounds of inoculant-grade compost per acre. But wait–you might ask–how do we spread five pounds of compost over a whole acre? That’s where liquid inoculation comes in.

The next article will dive deep into how to make and apply liquid amendments and inoculations. Continue Reading: All About Inoculants Part II

Navona Gallegos is a soil ecologist and lover of life. Her passion for the wholeness of nature led her to study terrestrial ecology and specialize in soil biology. Navona’s educational background includes a BA in Environmental Science from the University of Virginia, Starhawk’s Earth Activist Training Permaculture Design Course, and Dr. Elaine Ingham’s Soil Food Web School as well as lived experience as a farmer and informal learning from a wide variety of land stewards from across the United States, Caribbean, and Central Africa.