That article on successional continuum patterns is excellent. You have really nailed it, and I will read it again and again. If you have any other comments or suggested reading on this subject please feel free to share it.

Kevin Gay

If you’ve ever been to an old growth conifer forest and looked at the forest floor you’ll see lots of fruiting fungi in the form of mushrooms. Dig into that old growth forest soil and you’ll see kilometers of mycelium strands (no wonder Paul Stamet who lives in the Pacific Northwest USA old growth forest country sees fungi as key to ecological restoration). In contrast, if you go to a drylands environment you’ll tend to not see fungi fruit (mushrooms) or visible mycelium strands in the soil. Or similarly, I think most of us would be surprised to see mushrooms growing in tilled agricultural farm fields. Why is this?

Its because there is an underlying ecological microbiological pattern to all soils which is at its base the ratio of fungi to bacterial biomass in the soil. Over the past several decades soil microbiologists have been able to pin down a pattern of soil microbe and plant changes through time and space that they call “succession.” I think understanding microbial/plant succession is vital for any permaculturalist. Microbe/plant succession is a continuum. If you start on the extreme left of the continuum you get very “early” successional soils and plant communities where weeds (in the biological sense of weeds: annual, fast growing, tons of seeds, nitrate loving plants) thrive. A key to understanding why the weeds thrive is that the soil microbe population is bacterial DOMINATED. The fungi to bacteria biomass ratio is 8-10 more bacteria than fungi. The bacteria produce a sugar substance loosely called bacterial slime that is alkaline (slime is for protection, transport corridors, etc.) The slime drives the soil alkalinity here. So these bacterial dominated soils have a high ph. Without going into the details of microbial nutrient cycling, let me cut to the chase and say that the alkaline soil conditions results in a bunch of specific bacteria converting microbial cycled soluble Nitrogen into Nitrate. This process is called nitrification. And the bacteria that do it require high (alkaline) ph for the enzymes they use to convert Nitrogen into Nitrate to work. Weeds love high Nitrate levels. They’ll out compete “higher successional” plants any day if there’s tons of Nitrate in the soil.

If you move to the right on the succession continuum you get a tighter fungi to bacteria ratio. In a soil where the fungi to bacteria biomass ratio is more balanced, lets say 1 to 1, you’ll find that “higher” level plants thrive and out compete the weeds. Higher level plants means in the context of the successional pattern and include annuals like tomato, cucumber, pepper,corn, etc. All these plants like to have their Nitrogen in two flavours: Nitrate and Ammonium. In soils that have a more balanced mix of bacteria to fungi the soil alkalinity is more neutral. This because fungi, as part of their ‘eating’ secrete acids as well as enzymes. The extra fungal biomass means extra acids being generated in the soil resulting in a soil ph change. As the ph moves closer to 7 (neutral) the nitrifying bacteria I mentioned above can’t as easily convert Nitrogen into Nitrate. Just a note, the soluble Nitrogen I originally mentioned above is actually Ammonium. So, if less Ammonium is being converted to Nitrate, the Ammonium to Nitrate balance is changing. Weeds like high pulse levels of Nitrate. When they don’t get it, they are on a diet and can’t thrive. In contrast the tomatoes and peppers do well with a bit of both Nitrate and Ammonium. Its important to understand, that the soil microbes are responsible for this. If we leave an area alone, the plants that grow will tell us the story of the fungi to bacteria ratio in the soil.

If we move more to the right on the successional continuum we get ever increasing fungal biomass and ratio dominance. In a healthy vineyard soil you might see 3-5 times the fungal biomass compared to bacteria. A fruit orchard, 5-10 times fungi. A healthy old growth conifer forest 100-1000 times as much fungi to bacteria. Think about the plant systems we just moved into, they’re perennials. As we jumped from annuals to perennials we jumped into a fungal dominated soil. As the soil gets increasingly fungal dominated we get more and more fungi secreted acids entering the soil, resulting in an increasingly lower soil ph (acidity). When the soil ph drops below a nuetral 7 and becomes acidic, those nitrifying bacteria I mentioned above can’t convert Ammonium into Nitrate. The enzymes they use to do this just don’t function in acidic soils. So, the form of soluble Nitrogen in the soil becomes ever increasingly Ammonium. As we move into more advanced (in terms of succession) Perennials we get a greater love of Ammonium as the desired plant Nitrogen food. If you give any organism its preferred food it will out compete other organisms that have different preferred foods. Broccoli, for example, likes a more Nitrate rich soil with a fungi to bacteria ratio of .3-.7 approx (bacterial dominated). Chances are slim that you’ll find a ‘natural’ growing broccoli plant in an Ammonium rich old growth forest in California. Likewise, chances are slim that you’ll find a confer thriving in a tilled broccoli field. The soil fungi to bacteria ratios are different and give a competitive advantage to different above ground plants groups.

When thinking about your field ask yourself some questions about the type of grasses you WANT to grow and the types of plants that seem to thrive there. Do you want to grow perennial grasses but lots of high seed yielding annual weeds thrive there? If so, you’ve got a bacterial dominated soil. Most of the time, we do need to move our soils along the successional continuum pattern and get more fungi in there, especially if the soils have been farmed (chemically or often organically). Farming practices of tilling, chemical ferts, pesticides and evening dumping manure on fields either kill fungi or overfeed bacteria. Both result in bacterial dominance.

But here’s the rub. If I live in an old growth conifer forest in say Oregon, USA, and I want to grow a veggie garden I’ll most likely need to make my soil more bacterial. I mention this to point out that understanding the successional continuum pattern and what your specific soil microbe population is in relation to the plants you what to grow is always the first step. Getting fungi into many soils is often the first answer, but not always. Then once we’ve got the right fungi:bacteria ratio for the plants we want to grow, we need to look at the other members of the soil food web (ciliates, flagellates, amoebae, nematodes, micro/macro arthropods) and ensure their numbers and diversity are OK. But, first step, is the fungi-bacteria ratio.

All the best,

Darren Doherty

The 2009 summer in Mexico has had worst grought in 68 years. Most dryland corn in Mexico has died or is extremely stunted and non-productive, stressed stalks plagued with worm bores. The results with the Soil Foodweb aprroach have been quite dramatic. Here’s a link to my website in case you want to take a look. http://soildoctor.org The second article on my site has a link to my initial corn results. Its a rough presentation doc without a lot of explanation. I’m right in the middle of the corn harvest (yields look great) and will be doing a video that should be on my website in a couple of months with More details and a background story. Permaculture folks might be interested to know that the swales in the corn field are a first year drylands food forest. We’ve planted over 120 backbone rhizobia Nitrogen-fixing native trees and some nut and fruit trees – next year a lot more. Since the corn and forested swales need a different bacterial:fungi soil ratio I’m using different aerated compost teas, fungal foods, woody much and fungal composts to create an ‘island’ of fungal dominance in the swale rows while maintaining a more even split of bacteria to fungi in the adjacent corn field. Thanks for the great article. I hope more permaculturalists learn about the incredible possibilities of Dr. Ingham’s approach.