Different types of plants have distinct preferences for the types of microbes with which they associate, and your composting strategies will help to determine whether your plants do badly, do well, or thrive. Knowing what type of organic material to add, and how to add it, will make your growing more efficient.
What type of Microbe?
In simplified terms, the longer that a plant is going to live for, the more likely it is that it will need a fungally dominated soil. That’s pretty logical really, as it takes time for the mycorrhizal fungi to develop a network of hyphae. If the plant is only going to be alive for a short time, it doesn’t make sense for it to share it’s surplus energy with fungi, and it will taylor the substances that it exudes to encourage the multiplication of bacteria. At the other end of spectrum, a tree that is going to live for centuries, is going to really depend on mycorrhizal fungi, and will actively encourage root associations with it.
In gardening and growing, the distinctions are relatively clear. The short lived plants, particularly brassicas, annual weeds, and some salad leaves, will need a bacterially dominated soil. Most of the commonly grown vegetables will prefer a mixed soil, containing a roughly equal ratio of fungi to bacteria, as do grasses. That may seem a little strange, but many of the vegetables that we grow are biennial, with a two year life cycle. The plant doesn’t know that we are going to use it earlier than that. Some plants that we grow, like Runner beans, and wild rocket, are actually perennials, and so need fungi. Soft fruit, shrubs, and the shorter lived trees prefer a fungally dominant soil, but associate predominately with endo mycorrhizal fungi, which penetrate the root of the plant, in order to exchange nutrients. The really long lived trees also need a fungally dominated soil, but associate with ecto mycorrhizal fungi, which form a sheath around the tree roots. One of the things that I find interesting is that the trees which are most resistant to honey fungus, seem to be those that associate with the ecto mycorrhizal fungi, and I wonder if the mycorrhizal fungi are providing some immunity to the fungus, or are actively resisting it.
The link below leads to a page which shows which type of fungi different plants need. Note at the bottom that brassicas and beets don’t need fungi. More on this later.
Influencing soil life
One of the ways that we can influence the type of microbes present in the soil is by selecting the right compost material. Vermicompost tends to be bacterially dominated. Hot composting, which is turned regularly, will have both bacteria and fungi, with the ratio dependent on the materials used. More grass/straw will give more bacteria, more wood/leaves will give more fungi. Cold composting, particularly with plenty of woody material, will give a more fungally dominated material, as will leaf mould, and charcoal. Actively Aerated Compost Tea can be brewed to be bacterial, fungal, or mixed, but the survival of the organisms in the soil will still depend on the material in the soil.
We can also influence microbes by the way that we garden, repeated digging, rotavation, ploughing etc. will destroy fungi, and leave bacteria dominant. This leaves the soil dominated by bacteria, which makes it better for annual weeds. No dig gardening will allow the fungal hyphae to remain intact. Bacteria tend to make soils more alkaline, whereas fungi tend to make soil more acidic. Adding lime will make conditions better for bacteria.
Fungi grow rapidly, and can move to a food source, whereas bacteria can hardly move, and so the food source needs to go to them. By adding organic material to the soil surface, we assist fungi, whereas incorporating it into the soil, we bring it to the bacteria, and assist them.
Putting it together
Much depends on how you garden. With a forest garden, or a really diverse polyculture, which incorporates lots of perennial plants, there may be little need to add organic matter.Where necessary, leaf mould, charcoal, or woody prunings, surface laid, will feed and maintain a fungally dominated soil. For more conventional annual vegetables, more input is needed. I add conventional compost to potatoes, and brassicas, and vermicompost to brassicas and salad leaves. These prefer bacterally dominated soils. The compost can be incorporated into the soil. I add leaf mould to my root crops. Although they prefer a slightly bacterially dominated soil, too rich a compost causes the roots to fork. For my perennial crops, I use leaf mould, charcoal, and sawdust, all surface laid, to create an environment which suits fungi. These crops include asparagus, rhubarb, wild rocket, mallow, etc.
Most crop rotations suggest following brassicas with potatoes, but I do it the other way around. All of the planting, earthing up, and harvesting of the potatoes will have a disruptive effect on fungi. So I grow most of my other crops first, building up fungi, and using the woody materials. The fungi create a more acidic soil, which is ideal for potatoes. I then incorporate hot compost into the soil for growing potatoes, and mulch with grass clippings and straw, which are bacterial foods. This creates ideal conditions for bacteria. The potatoes are followed by brassicas. They need a bacterial soil. They get hot compost in the Autumn, and the soil is limed at planting time.The soil is not disturbed when they are harvested, and the following season those beds get leaf mould prior to growing alliums, or root crops.
I mulch with grass and straw for the crops that prefer a bacterial soil, and use leaf mould, sawdust, and hedge prunings for the plants that need fungi. I use compost tea to help add the right microbes.Over time, each time a bed is used for growing potatoes, most of the woody, surface laid material is incorporated into the soil, and then is replaced by fresh material laid on the surface. The woody materials take a long time to be processed, giving a significant increase in organic matter in the soil. As the fungi digest them, they produce materials which can also feed bacteria.
Whilst my strategy may not be ideal for you, hopefully the two composting articles will help you to come up with your own plan.