HOW SOIL IS BORN
Over long periods of time, wind, water, earthquakes, and so forth break rocks into small particles. Oxygen, acid rain, and other chemicals also break down rocks. Physical and chemical weathering eventually creates sand or dirt. However, to be considered soil, biology needs to be involved.
When there is a lava flow on the Big Island of Hawaii (it happens often), the first thing to grow on the fresh lava after it cools is lichen. Lichen is a unique organism formed by a symbiotic relationship between fungi and algae (or cyanobacteria). The fungus provides structure and protection, while the algae or cyanobacteria carry out photosynthesis.
Lichens arrive carried by wind and water. They obtain nutrients through a combination of photosynthesis and absorption from their environment. Photosynthesis provides energy for both partners. They take in nutrients from their surroundings, from rain, dust, the air, and the substrate they grow on.
This combination of strategies allows lichens to survive in a wide range of environments, even in places where soil or other nutrient sources are scarce. Obviously, there is no soil on a fresh lava flow. Lichen are the ideal pioneer organisms for fresh lava flows.
The next two things that grow on the lava are ferns and, surprisingly, a tree called ‘ohia-lehua. ‘Ohia seeds are extremely tiny and carried by the wind. The ferns are primitive plants and reproduce by spores, also carried by the wind.
So here, in a new environment created from magma, recently upwelled from the earth, is a quickly formed forest system. This ‘ohia forest grows directly on lava rock.
The pioneering forest turns the lava rock into soil. How does it do this?
I’m using lava as an example of how soil forms because there is zero soil on lava when it comes from the earth, freshly cooled into hard rock. The ‘ohia trees, lichen, and ferns establish a forest on fresh lava flows because they are easily spread to new lava flows by the wind and are perfectly designed to pioneer this harsh landscape. They begin a process called biological weathering. The lichen grows directly on the lava. The trees and ferns start shedding leaves.
Plants, fungi, and microorganisms contribute to breaking down rocks by secreting acids and through physical actions (like root penetration). Then, the trees and plants start to shed leaves and branches once they die off. Trees and ferns are always growing new leaves and branches and shedding the old ones.
Once there is organic matter on the ground, insects and pioneering microbes move in. Dead plants, animals, and microorganisms decompose, forming humus. Earthworms, insects, and bacteria help mix the organic matter with the mineral particles, creating a nutrient-rich medium for plants. Soil is born.
RAMIAL WOOD—a Caution
Wherever you see plants growing in the wild, you will see leaves falling and soil building. When trees shed leaves and twigs as they grow, they produce their own soil. This fallen organic matter is called ramial, and it is different from the wood of tree trunks. Ramial wood is created by trees and shrubs to build soil.
Tree trunks and their main branches grow to resist being broken down in order to maintain strength and resiliency. Many trees grow to live for centuries, a few for over a thousand years. They need to be strong to live so long. Dead trees require hearty fungi and other biology to break down into organic matter. The strategy to build soil for itself is not for the tree to die. Ramial wood, which includes leaves, is what creates soil, not the trunks.
This is a good time to offer caution about using wood material in the garden.
The microbes involved in breaking down ramial wood and vegetative matter into soil are vastly different from those that break down fallen trees and other large woody materials.
Trees want their trunks and main branches to be as strong and resilient as possible. They grow to be resistant to decomposition and, therefore, require really strong fungi to break down their structure. When you put this kind of wood in your garden, you will bring in these strong fungal microbes, things like phytophthora, blights, and rusts.
As a Master Gardener, helping gardeners figure out their problems, I saw one clear pattern. People who praised the woodchips in their garden were the ones complaining about persistent fungal diseases. I saw the same pattern when I listened to syndicated gardening question-and-answer shows.
I get it. I do. Wood has a magical sponge-like property that is great. It helps retain moisture and gives a nice structure to the soil. The problem is dead wood attracts things that like to eat dead wood, including the fungal diseases just mentioned. My suggestion is to use ramial wood instead of log wood. Ramial wood is the way trees build soil for themselves. It will still have the sponge-like qualities but a better microbial community and, therefore, fewer disease issues.
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