Many of you may not know that I do not have a long history of farming. Just 5 years ago, I left my full-time employment, packed up my possessions and hit the road, eager to learn how to grow food. Everything I know now I have learnt through my farming mentors, reading and small workshops and I still have so much to learn! A few weeks back, myself and a few others from the co-op attended a short workshop on soil biology and nutrient cycling. We only skimmed the surface of this highly complex and poorly understood world but here’s a brief summary of what we covered:
Approximately 0.04% of the air we breathe is carbon (one of the greenhouse gases). Too much carbon in the air is contributing to climate change. Plants, the magical beings that they are, draw in this carbon and convert it to carbohydrates (sugars) for growth. They also use these sugars to attract bacteria to their roots.
Another greenhouse gas, Nitrogen, makes up about 78% of our atmosphere. Nitrogen is a nutrient integral to plants for building biomass, or organic matter (ie. Leaves, roots etc). However, plants cannot ‘breathe’ in nitrogen as they can with carbon. Most plants rely on bacteria called rhyzobia to perform this function. Only bacteria can capture nitrogen from the air. They then store it in the plant roots in exchange for the sugars mentioned above. This is why soil aeration (soil structure) is so important – these bacteria live underground but need access to air.
Phosphorus is generally in soils as inorganic/insoluble phosphate rock. Some plants, like perennial grasses, can exude acids that solubilise this rock to make it available. Most others, in a similar way to above, rely on a symbiotic relationship with fungus. Fungus can metabolise inorganic phosphate rock and transport it to plant roots in exchange for sugars. Fungus in the soil will also receive signals from the plant to selectively ‘mine’ other nutrients and transport them to the plant.
All of these processes are being used to grow the plant. This organic matter, which is essentially carbon plus nutrients, will eventually be returned to soil. Other bacteria and fungus eat this organic matter. Protozoa eat bacteria and fungus. Nematodes eat protozoa, and onwards. With eat step in the food web, these microorganisms are excreting more organic matter.
The carbon in this organic matter is released back into the atmosphere through oxidisation or respiration, the nutrients remain behind for re-entry into food web, and the cycle is complete. However, in healthy soils where there is enough carbon in relation to nitrogen, humification occurs. Humus is a complex substance that has benefits for soil structure and water holding capacity. Importantly though, humus acts a storage tank, binding nutrients in the soil in a stable form.
Understanding these processes is the crux of organic and regenerative agriculture. The holy grail for farmers is to see humification occurring, which means carbon and nitrogen are being sequestered from the air and stored in the soil, restoring the natural balance. Soil should not be seen as an inert substrate in which to grow things, but as a biologically active, self-sustaining ecosystem. As we are seeing now, upsetting this ecosystem or interrupting this nutrient cycle has big consequences.