Soil for gardeners, gardeners for the soil
Only 10 percent of the Earth’s surface is soil. It feeds all of us who grow and move around on the land and air. No soil, no food.
Good soil for plant life — including gardening — is about 45 percent minerals (broken down rock) and 4 percent organic material (decomposed plant and animal matter). The other half consists of water and air clinging to the surfaces of these particles and moving through the spaces in between them.
Within the soil lives a vast network of bacteria, fungi and the many creatures that eat them. They digest fallen plant and animal material and return essential nutrients to the soil. They also absorb and give off key elements from the air, such as carbon and nitrogen. In other words, the soil eats and breathes — it is alive.
Carbon is the basic building block of life and half of the organic material in the soil is some form of carbon. Through photosynthesis, green plants (and cyanobacteria) take in carbon dioxide from the air and combine it with water to make sugars. They exhale the leftover oxygen gas (O2), and use the sugars for their own energy needs. As plants are eaten, these sugars feed the rest of the biosphere. During the night, green plants exhale carbon dioxide back into the atmosphere (like us) and use sugar for energy.
Rising amounts of carbon dioxide in the atmosphere is a central issue in global warming. One way to reduce it is to protect the forests and plant more trees and other green plants.
Nitrogen is the key ingredient in forming the “working parts” of plant and animal cells, such as proteins and enzymes. Often it is the availability of nitrogen that controls how fast and large plants can grow. It is very abundant in the atmosphere (as nitrogen gas, N2), but most living things can only use it after it is converted to nitrate (NO3) or ammonia (NH3). This critical step depends on nitrogen-fixing bacteria in the soil; these bacteria live symbiotically with legume plants (peas, beans, and clover) which contain specialized nodules for them on the roots.
Nitrates and ammonia are taken up by plant roots and eventually recycled as microbes that decompose dead plants and animals. Finally, they are exhaled back into the atmosphere in the original form (nitrogen gas, N2) by “denitrifying” bacteria.
Nitrogen can be added directly to the soil by chemical fertilizers. But it is not recommended, as unfortunately they make the soil too acidic for the normal soil microbes. Eventually, the soil is made lifeless and the gardener dependent on continued use of the chemicals.
The decomposer microbes — mainly the bacteria and fungi — consume the plant and animal material that falls to the ground and take nitrogen, carbon and other nutrients into their bodies. Next, these creatures at the bottom of the food chain are consumed by ever-larger members, such as protozoans, nematodes, mites, springtails, earthworms, ants, termites, snails, pill bugs, centipedes, spiders, wasps, toads, gophers and moles, ground squirrels and badgers. This process finally releases the nutrients into the soil.
These underground creatures also aerate the soil, giving it the right texture for roots. Root-related fungi (mycorrhizae) branch out beyond the reach of their home base roots and collect nutrients, which they channel back to the roots and plant in exchange for sugars.
The end result of these processes is soil that is rich with nutrients and well-aerated. Well-fed soil returns the favor by feeding the plants, us, and the rest of the biosphere.
To be a successful gardener, be a good partner to your soil.
- Before adding manure and other organic materials to your garden soil, give the decomposers time (at least three months) to do their composting, so that nutrients are fully available for plant use.
- Avoid using chemical fertilizers because they harm the microbial community that makes your soil healthy.
- Minimize digging because it disrupts the fungal network that supports the roots.
- Whenever you touch the ground, remember that it is alive.
Master Gardeners are available to answer home gardening questions at local Farmers Markets and at their office Tuesday through Friday, 9 a.m. to noon. Call (530) 621-5512, or walk-ins are welcome, at 311 Fair Lane in Placerville. For more information about our public education classes and activities, go to our Master Gardener website at ucanr.edu/sites/EDC_Master_Gardeners/. Sign up to receive our online notices and e-newsletter at ucanr.edu/mgenews/. You can also find us on Facebook.