The Living Soil
What is the difference between dirt and soil? To me, “dirt” evokes an image of a dead substrate, while “soil” evokes something dynamic and alive. Indeed, millions of under-recognized organisms keep the substrate under our feet alive and evolving. Microbes comprise a world that works in synergy to create soil conducive to healthy organic agriculture. If you find this intriguing, please do read on.
First, a brief history of life on land. Aquatic plants gained a foothold on dry land some time about 450 million to 500 million years ago. Fungi have been on dry land more than 700 million years. How much longer is in debate as some scientist say more than 1,300 million years. Either way, fungi was on dry land before plants.
Bacteria were present on dry land even before the fungi. As aquatic plants began to try their luck at colonizing the terrestrial earth, from the start they had to associate with these two established orders of bacteria and fungi.
Symbiosis is a relationship that benefits all parties. These microorganisms have found ways to make their relationship with the rhizosphere, or plant root zone a mutually beneficial arrangement.
“Arbuscular” mycorrhizae invade root cells of a plant. They then feed on the plant’s available sugars, benefiting the fungi. In exchange, they increase the root zone by up to 50 percent, and breakdown soil nutrients such as phosphates, mining and converting to make them available for bio-uptake by the plant. (Simon, L.; Bousquet, J.; Lévesque, R. C.; Lalonde, M. (1993). Origin and diversification of endomycorrhizal fungi and coincidence with vascular land plants. Nature, 363 (6424): 67.)
The fact that arbuscular mycorrhizae is found in association with all plant groups and more than 3/4 of all vascular plants leads many to believe that this association has been a key part of plant evolution upon this planet.
As for bacteria, beneficial bacteria can play a huge role in the immune systems of plants. Some cyanobacteria fix nitrogen in the rhizosphere. Others boost the immune system of the plant by preventing disease.
An old-time farmer taught me that chicken manure, high in beneficial bacteria, will help with the disease and bug resistance come fall. I have had luck in side-by-side grows where the plants top dressed in chicken manure staved off infestation far better. How does this apply to my garden? Please read on.
Mycorrhizae and Beneficial Bacteria in Soils
The soil is alive and as it thrives, microbes are able to convert and store nutrients and minerals. In this way, a constant remediation is taking place, thereby making nutrients accessible for plants to uptake and benefit from. This process ties up nutrients in the soil where they are less likely to leach out. Bacteria and fungi make all of this happen. They each are generally present in different ratios depending on conditions and soil pH.
From a plant-growing range of acidic soils, 4.5 pH to a high alkaline soil of 7.0 pH, there is a trend of fungal dominance in soils below 5.0 pH and a trend of bacterial dominance in soils above 6.0 pH.
Perennial plant systems trend more toward acidity and fungal dominance and annual plant systems trend toward alkaline and bacterial dominant soils. So, depending on your crops, you can custom tailor your soil pH to reflect your plant audience.
Mulch Feeds Microbes
To foster a healthy microbial system, mulch can play a tremendous role. Mulch protects the soil from the sun’s rays and the evapotranspiration of a portion of the soil’s moisture during hot days. The mulch helps by retaining water and extending the wet period following each watering.
Mulch also helps to reduce weeds sprouting up through production crops. I recommend putting three to four inches of mulch down around all of your perennial crops.
Nitrogen Fixers Grow Soil
Plants of the family Fabaceae, otherwise known as the bean tribe of plans actually make the soil better by “fixing” nitrogen through root nodules. There is such an amazing range of bean and pea plants to be utilized (see list of Nitrogen-Fixing plants at bottom).
Not only is there a huge diversity of nitrogen-fixing plants, many of the annual varieties grow quickly and can be trained up vertically. Growing upward can be a great strategy for small spaces, such as the rooftop gardens I manage in downtown San Francisco, California.
I have had luck with seeding a blend of several bean, peas and lupines to create a “living mat” of green below my taller production fruit trees, berry bushes and annual veggies. This living mat increases the water-holding capacity of a food system while creating an added harvest in the margins of your main production crops.
Joshua’s List of Nitrogen-Fixing Annual Plants
Joshua’s List of Nitrogen-Fixing Perennial Plants
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