If John Slack had his way, farmers, especially organic farmers, would spend a lot less time worrying about the amount of soluble nutrients in their topsoil. The former mining prospector and current head of the Northern Organic Research and Development Co. (NORDC) spoke at the recent Ecological Farmers Association of Ontario symposium in Dixon's Corners. According to Slack, the topsoil and its soluble nutrients are only a small, and even relatively unimportant, part of a much larger picture of what's happening in the planet's most complex system - the soil.
While he acknowledges the usefulness some soil test measures -- cation exchange capacity and organic matter in particular - John Slack calls for a much more holistic approach to soil management. In particular, he wants us to pay attention to the geology and the microbiology of our soils.
The problem with traditional soil sampling, according to Slack, is in its limited, mechanistic approach to the both the soil and plant growth. The approach is largely a result of the modern reliance on chemical fertilizers. In this model, the top 6 or 8 inches of soil contain certain levels of easily-available soluble nutrients that are used by the plant during the growing season. Soil testing labs use a weak-acid extraction method to estimate the level of these plant-available nutrients and base fertilizer recommendations on these results.
A holistic approach recognizes that the soil is the result of the interaction of the atmosphere, hydrosphere, biosphere, and lithosphere - air, water, life, and rock. It is the lithosphere -- the parent rocks and the geological forces that created them -- that determines soil type, fertility, and microbial life. In other words, to understand the potential of a soil, one has to look 6 to 25 feet beneath the top 4 to 6 inches of topsoil.
This subsoil does not just act as a passive reservoir for the topsoil, either. Contrary to the popular belief that only the topsoil is suitable for plant growth, research stretching back to the early part of the 20th century, demonstrates that the subsoil plays a crucial role in plant growth. It is the younger roots of a plant that absorb the most nutrients and water. In a newly-sprouted plant, these roots are in the topsoil. However, as the plant matures, the younger roots are the deeper roots, and research shows that even when the topsoil is rich in nutrients, plants in later stages of development receive water and nutrients from the subsoil. Both protein and yield are significantly affected by subsoil nutrients.
Clearly, common soil tests do not account for the role of the subsoil. Nor do they account for the role of "bugs" in the soil. It is the microorganisms in the soil that are responsible for making nutrients available to plants. According to research presented by Slack, "these biological systems can accelerate geological processes by a factor of more than a million," and "the extraction of 'insoluble' nutrients from rock happens at an 'inconceivable' speed in the rhizosphere during the growing season." In other words, the bugs in the soil are breaking down minerals and making them available to plant roots at a speed and quantity entirely unaccounted for by standard soil tests.
Of course, this depends on an environment in the soil that encourages this microbial activity. Plant roots cannot penetrate through compacted soil to reach subsoil nutrients. Microorganisms need high levels of organic matter, good soil aggregation and structure, and the right amount of air and water in order to thrive. Using good crop rotations, cover crops, composted livestock manure, and limiting traffic and unnecessary tillage all contribute to healthy soil life. The good news according to Slack is that most soils in Eastern Ontario are rich in minerals, and once the soil system is operating well, nutrient availability becomes a given.
"We shouldn't be wasting our time looking at solubility factors," says John Slack, "we should be recognizing and encouraging the activity of the soil microorganisms that can extract the nutrients for us."
As always, I welcome feedback and suggestions. You can contact me by emailing wallbridge@ontarbio.com or by calling 613-724-9287.
Coming Events:
Homeopathy and Nutrition in Dairy Herds is the topic of the Ecological Farmers Association of Ontario's first kitchen table meeting for 2006. This informal event will be held at the farm of Michael and Heidi Krol and will feature guest speaker Gudula Beythein. It happens on Tuesday, January 10th, from 10am to 3:30pm; soup and coffee are provided, but bring your own lunch. For more information, contact the Krols at 613-347-7561 or email krolmh@glen-net.ca
From the Archives:
News, December 2005
