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Soil Fundamentals: A Quick Review Soil Science and Preparation
by Don Boekelheide
Natural soils are made up of four components:
minerals, organic materials, water and air.
Although naturally occurring “peat” or “muck” soils largely composed of organic materials exist, the vast majority of garden soils in the Carolina Piedmont and elsewhere are mineral soils. On average, simplifying greatly - healthy garden topsoil is about 45% mineral, 5% organic matter, and a balance of 25% water and 25% air. The water and air percentages change in response to environmental conditions. The key point is that a healthy soil has open space where water, air and roots can penetrate.
Soil scientists divide the mineral fraction of soil into three categories by the size of particles: sand, the largest particles; silt, smaller and often with a flat shape; and clay, the tiniest microscopic rock particles. Locally, clay predominates. The percentage of sand, silt and clay in a given soil defines the soil’s texture. A loam has a balanced percentage of each, roughly 40-40-20.
Also true for Mecklenburg County, NC: “About that 45%, 5%, 25%, 25% ideal soil…. I always point out that gardeners will never be bothered by ideal soil as long as they stay in Cabarrus County, NC. I suspect our good soils here are closer to 78% mineral, 2% organic matter, 10% water and 10% air.” David Goforth Extension Agent, Agriculture Cabarrus County NCSU Cooperative Extension Service
Soils are naturally arranged in layers called horizons. The O horizon on top is a shallow layer of organic matter. The A horizon, or topsoil, is normally about 10 to 30 cm deep (rarely more). It is the prime medium for root growth, and garden plants absorb most nutrients from the top 30 cm of soil. Below the A horizon, the B horizon, or subsoil, extends to about one meter. It is markedly poorer in organic matter and nutrients than the A horizon. Beneath that is the C horizon, rock in the beginning stages of soil formation, and finally bedrock, or the R horizon.
With the exceptions of oxygen and carbon, plants rely on the soil for nutrients, absorbing them through their roots from water in the soil (the soil solution). Organic matter helps keep nutrients in the topsoil where plants can access them. Soil organic matter is complex, made up of fresh materials, humus that resists further breakdown, and a vast population of living organisms from bacteria to earthworms. A healthy, balanced soil ecosystem is a key part of a healthy, productive, and sustainable garden soil.
In the series of three articles that accompany this soil science topic we’ll take a closer look at active zone soil preparation: the nuts and bolts of digging a vegetable bed, doing transplant preparation and dealing with buying soil.
© Don Boekelheide 2010 all rights reserved
Don Boekelheide
For the past five years, Don Boekelheide, has taught a hands-on food gardening class at Central Piedmont Community College, modeled on his Peace Corps training. Don holds a Master of Science degree in agriculture from Cal Poly and formerly served as a Peace Corp Ag teacher in Togo. He is a former Extension Master Gardener Volunteer for Mecklenburg County, NC.
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