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Happy Garden Soil is Rich in Microbes

014Behind (or rather, below) every happy garden is a thriving population of bacteria (and fungi and other microbes). Every ounce of productive garden soil contains hundreds of millions of these microscopic critters, which are its very life. Feed the soil with organic matter (think compost), and the soil’s micro-life will respond in kind, converting nitrogen-containing compounds and other nutrients into forms available to plants (and humans).

Conversely, a soil without microbes is barren. That sweet juicy ruby-red tomato; those amber waves of grain; the gardens at Versailles; civilization itself – they all owe their existence to these humblest of organisms.

Soil Food Web

The soil food web comprises countless species interacting in a myriad of mind-numbingly complex ways. Fungi often take the lead, feeding on larger, tougher plant debris (such as wood) and breaking it down into smaller particles. Many fungi, in turn, require nutrients (such as amino acids) synthesized by other microbes. Bacteria – the most abundant soil micro-organisms – consume all manner of organic substances including each other. Protozoa roam the soil’s watery pores, gobbling up bacteria and releasing their nutrients. It’s a jungle down there.

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Every ounce of productive soil contains hundreds of millions of these microscopic critters.

Ultimately, this complex bustle of microbial activity feeds the plants that fed it. Bacteria release the nitrogen locked up in organic matter by converting it into nitrates and other inorganic forms required by plants. Beneficial fungi (known as mycorrhizae) thread their way around and into roots, exchanging nutrients from the soil for sugars synthesized in the plants’ leaves. Symbiotic nitrogen-fixing microbes (such as the bacteria that inhabit the roots of beans and other legumes) pluck nitrogen molecules from the air and split them into individual atoms, making them available to their plant partners. And humus – organic matter that’s been fully broken down by the microbial food web – bonds soil particles into fluffy, moist, well-aerated, nutrient-holding clumps, creating that enviable texture known as good tilth. This is the sort of soil that avid gardeners prize.

Soil Organic Matter

Organic matter is the key to a healthy, happy soil microbial community, and to the fertility and good tilth that come with it. For gardens blessed with good soil, annually apply enough compost or other organic matter to compensate for what the garden produces during a growing season. A relatively light, rapidly decaying compost such as Fafard Premium Natural & Organic Compost Blend is best for short-lived plants (e.g., annuals and vegetables). Trees and shrubs, on the other hand, benefit most from a coarser compost with a higher bark content such as Fafard Ultra Organic Planting Mix.

Natural and OrganicHeavy, sandy, or sterile soils can use all the organic matter they can get. Top dress as often as possible with suitable materials: leafy mulches for herbaceous plants; more woody mulches for shrubs and trees.

Fertilizing Soil

Fertilizers – especially fast-acting chemical fertilizers – are never a substitute for adequate organic matter and a healthy microbe community. Soils low in organic matter have little nutrient-holding capacity, resulting in rapid fertilizer nutrient loss (and likely runoff into neighboring bodies of water). Additionally, too much nitrogen can reduce the amount of organic matter by stimulating soil microbes to consume more carbon. Likewise, avoid the use of fungicides and other chemicals that are potentially toxic to soil microbes. Their health is critical to your garden’s (and the planet’s) health!

About Russell Stafford


Hortiholic and plant evangelist, Russell Stafford, transplanted his first perennial at age 7 and thereby began a lifelong plant addiction. He is the founder and custodian of Odyssey Bulbs (and Odyssey Perennials), an online nursery specializing in cool and uncommon plants. Russell also works as a horticultural consultant, freelance writer (Horticulture and The American Gardener magazines), and garden editor. He formerly served as Curator and Head of Horticulture at Fernwood Botanic Garden in Niles, Michigan and as the Horticultural Program Coordinator at the Center for Plant Conservation, then located at the Arnold Arboretum in Jamaica Plain, Massachusetts. His academic degrees include a masters in forest science from Harvard University.

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