December 13th, 2011

Sugar as a Soil Supplement

Someone told me that sugar is good for plants. I sprinkled some around my vegetable garden this year but didn’t notice anything different. How much should I apply?

blog-dougPlants don’t need you to add sugar to their soil; they make their own. Through the process of photosynthesis, which is powered in nature by energy from the sun, plants turn water and carbon dioxide into sugars. Plants use their self-made sugars as a fuel for growth and reproduction.

Sugar you add to the soil will instead feed soil microorganisms, such as bacteria and fungi. These naturally occurring microbes are nature’s recyclers; they help to nourish plants by breaking down the bits of organic debris in soil into their nutrient components—including the potassium, magnesium, nitrogen, and other elements that are essential for plant life. In this sense, sugar could benefit the plants in your garden by boosting the microbial population, thereby speeding up the rate at which nutrients become available.

But plants already have a process for encouraging microbial life. Soil scientists have discovered that plant roots exude sugars—sugars produced by photosynthesis—as a way of developing mutually beneficial relationships with microbes. By controlling the types and amounts of sugars they release, plants can select which kinds of microorganisms will colonize the soil around their roots. Not coincidentally, plants choose to feed the microbes that will provide them with the nutrients they need most. Compared to this sophisticated à la carte system, a sprinkle of processed sugar from your pantry is just junk food for bacteria.

By the way, there’s another reason some gardeners haul the sugar canister out to the garden. Sugar added to the planting holes of vegetable transplants is said to discourage root knot nematodes, a destructive soil-dwelling parasite that plagues many Southern gardens.  —Doug Hall

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Comments

    Hi, There is yet another reason plant ecologists add sugar to their experimental plots. All that increased microbial activity temporarily fixes the soil nitrogen and makes it unavailable to plants. It can help ecologists form nitrogen gradients to study the interactions of native and invasive plants under different nutrient regimes. Pollution often releases nitrogen from agriculture, urban runoff, and car exhaust into the environment. For a gardener this could mean temporarily decreasing growth- but to have much of an effect on nitrogen you have to use an awful lot of sugar. Also because gardeners aren’t concerned with impurities- in situations like native plant gardens- we usually use wood chips or other carbon rich mulch to accomplish the same thing for less money.

    Would adding sugar water to shaded plants make up for the loss of sugar from decreased photosythesis?

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