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August 22nd, 2012

Nitrogen-Depleting Mulch

Right after I finished mulching my rose garden with free wood chips I got from a tree service, I read that wood chips suck all the nitrogen out of the soil. Should I fertilize now?

blog-dougAs soon as you spread mulch, there’s a population boom of bacteria, fungi, and other soil microbes that work to decompose organic matter. These beneficial organisms consume some of the nitrogen present in the soil. The nitrogen loss is temporary, however, because the nitrogen is once again available to plants when the microbes die.

But in the meantime, the thin layer of soil that is in direct contact with the mulch will relinquish some of its fertility. The depletion of nutrients does not extend deep into the soil, so it won’t likely affect anything other than the shallowest-rooted annuals.

The benefits of organic mulch outweigh the possibility of a temporary nitrogen shortage. Mulch conserves moisture and contributes to a healthy root zone. It also helps to suppress weeds. As mulch decomposes, the nutrients held within it—including a small amount of nitrogen—are released.

For roses and other plants with high nutrient needs, it’s a good idea to keep an eye out for nitrogen deficiency, whether you’ve mulched with wood chips or not. Plants lacking nitrogen grow slowly and exhibit pale or yellow-green leaves; in extreme cases, growth may be stunted.

To preclude the chance of nitrogen deficiency, many gardeners make a habit of distributing a small dose of slow-release nitrogen fertilizer every spring to roses, perennials, edible crops, and other heavy feeders. Good organic sources include alfalfa meal (about 5 percent nitrogen by weight), fish meal or fish emulsion (5 percent), blood meal (12 percent), and feather meal (15 percent). Compost and worm castings also contain small amounts of nitrogen.  —Doug Hall

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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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