Columns, In the Garden

Benefits of Organic vs. Chemical Soil Treatment

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A selection of my homegrown tomatoes.

by Henry Homeyer

CORNISH FLAT, N.H. – On a cold and snowy day I paused to think back a few years to a conference I attended run by the Ecological Farming Association in Pacific Grove, Calif. There were several sessions by scientists presenting research confirming what organic gardeners have always known: organic techniques yield plants that resist disease and insects better, and produce better quality and healthier vegetables. There was even data presented indicating that organic practices can reduce weed pressure. I dug out my notes, and would like to share some of what I learned.

Dr. Larry Phelan, a research scientist at Ohio State University, explained that he wanted to see if organically grown plants attracted insect pests differently than those grown using conventional techniques. He collected soil from two farms that were across the road from each other. The soils were identical except for how they had been tended for the past several years. One farm was organic, the other conventional.

To reduce other variables, Dr. Phelan brought the soil to his greenhouse, and potted it up in large containers. He then grew corn in containers, adding chemical fertilizers in some, fresh cow manure in some, and composted manure in others, using both types of soil for each method. When the corn was at the appropriate size, he released corn borers into the greenhouse, and watched what happened.

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Organic Corn from my Garden never gets treated with chemicals.

Not surprisingly, the corn borers preferred the corn grown conventionally. Not only that, the long-term history of the soil mattered. The soil from the organic farm had higher levels of organic material in it, and consistently was less attractive to the borers, even if used with chemical fertilizers.

Why should this occur? Dr. Phelan explained that plants evolved over the millennia getting their nutrients through the soil food web, depending on the symbiotic relationships between plants and microorganisms. Chemical fertilizers are imprecise, providing nitrogen for fast growth, but often giving too much nitrogen, or providing it all at once. Soils rich in organic matter provide nitrogen and other needed nutrients in a slow, steady stream the way Mother Nature does it.

He said that when a plant gets too much nitrogen, the excess is stored in the form of amino acids, the building blocks of protein. For insects, this is like candy for kids or drugs for addicts: they can detect it, and go to the source.

In another experiment, Dr. Phelan grew soy beans hydroponically, varying the amount of nutrients present. The soy bean loopers preferred plants that were out of balance nutritionally. But not just nitrogen mattered. Iron, boron and zinc levels were important, too. And of course, those elements are not present in conventional fertilizers. Chemical fertilizers only offer nitrogen, phosphorus and potassium. Good soil enriched with compost should have everything your plants need.

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This artichoke from my garden was grown without chemicals.

Dr. Autar Mattoo of the United States Department of Agriculture Research Station in Beltsville, Md., also presented some very interesting findings. He compared the health of tomatoes grown with chemical fertilizer on black plastic versus that grown organically using a mulch of hairy vetch, an annual cover crop. He found that tomatoes grown with hairy vetch was dramatically better at resisting fungal diseases, especially those that cause blackening and dropping of leaves, which is often the bane of gardeners.

Dr. Mattoo explained that the vetch fixes nitrogen when growing. Which is to say, it extracts nitrogen from the air and turns it into a form that plants can use. It was mowed down before flowering and allowed to stay on the surface of the soil, producing a considerable biomass to nourish soil microorganisms.

Compared to chemical fertilizer and black plastic, Dr. Mattoo found a 25% to 30% increase in yield using vetch. He explained that eventually the organic tomato plants would develop fungal diseases, but that for the first 84 days after transplant (late August for us), there was virtually no leaf blackening. At the same time, the tomato plants grown conventionally were severely damaged.

He attributed much of the difference to hormone signaling. Anti-fungal proteins can be produced when specific genes are activated, protecting leaves. He explained that depending on the environmental conditions specific genes are turned on or off. He was able to show this by photographing specific genes in the leaves of the tomatoes to see their size and thus their levels of activity. It appears that something in the vetch stimulated the tomatoes to produce those anti-fungal proteins.

What does all this prove? Being an organic gardener has many benefits, and scientists are just catching up with us. So as you plan your garden projects for the spring, think about giving up your use of chemical fertilizers. There are plenty of organic fertilizers made from natural, biologically-created ingredients like oyster shells, peanut hulls, cotton seed meal and naturally occurring minerals like rock phosphate and green sand. And of course, compost is a terrific way to increase biological activity in your soil.

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