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Your Soil's Secrets
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Your Soil's Secrets
Soil can kill or nurture a landscape
Which is yours doing?


By George Weigel, Pennsylvania Certified Horticulturist (PCH)

One common trait of experienced gardeners is that they've learned not to treat their soil like dirt.

While most people think of soil as little more than the hard, brown stuff you walk on, gardeners know it holds the key to any landscape's success.

Loose, well-drained and organic-rich soil encourages healthy roots that produce healthy plants. Lifeless, heavily compacted clay or rocky "soil," on the other hand, is a virtual death trap for all but the toughest of plants.

"Soil is a living, breathing thing," says Patti Olenick, a soil scientist and organics recycling coordinator for the Pennsylvania Department of Environmental Protection. "I like to think of it as an organism."

That becomes apparent if you look at a handful of good soil under a microscope. You'll see a living city of activity — strands of fungi swirling around plant roots, various bacteria and nematodes breaking down bits of organic matter, tiny mites feeding on other microbes and so on.

It's essentially a factory of life, and all the moving parts fuel one another. When people do things to disrupt that process — say, by stripping and regrading soil to build a house or by routinely dumping fungicides and insecticides on the lawn "just in case" — the whole machine sputters.

"Any good garden starts with the soil," says Tom McCloud, a Pennsylvania Certified Horticulturist and owner of Appalachian Nurseries in Franklin County, Pa. "You have to know the limitations of your site. For example, if you live in a typical subdivision where the soil has been stripped away, you have to realize you can't take a plant like a dogwood and expect it to perform well. You can buy the best quality of dogwood in the world, but plant it in our yellow clay, and it won't work."

Matching plants to your soil
With effort, poor soils can be improved to make them more hospitable to the plants you'd like to plant. But most horticulturists suggest that it's better to match plants to the intricacies of the site rather than try to alter the site to suit a desired plant.

A good place to start is by assessing the kind of soil you've got. That's something very few people do in their haste to get the yard planted, says Dr. Jim Sellmer, associate professor of ornamental horticulture at Penn State University.

"Don't jump the gun in planting a new landscape or in improving an existing one," he says. "Observe the site over various conditions to see the trends and hot spots. You can learn a lot about a site simply by watching it through rains and dry periods, such as where is the water puddling? Where is it running off? What spots are perpetually dry or damp? Observation is a tool we do not employ enough in our landscape. We often get too caught up in what we want, not what we have."

That bit of patience and observation up front can save a lot of needless plant replacement later. Most yards have a variety of environmental conditions, and by matching a plant's preferences to those exact sites, the odds of success go way up.

A winterberry holly or a Virginia sweetspire, for instance, would have a much better chance of thriving in a wet spot than a butterfly bush or juniper.

When it comes to the soil, there's more to it than just adding fertilizer or lime every now and then. Widely overlooked is the underlying structure or quality of the soil.

Soil is a blend of minerals, air, water, organic matter and microbes that keep the "life factory" ticking.

The mineral particles come in three types — sand (the biggest particles), clay (the smallest) and silt (mid-sized). An ideal soil has a nearly equal mix of the three with the spaces between the particles ideally filled about half with water and half with air.

Why should you care about that? Because if your soil is too clayish, the small particles pack closely together and have limited pore space. Some homeowners have such compacted soil that it tests out near the denseness of brick — certainly not a great medium for shrub-growing.

In rainy weather, water drains slowly through clay, fills the pore spaces and deprives the plant roots of oxygen. That means your plants suffocate and rot.

In sandy soil, on the other hand, the spaces are larger, water drains quickly, and the roots may dry out when rain or watering doesn't happen regularly.

An accurate way to determine your soil's makeup is to send off a soil sample (or samples) to a soil-test laboratory. Penn State do-it-yourself test kits are available at county Extension offices and most garden centers. Numerous private labs also offer the service. Soil particle tests usually require an extra fee beyond the basic fertilization test. (See the sidebars to the right for three at-home tests you can do to determine your soil's structure.)

Improving your soil
Olenick says the best way to improve soil structure is to work compost or other organic matter into it. In clay soils, organic matter adds pore space to improve drainage, while in sandy soils, it acts as little "sponges" to slow drainage.

Reducing soil compaction not only prevents plant roots from rotting, it slows runoff during heavy rain, thereby reducing the load (and potential pollution) going into storm sewers.

Even more earth-friendly — some of the best soil-improving material can come from your own recycled yard waste.

Grass clippings, fallen leaves and even many weeds make excellent compost while keeping those materials out of the waste stream. (For more details on making your compost, click here.)

Chipped-up branches can be used as mulch to keep soil from washing away while reducing herbicide use by smothering weeds instead.

Even topping beds with chopped leaves gradually adds nutrients and organic matter as earthworms pull them into the soil and digest them.

Other good sources of organic matter include decomposed leaves from municipal recycling programs, commercial compost, mushroom compost, well-otted manures and peat moss.

Olenick says core-aerating a lawn and giving it a light top-dressing of compost twice a year is a good natural alternative to the more common four-step chemical programs.

She adds that an important benefit of using natural materials is that these also encourage and feed the soil microbes and earthworms that are critical to good plant health.

"If you build up the soil," Olenick says, "you won't see immediate effects, but over the long term, the soil will be healthier, and healthy soil leads to healthy plants that are better able to fight off bugs and diseases."

Working 2 to 4 inches of organic matter into the top 12 to 18 inches of planting beds greatly improves poor soil. Layering compost, leaves and/or bark mulch over beds each year continues to improve the soil without the need to dig it in.

Besides helping soil quality, these organic materials also add nutrients to the soil. That reduces and even eliminates the need for additional fertilizer.

Fertilizer — less is more
Sellmer says most trees and shrubs need no supplemental fertilizer, yet homeowners often add fertilizers routinely to all their beds. If some is good, more must be better, right?

Actually, over-fertilizing is bad for several reasons. Apply too much water-soluble chemical fertilizer and you can burn the roots. The most obvious example is the brown spot that appears in the lawn where you spilled fertilizer while dumping it into the spreader.

Applying more fertilizer than plants use means some of the excess washes off into waterways or leaches into groundwater. Excess nitrogen and phosphorus are main contributors to water pollution.

More insidious and less well known is that overly fertilized landscape plants are more desirable targets for bugs, which increases the use of insecticides and leads to even more water pollution.

And then there's simple economics — it's a waste of time and money to apply products that plants really don't need.

Sellmer says there's really only one accurate way to figure out whether you need to fertilize and what kind to apply if you do — a soil test. Without a test, you're just guessing, he says.

Basic tests report on the big three nutrients of nitrogen, phosphorus and potassium and also the soil's acidity level (pH). The report then tells exactly how much of what kind of fertilizer or pH-adjusting material is needed, if any.

That information is another important piece of the plant-selection puzzle, says Sellmer.

"If your soil tests at pH 7 (neutral), the work needed to acidify the soil enough for azaleas or rhododendrons and to maintain that acidic condition is more than is necessary for a beautiful landscape," he says. "I would reconsider my plant selections to better suit my site rather than spend the rest of my life trying to fight the inevitable upward rise of my pH."

To find out your soil type and underlying rock structures that influence the soil acid level, visit either http://soilmap.psu.edu/metadata/Soils.htm or http://websoilsurvey.nrcs.usda.gov/app.

For more information on maintaining healthy soil in the landscape, Penn State's College of Agriculture offers a free online publication called "Soil Management in Home Gardens and Landscapes." It's accessible at http://pubs.cas.psu.edu/Publications.asp.


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Three at-home tests to help you gauge your soil's structure and drainage capacity

1. Drainage test. Before planting, dig a hole about as big as one of the rootballs of the plants you plan to plant. Fill it with water and give it 24 hours to drain. Then fill it again and watch to see how many inches it drains per hour. If it's not going down by at least 1 inch per hour, you've got some "uncompacting" to do.

2. Soil-texture test. Dig up a tablespoon or so of soil and add enough water that you can roll it into a ball. If you can't form a ball, the soil is sandy. Next, squeeze the ball between your thumb and index finger to make a ribbon. The longer the ribbon goes before cracking, the more clay you've got. Less than 2 inches is a pretty good composition. More than 2 inches means the soil is clayish. The feel alone can also give you a clue ... sand feels gritty, clay feels sticky, and silt feels velvety slick.

3. Jar test. Dig 2 to 3 cups of soil from 6 to 8 inches deep in your soil. Let it dry on newspaper for 24 hours. Use a sieve or old metal colander to sift rocks, roots and other debris out of the soil. Crush lumps of soil to sift them through. Pour 2 cups of the sifted soil into a quart Mason jar or clean mayonnaise jar and add 1 tablespoon of powdered detergent. Then fill the jar with water, seal and shake vigorously for 3 minutes.

After 1 hour, the biggest sand particles will settle out into a bottom layer. After 2 hours, the slightly smaller silt particles will settle out into a second layer. And after 24 hours, the smallest clay particles will settle out into a third layer.

Measure the thickness of each layer and the total depth. To figure the percentage of each layer, divide that layer's thickness by the total depth. (Example: If all three layers total 3 inches and 2 inches of that is the clay layer, then about 66 percent of your soil is clay.)

Ideally, all three layers will be about the same. When any of the three exceed 60 percent, that type is becoming undesirably dominant, and amending is advised.

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