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Down and Dirty: Making Your Soil Suitable for Gardening
by Larry Caplan - posted 05/23/12

Poor soil is the leading cause for landscape plant problems. But if you make the effort before starting your lawn or planting your landscape, your gardening skills will be the envy of the neighborhood.

 


Garden and landscape plants will struggle to survive in poor soil.

A successful yard or garden begins with healthy soil. Unfortunately, the process of home construction usually destroys the soil. Topsoil is frequently removed or buried. Heavy equipment compacts the soil, preventing the movement of air, water and roots. Spending time and money preparing the soil isn’t as glamorous or as much fun as actually planting trees, shrubs and flowers. But if you take the time to improve your garden soil, your plants will be healthier, less prone to growth and pest problems, and will require less water and fertilizer in the long run.

Before you begin any major excavation work, be sure to “Call Before You Dig.” Dial 811 and schedule a free visit to have your underground utilities marked.

Test the Soil

Once you know it’s safe to work in that part of your yard, collect a representative soil sample and bring it to a reputable lab for testing. A soil test will provide information on the soil’s pH (level of acidity), nutrient levels, amount of organic matter and texture. Contaminants such as lead should be tested for if edible crops are to be planted. Only about 2 or 3 cups of soil are needed for the sample.

Collect seven or eight scoops of soil from around the garden. If you are testing a larger area, such as for a lawn, you may need as many as 10 or 15 scoops. Dig down 3 to 4 inches for lawn areas; 6 to 8 inches for gardens, flowers and shrubs, and 8 to 12 inches for trees and orchards. Mix this soil in a plastic bucket and then transfer it to a clean container or plastic bag to send to the lab. Be sure to properly label which area of your yard each sample came from.

Diggin’ the Drainage?


Check soil drainage with a percolation test. If water empties out at less than ½ inch per hour, this site is poorly drained and needs to be amended before planting.

While you’re waiting for the lab results to come back, you can perform a percolation test to see what your soil’s drainage actually is. Dig a hole about 1 foot wide and 1 foot deep. Fill it with water, and let it drain for 24 hours. Then, fill it again and record how long it takes for the water to drain out. Sticking a ruler into the hole will allow you to measure the progress of the water. If the water level drops less than 1/2 inch per hour, your soil is considered poorly drained and will need major work. If the level drops about 1 inch per hour, your drainage can be considered good; you should be able to plant pretty much anything you want. If the water drains faster than 2 inches per hour, it may be too well drained; plants that prefer moist soils will have a hard time here.

Add Amendments

The single best amendment you can add to your garden soil to improve drainage is organic matter. Compost, well-rotted manure and peat moss are all excellent products. Just be aware that while they will help break up clay soils, there are very few actual nutrients in compost and peat moss.

Before spreading the organic matter, break up the native soil as deeply as possible. Most garden rototillers can only dig down about 4 to 6 inches, which may not be deep enough for soils that are compacted or have large amounts of clay. You might need to hire someone with a tractor-mounted tiller to loosen the upper 12 inches, or double-dig the bed. Once this is done, spread 3 to 4 inches of your organic material over the bed, and then till this in as deeply as you can.

Sand does not work as a soil amendment. You cannot add enough sand, nor mix it well enough, to actually improve the drainage. Usually, all you will wind up doing is coating clods of clay with a thin crust of sand, which doesn’t benefit the plants.

Importing Topsoil


Deep tilling before planting breaks up soil compaction and mixes organic matter throughout the root zone.


Master Gardeners adding shredded leaves and compost to garden soil. Rotted manure or peat moss can also be tilled into the soil to improve aeration and drainage.

Topsoil can be brought in to fill in low areas, but it doesn’t help the drainage if added as an amendment. True topsoil should contain large amounts of organic matter, earthworms and microbes, and have moderate to high levels of fertility. It should be dark in color, have good tilth (is easily worked) and have sufficient pore spaces for easy movement of air and water. This is not what you will be buying, however.

When you purchase a truckload of bulk topsoil, you really don’t know what you’re getting. That’s because there is no legal definition for “topsoil.” In some parts of the state, you would be getting river bottom silt. In other areas, excavated subsoil from lake construction can be sold as topsoil. These materials may have large amounts of clay or silt, as well as weed seeds or contaminants. Examine the material carefully before purchasing it. You will almost certainly want to mix organic matter into this topsoil material before adding it to your garden.

Bringing in topsoil can be harmful to existing trees. Adding more than 1 or 2 inches of soil over the surface can change the amount of oxygen reaching the tree’s roots. Piling soil against the tree’s trunk can promote crown rot. Less is best when working around tree roots.

Obey Soil Test Results

By this time, your soil test results should be in. While all of the numbers might be a bit confusing, the lab should have made a recommendation at the bottom of the report to let you know what fertilizer to add. Also, if the soil pH is not correct for the type of plants you want to grow, there should be instructions for adjusting that as well. Generally, if your soil is too acid and you need to raise the pH, you will need to add limestone. If the soil is too alkaline and you need to lower the pH, you will need to add sulfur or a sulfur-containing product, like ammonium sulfate. Never add lime or sulfur without first getting a soil test.

Pick Perfect Plants

Your final step is to match landscape plants with your soil conditions. Many universities have publications that list plants that will tolerate problem soils such as those found in sandy, acid, dry or wet sites. Take these lists with a grain of salt, though. For example, most plants that will tolerate wet soils still require decent drainage. Other than a few plants native to boggy wetlands, a wet, poorly drained soil will not hold enough oxygen to allow root survival.

Poor soil is the leading cause for landscape plant problems. It is costly, messy and time-consuming to make improvements. But if you make the effort before starting your lawn or planting your landscape, your gardening skills will be the envy of the neighborhood.

 

Attributes of a Suitable Garden Soil

• Organic matter content between 1.5 to 10 percent

• Soil pH 4.5 to 5.9 for acid-loving plants such as rhododendrons and azaleas

• Soil pH 6.0 to 6.8 for most other plants

• Soil textures of sandy loam, silt loam or loam

• Soil is free of broken glass, paint chips, plastic or construction leftovers

• Free of weed seed or perennial weed roots

 


Larry Caplan has served as an Extension horticulture educator for more than 25 years. He has won several national awards for his work with disabled gardeners, teaching about alternatives to pesticides and for his weekly column with the Evansville Courier and Press.