Why Test Your Soil?

[This article was first published in The Gateway Gardener September 2006 issue.]

By Dr. Steven Cline

You hardly think of gardening as an absolute science, but the one place where it helps to know as much as possible is when it comes to your garden soil.  After all, if you expect your plants to last a long time, then would you not also want the planting site to be the best possible?  Whether you are establishing a new planting bed, expanding the lawn area, or trying to figure out why your plants are growing poorly, start with a soil test – it’s a good investment, and good “dirt” means healthy plants.  Environmentally, soil tests avoid the guessing and over-application of fertilizers leading to nutrient loading of lakes and streams. Here is what a soil test will give you.

The soil pH tells you how acid or alkaline the soil is and ranges from 5.0 to 8.0 with a pH of 7.0 being neutral.  Values are multiples of a ten-fold difference from each other so a pH of 5 is one hundred times more acidic than 7.  Soil acidity affects the availability of nutrients in the soil, not their presence.  Plants like pin oak, sweetgum and rhododendron may show yellow leaf color because the pH is too alkaline.  These plants don’t absorb some nutrients unless the soil pH is acidic.  That’s why it is important to figure this out first, then incorporate either sulfur or lime to balance the soil pH.  In St. Louis about one-third of the soils tested are either too acidic or too alkaline for good plant growth.  Only a soil test can give you an accurate pH assessment.

The most deficient nutrient in our area is potassium.  One third of all soils tested have levels below adequate amounts for lawns and gardens.  Potassium levels in the soil drop as water leaches through the soil, as vegetables are harvested and lawn clippings removed.  Of course, there is no real reason to remove grass clippings since they do not contribute to thatch.  Mow tall and invest in a mulching mower.  If a soil test indicates that you need potassium, never apply more than 1.5 pounds per 1,000 sq. ft.  Excessive amounts will burn young plant roots.  If you test now, there will be plenty of time to apply it to a new or renovated fall lawn.  September is lawn month and this year you may need some help after the drought.

Out of 100 soil tests, only 5 will show that phosphorus is deficient.  This is because we blanket- apply complete fertilizers and phosphorus tends to stick to the soil and not move far from where it is applied.  Over time, you will see a build up of this nutrient as fertilizers are annually applied.  The time to be concerned is when it is time to establish a new planting.  Young plants with small root systems do better when starter fertilizers with high levels of phosphorus are applied.  This goes for lawns, too, so if you are seeding or sodding this fall, be sure to consider this as you prepare the soil.

Nitrogen is the main reason we apply fertilizers.  Because it leaches with rainfall, each year a certain amount, depending upon the plant, should be applied close to the time when plants begin to grow.  Lawns should be fertilized in the fall when root growth is favored.  Gardens, tree and shrub beds should be fertilized in the early spring.  Nitrogen is the only nutrient we do not measure in a soil test, since amounts go up and down with temperature, microbial activity and moisture levels in soil, making it hard to evaluate accurately.  Assume you will need some and contain yourself from over-application.  Time-release fertilizers are the best.  Demand your garden center stock these so all the nutrients you apply won’t go down the street.

Fall is the best time to test and prepare your soil.  Randomly choose 3 to 4 spots in the lawn or garden area (2-3,000 sq. ft.), pull off the top one inch of soil, plant growth, weeds etc.  Sample the soil by cutting off a wedge down to 6 inches deep.  This is the rooting zone for most plants.  Pool all of your samples together and draw out one cup for testing.  This will give you an average assessment.  If your landscape is large and you suspect soil differences, sample these areas separately.  A second sample might help you diagnose a problem. (Ed. note–see our video on “How to Take a Soil Sample.”

You can ask your local nursery if they do soil testing, or bring samples to the Kemper Center for Home Gardening at the Missouri Botanical Garden and expect results in 2 weeks.  Call the Center at 314 577-9440 between 9 a.m. and 5 p.m. to determine the cost.

Steven Cline, PhD., retired in 2011 as the director of the Kemper Center for Home Gardening at the Missouri Botanical Garden. He also operates a soil testing business, EarthCo.