How To Complete A Soil Test The Right Way

Adding fertilizer or lime without a soil test is guesswork. Too little and plants go short; too much wastes money, can burn roots, and washes nutrients into streams and groundwater. A test from a soil laboratory tells you what is already there and how much, if anything, to add.

What the test measures

A standard garden test reports soil pH, which controls how available nutrients are to roots, along with levels of phosphorus, potassium, calcium and magnesium. Many labs add sulfur, some micronutrients and organic matter, sometimes for a small extra fee. Nitrogen is usually left out of routine garden tests because it changes too quickly in the soil to give a useful single reading.

The report comes with recommendations: how much lime to raise pH or sulfur to lower it, and how much of each nutrient to apply for the plants you list. Naming the crop matters, because a lawn, a vegetable bed and a row of azaleas want different things.

Where and when

Land-grant university labs test soil for home gardeners, and county extension offices supply bags, forms and instructions. Private labs offer the same service. Fall is a good time to sample. Labs are less busy than in spring, and lime, which acts slowly, has the winter to start working before planting. Most beds need retesting only every few years, or sooner if plants show problems the last report does not explain.

How to take the sample

A lab analyzes only a small portion of what it receives, so the sample has to represent the whole area. The method recommended by the Alabama Cooperative Extension System is typical: collect a composite of 15 to 20 cores or slices from random spots across the area in a zigzag pattern, mix them well in a clean bucket, and send about a pint of the mixture.

Separate the areas

Sample the vegetable garden, the front lawn, the back lawn and the shrub beds separately if they are managed differently or the soil looks different.

Sample at the right depth

In gardens and other dug beds, sample to the depth the soil is worked. In established lawns, take cores 2 to 3 inches deep. Around trees and shrubs, sample about 6 inches deep near the drip line, the outer edge of the branches.

Keep it clean

Use a clean spade, trowel or soil probe and a plastic bucket. Galvanized or brass tools can add zinc or copper to the sample. Scrape away mulch and thatch before digging, and skip odd spots such as old fire pits, compost piles or places where fertilizer was just spread.

Dry it and label it

If the soil is wet, spread it on paper to air-dry at room temperature. Do not heat it. Label each bag with a waterproof pen and fill in the form completely.

When the results arrive, apply only what the report calls for. Garden soils fed for years with all-purpose fertilizer often test high in phosphorus already, and for those beds the most useful thing to add is nothing at all.