Not all gardening soils are created equal. Soil that may look healthy and strong may prove to be unsuitable for the plants you as a gardener wish to plant. The only way to determine a soil's fertility is by having a soil analysis test carried out by a university extension, or with store-bought test kits.
Soil Sample Materials
Most university extension services provide soil sample containers and instructions specific to the soil testing laboratory. Soil samples are taken from several areas of the garden or lawn to determine differences between areas. You can do a soil analysis test any time of the year, but to adjust soil before planting, do a test months before planting begins.
How to Take Samples
Soil tests usually require a 6- to 7-inch core sample of the dirt in your garden or lawn, notes the University of Missouri Extension. Coring devices are available at most garden centers for the purpose of taking core samples from the soil. Alternatively, use a power drill for hard or rocky soil. Take a plastic container with a hole through the middle, and drill into the soil through the hole. Dirt is pulled up into the container as the drill auger comes out of the soil.
Soil pH is the main factor determined in a soil test. The pH is a measurement of the acidity or alkalinity of the garden soil. It is measured on a scale or 0 to 14, but most soils fall between 5 and 8 on the scale. A reading of less than 7 means the soil is acidic, while a reading above 7 is alkaline. Soil that is too acidic or too alkaline will not allow plants to absorb nutrients properly.
Nutrients in the soil are also measured during the soil analysis. Nutrients are broken up into three separate categories in a soil analysis, says Clemson University--primary, secondary and micronutrients. Primary nutrients include the main nutrients needed for proper plant growth, which are nitrogen (N), phosphorous (P) and potassium (K). Plants need these in large amounts. Secondary nutrients such as calcium (Ca), magnesium (Mg) and sulfur (S) are needed in smaller amounts. Micronutrients are only required in the smallest of amounts.
Along with the soil analysis, extension offices will give you nutrient and fertilization recommendations to help you properly adjust your soil to grow plants. Do a soil test every three years to ensure you are keeping the soil properly fertilized and that nothing has changed the soil pH.