Soil tests are used to determine what nutrients are in the soil where you are gardening. All plants need nutrients, but too much of some nutrients is as bad as not enough of them. A soil test will tell you precisely what is in the soil and what you need to add in order to grow the desired plant in that location.
In order to do the soil test, you first need to collect some soil. Collecting soil from several locations in the garden delivers better results than only collecting it from one location. The easiest way to do this is to get a bucket and a shovel. Sample five or six places within the garden bed. Each sample should be a shovel of dirt down to a depth of six inches from the soil surface. Place each sample in the bucket.
Mix the dirt in the bucket well and fill the soil test sample bag completely. These bags are available from your Extension agent, generally for free. Next you need to chose the tests you want run on your soil.
Generally, the routine tests determines the soil pH, salinity, nitrates, and the levels of phosphorus, potassium, calcium, sodium, and sulfur available to the plants. Most people will receive all the information they need from this test in order to effectively fertilize their garden.
However, sometimes you want more information. Most soil labs have an extended test available that measures the levels of zinc, iron, manganese, and copper that is available to plants. This test is important for soils with a high pH where plants such as corn or sorghum will be grown.
The rest of the available tests are generally more information than home gardeners need. They test for boron, a detailed salinity panel, lime requirements, and texture and organic matter content. Such tests are required for some building permits such as a septic system, to detect problems cause d by irrigation, or other specialized applications.
When you mail your soil test off to the lab, it takes about two weeks to receive your results. Most labs display the results in a graph that explain the amount of each nutrient tested for and whether it is low, moderate, high, or very high. It is important when sending off the samples to tell the lab what crops you are growing in the space tested so they know how to calibrate their recommendations for fertilizer. If you send off several samples at the same time, it is also important to keep a list of which sample goes where. The sample results will come back labeled “sample 1, sample 2, sample 3,…” and guessing which sample goes where is no fun.
The amount of fertilizer you need for each nutrient will be expressed in pounds of nutrient per 1000 square feet, the size of the average front yard. Your Extension office can help you translate that into how much fertilizer you should put in a given area.