For Release On Or After 03/11/11

By Dan Gill
LSU AgCenter Horticulturist

The condition and type of soil in which you garden has a profound effect on the health and growth of your plants. One of the most common mistakes novice gardeners make is putting too little effort into learning about their soil.

Soil is the primary source of water and nutrients for plants and must also provide sufficient air – oxygen – to their root systems. Ideally, a soil should be about 25 percent water, 25 percent air and 50 percent solids, which include minerals – sand, silt and clay – and organic matter.

Proper soil fertility keeps lawns lush, flowers blooming and vegetables producing. And the key to proper fertilizing, whether you use commercial or organic fertilizers, is a soil test. Adding nutrients that are already available in adequate amounts is wasteful and may contribute to environmental problems such as polluting surface water.

Around Louisiana, soil types vary greatly from light sands to loams to heavy clays. A thorough knowledge of the characteristics of your soil is necessary to make proper decisions about vegetable gardening, ornamental beds and landscaping. This information is vital to appropriate soil improvement, as well fertilizing.

A great place to start is your parish LSU AgCenter office. Your county agent can familiarize you with the general characteristics of the soils in your area. For more precise information, testing your soil is an important step in learning about the soil where you garden. You can have your soil tested by the AgCenter’s soil testing laboratory in Baton Rouge for $10 (plus $5 postage per box).

Kits to submit soil samples for analysis are available at your local AgCenter office. These kits include a self-addressed, postage paid box containing instructions for taking soil samples, bags to submit the samples and the form you will fill out. They may also be available at nurseries in your area. You can find out if nurseries in your area carry the soil test kits at the LSU AgCenter Soil Testing Laboratory website www.lsuagcenter.com/soiltest.

A soil sample should be submitted for testing from each unique area of your landscape, especially if the soils may be different because of past treatment, location or type of plants being grown. For instance, one sample could be submitted from your front lawn area, one from a shrub bed and another from an annual flowerbed in the front yard. A separate soil sample would also be submitted from a vegetable gardening area.

When taking a sample, take soil from several different spots in each bed or area to be tested. Dig down about 3-6 inches to take a sample, and remove any rocks, mulch, grass, roots or other material, leaving only soil. Put all the soil collected from several spots in one area in a plastic bucket and blend it together. Use this soil to fill the plastic bag in the pre-addressed mailing box. Each mailing box has three plastic bags, so you can send up to three separate soil samples in one box.

Along with the samples, you will submit a form that includes pertinent information such as the plants you are growing or intend to grow in the area and a check to cover the costs.

The test results, which you will generally receive in about three weeks, will tell you the texture of your soil – the relative proportion of sand, silt and clay. Soil texture is important in how easy the soil is to work and influences drainage. Heavy, clay soils that are difficult to work and drain poorly may benefit from the addition of sand and organic matter.

You also will learn the soil’s pH, which reveals how acid or alkaline it is. A pH of 7 is neutral, lower numbers indicate an acid soil while higher numbers mean the soil is alkaline. Generally, a pH from 5.5 to 7.5 is acceptable for most plants. If necessary, the pH of the soil can be adjusted higher by the addition of lime or lower with the addition of sulfur. The soil test results will indicate the lime or sulfur requirements based on your soil and the plants to be grown in the area.

The test results indicate the soil fertility by the levels of phosphorus, potassium, calcium, magnesium, sulfur, copper and zinc. These are given in parts per million, which can be somewhat confusing. Each level, however, is interpreted for you and is indicated as being very low, low, medium, high or very high. Ideally, the levels should be medium to very high. Fertilizer recommendations you receive with the test results are based on these levels and the type of plants you indicated you are growing or intend to grow where the soil sample was taken.

The amount of sodium in the soil is also shown. An excessive amount of sodium in soil is detrimental to plants, so the level should be low or very low. This can be important to gardeners along the Gulf Coast or those using irrigation water high in sodium.

A soil test will only resolve issues that relate to soil characteristics such as fertility, pH or sodium levels. If you suspect these soil characteristics are causing the problems, a soil test will help you determine if, in fact, they are. Soil tests are not useful if the plants are having problems with insects, diseases or how you manage your plantings, or for testing for pesticide or chemical residues.

Soil testing is an inexpensive way to improve your gardening success. The information you receive will influence what plants you grow, how you fertilize and what fertilizers you choose to use. Everyone should have their soil tested at least once to know garden-related characteristics of their particular soil.