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 and what is needed for proper bed preparation.
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% water, 25% air, and 50% solids which includes minerals (sand, silt and clay) and organic matter.
There are many different kinds of soils in our state, 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 as watering and fertilizing.
A great place to start is your parish LSU AgCenter Extension office. Your county agent will be able to familiarize you with the characteristics of the soils in your area. In addition, you can have your soil tested by the AgCenter's soil testing laboratory in Baton Rouge for $7.00 (make check to LSU Soil Test Lab). Kits to submit soil samples for analysis are available at your local Extension office.
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 non-point source pollution.
A soil sample could be submitted for testing from each area of your landscape, especially if the soils may be different because of past treatment or location. For instance, one sample could be submitted from your front lawn area and another from a flower bed in the front yard, since the soils would have been treated differently over the years. A soil sample from the back lawn area would generally be very similar to the front lawn, and it would usually not require a separate test.
It's a good idea to take soil from several different spots in each bed or area you wish to be tested. Dig down about 6 inches to take a sample, and remove any rocks, mulch, grass, roots or any other material, leaving only the soil. Put all the soil collected from several spots in one area in a bucket, blend it together, and remove about one pint to submit to the soil testing laboratory. The sample may be spread out and air-dried for several days at room temperature to reduce its mailing weight.
Along with the soil you will submit a form that includes pertinent information such as the plants you are growing or intend to grow in the area. Depending on which parish you live in, you can bring samples to your local Extension office or mail them directly to the soil testing laboratory.
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. The soil texture is important in how easy the soil is to work and influences drainage. Heavy, clay soils, which are difficult to work and drain poorly, may benefit from the addition of sand and organic matter.
You will also learn the pH of your soil; it reveals how acid or alkaline the soil is. A pH of 7 is neutral, lower numbers indicate an acid soil condition and higher numbers mean the soil is alkaline. Generally, a pH between 5.5 and 7.5 is acceptable for many plants. If necessary, the pH of the soil can be adjusted higher by adding lime or lowered by adding sulfur.
The fertility of the soil is indicated in the test results by the levels of phosphorus, potassium, calcium and magnesium. The levels 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 resolve only 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 cultural problems, or for testing pesticide or chemical residues.
An optional test, which generally should be done, will determine the percentage of organic matter in your soil and costs an additional $1.00. Adequate amounts of organic matter are important for plant growth and health. And, organic matter is the primary way nitrogen is stored in the soil. Levels of 2% to 5% are considered minimally adequate; more is better for home gardens.
Other optional tests may be run at additional cost for the levels of various heavy metals (including lead), salts and oil residues. Generally, soils do not need to be tested more than every three years.