One of the most important needs of plants is good soil for them to grow in. A soil test from the University of Georgia is the most accurate and effective way to assess the nutrient status and the relative acidity of the soil (pH). Applying fertilizer without a test can lead to applying too much or too little lime and fertilizer needed for optimum plant growth.
To collect the soil, take a minimum of ten samples randomly in the area that requires testing and mix the soil thoroughly in a container, like a bucket. Bring two cups of the soil to the Extension office for testing. You will then place the soil in a small bag, and fill out some information on the side of it. For a fee of $8.00 per sample, the Extension office will mail it to the University of Georgia Soil Testing Laboratory with the results being sent to you in seven to ten days.
Soil tests measure the level of several nutrients of importance to plants: phosphorous, potassium, calcium, magnesium, zinc, manganese, and pH. The information on the nutrient levels, the pH, along with the crop to be grown, is used to determine the necessary nutrient requirements. Nitrogen is not routinely a part of the normal soil-testing regimen since nitrogen is quite mobile in the soil and may be leached out before planting. Recommendations given are based on the amount of nitrogen required for the plant growth in a year.
Here are a few basics of soil chemistry for interpreting the test results: The measurement of the acidity of a soil (the pH) is one of the most important factors in determining a soil’s relative fertility. The pH scale ranges from 1 to 14 with 1 being the most acidic, 14 being the most alkaline, and 7 being neutral. Soils with low pH, that is, acidic soils, restrict the availability of many nutrients that are needed in large quantities. It also increases the availability of others nutrients needed in very low quantities leading to toxicities. The preferred pH for our soils in Georgia is 6.0-6.5; however, the average pH is 4.8, which is acidic. The addition of calcium in the form of lime is the preferred way of increasing the pH of the soil. Since changes in the pH take time, applications of lime are best done months ahead of planting. In some cases the soil is too alkaline, above the pH of 7. Adding acidic soil amendments, like pine bark or peat moss, to the soil is one way to reduce the pH. Sulfur and ammonium sulfate can be used, but do so with extreme caution since they can cause harm to plants. Different plants have differing soil pH and nutrient requirements. Most plants grow at a pH between 6.0 and 6.5; however, some plants prefer a more acidic soil since they have higher iron requirements, such as blueberries, azaleas, rhododendrons, and mountain laurels. They need a soil pH of 4.0 to 5.5.
The soil test report shows available soil nutrients and pH, and recommendations for improving pH and nutrient levels. Lime recommendations are given in pounds per square feet and are self-explanatory. Nutrient recommendations are also given in pounds (lbs) per square feet.
The three numbers on fertilizer bags represent nitrogen (N), phosphorous (P), and potassium (K) respectively and denote the percentages in the bag of each particular nutrient. For instance 10-10-10, the most common fertilizer has 10% nitrogen, 10% phosphorous, and 10% potassium. In other words, it is in a 1:1:1 ratio. The other 70% of the bag is inert ingredients that are used to carry the three nutrients. A forty-pound bag of 10-10-10 has 4 pounds of N, P, and K respectively since 10% of 40 is 4. So, if the soil test report recommends 60 pounds each nitrogen, phosphorous, and potassium per acre, one may use 10-10-10, 20-20-20, or any other fertilizer that has a ratio of 1:1:1. If you use 10-10-10, you would need 600 pounds of fertilizer (60 x 10). Divide the 600 pounds of fertilizer by the number of pounds in the bag to find out how many bags of fertilizer you would need.
Soil test results and fertilizer recommendations can make sense if you take the time to analyze them. Sometimes, however a homeowner or a business will have peculiar situation or set of circumstances and will need help wading through the numbers. This is where County Extension agents and personnel can come into the picture and provide help. Feel free to call your local office anytime.