The Alabama Cooperative Extension Publication number ANR-6-A, titled Home Soil Testing: Taking a Sample, describes the procedure of how to collect and send off the soil sample to Auburn University. From this simple procedure you will get recommendations of what nutrients (such as nitrogen, phosphorus, and potassium along with possible lime requirements) are needed in the soil.
When your soil test results come back, it will tell you how much nutrients to apply on a per acre basis. The report will also tell how much to apply per 1000 square feet for those with less than one acre plots. This article is written for those calculating fertilizer requirements on a larger scale.
What do the numbers such as 8-8-8, 10-10-10, 13-13-13, and 17-17-17 mean when they are written on fertilizer bags? Those numbers represent the percentages of nitrogen, phosphorus, and potassium that are in that bag. A 50 pound bag of 13-13-13 does not have 50 pounds of nitrogen, phosphorus and potassium in that bag, but only 6 ½ pounds of each. (50 x 13% = 6.5)
Let’s go over a couple of examples. If your soil test results recommended 80 pounds of nitrogen, 80 pounds of phosphorus, and 80 pounds of potassium per acre, what fertilizer materials should be applied? Since nitrogen, potassium, and phosphorus are all needed, a complete fertilizer such as 8-8-8, 10-10-10, 13-13-13, or 17-17-17 will work, but different amounts of each are needed.
Simply divide the pounds of fertilizer needed, in this case 80 pounds, by the percentage of the nutrient in the bag. If 8-8-8 is used, dividing 80 pounds by 8% will give the answer of 1000 pounds. That means 1000 pounds of 8-8-8 is required to fertilize one acre with 80 pounds of nitrogen, phosphorus and potassium. If 13-13-13 is used, dividing 80 pounds by 13% will give the answer of 615 pounds.
That means 615 pounds of 13-13-13 are required to fertilize one acre with 80 pounds of nitrogen, phosphorus, and potassium. If you were to calculate this with 10-10-10 and 17-17-17 you would work them the same way. It would take 800 pounds of 10-10-10 or 471 pounds of 17-17-17 to obtain the desired 80 pounds of nitrogen, phosphorus, and potassium per acre. (80/10% = 800 and 80/17% = 471)
These calculations will tell you not only how much to apply, but will also help you decide on the most economical fertilizer. For example, it would take twenty 50 pound bags of 8-8-8 per acre to add the 80 pounds of nitrogen, phosphorus, and potassium. (1000/50 = 20) It would take 12.3 bags of 13-13-13 per acre to add the 80 pounds of nitrogen, phosphorus, and potassium. (615/50 = 12.3)
Try another example. Let’s say the soil test recommended 120 pounds of nitrogen and no phosphorus and no potassium. It is common to have a soil test with this type of recommendation. This means the soil has proper amounts of phosphorus and potassium and does not need any more of these elements for proper plant growth. Applying fertilizers such as 8-8-8 or 13-13-13 would be applying unneeded nutrients.
Applying more than needed will not cause the plants to grow any better but will cost more money. A fertilizer with nutrient content of 34-0-0 is easy to find and just what this example needs. Divide the pounds of fertilizer per acre needed, in this case 120 pounds, by the percentage of nutrient being used. If 34-0-0 is used, dividing 120 pounds by 34% will give the answer of 353 pounds. (120/34% = 353). This field would need 353 pounds of 34-0-0 per acre to obtain the recommended 120 pounds of nitrogen.
To calculate fertilizer on smaller areas simply divide the amount of fertilizer needed per acre by 43560 (square feet in one acre). This will give you the amount needed per square foot then multiply that by how many square feet are in the small area to be fertilized.
Call around and find out what fertilizer materials are selling for in your area. Then calculate the price of fertilizing an acre, not prices per bag. Since more bags of one product may be needed than another, the cheapest price per bag may not be the most economical. On a larger scale the cheapest price for fertilizer may be purchasing it in bulk or per ton.
The soil test will also tell you how much lime is needed per acre or per 1000 square feet and when to apply the fertilizer during the year. If 120 pounds of nitrogen was needed per acre we would not apply that much at one time. If lime is needed in small plots, 50 pounds of lime per 1000 square feet is equivalent to 1 ton per acre. If the soil test calls for 2 tons per acre of lime, you could apply 100 pounds per 1000 square feet and so on.