Ferguson: Check chemical concentrations to get the best deal

Published 2:33 pm Friday, August 27, 2021

Sometimes people wonder if, for example, 8-8-8 or 13-13-13 fertilizer is best for a certain situation. I’ve heard comments along the lines of 13-13-13 being too “strong.” However, when comparing fertilizers with the same nutrients and the same ratios of one nutrient to another, a variety of fertilizers can be used to provide the same amounts of actual nutrients, as long as you adjust the application rate to account for their concentrations in the fertilizer.

In the case of 8-8-8, 10-10-10, 13-13-13, and 17-17-17, all fertilizers have a 1:1:1 ratio of nitrogen (N): P2O5-equivalent (“phosphate”) : K2O-equivalent (“potash”). (The fertilizer 8-8-8, for example, is 8 percent nitrogen, phosphate, and potash.) If you need one pound per 1,000 square feet of nitrogen, phosphate, and potash, any of the following will provide those: 12.5 pounds of 8-8-8, 10 pounds of 10-10-10, 7.7 pounds of 13-13-13, or 5.9 pounds of 17-17-17.

When I’m choosing among such options, I generally calculate which is less expensive for the same amounts of actual nutrients. Consider an example in which a 50-pound bag of 8-8-8 costs $11, and a 50-pound bag of 13-13-13 costs $14. The 50-pound bag of 8-8-8 contains four pounds of nitrogen, phosphate, and potash. At $11 per bag, that’s $2.75 per pound. The 50-pound bag of 13-13-13 contains 6.5 pounds of each. At $14 per bag, that’s $2.15 per pound. So, even though the 13-13-13 is more expensive, in this case, it costs less to get the same amounts of nutrients from the 13-13-13 than the 8-8-8.

The same principle applies to pesticides (herbicides, insecticides, fungicides, etc.) and their active ingredients.

For example, pesticides for home use are sometimes sold in ready-to-use and concentrated versions. One ready-to-use (RTU) insecticidal soap product has a 1-percent concentration of “potassium salts of fatty acids,” which is the active ingredient in many insecticidal soap products. This product is sprayed directly from the bottle, without dilution.

One concentrated insecticidal soap product has a 47-percent concentration of potassium salts of fatty acids. Its label says to mix four teaspoons (0.67 fluid ounce) per quart of water, so one 16-fluid-ounce container of concentrate can be used to mix about 24 quarts of spray solution.

If it costs $9 for 32 fluid ounces (one quart) of the RTU product and $18 for 16 fluid ounces of the concentrate (which can be used to make 24 quarts of spray solution), the concentrate is a better deal if you expect to use more than a couple of quarts of spray over time. If you just expect to use a small amount of spray solution, the smaller volume and convenience of the RTU product may make more sense for you.

While more concentrated products often cost less per amount used, that isn’t always the case, so compare product prices and concentrations of active ingredients (in pesticides) or nutrients (in fertilizers) for yourself and see what makes the most sense for your situation.

There are also other things to take into account when choosing a pesticide. For example, one herbicide product may include a surfactant, while a surfactant may need to be added to another product with the same active ingredient. Also, some pesticides with the same active ingredients are not labeled for the same purposes. This type of information is available on the product label. Read and follow label instructions when using any pesticide.

Let me know if you have questions.

Dr. Mary Helen Ferguson is an Associate Extension Agent with the LSU AgCenter, with horticulture responsibilities in Washington and Tangipahoa Parishes. Contact Mary Helen at mhferguson@agcenter.lsu.edu, 985-839-7855 (Franklinton) or 985-277-1850 (Hammond).