An Exercise in Calibration: Accurate Applications of Granular Products Without Rotary Spreader Calibration

If one knows their lawn area AND doesn’t mind a little exercise, granular fertilizers or pesticides can be accurately applied to lawns with a rotary spreader without going through all the steps and calculations of spreader calibration. This podcast will guide you through the process of applying these products.

A great question for Do-It-Yourselfers was recently raised at a Master Gardener Volunteer training event: Can a homeowner accurately apply products to a lawn with a rotary (broadcast) spreader and NOT go through the time and effort of calibrating the spreader to make the application of a granular fertilizer or pesticide product? The answer is an emphatic YES in what I usually call “lawn care exercise.” Let’s talk about how to do it and where the “exercise” comes in.

The most important thing to utilize this strategy of application, without calibration, is to first accurately measure/know the area of your lawn. There are several smart phone apps that are fairly accurate in measuring lengths and widths, and there are numerous GIS-derived maps on the Web that can be used if they are accompanied by an appropriate scale. The picture that accompanies this podcast shows how to do it the old-fashioned way by using basic mathematical formulas for calculating the area of rectangles, squares, circles, etc. You simply divide your lawn and landscape areas into the shapes that best fit different areas (as shown in the picture that features a lawn comprised of a rectangle, a square, a triangle, two trapezoids, and two circles). Now, it is highly likely you probably don’t remember a lot of the formulas off the top of your head, and you might need to remind yourself just what the value of π is, so don’t be too prideful to revisit your (or your kids’/grandkids’) math books for a refresher in the calculation of areas of various shapes, or check out a website such as Math is Fun that also includes a handy area calculator as part of their offering. You can take your measurements with a walking wheel, a measuring tape, or you can even get pretty close by pacing the area off if you practice a little in determining how long your pace is as you walk the area and record the various lengths and widths. 

Now, once you know your area to be treated, you determine how much product you need. Let’s say that the total area of lawn to be treated in a front yard is determined to be 5,000 square feet. Your goal is to apply 1 pound of nitrogen per 1000 square feet, so you need 5 pounds of nitrogen to treat the entire 5,000 square feet. The fertilizer source you have selected is 29-0-6, a source that is 29 percent-by-weight nitrogen (N), 0 percent-by-weight phosphate (P₂O₅), and 6 percent-by-weight potash (K₂O).  The nutrient of importance is N, so to determine how much product you need to deliver 5 pounds of total N to 5,000 square feet of area, you take the amount of total N required (5 pounds) divided by the percentage of N in the fertilizer product (in this case, 29 percent). 5 ÷ 0.29 = 17.24 pounds of 29-0-6 fertilizer. 

You will next weigh out approximately 17.25 pounds of the fertilizer and place it (or a fraction of it if you are using a small, hand-held broadcast spreader) in your spreader, being sure that you have selected a very low setting on your spreader that is not going to be delivering most of your fertilizer in one or two passes across the lawn! This is where the exercise comes into play: You keep walking back and forth across your lawn area in many different directions until the known amount of product has been delivered to the known area. It is not the most efficient way to use your time in applying fertilizer (i.e., a lawn care professional who treats many lawns is going to be using calibrated spreaders and sprayers), but for an individual homeowner not interested in calibrating spreaders, it is a pretty effective and precise means of delivering recommended levels of dry products to known areas of lawn.

This strategy works equally well with dry pesticide products that are going to be spread with a rotary (broadcast) spreader. Similarly, once you know your area and the amount of product to apply, you perform the same steps as described above with your chosen pesticide. The concept can even work with some liquid applications when using a backpack sprayer, but note that some chemical applications that use water as a carrier have specifications about required volumes of water to deliver the targeted rate of product to an area.  

If this all sounds like too much exercise (or time) for you, there are a lot of resources available to instruct one in how to calibrate spreaders and sprayers. Go to the Virginia Cooperative Extension website and search for Fertilizer Applicator Certification Training. Enrollment in this free training and testing program gains one access to a large variety of educational offerings in the steps and mathematics of environmentally responsible chemical applications.