Mathematics makes for an efficient landscaping project

March 17, 2005
Victoria County Master Gardener Intern

The Victoria County master gardeners will devote educational project time and coverage to turf and water issues over the next few months. Look for this information on a periodic basis in upcoming issues - and contact us at with questions and comments.

Every gardener needs to be at least part mathematician because much of successful gardening calls for an understanding of weights and measures. You might say, "Oh, I can't do math!" But simple calculation of numbers is essential for a safe and productive landscape or garden, whether applying organic or chemical pesticides, fertilizer or water.

The area of a lawn or garden in square feet is the most used and useful measurement of all. With most all lawn and garden product application rates given per 100 or 1,000 square feet, determining size or area to be treated is extremely important and rather easy to calculate. Using a measuring tape, determine the length and width of your property, then multiply the length times the width to get the total square footage. Next, measure areas on your property that are not to be treated, deducting them from the total square footage. The result is total lawn area.

Follow the example included on the accompanying diagram for a typical 80-foot x 100-foot home lot. Note that you start with 8,000 square feet but only have 4,030 square feet of treatable area. Complete the same calculations for your landscape area and record the results, putting them in a safe place for future reference.

Now that your math is complete and landscape size is determined, applying the numbers further will illustrate how useful they can be. Although mid-April, and not Easter, is the appropriate time to fertilize the grass, it is a good idea to plan ahead so the proper amount can be purchased. A 21-7-14 fertilizer is recommended for our area. The maximum recommended rate is 1 pound of nitrogen (the first number on the bag) per 1,000 square feet, with less being better as excessive rates can lead to disease problems.

A simple formula to be used with the example provided is 1 (pound of desired nitrogen per 1,000 square feet) x 4,030 (area to be fertilized), divided by 0.21 (percent of nitrogen in fertilizer) x 1,000 (square feet). The only numbers you may need to change are the area to be fertilized and the percent nitrogen in the fertilizer.

So these calculations call for only 19.19 pounds of fertilizer for your entire lawn, or 4.76 pounds per 1,000 square feet. This means that a 40-pound bag should last for two applications, in April and possibly in June or again in October. Buying and putting it out beyond this rate is excessive and costly.

So now that we know the pounds of fertilizer needed, let's next look at the two most likely application spreaders. A drop spreader has the advantage of applying a fairly exact pattern since the area of application is between the wheels. The cyclone (rotary or centrifugal) spreader has a wider and less uniform pattern compared to a drop spreader.

Spreader calibration involves measurement of the fertilizer output as the spreader is operated over a known area. Start by checking the spreader to make sure all the parts are working properly. Items you will need include fertilizer, measuring tape, a scale, a container to hold the fertilizer, broom and dustpan, calculator or pen and paper. Know your desired application rate - remember from the figures above that we calculated 4.76 pounds per 1,000 square feet.

First, measure the width of the drop opening on your spreader. Next, mark off a distance on a flat, smooth surface, which when multiplied by the width of your spreader will give 100 square feet - such as 2 feet x 50 feet = 100 square feet. Because 100 square feet is 1/10 of 1,000 square feet, watch to collect 1/10 of 4.76 pounds (0.476) or just under 1/2 lb. of fertilizer. Fill the spreader with the material you wish to apply and begin application at the lowest setting and proceed to higher settings.

To begin calibrating, open the spreader just before you enter the calibration area and close it after you pass the other end of the calibration area. Sweep up or collect on a plastic sheet all of the material dropped within your calibration area and weigh and record for that particular setting. Repeat this at different settings until you find the correct setting for the amount of material that you need to apply.

The easiest method, and one avoiding having to sweep, is to attach a catch pan under the spreader opening and catch the material in the catch pan during the test runs, weighing the material after each test run for different settings.

For rotary spreader calibration, first determine the width of throw pattern for the rotary spreader. This is done by walking at a normal speed and opening the hopper on the spreader with the desired material inside. Fill the spreader about half full with the material you plan to use. Make the application on plastic or a hard surface where the width of throw can be measured. Since rotary spreaders do not apply a constant amount of material across the entire width of application and the effective coverage width is about two-thirds the full width, a general recommendation is to overlap all passes by one-half the width of the distribution.

To calibrate a rotary spreader, mark off a distance, which when multiplied by the effective width (2/3 full width) will give you a 1,000 square foot area. Fill the hopper with a known weight of material and adjust the spreader to the lowest setting, which will allow the material to flow. Push the spreader down the middle of the test area, opening the hopper at the start line and closing the hopper at the finish. Collect the dispersed material or weigh the material left in the hopper and deduct that weight from the beginning weight to determine the amount used for 1,000 square feet.

As before, repeat at different settings and record the material applied at each setting. Select the setting that most closely applies the desired rate of material. To obtain a more uniform spread of material, set the spreader at half the desired rate and make two passes at 90-degree angles to each other. This will give a fairly consistent rate of application over the entire area. Strive for proper spread overlap during application as illustrated in the bell-shaped graph that accompanies this column.

These same calibration procedures can be used for applying other materials. Since the quantity applied depends upon the material's physical properties, the same settings cannot be used for different materials, even if the pounds are the same. Once the spreader is calibrated and set for the proper rate, any size area can be treated accurately but it is always good to double-check.

By following these tips, you will hopefully apply a little lesson in math to your lawn. For further information, contact the Extension Office for the Spreader Calibration for Turfgrass brochure No. L-5330. Stay tuned for next week's article on when to water your turf.