How To Calibrate Your Spreader
Calibrate Your Spreader
The following is meant to serve as a simple, yet accurate approach for the use of material spreaders in general turf and landscape applications. This is written for the groundskeeper who needs to apply product properly without spending excessive time becoming laser-focused on the details. More scientific (and a little more accurate) approaches can be found in the resources at the end of the article.
Why Take the Time to Calibrate
You’ve gone through the trouble and expense of buying your product. Apply it incorrectly, though, and you are wasting both money and time. Even worse, you could do a great deal of harm. Too much fertilizer will burn your turf; too little pre-emergent will still allow weeds to grow; too little grass seed won’t produce the results you were hoping for; too much seed can do the same. Considering the risks, taking a little bit of time to know how much material you need and how you are going to deliver that material accurately will be well worth it.
And no, you can’t just place your spreader at the setting recommended on the bag, or by your sales rep, or by your cousin who runs his own lawn care company. There are too many variables; material size and shape, traveling/walking speed, PTO speed for mounted spreaders, humidity, equipment differences, etc.
Know How Much Material You Need Per Area
There are two different types of areas you will be applying material to; the defined area that is easily measured, and the undefined, open-ended space with all kinds of nooks and crannies that you haven’t accurately measured. The first is likely an athletic field or an annual bed. The second would be general turf areas on your property. While the calibration of the equipment will be the same for both approaches, your understanding of how much material needs to be applied to either area will be slightly different.
Athletic Fields or Landscape Beds
You should be able to very accurately measure the area of your soccer field or your annual beds, and therefore know exactly how much material to apply to each.
If a soccer field is 300 ft long by 180 ft wide, you have 54,000 sq ft (300 ft X 180 ft) to apply material to. Once you determine how much material to apply per 1,000 sq ft (more on that below) you take that amount x54 and make sure that amount gets evenly distributed across your field.
Other areas won’t be so well defined, or maybe you don’t want to take the time to measure all the little flower beds or turf areas on your property. That’s alright. Take rough measurements or your best guess when you order. You will still figure out how much material to apply to 1000 sq ft and then apply that rate throughout the area.
Some material, like grass seed or a granular pre-emergent will have labeled rates of application. Snapshot 2.5 TG, for example, has a label rate of 2.3 to 4.6 lbs. per 1,000 sq ft. Easy enough – I’ll tell you what to do with that rate in a minute.
Other material rates aren’t so straight forward. Fertilizers can be particularly tricky (please seek qualified advice from your local extension office or other trustworthy source to determine the nutrient needs of your target plant at your specific site). In turf applications, you will usually be measuring the Nitrogen content of a fertilizer. Most fertilizers will indicate a three-number code that identifies (1) the percentage of nitrogen as elemental N, (2) percentage of phosphorus (P) as phosphate, and (3) the percentage of potassium (K) as potassium oxide.
Lets say, for example, that you want to apply 1.0 lb. of N per 1,000 sq ft of turf. Your fertilizer bag has a formulation of 25-0-15. That means that 25% (or .25) of that fertilizer is nitrogen (N). Or, one lb of fertilizer has .25 lb of N. To determine the amount of material per area you calculate:
lb of N per 1,000 sq ft ÷ % of N = lb of fertilizer per 1,000 sq ft
Or in our example:
1.0 lb of N per 1,000 sq ft ÷ .25 = 4.0 lb fertilizer per 1,000 sq ft.
Great, now you know you need to apply 4.0 lbs. of material for every 1,000 square foot of turf. Now you have to get your spreader to do just that.
Note: If you want to apply fertilizer in two different directions to minimize mistakes, cut your application rate in half (2.0 lb fert./1,000 sq ft in example above).
Know Your Spread Patterns
Before you calibrate, it is important to know how your spreader is delivering the material to the ground. These are important nuances and, if not understood, could result in misapplication.
The most straight-forward spreader. It drops the same amount of material across its entire width.
The most common type of spreader, utilized as walk behind, hand-crank, and equipment mounted. Because these use a rotating disk to spread material in an arc, more material is delivered to the ground closest to the spreader, while the amount of material decreases on the edges of the swath-width.
Their unique design allows pendulum spreaders to deliver a consistent amount of material across their entire swath-width. In this way, the spread pattern is very similar to the drop spreader, with material being delivered beyond the edges of the spreader.
Test and Measure
Now, lets get that spreader to put down the right amount of material.
First you have to figure out your 1,000 sq ft
- Find your effective swath width. This is how wide your spreader applies material.
- Find a test plot – if you use a parking lot, sweep up the material and apply to an appropriate place later.
- Put some material in your spreader.
- Pick your ground-speed and RPMs (if applicable) and move forward.
- Open the hopper
- For drop and pendulum spreaders: Mark the distance the material traveled on either side of the spreader.
- For rotary spreaders: Mark the edges where a decent amount of material landed, but do not include the very outer edges where only a few pieces of material land. Some material will travel beyond your effecive swath width with rotary spreaders.
- Measure your effective swath width. How far is it between markers?
- Divide 1,000 sq ft by your measurement in step 2 (remember, we are trying to find out how much material per 1,000 sq ft). For example, for an effective swath width of 15 ft (1000 sq ft ÷ 15 ft = 66.66 ft).
- Measure a course the length of your answer in step 3. For my example above, I have two cones set 66.66 ft apart.
Now that you have your course, let’s make sure we are placing our 4 lb of material down within that area.
- Set your spreader to the recommended setting on your product’s label or to the setting you believe will be closest to your target rate.
- Weigh a specific amount of product and place in the hopper. For my 4 lb/1000 sq ft example, I would place 8 lb of product into the hopper.
- Approach your first cone at the same speed you would take when applying the material to the rest of the property.
- Open the hopper when you arrive at the first cone.
- Close the hopper when you arrive at the second cone. You just applied material to 1000 sq ft. But how much?
- Weigh the remaining material in the hopper. In my example, if I had 3 lb left in the hopper, I would have applied 5 lb…1 lb too many.
- Adjust and re-test. In my example, I need to close my gate a bit and try again (if applying to turf, move the course to a new location keeping the same dimensions)
- Once you’ve run the course while applying the correct amount of material your SPREADER IS CALIBRATED!
Once you do this once or twice it really becomes fairly simple. Re-calibrate every time you change material or spreaders.
If your spreader model comes with a collection tray (especially useful for equipment mounted units):
- Record how long it takes you to move through your course
- With the tractor parked and the RPMs at the proper level, open the hopper for the time measured in the previous step.
- Weigh the material in the collection tray.
- Return material to hopper and repeat until the target weight is achieved.
Now you can apply with confidence that you are getting the right amount of material on the ground.
You still need to keep a few things in mind to protect yourself from under or over applying material
- Rotary Spreaders: Because the percentage of material applied decreases further from the hopper, it is necessary to overlap passes to achieve full uniformity. A rule-of-thumb is generally to overlap passes by 33% (or 1/3 of your current pass should overlap 1/3 of your last pass).
- Pendulum and Drop Spreaders: The opposite challenge exists for these spreaders. Any overlap at all will result in a 200% application. I suggest calibrating these spreaders at 50% target application rate and then making two applications in two different directions.
- Always maintain a constant speed, identical to the speed used when calibrating. If using a PTO driven spreader, keep a consistent RPM.
- Always open the hopper while moving to distribute material evenly.
- When using a walk-behind spreader, use the cover provided. I’ve spilled fertilizer next to a putting green before. Replacing sod was much more work than placing a cover over the hopper.
- Clean your spreader well after every use.
If you are wanting to dig deeper and be a little more scientific in your calibration, check out the following sites: