Calibrating a Fertilizer Spreader [video]

In this video, Extension field specialist, Margaret Hagen will describe how to calibrate a fertilizer spreader. See below for a full video transcript. 

Hello, my name is Margaret Hagen. I'm a Field Specialist for the University of New Hampshire Cooperative Extension. Today we're going to talk about calibrating your fertilizer spreader. And why do you want to calibrate your fertilizer spreader? There are a couple of different reasons. If you live near a body of water like a lake, stream or river, or if you live near a storm drain, calibrating your fertilizer spreader will ensure that you're putting down the exact amount of fertilizer you need without allowing runoff into a water body, or fertilizer leaching into groundwater. Calibrating a fertilizer spreader will also help keep you from applying excess fertilizer, and that will help you to save money.

The setting on which you put your fertilizer spreader is going to depend on the spreader itself, the product that you are using, and the speed at which you walk. There are two basic types of lawn spreaders. The first is a rotary spreader and the second is a drop spreader. When you're using a rotary spreader the advantage is that you can cover large areas at a very short period of time. This spreader throws the fertilizer in front and out to both sides. The drop fertilizer spreader drops the fertilizer straight down. The advantage to this is if you have a small space, or you're fertilizing along the edge of pavement or a driveway you won't put excess fertilizer where it's not wanted.

The easiest way to calibrate your fertilizer spreader is to choose a spreader and a bag that match. So, here we have a Scotts drop spreader. And, on every fertilizer bag you're going to find a little chart that lists spreader brands and names and it also tells you what spreader setting to use. So, for all Scotts drop spreaders we know that we want to set our spreader at 7.5. This one is just a little bit low right now so we're going to put it up until it reaches 7 1/2. Today we're going to calibrate this fertilizer spreader to apply 9 /10 of a pound of actual nitrogen per 1,000 sq. ft.

When you go to the store to buy your fertilizer bag it's hard to know how many bags you actually want to buy, so before you do that you need to know what the square footage of your lawn is. To know that you can use a fact sheet 'Does my Lawn Measure Up'  to help you do that. This bag will cover about 10,000 sq. ft. which is about a quarter of an acre. So based on how much square footage your yard has you can calculate how many bags you need to buy. The analysis on this fertilizer is 18-0-3. That means that there is 18% nitrogen, no phosphorus, and 3% potassium. The other thing to know when you buy your fertilizer is what percentage of the fertilizer analysis on nitrogen is slow-release. In the Spring and the Fall you want at least 25% of the bag to be slow release nitrogen. It will release over a time period of 6 to 15 weeks and ensure that you have a long feed on your lawn.

The first thing to do is get a soil test done. For a reasonable fee UNH Cooperative Extension will analyze your soil. The results will contain your soil pH and certain nutrient levels from which lime and fertilizer recommendations can be made for your lawn. To calibrate any fertilizer spreader you need to know how many pounds of fertilizer you want to put down per 1000 sq. ft. We know in New Hampshire that we can put down 0.9 or 9/10 of one pound of actual nitrogen per 1000 ft. in any given application. So we take that 0.9 and divide it by the 18% nitrogen in the fertilizer bag. So 0.9 divided by 0.18 equals 5. Now we know that we can put down 5 lbs. of fertilizer per 1,000 sq. ft.

Next, we need to weigh out the fertilizer to be used to calibrate the spreader. 10 lbs. in this case. Then, load the fertilizer into the spreader. Next we need to establish our test area.Usually it's good to have a large test area to get the most accurate results. So a 50 or 100 ft. long strip is best. In this case we've used bricks at the beginning and the end to mark a 50 ft. long strip. The next thing you want to do is make sure that you stay back from the beginning, or your start line,  and start walking there. And only open your fertilizer spreader when you reach that line. At the end of your test strip you want to close the spreader as you cross the line and then walk a few more steps. Here we go.

When using a rotary spreader like this one, which throws to both sides as well as in front, you need to measure the width of the fertilizer that's dropped in order to determine your ultimate square footage. So, what you do is just take your tape measure. And you can see lots of fertilizer granules here. You just keep measuring until you don't see any more on your left. In this case the width turns out to be about 10 ft. Remember, we want to apply 0.9 or 9/10 of a pound of fertilizer per 1,000 sq. ft. using a fertilizer with 18% nitrogen. If we do the division we get 5 lbs of actual fertilizer that we want to apply over 1,000 sq. ft. We know that we have a 500 sq. ft. test area which is approximately half of 1,000 sq. ft., the area over which we want to apply the fertilizer. If we know, after weighing the fertilizer that's left,  that we applied 2 1/2 pounds of fertilizer in the 500 sq. ft. test area,  then by multiplying our fertilizer rate by 2, we'll find out how much fertilizer we would be spreading in a 1,000 sq. ft. area at this fertilizer setting. Two and one half times 2 equals 5. The amount of fertilizer we want to spread per 1,000 sq. ft. So this setting is spot on and our spreader is calibrated. So now we know that using this particular fertilizer setting and this particular fertilizer product and this spreader that we're putting down 9/10 of a pound of actual nitrogen fertilizer per 1,000 sq. ft. So we're spot on.

If you weigh out, and you have a little bit too much fertilizer then you want to back your setting down just a little bit. And if you find out that you haven't put down enough fertilizer then you can up your setting just a little bit. Usually it only takes 2 or 3 tries at the most to get your fertilizer spreader calibrated. One more thing --- if you're nervous about applying too much fertilizer you can apply 1/2 the recommended rate at one time. Wait for the response on your lawn for 3 or 4 weeks, and if it's not green enough where it doesn't satisfy you, then you can apply the other half of the recommended rate. This is especially useful to know if you do live near a water body or a storm drain.

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