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Features Manure Minute
Manure basics: calibrate your manure spreader


December 27, 2019
By Chryseis Modderman

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Whether you apply liquid or solid manure, applicator calibration is an important step that is often overlooked. Taking a few extra moments – despite a busy schedule – to adjust your speed and settings will greatly improve your confidence that you haven’t over- or under-applied nutrients – and eyeballing it isn’t going to work. A study from Michigan State University found that only a quarter of farmers who didn’t calibrate were within 20 per cent of their desired application rate. If you’ve already done the work of soil sampling, manure nutrient testing and calculating application rate, why wouldn’t you calibrate your spreader to ensure you’re actually applying the amount you had planned?

You should calibrate every couple of years, or whenever manure consistency or equipment has changed. Note that the application methods described here are for broadcast applications of solid and liquid manure; manure injectors tend to rely on flow meters for accuracy.

How to calibrate

Calibration is a fairly simple process. The easiest method that works for both solid and liquid manures is to weigh the spreader before and after application to a small area. Measure the length and width of the area to which manure was spread.

If you don’t have access to drive-on scales, the tarp method can be used for solid manure. You’ll need a scale, a five-gallon bucket, and a tarp. First, weigh the empty bucket and tarp. Then, lay the tarp in the field with bricks or rocks to hold down the edges. Measure the length and width of the tarp to get spread area. Next, apply manure normally to the area with three passes (one directly over tarp, the other two on either side of the tarp) to account for overlap. Then, fold up the tarp and place in bucket. Weigh the bucket and tarp full of manure.

Calculations

Math time! For both the weight and tarp method, you just need to extrapolate the amount spread over the small area/tarp to amount per acre. To do this, you first need to figure out how much manure was spread over the small area/tarp. Subtract the after-application spreader weight or empty bucket/tarp weight (in pounds) from the full spreader or full bucket/tarp weight to get pounds of manure spread. Then, divide that number by the area spread/tarp area (in square feet). Multiply by 21.8 to get tons of manure applied per acre. Note that 21.8 is derived from 43,560 square feet per acre divided by 2,000 pounds per ton – a different number will need to be used for metric units. This method will work for solid manure.

For liquid manure, since it is applied in gallons per acre, not tons, you’ll need to do an extra step. To convert tons per acre to gallons per acre, you’ll need to find the density of your manure – yes, it’s different than the density of water. This is also fairly simple. Just weigh an empty five-gallon bucket (or any container of known volume), and then weigh it again full of manure. Subtract those numbers to find how much manure occupies a five-gallon space. Divide five gallons by the manure weight in pounds, and then multiply by 2,000 to get gallons per ton. Finally, multiply that number by the tons per acre you calculated earlier to get gallons per acre.

From here, you can decide to adjust speed or equipment settings to spread more or less
per acre.

It is a good idea to repeat the calibration process a few times to get a good idea of application rate. Make sure to record your settings for future reference. Happy spreading!


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