Formulas

Rotary Spreader Calibration

In order to properly calibrate a fertilizer spreader you must keep a constant speed when either pushing or pulling the unit. Most fertilizer rates are based on the percent of nitrogen that makes up their analysis. For example, on any given back of fertilizer you will find an analysis that look like the example given.

Ex. 46-0-0 Urea    or    28-3-4

First you must determine the amount of nitrogen that is needed during application. The numbers on a bag of fertilizer tell the percentage of certain elements that are contained in the bag. The three major nutrients are Nitrogen, first on the analysis, Phosphorus, second on the analysis, and Potassium, third on the analysis.  The rate of application can be obtained from either soil sample recommendations, recommendations from an agronomist or recommended rates on the fertilizer bag. After the rate of nitrogen is determined, the amount of actual fertilizer applied can be determined. This is done by taking the desired nitrogen rate and dividing it by the total percentage of nitrogen contained in the bag, taken from the analysis.

Ex. If a bag of fertilizer says the desired rate is 1.5 pounds of nitrogen per 1000 ft² for a 46-0-0 urea fertilizer, then the amount of actual product to be spread is determined by taking 1.5 and dividing it by .46. This is done based on the percent of total nitrogen in the bag of fertilizer. You should be spreading a total of 3.26 pounds of fertilizer per 1000 ft² to recieve the recommended rate of nitrogen.
    For the 28-3-4 fertilizer at a 1 pound of nitrogen per 1000 ft² rate, you take 1 pound of nitrogen and divide it by .28, which is the percent of nitrogen found in that particular bag of fertilizer. You should recieve a determined rate of 3.57 pounds of actual fertilizer spread to recieve the recommended rate of nitrogen.
    For either of these, the rate of nitrogen per acre can be determined by simply multiplying the determined pounds of fertilizer to be spread by 43.56, this number is based off of the total number of times 1000 can go into 43560, the total square feet in one acre.

Now that you have deteremined the amount of fertilizer you need to spread, mark off a desired area of 1000 square feet. The easiest way to do this is to determine how wide your spreader is throwing fertilizer. The width of spread is divided by 1000, the total area needed for calibration. The number that you get from this is how far you need to measure from your starting point to finish point.

Ex. If your spreader is throwing fertilizer 8 feet wide divide 1000 by 8 feet to determine the length you need to measure in order to get a 1000 ft² area to ajust spreader settings. This should be 125 feet long. Since the spreader is throwing 8 feet wide you have a total area covered per pass of 1000 square feet.

Now that you have determined how many pounds of fertilizer you need to spread and have a 1000 ft² area measured out, you can make pass to determine how much fertilizer is being spread. You can either dump a pre-weighed amount of fertilizer in the spreader, make a pass with the spreader and then reweigh what is left. Or you may pour fertilizer into the spreader and weigh the entire spreader before and after each pass to determine how much is spread each time. Adjust the spreader accordingly until the determined amount of fertilizer is obtained each time you make a pass.