Calibrating Fertilizer Applicators
Fertilizers, which are mainstays of modern agriculture, are applied in liquid, gaseous, or granular form. In this section we will discuss the calibration of granular
applicators. The application of liquid fertilizers follows the same principles as spraying chemicals and sprayers are discussed in a separate section. Gaseous fertilizers require specialized equipment and are very hazardous to use. The equipment to dispense gaseous fertilizers is not included in this text.
Most granular fertilizer applicators will be one of two designs, broadcast or gravity flow. As with any agricultural machinery, designs vary, but a common tractor powered spreader consist of a hopper that holds the material, a metering mechanism, and a spreading mechanism, Figure 8.1. The hopper capacity will vary depending on the size of the machine. Metering usually is accomplished through the use of a variablespeed chain belt in the bottom of the hopper and an adjustable opening. The chain belt generally is operated by a drive train powered by one of the wheels of the spreader. Because a ground wheel drives the chain belt, the amount of material dispensed changes as the speed of the spreader changes. The impellers are usually powered by the tractor PTO and must turn at a constant speed for the spreader to have a consistent dispersal pattern.
In this design the application rate of the material is set by changing the speed ratio of the drive train that powers the metering device and/or changing the size of the opening at the metering gate. The process of using a variable opening for metering is called bulk metering. Bulk metering units meter the volume of material applied. The material usually is spread as it drops onto one or more rotating disks, impellers, which sling it out in a wide pattern either behind or in front of the spreader. An alternative design powers the impellers from one of the ground wheels, eliminating the need for a PTO or additional power source.
Gravity flow applicators also use a hopper, Figure 8.2, but in this type of applicator the hopper extends across the width of the machine. The material is dispensed through adjustable openings along the bottom of the hopper. The rate of material flow is set by the size of the opening. This design uses an agitator along the bottom to prevent the material from bridging across the hopper and to break up lumps, thus improving the uniformity of flow. The material flows through the openings and onto the soil surface.
At one time gravity flow fertilizer spreaders were popular in agriculture, but limits on practical widths has almost eliminated agricultural uses. Small ones are still used for lawn care and other situations where 18 to 24 inch widths can be effective.
Both types of spreaders may be calibrated using either the stationary or mobile method. For both methods, the application rate, R, (lb/ac) is equal to the pounds of material collected per revolution of the drive wheel, divided by the area covered. The area covered is determined by multiplying the width (w) x distance traveled. The distance traveled is determined by calculating the perimeter of the wheel (2 nr) and multiplying it by the number of revolutions of the drive wheel (nr). In equation form this relationship is:
R = nL. This can also be expressed as: R = W x —
n where R = application rate (lb/ac); W = weight of material (lb); nr = number of revolutions of drive wheel; A = area (acres).
To calibrate the spreader using the stationary method, the drive wheel for the metering mechanism is elevated and the material is collected as the drive wheel is rotated a predetermined number of turns. The weight of the material collected divided by the number of turns of the drive wheel equals the pounds per revolution. Acres per revolution are determined by multiplying the circumference of the drive wheel by the number of turns and the effective width of the spreader, and then converting this area to acres, or:
Wr(rev) 43,560 — ac where n = 3.14; r = Effective radius of drive wheel (ft); nr = number of revolutions of the wheel; w = Effective width of the spreader (ft); 43,560 = Number of square feet per acre. Combining the unchanging units to form a constant gives:
—— =x r x w = 0.0001442 x r x w or 1.442 E  4 x r w nr(rev) 43,560
Therefore, when a spreader is calibrated in a stationary position, the pounds per acre can be determined by:
where R = application rate (lb/ac); W = weight of material collected (lb); A = area (ac).
ac 1.4420 E  4 x r (radius) x w (width) x n (turns)
Problem: You set your neighbor's spreader to apply 1,200 lb/ac of fertilizer. During a stationary calibration of the applicator 160.0 lb of fertilizer were collected as the drive wheel was turned 15 revolutions. The effective radius of the drive wheel is 21.0 inches and the effective width of the spreader is 30.0 ft. Is the application rate correct?
Solution:
160.0 lb
1 ft
1 rev
0.1135575 ac
In this problem the spreader is applying 200 lb/ac more than the desired amount. Is this an acceptable level of accuracy? Should the spreader metering unit be adjusted and rechecked? No consistent standard has been established to determine the acceptable error for broadcast fertilizer spreaders. One way to evaluate the error is to calculate the percentage (%) of error. In this example the amount of error was 17% [(200 lb/1200 lb) x 100]. It would be up to the owner/operator to determine if 17% was an acceptable level of error.
The one major disadvantage of the stationary method of calibrating broadcast applicators is the problem of collecting the fertilizer during the calibration process (160 lb in this example). The quantity of material that must be collected will be greater if the desired application rate (lb/ac) is increased, or if the number of revolutions is increased. The primary advantage of this method is that it can be performed when it is impossible to have the applicator in the field, for instance, because of muddy ground or inclement weather.
The alternative is to complete a mobile calibration. This can be accomplished in two ways. In the first method, collectors (flat pans or pieces of plastic or tarp) are randomly placed in the distribution path of the spreader or an alternative is to place the collection devices in a row perpendicular to the line of travel of the spreader. After the spreader is driven over the collectors, the material is weighed. The application rate (lb/ac) is computed by dividing the amount of material collected, by the area of the collectors, and then converting the answer to the desired units of volume weight or mass divide by the area. An advantage of the perpendicular row of collectors is that the distribution pattern can be evaluated by plotting the amount collected from each collector on a graph.
Problem: What is the application rate of a spreader that deposited 23.5 lb of material on two tarps, each measuring 10.0 ft x 12.0 ft?
Solution:
ac^ A (ac) 2 x 10.0 ft x 12.0 ft x 1ac °.°05509 ...
43,560 ft2
[Note: This is an example where the number of significant figures in the answer could be ambiguous because of leaving a zero when rounding. To clarify the number of significant figures, the answer should be written as 4.260 E3.]
This method does not require a spreader wheel to be elevated, but you must be able to drive the spreader in the field or over an area as wide as the dispersal pattern. If the spreader is dispensing an excessive amount of material, the test area will be over fertilized.
Problem: Determine the application rate and distribution pattern for a broadcast spreader that was calibrated by placing 202 ft2 collectors perpendicular to the direction of travel. The desired rate is 800 lb/ac. Table 8.1 shows the pounds of fertilizer collected.
Solution: The first step is to determine the application rate by summing the quantity of fertilizer in the 18 collector pans (nc) containing fertilizer Wt. Then determine
Collector 
lb 
1 
0.000 
2 
0.008 
3 
0.014 
4 
0.016 
5 
0.020 
6 
0.023 
7 
0.030 
8 
0.038 
9 
0.050 
10 
0.062 
11 
0.062 
12 
0.060 
12 
0.057 
14 
0.052 
15 
0.048 
16 
0.041 
17 
0.035 
18 
0.026 
19 
0.017 
20 
0.000 
the total area of the collector pans containing fertilizer (At). The application rate is: lb\ Wt
Hu X Aq
0.659 lb
ac ft2 1 ac
18 collectors x 2
'collector 43560 ft2
0.659 lb
The spreader is applying 797 lb/ac. [Note: Only 18 collectors were used to determine the area because two collectors did not have any measurable material.] In this case the percentage of error was
This is an acceptable level of error. The calibration is completed by plotting the distribution pattern. A plot of the distribution is shown in Figure 8.3.
In the graph of the distribution the dashed line represents the ideal or acceptable distribution for an overlapping distribution pattern. The solid line is the distribution for this spreader. It is clear that this spreader has a problem. Even thought the
Fertilizer distributions
Fertilizer distributions
1 2 3 4i 5 6 7 8 9 10 11 12 13 14 15 16 17 16 19 20
Collector
FIGURE 8.3. Distributor graph for spreader.
1 2 3 4i 5 6 7 8 9 10 11 12 13 14 15 16 17 16 19 20
Collector
FIGURE 8.3. Distributor graph for spreader.
application rate is acceptable, the distribution of the material is not. The cause of the poor distribution should be determined and corrected before it is used.
Another mobile method can be used if it is certain that the spreader pattern is reasonably accurate. First, weigh the spreader, and then drive it over a measured area. Then reweigh the spreader. The difference between the before and after weights is the amount of material applied. The area is already known, so it is easy to find the application rate using mobile calibration equation.
Both of these methods can be used to calibrate a fertilizer spreader. The choice and the materials used are up to the operator.
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