All row crop planters are easy to calibrate using the mobile method because for each row spacing and planting rate there is a unique spacing for the seeds in the row. To complete a mobile calibration of a row crop planter seeds are added to the hopper, the planter is lowered into the ground and driven for a short distance to insure the system has stabilized. Next carefully uncover several seeds in a row and measure the distance between the seeds. The average distance is used to determine the seeding rate.
Problem: The calibration of a row crop planter, set for 20 inch rows, determined soybean spacing of 2.24, 2.23, and 2.25 inches. What is the planting rate (sd/ac)?
Solution: First determine the mean distance between seeds in the row.
/sd\ 1 sd 12 in 43560 ft2 1 row 12 in R — = - x - x - x - x ac 7 2.24 in 1 ft 1 ac 20 in 1 ft
Sometimes it is useful to know the in-the-row seed spacing for the planting rate. This can also be determined using by using units cancellation.
Problem: What is the spacing in the row (in/seed) for 40,000 plants per acre when the planter plants rows 28.0 inches apart?
Solution: Using units cancellation:
in 1 ac 43,560 ft2 144 in2 1 row x - x -t— x seed 40,000 seed 1 ac 1 ft2 28 in
Is this the correct answer? The units in the answer do not match the desired units. It is correct because it is common practice to drop off the unit of row in the answer and just use in/sd.
When doing a few calculations it would be appropriate and preferred to complete the problem as in this example by using the units cancellation method. This would not be true if the in-the-row spacing needed to be calculated many times. In this situation it is easier to use an equation. In Chapter 1 the source of many equations was identified as units cancellation calculations where the consistent numbers were reduced to a constant. Combining the consistent units in the previous problem (43,560 ft2 x 144 in2) produces the constant 6.273 E6. The seed spacing in the row can be found by the equation:
POP X RS
where SS = Seed spacing in the row (in); POP = Population, planting rate (seeds/ac); RS = Row spacing (in).
If the seeding rate is not correct, further evaluation of the planter must be made. Because the metering unit is ground driven, the source of the error may be in the drive train ratio between the drive wheel and the metering unit.
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