# How to Calibrate a Grain Drill: Getting the Right Calculations for Planting

In this installment, farmer Jason Carter walks through his calculations step-by-step, after collecting and weighing the seed dispensed during 20 turns of the drive tire.

This is the fifth installment in our “How to Calibrate a Grain Drill” series where farmer Jason Carter walks through his method for calibrating a grain drill for multi-species cover crop seed. Visit our previous blog installments to find the introduction, supplies, and initial steps of Carter’s process.

After determining the circumference of the drive tire in feet in Video #2, he multiples that by the row spacing, then by the 20 revolutions turned on the drive wheel, which will equal the size of the test area in square feet.

1. Square Footage of Test Area=

**Circumference of Drive Tire (ft.) x Row Spacing* (ft.) x # of Drive Wheel Turns During Metering Test = sq. footage of test area**

*Row spacing in ft. = row spacing in inches / 12 inches per foot

“Our row spacing is seven and a half inches, but we need to convert that to feet. So, that would be 0.625 – we multiply that by our drive tire circumference of eight feet, and then we will multiply that by the 20 revolutions that we turned on the drive wheel, and that will equal the size of the test area in square feet. After we do those calculations, it comes up to 100.”

Jason then takes that number and multiplies it by the desired pounds per acre determined from the chart on his drill. He then divides that number by the amount of square feet per acre to get the lbs of seed per 100 sq. ft., then multiplies it by 454 g/lb to get the total weight (in grams) of seed needed to be collected to be equivalent to 25 lb/ac.

2. Calculate grams for the above Test Area

**Test Area (sq. ft.) x Desired Seed Rate (lb./ac.; determined in previous video) ÷ 43,560 sq. ft./acre = lbs. seed per 100 sq. ft. x 454 grams/pound = total grams of seed for 20-turn test area to be equal to 25 lb/ac.**

“We will take that 100 and multiply it by the desired pounds per acre which is 25. That comes out to 2500, then we'll divide it by 43,560 which is the amount of square feet per acre, and then that comes out to a .057321. To get a better number to work with, we're going to convert that into grams– so we'll multiply that number by 454 and that comes up to 26.05. So that means we need to collect 26 grams of cover crop seed in 20 turns of the drive tire to equal 25 pounds per acre.”

Though these calculations have come out to nearly a one-to-one ratio, Carter says this isn’t always the case – factors like differences in tire size can cause variation.

Next, he weighs the seed he collected in grams. In this instance, they weighed in at 27. Carter says that though this isn’t precisely 26, the number gleaned from their earlier calculations, that he wouldn’t try to adjust it.

“What I've noticed when you try to start getting in by one gram plus or minus, that it's very tough. We could do that same calibration or turn the wheel 20 more times, and the next mix might be 27, it might be 28; so I would leave it there. Now let's say it was 30 or let's say it was 22– we would either need to increase or decrease how far our metering roll is open at the end to try to get as close as we can to that 26 grams. So, remember, we're shooting for 26 to get to 25. I would like to always be maybe a little over then a little below, because usually when I order my cover crop seed, I usually get maybe five to ten percent more, cause I would always like to shoot for being on the high side than the low side.”

In Carter’s weigh-in, the weight of the seed collected was relatively accurate on the first try, but some farmers will need to adjust by increasing or decreasing their seeding rate. All drills are different, but on Carter’s Landoll drill, there is a nut on the far side that he can turn connected to a threaded rod that pushes the rod either inward or outward. When he threads the rod toward himself, the seed-per-acre increases, and to decrease, he turns it the opposite way, so that the rod is moving in toward the machine. On his drill, once reaching the desired rate, he closes down the jam nut to hold the adjusting rod in place.

**To watch the full "How To Calibrate A Grain Drill" Series ****CLICK HERE**

Keep following along at www.SoilHealthLabs.com for more up coming blogs discussing Carter’s final tips to calibrate a grain drill to multi-species cover.