NRCS – Texas 1 

Technical Note No: TX-PM-10-04 

United States Department of Agriculture

Natural Resources Conservation Service

Technical Note No: TX-PM-10-04 

June 2010

Calibrating a Seed Drill for Conservation Planting

Plant Materials Technical Note

 Photo by Shelly Maher, E. Kika de la Garza PMC

Introduction

Many species of grasses, forbs and legumes are planted to help solve resource problems. Some 

examples include seeding highly erodible cropland to permanent vegetative cover, seeding 

forages for hay or pasture, establishing permanent perennial vegetation on rangeland, and 

planting critical areas such as roadsides or grassed waterways. To achieve a successful seeding, 

an important step is the calibration of the seeding equipment so that the recommended 

amount of seed is uniformly planted.

Seeding rate recommendations are often given in pounds of Pure Live Seed (PLS) per acre. PLS 

seeding rates must be converted to bulk pounds per acre in order to calibrate planting 

equipment. Planting equipment varies considerably among cooperators so it is important to 

become familiar with the equipment being used.
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Technical Note No: TX-PM-10-04 

General rules of thumb are used in setting PLS seeding rates. Seed mixtures commonly have 

both large and small seeded species. The following rules are used to establish those rates:

Large seed <500,000 seeds /lb calibrate at 20 seeds/sq. ft.

Small seed >500,000 seeds /lb calibrate at 40 seeds/sq. ft. 

Critical area plantings use 2 times the drill seeding rates shown above.

The following are several different methods for calibrating a drill:

• Determine the seeds per foot of row. 

• Run the drill a given distance, weigh the seed dropped, then convert to pounds per acre.

• Determine the drive wheel circumference, turn it a given number of times, weigh the 

seed dropped, then convert to pounds per acre.

• Fill the drill with a weighed amount of seed, seed a measured area, reweigh what is left 

in the drill, then calculate the pounds per acre.

• Consult the seed chart on the inside of the drill cover.

Most people try to use the handy chart on the inside cover of the drill because it lists some 

species along with their seeding rates and drill settings. The charts work well for crops like

wheat and sorghum. However, the charts DO NOT work well for conservation mixtures. These 

mixtures have different sized seeds which flow differently and also tend to stratify. This 

requires combining species of similar size in each seed box as well as adding a carrier such as 

rice hulls, sand, or kitty litter in order for the drill to operate properly.

Preliminary Steps Before Calibration

Become familiar with the drill being used. Perform necessary maintenance and check to ensure 

the drill is in proper operating condition. You can expect to find rusty openers as well as 

overzealous greasing and oiling that will interfere with seed flow or the tubes may be clogged 

with nesting materials from field mice. Check drill gates or flaps to ensure they are in the 

proper position for the size and kind of seed being planted. Start calibration from an open 

position before closing down to the desired position.

Determine the bulk seeding rate per acre. PLS information (purity and viability) will be on the 

seed tag or seed analysis report. Based on the PLS seeding recommendation from the Texas 

Field Office Technical Guide, determine the PLS seeding rates for the mixture.

To calculate bulk seeding rate use the following formula:

PLS pounds per acre divided by percent PLS = Bulk seeding rate per acre
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Technical Note No: TX-PM-10-04 

Example:

A seeding recommendation for a switchgrass/purple prairie clover mix is set at 2 pounds PLS/ac 

switchgrass and 2.5 pounds PLS/ac purple prairie clover. Percent PLS for switchgrass as 

determined from the seed tag is 85% and for purple prairie clover it is 90%.

Switchgrass:

2 pounds PLS per acre divided by 85% PLS = 2.4 bulk pounds per acre

Purple prairie clover:

2.5 pounds PLS per acre divided by 90% PLS = 2.8 bulk pounds per acre.

SEED WEIGHT-DISTANCE

Calibration Methods

First compute the area of a 100 foot test run based on the width of the drill to be used for the 

seeding.

Drill width (ft) x 100 foot test run

 43,560 sq.ft./ac

= acreage of test run

Example:

10 feet drill x 100 foot test run

 43,560 sq. ft./ac

= 0.02 ac

Next calculate the amount of seed mixture required for test run

Purple prairie clover 2.8 bulk pounds per acre

Total 5.2 bulk pounds per acre

Switchgrass 2.4

5.2 x .02 ac = 0.1 pounds of seed mix for test run

Then

Pounds of mix for test run

Number of tubes on drill

= pounds of mix per tube

0.1 pounds of seed mix for test run

 10 tubes on drill

= 0.01 pounds of mix per tube

Since a small amount of seed is being weighed, it is desirable to convert the amount into grams 

(0.01 pounds x 454 grams/pound = 4.54 grams per tube for a 100 foot test run).

Measure and stake 100 feet of linear distance. Run the drill for at least 6 feet before initiating 

the test run in order to “prime” the drop tubes. Remove the drill tubes from the double disk 

openers and place a baggy or whirlpack under the tube spout to collect seed. Drive at a 
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Technical Note No: TX-PM-10-04

constant speed (the same speed that seeding will take place) during the test run. Measure a 

minimum of 2 tubes on each side of the drill. Stop and weigh seed from each tube and 

compare to calculated amount. Make necessary adjustments to the drill openings. Repeat the 

above procedure until seed weight coming from the tube is within 10 percent of the desired 

rate. Repeat the test run at least twice once calibration is set to desired setting. 

A variation to this method, known as the catch-all method, involves removing all the tubes and 

collecting seed from all of the spouts during the calibration trial. The target rate would be the 

total pounds of seed mix for the test run as shown above (0.1 pounds).

WHEEL CIRCUMFERENCE

This method involves measuring the drill drive wheel circumference in feet and determining the 

number of revolutions the wheel must turn to cover a pre-determined length of run (usually 

100 feet). This method only works on drills where the drive wheel can be turned by hand 

without pulling the drill. A variation of this method that also works quite well is to count the 

number of revolutions the drive wheel turns in 100 feet. It is extremely important that the air 

pressure in the tires on the drill be maintained at recommended levels during both the 

calibration and the actual seeding. If a consistent air pressure is not maintained, actual seeding 

rates can be different than the rate calibrated.

Determine the area covered in 100 feet:

Drill width (ft) x a 100 foot test run 

43,560 sq.ft./ac

= acreage of test run

Example:

a 10 foot drill x 100 feet

43,560 sq. ft./ac

= 0.02 ac

Once the distance of the test run is determined, you will need to measure the circumference 

(perimeter) of the drive wheel and determine the number of revolutions the drill drive wheel 

must turn to equal the distance of the test run. The calculation for determining the number of 

rotations is:

Distance of test run (feet)

Wheel circumference (feet)

= number of drive wheel rotations for test run

Example: 

100 feet

4.7 feet

= 21.3 wheel rotations 

Note: measurements of test run length and wheel circumference should be to the nearest tenth 

of a foot to insure reasonable accuracy.
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Technical Note No: TX-PM-10-04 

Remove tubes from the double disk openers and turn the drive wheel until all of the tubes are 

dropping seed uniformly in order to prime all the drop tubes. Place a baggy or whirlpack under 

the tube spouts to collect seed. Turn the drive wheel the number of rotations that were 

calculated. Remove baggies and weigh collected seed. 

Calculate the amount of seed mixture required for test run

Purple prairie clover 2.8 bulk pounds per acre

Total 5.2 bulk pounds per acre

Switchgrass 2.4

5.2 x .02ac = 0.1 pounds of seed mix for test run

Determine the amount of seed that should be delivered through each tube during the test run.

Pounds of mix for test run

Number of tubes on drill

= pounds of mix per tube

Example:

0.1 pounds of seed mix for test run

 10 tubes on drill

= 0.01 pounds of mix per tube

Since a small amount of seed is being weighed, it is desirable to convert the amount into grams 

(0.01pounds x 454 grams/pound = 4.54 grams per tube for a 100 foot test run). Compare this 

amount to the amount collected and weighed during test run.

As with previous method, make adjustments to the drill and repeat the above procedure until 

seed weight is within 10 percent of the desired rate. Check results with two or more trials at 

the same setting. If during the calibration process one species appears to be consistently short, 

check to make sure the seed mixture is thoroughly mixed. Be sure to also record the drill 

settings for future reference.

For additional information please contact: 

E. “Kika” de la Garza Plant Materials Center 

3409 N FM 1355

Kingsville, TX 78363-2704

361-595-1313

www.tx.nrcs.usda.gov/technical/pmc/kingsville.html 

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