young soybean plants in the field
WHAT'S THE STAND? Can you tell the stand count from this snapshot? It could be anywhere from 50,000 to 200,000. Using a hula-hoop, this stand was determined to be about 106,500 plants per acre.

Get quick soybean stand counts in the field

A new cellphone app calculator makes coming up with populations easy.

You’re not sure if soybean stands are what you anticipated. How can you estimate them?

There are two tried-and-true methods. One involves using a child’s toy: a hula-hoop. The other method boils down to counting plants within certain lengths of row.

Both methods require math. Using a hula-hoop requires access to tables that have factors for multiplication based on hoop diameter.

The new cellphone scouting apps released by Purdue University’s Crop Diagnostic Training and Research Center feature calculators that simplify several processes, including estimating soybean stands. Corey Gerber, center director, says the apps are available for both iPhones and Android models. There’s a Soybean Field Scout app and a Corn Field Scout app. Each is available at the app store for $5.99.

Here is a closer look at the two methods for estimating stands:

The hula-hoop method breaks down into these steps: Toss the hoop in at least five randomly selected locations; count the number of plants within the hoop at each location and find an average number of plants per hoop; multiply by a factor that represents the fraction of an acre inside the hoop.

You can find the correct factor in the Purdue Corn & Soybean Field Guide, still published in print form, Gerber says. Here are the factors for common diameters: 18 inches, 24,662; 21 inches, 18,119; 24 inches, 13,872; 27 inches, 10,961; 30 inches, 8,878; 33 inches, 7,337; and 36 inches, 6,165.

For example, if you find 12, 14, 14, 12 and 8 plants within a 30-inch-diameter hoop at five locations, the plant average per hoop is 60. Average plants per hoop is 12. Multiply 12 by 8,878, and your estimate is 106,536 plants.

If you have the Soybean Field Scout app on your phone, tap on "calculators" in the menu, select the "determining plant populations" calculator, and then choose the hula-hoop method. Enter a hoop diameter of 30 inches. Enter 12, the average number of plants. Tap the evaluate button. Your answer is 106,542 plants. You’re done!

Note that the two answers are off by a handful of plants per acre. That may involve rounding decimals. For all practical purposes, the estimates are the same.

The linear-row method is offered in the Soybean Field Scout app calculator. The app requires you to fill in row width, inches of row, number of rows counted and number of plants counted in that area. Suppose you’re in 15-inch rows. You are counting 24 inches of row across four rows. You count a total of 25 plants in this 2-foot section of four rows. Hit "evaluate." The estimate is 108,900 plants per acre. 

Repeat this process in other areas at random. Even a difference of one plant per area can impact population. For example, if you enter 24 vs. 25, the estimate is 104,544 plants per acre, or more than 4,000 plants per acre less.

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