Chubby Checker twisted his way to fame with a song 'Let's Do the Twist' decades ago. The hula-hoop became a stable as a toy and activity for kids. About two decades ago when farmers converted to narrow-row soybeans, it also became a staple for use in estimating soybean stands. This was before yield monitors, auto-steering and many GMO advancements. Yet the inexpensive, department story, garden-variety hula-hoop is still worthy of a spot in your pickup truck bed this time of year.
You'll also need a copy of Purdue University's Corn & Soybean Field Guide to help you interpret what the counts mean once you roll the hoop several times in a field, let it fall, and then count the seedlings emerged within the hoop. This popular Guide, published by the Purdue Diagnostic Training Center, has carried information on the hula-hoop and how to use it to calculate soybean stands for many years.
First, you need to know the diameter of the hoop in inches. A 30-inch hoop is standard, but you need to check it. And if you no longer have kids and hula-hoops around the house, you can make a circle out of stiff wire and do the same thing. Just make sure that you know the diameter. The Guide contains conversion factors to take number of plants counted to final population for hoops ranging from 18 to 16 inches in intervals of 3 inches.
Here's how it works. Roll the hoop at random in several areas within the field. Once it lands, count the number of plants within the hoop. Use your own judgment based on Shaun Casteel's advice about stand establishment to determine if a soybean just emerging should count if the rest are already at least a growth stage ahead. Casteel is a Purdue Extension soybean specialist.Suppose you find 24 plants as an average in the field using the 30-inch hula-hoop. The conversion factor according to the Guide is 8,878. Multiply the average number of plants found inside the hoop by the factor. The total is roughly 213,000 plants per acre. By today's standards, that's a very thick sand in narrow rows. If you find a stand that thick, Casteel would likely suggest that you reconsider if you need to be planting that thick.