If you want to know how healthy your soybeans are at this stage, take a shovel or large garden trowel and head to the field. Dig up some plants carefully and examine the roots. You should find nodules. That's where Rhizobia bacteria live that pull nitrogen out of the air and provide it to the plant. In exchange, the plant provides nutrients to help the bacteria survive and thrive.
First, determine If you have an adequate number of nodules on each plant. You may want to check several plants by digging them up and inspecting the roots. "Determine what growth stage the plant is at first," says Shaun Casteel, Purdue University Extension soybean specialist. "If the plant has two sets of trifoliates unrolled with the tips not touching, then it is a V3 plant. The 'V' simply stands for vegetative."
If it has three trifoliates, it has three sets of leaves and is at V3 stage, he adds. "You should find twice the number of nodules on the roots as the number of trifoliates above ground," Casteel says. "So if you have a three trifoliate plant, you would expect to find at least six nodules on roots. On a V5 plant there should be 10 nodules on roots."
Sometimes you will find many more than just twice the number. Because you find them doesn't mean they are all working yet, pulling in nitrogen from the air, Casteel cautions.
If nodules are brown and mushy, he says, they are dead. That's not a good sign, although he has found some in fields slow to take off due to weather conditions so far this growing season.
Nodules which are developing but still immature should be white inside. That means they are healthy and working toward becoming active.
"What you want to see are nodules that are pink when you crush them," he says. "Nodules that are pink inside are producing nitrogen for the plant."