It takes lots of time to take some 7,000 chlorophyll readings by hand with a hand-held chlorophyll meter. Yet that's what Don Robison and his brother, Dave, did this summer on an elaborate cover-crop plot on their farm near Greenwood. Both brothers also have full-time jobs off the farm.
"We wanted to see if the cover crops, once they were killed and broke down over the season, returned more nitrogen to the crop than just applying a set rate of commercial N by itself," explains Don, now with Beck's Hybrids.
What they found was that for most cover crops, you can detect an increase in both plant height and increased nitrogen level well into or completely through the growing season compared to the check plots where no cover crops were grown. This confirms the theory that once cover crops begin to break down, the nitrogen thy have captured from the soil will be released to the crop. It's an important finding since many farmers planted cover crops this fall, specifically to capture nitrogen left behind by a corn crop which couldn't reach it.
There were a couple of cases where the cover crop mixture by didn't cause increased chlorophyll readings later in the season during grain fill. But for the most part, there was an increase, Robison says. Chlorophyll meters actually measure the intensity of green color, which indirectly can be correlated to the amount of nitrogen in the plant. The greener the plant, typically the higher the nitrogen levels in the plant.
The chlorophyll levels and plant height were tracked in the plot from June 20 through July 26. Since the plot was planted in mid-May, this was the key time for pollination and early grain fill.
The bottom line is that cover crops will return extra N above what you apply to the soil as they break down, Robison concludes.