We tested them several times. They did detect plant stress. The idea was if the plant was stressed or chlorophyll levels were lower It would apply more nitrogen. we determined in our testing was that 75% of the time or more nitrogen was not the answer. It was other environmental issues, causing the stress in the plant and was just not able to uptake nitrogen that was already there. Adding more N didn’t pay and cutting N based on positive readings cost us by season end.