The three leaves in this picture came from three distinctly different situations. The leaf on the far left was dark green. The leaf in the middle was form a plant that was pale green. The leaf on the far right was actually from a plant that was already tuning yellow. Once leaves are pulled and compared side- by -side they don't look as different as they did on the plant.

Big difference: The leaf on the right measured much lower in nitrogen based on extrapolations from detecting green shades of color in the leaf.

Jeff Phillips, Tippecanoe County Extension educator, examined these leaves with an app form Spectrum Technologies. The Green Indexing app costs $100 – a shock to teenagers and young adults used to getting apps for free. But it and a $50 reference board also available from Spectrum Technologies combine to let you gather similar information to what you can get with a spectrometer costing more than $2,000.

Related: One Way to Determine Nitrogen Levels in Corn Fields

The Green Indexing method indicated that indeed there was considerable difference in green to yellowish color in the leaves, more than it appeared with them laying side by side. The system picks up a whole array of green colors, and sorts them out for you. The idea is that the lighter the color, the less nitrogen in the leaf. That likely means less chlorophyll and less photosynthesis going on in the plant.

Even though the leaf on the right wasn't yet showing signs of nitrogen deficiency, it was likely low on nitrogen, Phillips concluded. Classic symptoms include yellowing of the leaf which starts at the leaf tip and works down the midrib of the plant back toward the base of the leaf. As the shortage worsens, the yellow portions become brown.

The point is that there can be differences in color of leaves that you can pick up with instrumentation that may not look that different to the naked eye. This is a good time to be looking for signs of nutrient deficiency. If you see it now above more than the very bottom leaf, you may want to monitor it, and reconsider if you applied enough N or not.