In general I consider most useful to obtain an estimate of chlorophyll concentration per unit leaf area, either in mass or molar units. Calibration seems to depend more on the instrument itself than on plant species, and calibration is usually stable in time. Consequently you could do a calibration curve a posteriori of measurements. What you do need to be careful with is that chloroplast movements caused by high irradiance can alter this relationship. This means that SPAD measurements for calibration and actual measurements must be done on similarly illuminated leaves. By calibration I mean making a series of measurements with SPAD and an extraction method quantification by for example spectrophotometry on the same leaf location.

Getting an estimate of concentration in units that are independent of the instrument used makes comparisons and interpretation much easier. There are other possible sources of difficulties like pubescence which should not be a problem with corn.

Of course, in the case of fertilizer application decisions in the field, the direct use of SPAD units is the easiest.

I have linked a couple of papers that I have found useful.

http://link.springer.com/article/10.1007/BF00032301

http://aob.oxfordjournals.org/content/82/3/389.short

In general I consider most useful to obtain an estimate of chlorophyll concentration per unit leaf area, either in mass or molar units. Calibration seems to depend more on the instrument itself than on plant species, and calibration is usually stable in time. Consequently you could do a calibration curve a posteriori of measurements. What you do need to be careful with is that chloroplast movements caused by high irradiance can alter this relationship. This means that SPAD measurements for calibration and actual measurements must be done on similarly illuminated leaves. By calibration I mean making a series of measurements with SPAD and an extraction method quantification by for example spectrophotometry on the same leaf location.

Getting an estimate of concentration in units that are independent of the instrument used makes comparisons and interpretation much easier. There are other possible sources of difficulties like pubescence which should not be a problem with corn.

Of course, in the case of fertilizer application decisions in the field, the direct use of SPAD units is the easiest.

I have linked a couple of papers that I have found useful.

http://link.springer.com/article/10.1007/BF00032301

http://aob.oxfordjournals.org/content/82/3/389.short

The following paper may be useful for your case:

Respected sirs,

Thanks for the information.