|MadSci Network: Chemistry|
Ok Aram, I am going to take a stab at this question since it wasn't answered earlier.
I have some experience in assessing nitrogen content in marine algae, perhaps some of the techniques would apply. The challenge to you is to figure out how to get the organic nitrogen and phosphorus into the form of nitrate and phosphate. You may be able to do this by chemical oxidation. For organic carbon measurements, oceanographers (and others of course) oxidize organic material with strong acids such as perchloric or chromic. These are not trivial to work with and may be beyond the scope of a high school course, you will have to decide. Once organic nitrogen is oxidized, (presumably to nitrate) it will have to be converted to nitrite. This is done by passing the solution containing the nitrate through a specially treated cadmium column. The cadmium column also has special handling requirements. These techniques are available in Standard Methods books for water and seawater analysis. Then nitrite is analysed colorimetrically by formation of an aniline dye with N-Napthylene-diamine dihydrochloride and Sulfanilimide in a spectrophotometer set at 543nm.
Alternately, you may be able to burn the material, but I don't know what the recovery would be. I have used very high temperature combustion for sediment and particulate organic samples. This requires special (and expensive) equipment called an Elemental Analyzer which separates the combustion products into CO2, NO2 and H2 gases. You may be able to convince a lab to run the analyses for you and let your students use the data to scale the results up from a few milligrams of grass to an acre, this would at least exercise their math skills.
Finally, you can use stoichiometry. There are many sources to look for this information, but with a little digging, you should be able to find a ratio of nitrogen per gram dry weight, gram carbon or carbohydrate (this can be analyzed with a phenol-sulfuric acid method and most of the carbon in grass is probably in the form of carbohydrates) Chemical stoichiometry is an often used concept in aquatic sciences.
Try the links in the MadSci Library for more information on Chemistry.