What I think you may be looking for a VLOOKUP()
Lets say you have a separate sheet for nitrates (Name the sheet "Nitrates"), 2 columns with date and reading
ie;
04/10/09 - 5ppm
04/11/09 - 4ppm
etc.
Now on your 'report' sheet, you can do:
=VLOOKUP("04/10/09", Nitrate!$A$1:$B:$B1000, 2, FALSE);
And of course instead of entering the date, you can reference a cell that contains a date.
Now let's do a little error correcting, if you don't have a result for a specified date:
=IF(ISNA(VLOOKUP("04/10/09", Nitrate!$A$1:$B:$B1000, 2, FALSE)), VLOOKUP("04/10/09", Nitrate!$A$1:$B:$B1000, 2, FALSE), "No data")
Hope that points you in the right direction, and my explanation makes sense. I have no formal excel training, but consider myself an excel guru somedays
Oh yeah, the $A$1 sets those value as static references, so lets say if you drag the formula down to populate more cells, the values do not increment.