Commercial laboratories conduct a wide range of analytical methods for a number of different applications. It is vitally important the laboratory knows what you wish to be analyzed and how you want the results to be reported. For example if you do not specify your needs, the laboratory may report your inorganics as percent wet weight when you need the results in mg/kg dry weight; or they may report your nitrogen as “ammonium” (NH4+) when you need them to report it as “ammonium-N” (N). Laboratory results that state “ammonium” are not the same as “ammonium-N”. In addition total nitrogen is not the same as plant available nitrogen (PAN). These simple differences can have significant ramifications in your land application calculations and regulatory compliance. It is also important to ensure the laboratory is using the approved analytical methods for biosolids as specified in 40 CFR 503.8
To help address some of these issues, the Pacific Northwest Extension Service produced a worksheet for calculating biosolids application rates.
There are also spread sheets that can run these calculations for land application rates, but if you enter the wrong data (wet weight results or NH4+ instead of dry weight results or N), you will get the wrong answer and could be out of compliance. Be sure you know what the spread sheet or formula requires and produces or you may be relying on the wrong answer.
It may be worthwhile to have a professional agronomist calculate your application rates or at least review your work. Certified Crop Advisors are professional agronomists accredited by the American Society of Agronomy