Do you mean elemental (N,P,K,S,Mg,Ca,Fe,Cu,Mn etc.) analysis of plant extracts? Agricultural scientists estimate plant elemental composition by spectrophotometric, titrimetric, flamephotometric, turbidimetric etc. analyses. Autoanalyzers are also available. Atomic absorption spectroscopy is employed for trace elements. For plant elemental analysis, powdered plant biomass is used. Any standard textbook on plant elemental analysis provides details of analysis.
Or do you mean by proximate analyses such as ash content, moisture content, carbohydrates, proteins etc.?