I used a procedure which was developed from this article, Check the PDF please,

file:///C:/Users/qianlabgst/Downloads/aj-56-2-AJ0560020133.pdf  

For N analysis, dried and ground grain and leaf samples are first digested in conc. sulfuring acid with catalyst mixture. Ammonium N in the digests are then distilled with conc. alkali (NaOH) solution and ammonia is collected in boric acid solution with mixed indicator. N collected in boric acid solution is determined by titration with dilute acid of known strength.

For analysis of P, K, Ca, Mg, Se and Fe, dried and ground grain and leaf samples are first digested in tri-acid mixture (sulfuric+nitric+perchloric acid). Digests are filtered and fitrates are analysed. P is determined colorimetrically by vanado-molybdate method. K in the filtrates are diluted with distilled water and measured by using flame photometer. Ca, Mg, Se and Fe in the filtrates can be measured by using atomic absorption spectrophotometer.

To estimate the total S in plant materials, it is necessary to convert all the S to one form that can be determined accurately. Most workers have concluded that conversion of total S in plant materials can be accomplished by oxidizing the organic S to SO42- by dry ashing or alkaline digestion with NaOBr. The SO42- thus formed is determined gravimetrically, turbidimetrically, or colorimetrically.

see - Dr  Kundu

The grain has to be grinded and the samples ashed and dissolved in 6N Hcl, filtered.Ca, Mg, K,Se and Fe will be measured by AAS. Phosphorus by Uv Visible spectrophotometer. Nitrogen by kjeldahal digest.

I also have estimated leaf nutrient status in strawberry by using these methods as mentioned by Dr Kundu .....

 Analytical procedures are well laid out, can be referred through any laboratory  manual  .  For annual crops , it is ok. But ,most important of all , in such analysis , especially when perennial crops are involved ,is  the correct sampling of index leaves . And , the leaf analysis values will change abruptly, if right stage of sampling , right position of leaves on shoots , leaves whether emerging from fruiting or non-fruiting terminals  , whether correct number of leaves are sampled, at what height of the plant canopy, , leaves sampled represent which direction oriented to sunshine...etc .etc .. are taken into consideration  while sampling leaves . These parameters  will collectively  dictate the diagnostic  values , and the diagnostic values...which probably has a better utility in the context of correct diagnosis of nutrient constraints. 

Refer to article hope will be useful

Thank you all for these very nice  deliberations.

I would also like to know how to measure Iodine in grains and leaves