The forms of nitrogen that are detected using the (Kildahl) device are total nitrogen, nitrogen in the form of nitrate NO3, and nitrogen in the form of ammonium NH4.

Ammonium and nitrate (KCl extract) are referred to as available N and there are incubation techniques to estimate mineralisable N. Conventional Kjeldahl using K2SO4 + Se digestion at ~400 degrees recovers ammonium and org N but not nitrate. Nitrate can be included by a preliminary treatment. The details can be found in standard texts.

Modified Kildahl method

This standard specifies a method for determining the total nitrogen content of soil. This includes ammonium nitrogen, nitric nitrogen, nitric nitrogen and organic nitrogen. The determination is partial (incomplete) for bonded compounds.

Kjeldahl is for Total Nitrogen

Mr. Subham Chandra Mondal,

Inorganic N ( NH3+, NO3- and NO2- ) can be evaluated according to:

Bremner, J.M. Nitrogen total. D.L. Sparks (Ed.), Methods of Soil Analysis Part 3: Chemical Methods, Soil Science Society of America, Madison, United States, pp. 1085-1122, 1996. doi: 10.2136/sssabookser5.3.frontmatter

Organic N can be evaluated according to:

Nelson, D.W.; Sommers, L.E. A simple digestion procedure for estimation of total nitrogen in soils and Sediments. J. Environ. Qual., v.1, p. 423, 1972. doi: 10.2134/jeq1972.00472425000100040020x

Mr.Mondal, first clarify whether you estimated total soil N or available (mineralizable) soil N by Subbiah and Asija (1956 ) method/alkaline potassium permanganate method.

Kjeldahl convert all forms of nitrogen into ammonical and read it

Kjeldahl Method --- Organic + Amide + Ammonical N

Devarda's Alloy Reduction Method ---- Nitrate (NO3 + NO2) and Ammonical N (NH4) used for fertilizer analysis

Sulfuric Acid Hydrolysis --- Amide + NH4

Ammoniacal and nitrate forms of nitrogen are measured by Kjeldahl method.

Kjeldahl convert all forms of nitrogen into ammonium form

The procedure is summarized as follows:

Organic N + H2SO4 -------------------------------- (NH4)2SO4+CO2 +SO2+H2O digestion

(NH4)2SO4 +2NaOH ----------------------------------- 2NH3 +Na2SO4 Steam

2NH3+2H2O -------------------- 2NH4OH distillation

NH4OH + HCl ------------- NH4Cl + H2O titration

As Kamal said - all forms of N

I agree with the answer given by the Annangi Subba Rao, the former director of Indian Society of Soil Science, Bhopal and it is well explained and differentiated between total and available nitrogen.

Annangi Subba Rao Sir, I have estimated the available N following the procedure given by Subbiah and Asija (1956 ) method/alkaline potassium permanganate method.