Forms of Soil Nitrogen
The total N content in soil ranges from 0.03 to 0.4%. Ninety five percent or more of the N in surface soil usually occurs in organic forms.
Organic Compounds (Nitrogen):
The organic forms of soil nitrogen occur as consolidated amino acids or proteins or free amino acids, amino sugars and other complexes generally unidentified compounds.
Inorganic Nitrogen Compounds:
The inorganic forms of soil N includes ammonium ( NH4+) , Nitrate ( NO2), Nitrate ( NO3), Nitrous oxide ( N2O), Nitric oxide ( NO) and elemental nitrogen.
Initial Inorganic N Test:
The potential of soil for mineralize N as measured by N availability index is fairly constant form year to year.
A. Biological Methods for Initial Inorganic N:
The biological methods of testing for initial inorganic N differ from the chemical methods in the following ways.
1. The amount of biological growth is measured rather than directly measuring N.
2. It is impossible to determine if NO3 N or NH4 N is present using the biological method.
1. Aspergillus Niger test:
The fungi and algae have nutritional requirement that are similar to higher plants and attempts have been made to use the growth of these organisms as a measure of available N.
2. Cunninghamella Blackscllena Test:
1. Total A:
Conversion of organic nitrogen to ionic forms by digesting the sample in concentrated sulphuric acid. For acceleration of digestion salt mixture CuSO4, one part selenium powder and one part of this mixture with 20% un hydrous sodium sulphate. The Na2SO4 is added to raise the boiling temperature of concentrated sulphuric acid in digestion. Forty percent NaOH is added during distillation for liberation of ammonia which is absorbed in known excess standard sulphuric acid. Excess std. H2SO4 is titrated with std. H2SO4 utilized for neutralizing ammonia.
2. The Principle Involved in Determination of Available N2 by a Alkaline Permanganate Method.:
Mineralizable N by alkaline permanganate soil with alkaline KMNO4 solution and determination of ammonia liberated which serves as an index of the available or biological Mineralizable nitrogen status.
3. Exchangeable N:
The ammonia ion held in exchangeable form in soil extracted by KCL or NaCl interment shaking for 30 minutes. Then it is filter with Whatman no.42 filter paper. The leachate is then distillated in alkaline medium by adding 40% NaOH. The ammonia liberated is absorbed in std. Excess H2SO4.
Nitrate N Determination:
After distillation of NH4 N add Devarda’s alloy (Cu 50%, Al 45% and Mn 5%) to convert NO3N to NH4N. ammonia liberated is absorbed in std. Excess H2SO4.