Chapter 2.6 Determination of Total, Available, Mineral, Non-exchangeable Ammonium and Potentially Mineralizable Nitrogen in Soil YADVINDER-SINGH, BIJAY-SINGH AND SANDEEP SHARMA Nitrogen (N) is an essential element for plant growth and plays a major role in agriculture. The soil N includes all forms of inorganic and organic N. The N taken up by the crops is derived from a number of sources, particularly from fertilizer, biological N fixation, inorganic N in soil, and mineralization of N from soil organic matter (SOM), crop residues, and manures (Keeney 1982). The total N content of soils in India varies from 0.02 to 2%. Generally, the total N content ranges from <0.02% in a sub-soil to 0.06-0.5% N in the A-horizons of mineral soils. The inorganic N includes its soluble forms (NO 3 - and NH 4 + ), exchangeable NH 4 + , and clay-fixed non-exchangeable NH 4 + . The nitrite-N is rarely present in significant amounts in the normal soils. Up to 90% of the total N in upper soil layers exists in the organic forms. The analysis of total N and inorganic N provides an insight into the N supply of a soil to plants. Potentially mineralizable N (PMN) in the soil can contribute to crop N supply in the range of 20-200 kg N ha -1 , depending upon the quantity of mineralizable organic N in the soil and environmental conditions (soil temperature and moisture) that control the rate of mineralization. The organic N content includes numerous identifiable and non-identifiable forms (Stevenson 1986) and can be determined by the difference between total soil N and inorganic soil N contents. 2.6.1. Determination of Total N in Soil The methods used for the total N analysis may be divided into three main types: (i) wet digestion (e.g., Kjeldahl method), (ii) dry combustion (e.g., Dumas method), and (iii) CHN analyzer. The wet digestion techniques involve conversion of organic and inorganic N into NH 4 + in acid and its subsequent measurement. Some salts (e.g., K 2 SO 4 ) and catalysts (e.g., CuSO 4 ) are usually added to increase the digestion temperatures and accelerate oxidation of organic matter (Bremner 1996). The dry combustion method