Soil Testing for Nitrogen

Content

Determination of Essentiality
Nitrogen Metabolism and Nitrogenous Constituents in Plants
  Nitrate Assimilation
    - Nitrate Reductase
    - Nitrite Reductase
  Ammonium Assimilation
    - Glutamine Synthetase
    - Glutamate Synthase
    - Glutamic Acid Dehydrogenase
    - Transamination
    - Amidation
  Proteins and Other Nitrogenous Compounds
Diagnosis of Nitrogen Status in Plants
  Symptoms of Deficiency and Excess
  Concentrations of Nitrogen in Plants
    - Concentrations of Nitrogen in Plant Parts
    - Ratios of Concentrations of Nitrogen to Other Nutrients in Plants
Nitrogen in Soils
  Forms of Nitrogen in Soils
    - Organic Nitrogen in Soil
    - Inorganic Nitrogen in Soil
Soil Testing for Nitrogen
  Determinations of Total Nitrogen
  Biological Determinations of Availability Indexes
    - Determination of Inorganic Nitrogen
      - Ammonium
      - Nitrate
      - Amino Sugars
Nitrogen Fertilizers
  Properties and Use of Nitrogen Fertilizers
    - Anhydrous Ammonia: 82% N
    - Aqua Ammonia: 21% N
    - Urea: 46% N
    - Ammonium Nitrate: 34% N
    - Ammonium Sulfate: 21% N
    - Nitrogen Solutions: 28–32% N
    - Ammonium Phosphates: 10–21% N
    - Other Inorganic Nitrogen Fertilizers
    - Organic Nitrogen Fertilizers: 0.2–15% N
References
 
Testing for plant-available soil nitrogen is difficult. This difficulty arises in part because most of the nitrogen in soil is in organic forms, which have varying rates of microbial transformation into available forms. Also, nitrate, the main form of plant-available nitrogen, is subject to leaching, denitrification, and immobilization. Many attempts have been made to develop availability indexes for release of nitrogen from organic matter and to correlate yields with tests for inorganic nitrogen in soils (93,101,112-114). Biological tests are time consuming and may give variable results if the methodology is not standardized among researchers. Chemical tests for estimating plant-available nitrogen have been empirical in approach and have had low correlations with production of mineral nitrogen and crop accumulation of nitrogen.