Methods for determination of plant available nitrogen in soil.
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Nitrogen is an important nutrient in the process of crop growth. The aim of this research was to develop several models to estimate the Soil Total Nitrogen Content (STNC) using soil reflectance values. For this, 138 samples for three different types of soil: loam, sandy loam, and silty loam with different nitrogen contents were prepared. After pre-processing of data, the most effective spectral bands for the STNC assessment were determined using two different approaches. These bands were 1415, 1465, 1475, 1895, 1925, 1955, 1985, 2025, 2215, and 2305 nm. Subsequently, several statistical models were developed using the selected bands and multivariate linear regression analysis. The root mean square error (RMSE) of the most accurate models for loam, sandy loam, and silty loam was 0.35, 0.38, and 0.57g/kg, respectively. Additionally, three models were developed regardless of soil types. These models were independent of the soil type and can be applied to any agricultural soil. The most accurate independent model had an RMSE of 0.54 g/kg and a correlation coefficient (R) of 0.97. Although these models are valid for laboratory measurements, but the extension of these models to satellite application is the objective of future studies in the Khavaran Remote Sensing Laboratory (KRSLab).
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Most studies on the forms of organic nitrogen (N) in soils are based on the use of hot mineral acids to liberate nitrogenous constituents from organic colloids and clay minerals. Identifiable organic N compounds are the amino acids and amino sugars. In steam-distillation procedure, the different forms of N in a soil hydrolysate are converted to, and estimated as, ammonia (NH3). The only special apparatus required is the steam distillation unit described later in "Special Apparatus." The ninhydrin-NH3 method ("Amino Acid-Nitrogen") is best suited for the analysis of soils and sediments containing relatively large amounts of amino acids. Methods based on the colorimetric determination of the blue-colored product obtained by carrying out the ninhydrin reaction at pH 5.0 have greater sensitivity, and they can be used for analysis of samples containing low amounts of amino acids, such as the B and C horizons of terrestrial soils and marine sediments.
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