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Soil sampling and chemical analysis
               Soil  testing  is  a  useful  tool  to  assess  the  nutrient  deficiency  in  soil  and  fertilizer
               recommendations are given for different crops based on soil testing. Application of secondary
               and  micronutrients  in  different  crops  and  cropping  systems  is  based  on  availability  of  these
               nutrients in soil. During this process soil test results are compared with critical vales of these
               nutrients in soil to assess their deficiency level.GPS based soil sampling and chemical analysis of
               soil samples is a better way since soil testing for secondary and micronutrients is difficult and
               expensive and it needs very high technical sophistication.
               The first step for performing soil testing is collection of representative soil samples based on soil
               types, crops and cropping systems and land holding capacity of the farmers’. Soil samples were
               collected from top 15 cm soil depth. More than 2,00,000 soil samples were collected from more
               than 500 districts of India.  During sampling, the geographical position of the sampling points
               was also recorded using a GPS.

               Collected soil samples were air dried separately and debris and stones were removed. Grinding
               of the samples were carried out (with the help in wooden mortal and pistil) to pass through 2 mm
               size  sieve.    Soil  samples  were  chemically  analysed  for  secondary  and  micronutrients  in
               laboratories  of  different  centres  of  AICRP  on  Micro  and  Secondary  Nutrients  and  Pollutant
               Elements  in  Soils  and  Plants,  ICAR-Indian  Institute  of  Soil  Science,  Bhopal.    Soil  pH  was
               measured in 1:2·5 (w/v) soil–water suspensions. The soil organic carbon content was analysed
               using the Walkey and Black method (Walkley & Black, 1934). Available zinc, iron, copper and
               manganese in soil samples were extracted using diethylene triamine penta-acetic acid extractant
               (Lindsay & Norvell, 1978) measured by atomic absorption spectrophotometer. Available sulphur
               content was extracted by 0·15% CaCl 2 (Williams & Steinbergs, 1959), and B was extracted by
               hot water (Gupta, 1967) and estimated by using spectrophotometer.

               Soil mapping
               The parameters of descriptive statistics for soil properties and available micronutrients database
               generated  in  the  project  as  well  as  borrowed  from  scientists  other  than  AICRP-MSPE  were
               obtained using SAS software (SAS, 2011). The minimum, maximum and mean value of each
               nutrient was estimated for each district.   The normality of dataset was verified by Kolmogorov-
               Smirnov test (at p < 0.05). Geostatistical analysis of available sulphur and micronutrients was
               carried  out  by  ArcGIS  10.4.1.  Semivariogram  for  each  soil  nutrient  was  calculated  from
               averaged values. Best fitted model (such as Spherical, Circular, Exponential, Stable, Gaussian
               and  K-Bessel  etc.).)  for  each  soil  parameter  was  selected  through  the  technique  of  cross-
               validation. Spatial dependence of each nutrient element was assessed by estimating Nugget/Sill
               ratio.  The  soil  parameters  having  Nugget/Sill  ratio  ≤  0.25,  0.25  to  0.75  and  >  0.75  revealed
               strong, moderate and weak spatial dependence respectively. Ordinary kriging (OK) was applied
               for interpolation mapping and kriging biasness and accuracy was tested by cross-validation.
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