Analysis of geochemical parameters

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2.2 MATERIALS AND METHODS

2.2.4 Soil samples

2.2.4.1 Analysis of geochemical parameters

Soil parameters were analysed according to standard operating procedures (ASTM, APHA, EPA). Samples for analysis of geochemical parameters were air dried and sieved (≤ 2 mm).

2.2.4.1.1 Soil pH

Soil pH was checked with a soil and deionized water suspension of 1:2 ratio with a pH electrode (Make: Thermoscientific), calibrated at pH 4, 7 and 10.

2.2.4.1.2 Soil electrical conductivity

Electrical conductivity (EC) was checked with a soil and deionized water suspension of 1:2 ratio with a EC electrode (Make: Systronics), calibrated with KCl solution of strength 1000 µScm^-1.

2.2.4.1.3 Grain size analysis

Grain size of soil samples were analysed by Hydrometer Method (ASTM). Particle size for fine sand, silt and clay was analysed for soil samples.

2.2.4.1.4 Total Organic Carbon

Total Organic Carbon (TOC) was measured by Potassium Dichromate Digestion Method described by Jenkinson and Powlson (1976). Organic Carbon (OC) is oxidized

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in presence of excess of Cr2O72-

, the amount of reduced Cr2O72-

is linked to OC content, based on quantifying the amount of oxidizable carbon. This method was slightly modified, adapted from TOC analysis of MBC fumigated sample (oven dry basis). The digested sample was titrated against [(NH₄)₂ Fe (SO4)₂. 6H₂O] using ferroin indicator to obtain the Carbon content and also analysed for Carbon in CHNS elemental analyser (Make: Eurovector EA3000). The concentration of Carbon was obtained in mg/kg.

2.2.4.1.5 Total Nitrogen

Total nitrogen (TN) was estimated by Total Kjeldahl Method (APHA). Soil digestion and distillation was carried out in Kjeldahl Nitrogen Analyzer (Make: Velp Scientifica). Soil samples were digested with concentrated H2SO4 in presence of catalyst (CuSO4 + K2SO4) at a temperature of 420°C, followed by distillation in alkaline condition (presence of NaOH) and titration against H₂SO_4. The concentration of Nitrogen was obtained in mg/kg. Kjeldahl Nitrogen includes only organic and ammonium nitrogen, it excludes nitrates.

2.2.4.1.6 Total Phosphorus

Total Phosphorus (TP) was estimated by Ammonium Molybdate Method (APHA). As already mentioned in section 2.2.3.1.7, Organic Phosphorus in soil was converted into inorganic Phosphorus through acid digestion (concentrated HClO4 − HNO3).

Phosphorus in digested sample was then measured colorimetrically for in presence of SnCl2 as an indicator, at 690 nm in Cary 100 UV − Visible spectrophotometer.

Concentration of phosphorus was calculated from standard curve of absorbance versus

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concentration of K2HPO4 solution ranging from 0.5 mg/L to 5 mg/L of PO43- – P mgL^-1 at an interval of 0.5 mg/L. The concentration of PO₄^3- was obtained in mg/kg.

2.2.4.1.7 Soil nitrate

Soil nitrate (NO₃^-) was estimated by Phenol Disulfonic Method (APHA). The concept and method of determination of NO3-

is similar to nitrate in water samples. Soil solution for NO3-

estimation was prepared as 1:5 soil and nitrate extraction solution (CuSO₄. 5H₂O + Ag₂SO₄) suspension. The filtrate was treated with Ca(OH)₂ and MgCO3 to precipitate Cu and Ag prior to nitrate estimation. The concentration of NO3-

was obtained in mg/kg. (Refer section 2.2.3.1.4).

2.2.4.1.8 Soil ammonia

Soil ammonia was estimated by Micro Phenate Method (Clesceri, 1998; APHA) using sodium nitroprusside. The concept and method of determination of ammonia is similar to ammonia in water samples. Soil solution for ammonia estimation was prepared as 1:5 soil water suspension. The concentration of ammonia was obtained in mg/kg. The filtrate was used for ammonia estimation. (Refer section 2.2.3.1.5).

2.2.4.1.9 Soil nitrite

Nitrite (NO₂^-) in water samples was estimated by Diazotization Method (APHA). Soil solution for nitrite estimation was prepared as 1:5 soil water suspension. The suspension was filtered and analysed for presence of nitrites. Nitrite forms a diazonium salt with sulphanilic acid in an acid medium (pH 2.0 – 2.5), this complex combines

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with α – naphthylamine hydrochloride to form a pink coloured dye. The aborbance for pink coloration was measured colorimetrically at 520 nm in Cary 100 UV − Visible spectrophotometer. Concentration of soil nitrite was calculated from a standard curve of absorbance versus concentration of NaNO₂ solution ranging from 0 mg/L to 1 mg/L of NO2- – N mg/L at an interval of 0.1 mg/L. The concentration of NO2-

was obtained in mg/kg.

2.2.4.1.10 Soil sulphate Soil sulphate (SO42-

) was estimated by Turbidimetric Method (APHA). The concept and method of determination of sulphate is similar to sulphate in water sample. Soil solution for sulphate estimation was prepared as 1:5 soil water suspension. The suspension was filtered free of any kind of turbidity. The concentration of SO42-

was obtained in mg/kg. (Refer section 2.2.3.1.6).

2.2.4.1.11 Soil trace elements: Na, Ca, Mg and K

Digestion of soil samples for analysis of trace elements was performed with aqua regia (ASTM D3974, 2003). Trace elements (Na, K, Mg and Ca) were analysed in Flame Photometer (Make: Systronics) calibrated with standard reference material.

Concentration of trace elements was obtained in mg/kg on a dry weight basis.

2.2.4.1.12 Soil Cation Exchange Capacity

Soil cation exchange capacity (CEC) measures the cation exchanges on mineral components (especially clay) in soil and organic matter surfaces. CEC quantifies the

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amount of negatively charged sites on soil surfaces that can retain positively charged cations such Ca²⁺, Mg²⁺ and K⁺ by electrostatics forces and this preserves the easily exchangeable cations in soil in terms of fertility. CEC (cmolckg^-1) was calculated as (meq÷100 g or cmol_ckg^-1) = (ppm Ca÷200) + (ppm Mg÷120) + (ppm K÷390).