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7.4Coarse Fragments
The material greater than 2 mm including rock fragments of geologic origin, carbonate nodules/concretions & shells, but excluding gypsum crystals, are sieved and weight measurements are made and recorded on the 20to 75-mm, 5- to 20-mm, and 2- to 5-mm fractions, i.e., coarse, medium and fine gravels respectively. These coarse fragments are excluded from most chemical, physical, and mineralogical analyses, and are discarded, exception is for coarse fragments with carbonateor gypsum-indurated material or material from Cr and R soil horizons. Such special composition coarse fractions are recombined with fine earth fraction, crushed, homogenized and stored for laboratory analysis.
7.5Moisture Content
A known weight of air-dried soil is oven-dried (105 °C) overnight, and the difference in weight is reported as % moisture content. In case of gypsiferous soil, the moisture content must be adjusted by removing the crystal water of gypsum in soil (methods No. 3D1). Moisture content is required to convert laboratory analytical results from air-dry to oven-dry basis (a standard way of results presentation).
7.6Loss on Acid Treatment (LAT)
The LAT is determined to validate the quantity of calcium carbonate equivalents in the soil as measured by either back titration or calcimeter method. A known weight of air-dried soil sample is reacted with 1 M HCl to decompose carbonates, the treated sample is oven-dried and reweighed to calculate percent loss in weight after treatment.
7.7Calcium Carbonate (CaCO3) Equivalents
The calcium carbonate (CaCO3) equivalent (e.g., includes carbonates of Mg, Ca-Mg, Na, and K etc.) in soil is used in Kuwait Soil Taxonomy to identify calcic, petrocalcic horizons, and carbonatic mineralogy class (Soil Survey Staff 2014b). In
7.7 Calcium Carbonate (CaCO3) Equivalents |
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addition, presence of calcium carbonate induces soil buffering capacity and provides adsorption and precipitation reactions for phosphorous and micronutrients. The calcium carbonate is reported on fine earth fraction (<2 mm) and the 2 to 20 mm basis. In the laboratory calcium carbonate equivalent is determined by two methods,
(i) calcimeter and; (ii) back titration. In the calcimeter method, a known quantity of air-dried soil is reacted with HCl, the CO2 produced through calcium carbonate decomposition is measured manometrically (calcimeter), which is used to calculate percent calcium carbonate on oven-dry soil basis (method no 4E1a1a1). In the back titration method, a known quantity of air-dry soil is reacted with known volume of 1 N HCl, the leftover HCl is back titrated with 1 N NaOH solution in the presence of phenolphthalein indicator. The volume of HCl consumed during reaction with soil is used to calculate % calcium carbonate equivalent in soil.
7.8Gypsum (CaSO4.2H2O)
The quantity of gypsum (CaSO4.2H2O) present in the control section of soil profile is used to identify gypsic and petrogypsic horizons, and gypsum mineralogy class at the family level of US soil taxonomy hierarchy (Soil Survey Staff 2014b). Gypsum is reported on both a < 2 and a < 20-mm basis. To determine gypsum, a known weight of soil is mixed with water to dissolve gypsum. The soil suspension is filtered, and acetone is added to a portion of clear filtrate to precipitate dissolved gypsum. After centrifugation, the precipitated gypsum is dissolved in water and electrical conductivity of the gypsum solution is measured. The EC is used to calculate percent gypsum in soil.
7.9Extractable Cations
The extractable cations (Ca2+, Mg2+, K+, and Na+) from the 1 N NH4OAc (buffered-pH 7) extraction (method 4B1a1) are generally assumed to be sum of soluble and exchangeable cations. The extractable cations are measured by either Atomic Absorption Spectrophotometer (AAS) or Inductively Coupled Plasma (ICP) and expressed as meq/100 g (Thomas 1982) or SI unit [(cmol (+)/kg)]. In this method extraction of Ca2+ is overestimated due to Ca dissolved from free calcium carbonate and gypsum and K from dominance of illite/mica in soil (Thomas 1982).
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7.10Cation-Exchange-Capacity (CEC)
The cation-exchange-capacity is the ability of soil to hold and exchange cations in a plant available form (NH4+, K+, Ca2+, Mg2+ etc.). The CEC is also a measure of soil fertility level. The CEC is measured by two ways. Method 1: by saturating the exchange sites of soil with an index cation (NH4), washing the excess NH4 with 95% ethanol, and displacement of adsorbed index NH4 by 2M KCl, analyzed by steam distillation and titration to determine the NH4+ adsorbed on the soil exchange complex and reported as meq/100g or cmol(+)/kg (method no. 4B1a1a1a1). Method 2: In this method soil exchange complex is equilibrated with 1 N NaOAC (buffered at pH 8.2), the excess NaOAC is washed with 95% ethanol, finally the Na is extracted with 1 N NH4OAC (pH 7) and Na is measured using any of the three equipment (flame photometer, atomic absorption spectrophotometer, inductively coupled plasma) and CEC reported as meq/100 g or cmol (+)/kg.
7.11Exchangeable Sodium Percentage (ESP)
The ESP is used to identify soil sodicity problem, if ESP > 15, the soil is classified as sodic or alkali soil. In soil taxonomy Sodium Adsorption Ratio is used as criteria for natric horizon (Soil Survey Staff 2014b). The exchangeable sodium percentage (ESP) can be calculated by two calculation procedures.
Method 1: Compute the exchangeable sodium percentage (ESP) by dividing the exchangeable sodium (ES) by the CEC measured (meq/100 g) by NH4OAc, pH 7.0 (CEC–7) and multiplying by 100 (method no 4F3a1).
ESP ¼ ½Exchangeable Na=CEC 100:
Method 2: Compute the sodium adsorption ratio (SAR) value in the following formula given by U. S. Salinity Laboratory Staff (1954).
ESP ¼ ½100 ð 0:0126 þ 0:01475 SARÞ=ð1 þ ð 0:0126 þ 0:01475
SARÞ :
The SAR is calculated by using Na, Ca, Mg (meq/l) from extract of saturated soil paste.
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SAR ¼ Na=½ðCa þ MgÞ=2Þ 0:5
Where Na, Ca + Mg are reported as meq/l and the SAR (mmoles/l)0.5
7.12Saturation Percentage (SP)
The saturation percentage indicates available water content in soil. In addition, the SP is used to convert soil solution chemistry data (from saturation extract) to soil weight basis (e.g., meq/l to meq/100 g). A 250–300 g air-dried soil is used to prepare a standard saturated soil paste. A portion (30–50 g) of saturated soil paste is used to determine the moisture content (oven-drying at 110 °C for 12 to 16 h) to determine saturation percentage (SP) by method 4F2a1. The SP can also be calculated by using the volume of water to prepare saturated soil paste.
7.13Preparation of Saturated Soil Paste
A portion of 250 g air-dry soil is used to prepare saturated soil paste by adding deionized water and mixing, to reach to a level where soil paste glistens as it reflects light, flows slightly when the container is tipped, and slides freely and cleanly off the spatula. The saturated paste is covered and allowed to stand overnight. The saturation criteria are then rechecked.
7.14Saturation Extract Analysis
The extract collected from saturated soil paste under vacuum (Fig. 7.2) is used to
measure soluble cations (Na+, K+, Ca2+, Mg2+) and soluble anions (CO32−, HCO3−, Cl−, SO42−). The soluble Na+, K+, Ca2+, Mg2+can be measured by
AAS-ICP. Soluble HCO3− by titration with 0.011 N H2SO4 in the presence of methyl orange indicator, CO32− by titration with 0.011 N H2SO4 in the presence of phenolphthalein indicator, and soluble Cl−, NO3−, PO43-, and SO42- by ion chromatography. The Cl− can also be measured by chloride analyzer.