Preface This book is a compilation of some user friendly well accepted analytical methods for characterising soil physical and chemical properties. Primarily the book is meant to cater the need of students of agricultural universities and is expected to be helpful to the teachers and researchers of this related field. The text presented in the book is subdivided into 19 chapters and 15 appendices. Each method is discussed here under four subheads viz., principle, equipments and materials, procedure and calculation. The methods have been explained in a simpler language. Steps of calculation have been made easy understandable as far as practicable. Each chapter is ended with some relevant questions and their answers on that topic. Basic principle of different chemical analyses and working principle of the instruments used in soil analysis laboratories has been discussed in a very lucid manner. Inspite of the best efforts, the text may still contain some discrepancies. Any suggestion from the readers for improvement of the book in future will be highly appreciated. The author gratefully acknowledges the every source from where the technical matters are drawn.

Soil is a mass of large variability and thus its sampling is the most challenging problem for any soil analyst. Practically, soil sampling is a two step process which includes (i) collection of soil from homogeneous unit and (ii) processing which in turn includes drying, grinding, sieving, mixing, partitioning and storing. 1.1. Sources of Error in Soil Chemical Analysis Different sources of error involved starting from soil sampling to chemical analysis of a sample can be divided into three main groups: Sampling error : This error results from variability among the different samples drawn from same soil unit with respect to the objective of sampling. Sub-sampling error : This error is due to variability introduced among sub samples of the same soil sample. Analytical error : This error introduced due to variability from one chemical determination to another on same sub sample.

2.1. Particle Size Distribution Analysis Soil is composed of particles of varying size and shape. The expression of the proportion of the different size particles is termed as ‘soil texture’. It is one of the most important basic characters being little changed by cultivation or other practices. Particles are arbitrarily distinguished into following groups on the basis of their diameters.

3.1. Determination of Bulk Density of Soil 3.1.1. Determination of Bulk Density of an Undisturbed Soil Bulk density of soil is expressed as the ratio of the mass (weight) of soil particle to their total volume including the pore space between the soil particles. Alternately, it is the weight of unit volume of dry soil. It is usually expressed in the unit of gram per cubic centimeter (g/cc) or mega gram per cubic meter (Mg/m3). In fine textured soil bulk density varies from 1.00 to 1.60 g/cc, whereas in coarse textured soil from 1.20 to 1.80 g/cc.

In natural condition primary soil particles cohere to each other to form a cluster or an aggregate through the cementing action of various agents like organic matter, oxides of iron and aluminium, carbonates, etc. Aggregate analysis tells one about the structural status, pore geometry which in turn influences the movement of air and water in soil, amount of water stable secondary particles (aggregate) as well as the extent of finer particles aggregated to the coarser particles and the erodibility status of surface soil against wind and water. Methods of Analysis Among the several methods of aggregate analysis the commonly used methods practiced in agriculture and related fields are wet sieving and dry sieving method of aggregate analysis.

5.1. Determination of Soil Moisture 5.1.1. Determination of Soil Moisture Content by Gravimetric Method (Direct method) Traditionally water content in soil is expressed as the ratio of the weight of water present in the soil to the weight of dry soil. When this ratio is multiplied by 100, it becomes the percentage of water in the soil sample on dry weight basis.

6.1. Determination of Hydraulic Conductivity of Saturated Soil: Laboratory Methods Hydraulic conductivity of a saturated soil indicates the ease with which water is transmitted through the soil pores. If a constant water head (h) is maintained on a saturated soil column of length (L) and the volume of water (V) percolating per unit time (t) through per unit cross sectional area (A) to the other end, then according to Darcy’s Law the rate of flow of liquid or flux (q) is proportional to the hydraulic gradient (ΔH/L) across the length of soil column.

Soil consistency is a term used to designate the manifestation of different physical forces acting within the soil at various moisture levels. The consistency limits, also known as ‘Atterberg Limits’ (Atterberg, 1911; 12) are defined by the water content of the soil to create specified degree of consistency. These limits are – (i) Liquid limit or upper plastic limit, (ii) Plastic limit or lower plastic limit and (iii) Sticky limit. 7.1. Determination of Liquid Limit or Upper Plastic Limit Equipments and Materials Liquid limit device (Casagrande, 1932) and accessories, spatula, dish, oven, moisture box, balance, desiccator

8.1. Law of Mass Action Gulberg and Waage, the two Norwegian chemists in the year 1864 stated the law of mass action in this form: ‘The velocity of any chemical reaction at any instant at a given temperature is proportional to the active mass of each of the reactants at that instant present in the system, the active mass in a homogeneous system is defined as the number of gram molecule of the substance present per unit volume i.e., the molar volume’. Mathematical Formulation of the Law of Mass Action Let us take a simple case of a reversible reaction at constant temperature represented by the equation:

9.1. Balance and Weighing 9.1.1. Principle of Functioning of a Balance It is one of the important tools used in the laboratory and one should have thorough knowledge about its assembly, use and care, otherwise it may be the initial step for introducing error in any physical or chemical analysis. Every balance has its maximum load limit and sensitivity depending upon its construction. Essentially balance may be regarded as a rigid beam, BC having a central fulcrum, O and two arms of equal length, d1, d2 (Fig.9.1). The both ends of beam have prism edges upon which the balance pans are suspended by means of suitable suspension.

10.1. Determination of Soil pH The soil pH value is the measure of hydrogen or hydroxyl ion activity in the soil solution and it indicates whether the soil is acidic, neutral or alkaline in reaction. The pH is the most important property of soil as it controls the availability of plant nutrients, microbial activity and physical condition of soil. Since crop growth is affected both under very low (strongly acidic) and very high (strongly alkaline) soil pH, reclamation of these soils are necessary. Soil pH in fact indicates the extent of active acidity (activity of hydrogen ion in soil solution) of soil. Depending upon the purpose of measurement and soil condition, soil pH is measured in several soil-water or soil-salt solution ratios. The saturated paste is used for identifying specific soil problems like degree of acidity or alkalinity. For making fertilizer recommendation, 1: 2 soil-water suspension is generally adopted. To mask the variability in salt content of the soil, the pH is measured in soil-CaCl2 suspension (1:2 soil-0.1 M CaCl2 solution). The soil pH value is independent of dilution over a wide range of soil-salt solution ratio, thus more reproducible than those obtained with soil-water suspension.

11.1. Determination of Electrical Conductivity of Soil Solution The conductivity or more precisely the specific conductance of a soil at a given soil-water ratio is the reciprocal of the specific resistance offered by the solution at 25ºC between the electrodes of 1 sq cm cross section kept 1 cm apart. Thus, the electrical conductivity of any solution depends on the amount of soluble salt present in it. Knowledge on the extent of salinity is important for management of saline soil including the choice of the crop suitable for that soil. Soluble salt content in saturation extract is more reliable index of salinity level than in soil: water ratio of 1: 2 or 1: 2.5 because it simulates the extent of salinity experienced by crop in field moisture condition. Salinity class of soil is based on the electrical conductivity (EC) of saturation extract at 25ºC. However, EC of water extract from 1:2 soil-water suspension can be transformed into the EC of saturation extract following the formula (except for soil containing gypsum).

Carbon may present in soil in four different forms: Carbonates: Mainly as CaCO3 and MgCO3, active CO32- and HCO3- ions and gaseous CO2. Highly condensed elemental organic form: Charcoal, graphite. Completely decomposed resistant organic residues: Humus. Undecomposed and partially decomposed organic residues.

13.1 Exchange Capacity Soil colloidal particles carry both negative and positive charges on their surface which attract and hold equivalent amount of oppositely charged ions to their surface. The total number of negative charges present on a given mass of soil or capacity of a given mass of soil to exchange cations or total number of cation held by a given mass of soil is termed as cation exchange capacity (CEC). Similarly, the capacity of a soil to retain anions is known as anion exchange capacity (AEC). The ion exchange capacity of a soil depends upon the type and amount of colloidal particle (clay and organic matter) present in the soil. Ion exchange capacity (CEC / AEC) is expressed as milliequivalent ion (cation / anion, respectively) per 100 g of soil (meq / 100g). It is also expressed as centimoles of charge (positive / negative) per kg of soil [cmol (±) / kg]. Meq / 100g = cmol (±) / kg of soil.

14.1. Determination of Total Nitrogen in Soil (Modified Kjeldahl Method) Nitrogen mostly present in soil in organic form. Relatively small amount of nitrogen usually occur in ammonium and nitrate form, the available form. The Kjeldahl method of total N determination includes both organic and ammonium form and with modification nitrate form of N can also be included.

Phosphorus in soil present both in inorganic form and organic form. The inorganic P can be divided into different pools. The major active inorganic forms are: P bound to aluminium (Al-P), iron (Fe-P), calcium (Ca-P) and silicate minerals and relatively less active forms are occluded and reductant soluble forms of P. On the other hand the principal organic P compounds present in soil are (i) inositol phosphate, (ii) phospholipids, (iii) nucleic acid and other unidentified esters and phosphoproteins. 15.1. Determination of Available Soil P The availability of soil P to plant varies greatly depending upon the reaction, minerological composition and amount of the soil colloid. Very little P is present in the soil solution except in recently fertilized or sodic soils. The type of phosphate ions present in the soil solution depends on the soil pH. In soils of neutral to slightly alkaline pH, HPO42- ion is the dominant form. In slightly or moderately acidic soils, both H2PO4- and HPO42- ions prevail. At strongly acidic soil H2PO4- ion tends to dominate, while above pH 9.0, the PO43- ion becomes more important than H2PO4-.

16.1. Forms of Soil K Ignoring potassium (K) in soil organism, total K in soil may be classified as (i) water soluble K, (ii) readily exchangeable K (exchangeable K), (iii) slowly exchangeable K (non- exchangeable or fixed K) and (iv) mineral or lattice K. The different forms are in dynamic equilibrium with one another.

Sulphur (S) occurs in soil in both organic and inorganic forms. Inorganic S fraction may occur as sulphate (SO42-) and compounds of lower oxidation state such as sulphide (S2-), polysulphide (Sn2-), thiosulphate (S2O32-) and elemental sulphur (S0). In well drained soils inorganic S usually present as sulphate. Under anaerobic condition (reduced or waterlogged soil) inorganic S mainly occur as sulphide and under extreme condition as elemental S form. Estimation of S carried out in two steps:

18.1. Determination of Available Micronutrient Cations (Zn, Cu, Fe and Mn) in Soil The available micronutrient cations (Zn, Cu, Fe and Mn) can be estimated in a single extraction with diethylene triamine pentaacetic acid (DTPA) as proposed by Lindsay and Norvell (1978). DTPA has the excellent property to combine with free metal ions in the solution forming soluble complexes. The stability constants of metal-DTPA complexes make DTPA as a most suitable extractant for simultaneous complex formation with Zn, Cu, Fe and Mn. In calcareous soil DTPA extractant may result overestimation due to excess dissolution of CaCO3, thus releases occluded micronutrient cations which are not plant available. To avoid this excessive dissolution of CaCO3, DTPA is buffered at pH 7.3 using triethanol amine (TEA). At this pH major part (3/4th) of TEA is present as HTEA+ (protonation) and exchanges for Ca and Mg ion from the soil exchange site. This increases concentration of Ca and Mg in the solution phase and suppresses dissolution of CaCO3. The DTPA has the capacity to complex each of the micronutrient cations 10 times of its atomic weight and it ranges from 550 to 650 ppm depending on the micronutrient cation. The amount of cation in the extract is determined on an atomic absorption spectrophotometer (AAS). The experimental condition such as shaking time, DTPA concentration, pH and temperature during shaking influence the amount of Zn, Cu, Fe and Mn extracted. Increase in shaking time and temperature markedly influence the extractability of these cations.

19.1. Plant Available Fraction of Nickel, Cadmium, Lead and Chromium The DTPA method was originally developed to extract Mn, Cu, Fe and Zn from slightly acidic to alkaline soils (Lindsay and Norvell, 1978), but in some cases it can be used for other metals also such as Nickel (Ni), Cadmium (Cd), Lead (Pb) and Chromium (Cr). Among them Ni is considered as an essential micronutrient. The DTPA is used to extract soil metals by forming chelates with free metals in solution, which lowers their ionic activities so that additional quantities of metal are released from the soil until equilibrium is attained. Triehanolamine (TEA) is used to buffer the extracting solution at pH 7.3 in an attempt to optimize the extraction of various metals. Calcium chloride is included in the extracting solution to minimize dissolution of CaCO3.

References Allison, L.e., Bollen, W.E. and Moodie, C.D. (1965). Total carbon. P 1346-1366. In C.A.Black et al. (ed) Methods of Soil Analysis. Part 2. Agron. Monogr. 9. ASA, Madison, W.I. Arkley, T. H. (1961). Sulphur compounds of soil systems. Ph. D. diss. Univ. California, Berkley. Atterberg, A. (1911). Die Plastizitat der Tone. Int. Mitt. Bodenk., 1: 10-43. Atterberg, A. (1912). Die Konsistenz und die Bindigkeit der Boden. Int. Mitt. Bodenk., 2: 148-189. Beckett, P. H. T. (1964). The immediate Q/I relations of labile potassium in the soil. Journal of Soil Sci. 15: 9-23. Beer, A. (1852). Ann. Physik. Chem. (J. C. Poggendorff). 86: 78. Berger, K. C. and Troug, E. (1939). Boron determination in soils and plants. Ind. Eng. Chem. Anal. Ed. 11: 540-545. Bouyoucos, G.J. (1927). The hydrometer as a new method for the mechanical analysis of soils. Soil Sci., 23: 343 -353.