eChapters

Aim : To determine the Calcium in plant sample.

Principle

Carbonate (CO₃) and bicarbonate(HCO₃) ions are species of the same acid, carbonic acid. Their proportionate content is a function of pH. The CO₃ starts to form as pH rises above 8.4. Carbonate and bicarbonate are generally determined in soil saturation extract by titration with 0.01 N H₂SO₄ to pH 8.3 and 4.5, respectively.

Apparatus Required

Burette, pipette, conical flask and mechanical shaker etc

Introduction

Carotenoids are pigments found in plants that contribute to photosynthesis and provide various health benefits. Measuring carotenoid content in leaves can help assess plant health, productivity, and the effects of environmental conditions. The determination of carotenoids typically involves extraction followed by spectrophotometric analysis.

Materials and Equipments

• Fresh leaf samples

• Acetone, hexane, or a mixture of acetone and hexane (for extraction)

• Mortar and pestle or homogenizer

• Centrifuge and centrifuge tubes

• Spectrophotometer • Volumetric flasks and pipettes • Analytical balance

Chlorophyll content serves as a vital indicator of plant health and photosynthetic efficiency, with higher levels reflecting robust photosynthesis and overall plant vigor, while lower levels may signal nutrient deficiencies, diseases, or environmental stresses. Assessing chlorophyll content helps evaluate plant productivity, as chlorophyll is essential for absorbing light and converting solar energy into chemical energy. Additionally, the chlorophyll-a to chlorophyll-b ratio provides insights into how plants adapt to various light conditions, revealing their light absorption efficiency and ability to cope with environmental changes. Measuring chlorophyll on a fresh weight, dry weight, and leaf area basis allows researchers and agronomists to gauge the effectiveness of agricultural practices such as fertilization, irrigation, and crop management strategies. It also aids in monitoring the impact of stress factors like drought, salinity, pollution, or pest attacks, which is crucial for developing strategies to enhance crop resilience.

Reagents: 80% acetone: - 80 ml acetone + 20 ml water = 100 ml

Procedure for Chlorophyll Extraction from Leaves

1. Sample Collection: Collect fresh, healthy leaves from the plant to be analyzed. Ensure that the leaves are free from dust, pests, and diseases.

2. Preparation of Leaf Samples: Rinse the leaves thoroughly with distilled water to remove any surface contaminants. Pat them dry with a clean paper towel. Weigh an exact amount (usually around 0.1-0.5 grams) of fresh leaf tissue using an analytical balance. Record the fresh weight accurately.

Abstract

The geochemical signature and vulnerability of groundwater of the East Coastal aquifers of  India from Pondicherry to Nagapattinam were determined. The coastal aquifers are fragile and this situation becomes more intense in aquifer systems like that of the Pondicherry to Nagapattinam region. A total of 136 water samples were collected during pre and post monsoon of 2008 and 2009 respectively. Out of which 45 samples for each pre and post monsoon from shallow groundwater and 23 samples for each pre and post monsoon from deeper groundwater.. The samples collected were analysed for major, minor cations and anions. Selected samples were also analysed for stable isotopes (d¹⁸O and dD). The analysed samples were studied for; their utility for different purpose, the nature of the geochemical classification, their variations in ion ratios, extent of sea water intrusion and the percentage of mixing, The study reveals that chief mechanism controlling shallow groundwater is the salts precipitated in the pore spaces during the Pre monsoon were leached and dissolved during the subsequent monsoon apart from sea water intrusion. Southern part of the study area  is more vulnerable than the Northern part of the study area.

Breaking of soil aggregates and dispersion followed by a rapid drying leads to rearrangement and coherence of soil solids in the form of surface crust. The cohesion exhibited by soil upon drying is directly related to surface crusting and clod formation. The formation of soil crusts and hard clods is a major structural feature of soils. Seedling emergence of cereal crops is generally controlled by point resistance around the shoot tip (Richards, 1953). Penetrometer, which measures the resistance to vertical penetration in soil, is an effective instrument to evaluate the crust strength. A relationship between penetration resistance and seedling emergence would serve as a guide to ensure optimum soil condition for a desired plant density.

Apparatus and Accessories

Penetrometer, moisture box, trowel, electronic balance and oven.

Electrical conductivity of the soil is the quantity of electricity in coulombs that passes per second per cm3 of the soil solution under an applied potential difference of 1 volt. It is written as EC in abbreviated form and is expressed in millimohs cm-1 or dS m-1 (deciSiemen m-1). Current flows in the soil solution due to presence of soluble salts like Cl- , SO2- 4 and NO3 - of Na+, K+ Ca++ and Mg++. These salts fully dissociate in water to their constituent cations and anions and the current is carried by those cations anions in the soil solution.

Procedure

• Take 1 g soil in a 50 ml beaker

• Add 25 ml 1M KCL solution 

Reagents

• Buffer solution : Dissolve 67.5 g of Ammo. Chloride in 200 ml distilled water. Add 570 ml Ammonium solution. The volume make up to 1 ltr.

• EBT : 0.2 g in 25 ml methanol.

Calcium (Ca) and magnesium (Mg) are the two most abundant alkaline earth cations in soils. They both occur as soluble ions in soil solution, exchangeable, non-exchangeable and in organic material. Soils from arid regions are richer in Ca (1-2% CaO), while those from humid zones have a low content (0.01-0.02% CaO). On the other hand exchangeable Mg is found in very small quantities in calcareous soils. Sometimes, in acid soils, Mg²⁺ represents the largest quantity of the sum of exchangeable bases.

Principle

The method is based on the principle that calcium, magnesium and a number of other cations form stable complexes with versenate (ethylendiaminetetraacetic acid disodium salt, EDTA) at different pH. A known volume of standard calcium solution is titrated with standard 0.01N EDTA solution using murexide (ammonium purpurate) indicator in the presence of NaOH solution.

The end point is a change of color from orange red to purple at pH 12 when the whole of Ca forms a complex with EDTA. Calcium plus Mg are also present in the solution and can be titrated with 0.01N EDTA using buffer solution and a few drops Eriochrome Black T indicator. The end point is a change of color from red to blue at pH 10. Beyond pH 10, magnesium is not bound strongly to Erichrome Black-T indicator to give a distinct end point.

Carbonate is a natural constituent of many soils occurring as sparingly soluble, alkaline earth carbonate, chiefly as calcite and dolomite. Most carbonate minerals found in local soils are calcite and account 90% of total soil carbonates. The determination of total carbonate such as CaCO₃in soil is of great interest on account of high useful ess for diagnosing soil status in terms of structure, texture, biological activity or nutrient content. In calcareous soils carbonates exert a major influence on the chemical properties. Also carbonate content is used as a differentiating criterion for some classes at the family level of soil taxonomy.

Principle

A qualitative rapid test for free calcium carbonate is made by adding 10 mL of diluted HCl(1:3) to about 5 g of air dry soil on a watch glass and observing the intensity of effervescence which results from the evolution of CO Based on the . degree of effervescence the amount of calcium carbonate present in the soil can be broadly categorized into “traces or negligible, low, medium, high and very high”. Soils having traces to low amounts of calcium carbonate do not pose any problem. But those showing medium to very high degree of effervescence, a separate quantitative analysis should be carried out using Puri’s rapid titration method.

[Schoonover (1952) Method]

Introduction

Sodic (Alkali) and Saline-Sodic soils are characterized by the presence of large amounts of sodium, as high as 15% or more, on their exchange complex. As a result of this, the pH of soils increases beyond 8.0, causing nutritional imbalances, elimination of soil organic matter, deterioration of soil physical condition, etc., besides affecting the soil biotic community. Gypsum (CaSO₄. 2H₂O) is commonly used for the management of such soils, when applied in right amounts and size.

Principle

A given weight of soil is equilibrated with a known amount of Ca solution, and the amount of Ca, left in the solution is determined by EDTA titration. The difference between the amount of Ca added and the Ca left in the solution, gives the amount of Ca exchanged. In practice, gypsum of about 1/3 of the value, obtained by this method, is satisfactory in most cases.

Hydraulic conductivity is an important soil property that determines the ability of a soil to transmit water. A method of measuring hydraulic conductivity of saturated soil using undisturbed soil core is described in this exercise.

Principle

Flow of water across a soil layer or a sample of known dimension can be best described by Darcy’s law expressed as:

Infiltration rate refers to the volume of water entering into the soil per unit area in the unit time, when the soil is subjected to a shallow depth of ponding at the surface. Under continued ponding, the high infiltration rate in the beginning decreases to a constant value, known as final infiltration rate. The reduction in infiltration rate is due to combined effects of reduced hydraulic gradients, dispersion of aggregates, blocking of pores, cracks and channels, and swelling of colloids. Numerous techniques for measurement of infiltration rate have been developed to meet specific needs but use of double-ring infiltrometer has given consistent result in a variety of conditions.

Principle

In double-ring infiltrometer method, a small representative area in the field is bounded by two concentric metal rings (Figure 17.1) which permit impounding of water over the soil within the rings. The rate fall of surface of water ponded on the soil in the inner ring is taken as the rate of water intake or infiltration rate. In the cropped field, infiltration rate can be measured only within the rows and for which the diameter of the cylinders may be changed to meet the specific needs. The measurements under wetland conditions should be made from a distance of about 40-50 cm to avoid pressure effects.

Principle

When water is ponded inside the concentric metallic rings driven into the soil, the rate of recession (fall) of the water level, or the rate of water withdrawn from a supply source used to maintain a constant head of water on the soil surface, gives the infiltration rate of the soil.

During infiltration of water into the soil, appreciable lateral movement of water may also occur. To avoid the error due to this lateral movement, two concentric rings are used. Equal heights of water are to be maintained in both the rings. The infiltration is measured in the inner ring only. The rate of water intake per unit area varies markedly with the size of the rings and the depth of the soil to which they are driven into. Infiltration rate studies are preceded by the profile, initial soil moisture and any other problems of the region in which the soil is located.

Leaf Relative Water Content (RWC) is a key indicator of plant water status and is used to assess water stress, physiological health, and the efficiency of water use in plants. It reflects the amount of water in the leaf relative to its maximum water capacity and is a useful measure in plant physiology and stress studies.

Materials and Equipment

• Fresh leaf samples

• Analytical balance

• Oven or drying equipment

• Desiccator

• Caliper or ruler

• Distilled water

• Petri dishes or similar containers

• Weighing boats or paper

The lime requirement (LR) of an acid soil is the amount of lime needed to neutralize the acidity from an initial condition to some target soil pH, like 7.0. Different methods are used to calculate a LR for specific soils. Lime requirement tests will always recommend adding enough lime to neutralize all the acidity associated with soil pH, much of the salt-replaceable acidity, and some of the residual acidity.

[Shoemaker, Mc Lean and Pratt (SMP) Method, 1961]

Introduction

For proper plant growth, the soil should have a pH between 6.5 and 7.5, though there are certain plants which can grow satisfactorily at low pH, for example, tea, and at high pH, for example, sugar beet. In India, acid soils are located mostly in Eastern, Southern and South Central parts though some soils, at higher elevations, in North India are acidic.

In order to achieve maximum yield and sustained productivity through efficient soil management practices, it is essential to lime an acid soil, as it has considerable influence on soil environment, besides correcting soil acidity.

Several methods have been proposed to determine the lime requirement of acidic soils, to raise the pH around 6.5. The soil pH value alone, however, is not a good criterion for lime recommendation, because of variations in the exchange acidity of soil, and the nature of crops which may not require as high as this pH to produce maximum yield. Of these various methods proposed, the Shoemaker / SMP method was found satisfactory.

The recent trend of liming is to use it as a fertilizer rather than as amendment so as not to disturb heavily the natural environment while ensuring sustained productivity.

Aim : To determine the Magnesium in plant sample.

(Keen-Rackzowski/Hilgard Apparatus Method)

Principle

With the capillary rise of water, the soil immediately above a water table gets saturated. The height upto which the soil is saturated depends on the texture, structure and packing of the soil mass. The experimental technique discussed herein permits approximation of maximum water holding capacity while determining the amount of water held in the soil at the point of saturation.

All atoms can absorb light at certain discrete wavelengths corresponding to the energy requirement of the particular atom. When at ground state the atom absorbs light it is transformed into the excited state. It is the same atom containing more energy. This energy is measured in relation to the ground state and a particular excited state say for example in case of Na may be 2.2 eV (electron volts) above the ground state.

Each transition between different electronic energy states is characterized by a different energy and by a different wavelength. These wavelengths are sharply defined and when a range of wavelengths is surveyed, each wavelength shows as a sharp energy maximum (a spectronic line). These characteristic lines distinguish atomic spectra. The lines, which originate in the ground state of atom, are most often of interest in atomic absorption spectroscopy. These are called the resonance lines. The atomic spectrum, characteristic of each element, then comprises a number of discrete lines, some of which are resonance lines. Most of the other lines arise from excited states rather than the ground state. The lines of excited states are not useful generally in atomic absorption analysis as most of the atoms in a practical atomizer are found in the ground state.

The statement is still valid as long as it is understood that in an aqueous solution the substances involved are subject to dissociation, which changes the ionization state of the substances. 

Digestion Mixure : This is a mixture of two salt. That is potassium sulfate and cupper sulfate with 9:1 ratio (90 g Potassium sulfate + 10 g Copper sulphate).

The de    e plant growth, biomass composition, and energy potential.
(a) Nitrogen Determination by Kjeldahl Method
Theory
The Kjeldahl method is one of the most reliable and widely accepted procedures for determining organic nitrogen in biomass samples. It involves the conversion of nitrogen compounds into ammonium sulfate through digestion with concentrated sulfuric acid and a catalyst. The resulting ammonia is then distilled and quantified by titration. This method measures total nitrogen, excluding nitrate and nitrite forms, and is used extensively in plant, feed, and biomass analysis.
The process involves three major steps:
Digestion
Organic nitrogen in the sample is converted to ammonium sulfate by boiling with concentrated sulfuric acid and a catalyst mixture (usually potassium sulfate and copper sulfate) Organic-N+H2SO4?(NH4)2SO4+CO2+H2O
Distillation
The digested sample is made alkaline with sodium hydroxide, releasing ammonia gas, which is distilled and absorbed in a known volume of standard acid.

In the detection of soil organic carbon a known weight of soil is heated with an excess volume of standard K2 Cr2 O7 in the presence of Con. H2 SO4 . The soil is slowly digested at the low temperature by the heat of dilution of H2 SO4 and the organic carbon in the soil is thus oxidized to CO2 . The highest temperature attained by the heat of dilution reaction produced with the addition of Con. H2 SO4 is approximately 12o C which is sufficient to oxidize the active forms of the soil organic carbon but not the more inert forms of carbon that may be present.

Particle density of soil is the ratio of mass and volume of soil solids. Particle density of soil is an important physical characteristic because the interrelationships of porosity, bulk density and air space, and rates of sedimentation of particles in fluids depend on particle density.

Principle

The determination of particle density (ρ_p) requires the measurement of mass of the solid and its volume. The mass is obtained by weighing the oven- dry soil sample and volume by calculating from the mass and density of water displaced by the sample in a pycnometer. The pycnometer is a glass flask with a ground glass stopper having a capillary opening. The particle density can be calculated using the following equation:

Particle-size distribution is one of the stable soil characteristics, being little modified by agricultural management practices. The particle-size distribution expresses the proportion of various size particles in a soil, especially sand, silt and clay that determine soil texture (Hillel, 2004). Soil texture is closely related to soil physical properties such as water retention and release characteristics, surface area, swelling and shrinking, soil strength and tillage properties. There are two common methods being employed for particle-size (<2.0 mm) analysis namely international pipette method and the hydrometer method. Both the methods are based on Stokes’ law which establishes a relationship between particle size and the rate of sedimentation. Thus, particles are assessed by their settling velocities from suspension in a water solution that can be used to quantify particle size. Size range of soil particles (sand, silt and clay) according to two most adopted systems i.e. U.S. Department of Agriculture (USDA) and International Union of Soil Science (IUSS) formerly known as International Society of Soil Science (ISSS) are given below:

100 ppm ‘P’ : By dissolving oven dry KH2 PO4 salt. Take 0.4393 gm of KH2 PO4 and volume to 1 ltr that gives 100 ppm ‘P’.

Take 5 ml/2.5 ml aliquot in a 25 ml volumetric flask.

Soil aeration signifies the ability of a soil to supply oxygen to actively growing roots and soil organisms. Because of constant biological activity causing oxygen consumption and carbon dioxide evolution, the oxygen present in a soil pore or oxygen renewal by mass flow becomes inadequate to meet the plant demand. A continuous diffusion of oxygen from atmosphere to soil and to plant roots through the wetted pores plays an important role. The oxygen reaching plant roots has to diffuse not only through the gaseous phase in the soil pore and wetted pore neck but also through a water film that is present on the hydrated root surface.Lemon and Erickson (1952) suggested a platinum microelectrode method that simulates the field situation. In this method measurement of the rate of oxygen diffusing through the liquid phase to a reducing surface similar to the plant root.

Soil aggregates refer to a group of two or more primary particles which cohere to each other more strongly than to surrounding particles. Under field conditions, these units also cohere to some degree with neighboring aggregates and form larger aggregates. The aggregates form and break easily into smaller aggregates during tillage and by disruptive action of water and air. The aggregates maintaining their identity are those in which the cohesive forces among particles are greater than the disruptive forces. A quantitative characterization of aggregates is being done by determining aggregate stability and size distribution of aggregates. The aggregate stability refers to the resistance of soil aggregates to breakdown by water, air, and mechanical manipulations. Numerous methods have been proposed to determine size distribution and stability of soil aggregates (Kemper and Rosenau, 1986). The size distribution of wet and dry aggregates determines overall tilth, size of pores and susceptibility of aggregates to movement by water and wind.

Purpose

The bulk density varies indirectly with total pore space present in the soil and gives a good estimate of porosity of soil. Bulk density is of great importance than particle density in understanding the physical condition of soil.

Soil bulk density is defined as the ratio of the mass of the oven dry soil to its bulk volume.

Method of determination

Different methods are available for bulk density determination which differ how the soil sample is obtained and its volume is determined such as core, clod and excavation methods. There are other methods for the determination of bulk density where different principles are employed, for example, radiation method. Commonly used methods are weighing bottle, core and clod method.

Bulk density to which a soil can be compacted is dependent on water content and force of compaction. Adsorbed water on soil particles acts as lubricant and every addition of water increases ease of compaction to progressively a greater density. However, this increase in density will occur only to a certain value of water content beyond that it decreases with further addition of water in soil. The moisture content at which maximum dry density is obtained for a limited and fixed effort is called as proctor moisture and the density at that point is known as maximum or proctor density”.

Principle

A soil at varying water contents, compacted with a specific amount of compaction energy, would result in a bulk density-water content relationship. The maximum bulk density obtained at the proctor water content represents 100 % compaction. The test can be performed in laboratory on various soils using Abbot’s compactor

Soil consistency refers to the resistance offered by soil against forces that tend to deform it. The soil consistency varies with texture, kind of clay and moisture content. The efficiency of agricultural implements, animals and labour is largely affected by soil consistency as it directly inf luences the workability of a soil. Operations at suboptimal or supraoptimal water contents lead to clod formation, compaction, poor drainage and aeration. The common terms suggested by Atterberg (1912) to describe consistency are liquid limit, plastic limit, plasticity index, and sticky point. The physical basis of consistency limits is related to the interaction of water with soil. The water content at which plasticity is exhibited is the plastic limit or lower plastic limit of soil.

10.1 Liquid limit

Principle

The liquid limit refers to the water content at which resistance to f low becomes negligible and soil represents a semi f luid. A trapezoidal groove of specified shape cut soil held in a special cup will close in 25 taps on a hard rubber plate. The soil in the cup is 1.0 cm deep and the groove is 2.0 mm wide at the bottom with slopes outward at a 60^° angle horizontally. The soil in the walls of the groove f lows under the impact of the cup on the hard base and close the groove.

Electrical conductivity (EC) is a useful indicator of soil salinity. Relationships have been developed between EC and salinity classes for a 1:1 soil: water suspension. With increase in salt content in a soil, the EC increases. Thus, for salt affected soils, measurement of EC is vital as accumulation of salts in the root zone is detrimental to plant growth. EC is a measure of the ability of a soil solution to conduct electrical current.

Equipment and Reagents

1. Digital Conductivity meter
2. 0.01 M potassium chloride (KCl) solution: Dissolve 0.7456 g dry KCl in distilled water and make up the volume to 1.0 litre. This solution has an electrical conductivity of 1.413 dS m_-1 at 25ºC.

The variable amount of moisture contained in a unit mass or volume of soil and is the key factor affecting the growth of plants. Numerous other soil properties depend very strongly upon moisture content. Gravimetric (direct) and sensor based (indirect) methods are generally employed for soil moisture determination.

9.1 Gravimetric Method (Direct)

Principle

The gravimetric method is based on the principle that the water in the porous material is lost by evaporation on heating and the porous material undergoes an equivalent loss in its weight. However, the water loss may be limited by the internal operation within the complex porous material such as soil. This method is simple, routine, reliable, inexpensive, and easy to use. The major limitations of this method are that it is destructive, laborious and time consuming.

Principle

Soil moisture content is determined by drying a known quantity of soil sample in an electric oven at 105° C to 110° C and finding out the loss in weight.

This is one of the commonly used method to estimate organic carbon content in soil. In this wet digestion method, soil organic matter is oxidised with a mixture of K₂Cr₂O₇ and concentrated H₂SO₄ utilizing the heat of dilution of H₂SO₄. The unused K₂Cr₂O₇ is back titrated with ferrous sulphate or ferrous ammonium sulphate.

Soil Organic Matter (SOM) is the seat of nitrogen in soil and its determination is often carried out as an index of nitrogen availability. The top soil (surface soil) range from 0.5% to 3.0% organic carbon for most upland soils and less than 0.5% are limited to desert soils. Soil organic matter averages about 58% carbon, it follows that soils generally range from about 2 to 6 % organic matter (% O.M. = %C x 1.724, The factor 1.724 = 100/58). There is also a close relationship between carbon and nitrogen in soils. Most organic matter average about 5% nitrogen so that the N: C ratio is 1:11.6. Therefore by multiplying the soil organic matter percentage by 0.05 an approximate value for the soil nitrogen, percentage is obtained. In soil the chief source of some of the nutrients essential for plant growth is organic matter, such nutrients are N, S and boron is also largely derived from organic matter.

Particle density determination is based on the measurements of weight of solids and their volume. The weight of solids is obtained by weighing a sample of oven dried soil. The volume of solids is determined is determined by immersion. The method outlined here is “Pycnometer method”.

Principle

A given amount of dry soil when immerses in a definitive volume of water, expels air and results in the displacement of equal volume of water. The volume of soil particles is determined by measuring the volume of water displaced in the pycnometer bottle.

Determination of pH is actually measurement of hydrogen ions activity in soil-water system. It is defined as negative logarithm of the hydrogen ion activity. Mathematically, it is expressed

as:pH = - log a H⁺

The pH value of a soil is an indication of soil reaction i.e. acidic, neutral or alkaline. The nutrient availability is governed by soil reaction. It is maximum at neutral pH and decreases with increase in acidity or alkalinity. Thus, pH value gives an idea about the availability of nutrients to plants.

Principle

The pH is a measure of hydrogen or hydroxyl ion activity of the soil-water system. It indicates whether the soil is acidic, neutral or alkaline in reaction. Since crop growth suffers much under both very low (strongly acidic) as well as very high pH (alkaline) conditions.

Now a days, most of the pH meters have single combined electrode. Before measuring the pH of the soil, the instrument has to be calibrated with standard buffer solution of known pH. Since, the pH is also affected by the temperature, hence, the pH meter should be adjusted to the temperature of the solution by temperature correction knob.

Objective

To determine the pH of a soil sample, which provides insight into the soil’s acidity or alkalinity, influencing nutrient availability, microbial activity, and plant growth.

Materials Required

• Soil sample

• Distilled water

• pH meter or pH paper

• Beakers (100 mL)

• Measuring cylinder (50 mL)

• Stirring rod

• Filter paper and funnel (if needed) • Mortar and pestle (for soil grinding, if necessary)

Soil pores differ greatly in size, shape, tortuosity, and continuity. They are characterized generally in terms of total porosity, air-filled porosity, and pore-size distribution. The characterization of pore space is important in understanding the storage and movement of water and gases, the development of plant root systems, the flow and retention of heat, and soil strength. The observation that bulk density is considerably lower than the average particle density indicates that only part of the bulk volume is occupied by soil solids and the remainder is occupied by a lighter material, such as air and water.

Total Porosity

Total porosity (n) is the percentage of the bulk volume not occupied by solids and can be computed using following relationship:

Soil pH is the measure of hydronium ion (H₃O⁺) or more commonly hydrogen ion (H⁺) activity in the soil suspension. Soil pH can be measured in soil-water suspension, saturation extract or in 0.01 M CaCl₂ solution. Saturation extract is used to identify specific soil problems such as salinity or sodicity. Measurement of pH using 0.01 M CaCl₂ solution gives more reproducible results than measured through soil-water suspension. It also provides a good approximation of field conditions as the electrolytic composition of 0.01 M CaCl₂ is considered similar to that of soil solution. However, among the three methods, soil-water suspension is most commonly used in research laboratories as well as for fertilizer recommendation as it is simpler and more convenient.

(International/Robinson’s Pipette Method)

Principle

This method is based on Stoke’s law. According to this law, the rate of fall of a particle in liquid is directly proportional to the square of its radius.

Determination of soil texture mainly depends on the complete dispersion of soil aggregates into their individual primary particles without triturating the particles themselves, accurate fractionation of the sample into various separates, sedimentation of different particles in the viscous medium as well as measurement of individual particle by the hydrometer.

The method is based on the Stokes law which states that the rate of fall of particles in suspension is directly proportional to their size. Accordingly, larger particle (sand) will settle first, followed by silt and clay. The resistance offered by the liquid to the fall of the particles varies with the radius of the sphere.

Principle

This method is based on the principle of dispersion and sedimentation techniques, employed to a given weight of soil sample. Mechanical stirring disperses macro aggregates while chemical dispersing required for micro aggregates. The mixture of dispersed soil particles in water is called a soil suspension. Sedimentation refers to the settling rates of the dispersed particles in water, which is a function of particle size and is governed by Stoke’s law.

As water enters the soil, air is displaced and the soil pores in the upper part of the profile become saturated. A continuous supply of water leads to an increasing depth of saturation. If the supply of water at the soil surface is cut off then the rapid downward movement of water may essentially ceased within 48 hours. The soil is then said to be at its field capacity. At this stage macropores are drained off but micropores are still filled with water for use by plants. Since it is difficult to draw a boundary line between macro-and micropores and there is no sudden change in the profile drainage rate for many soils, it is not easy to specify a depth and time after which soil water movement is negligible. Therefore, measurements are necessary in each soil to specify the time and depth of sampling after wetting. Special care would be necessary in soils with impeded drainage and sand lenses, which produce perched water tables.

Proportion of solid, liquid (water), and gas (air) components in a soil mass indicates physical conditions such as soil structure, porosity and water content. A well Structured soil may have a high pore volume whereas a compacted soil may have little pore-space for other phases to occupy. In general, soil solids may occupy 50 % of the soil volume in a well-aggregated fertile loam and the remaining volume may be occupied by liquid and gases interchangeably.

6.25 HNO3 (for 2 ltr) : Take 181 ml of conc HNO3 and diluted to 2 ltr in a volumetric flask.