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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.

Theory

The specific gravity is defined as the unit weight of the product mass divided by the unit weight of distilled water at 4°C. The unit density is obtained by measuring the volume and weight of a unit product. Sometimes it is required to compare the density of the product to the density of water. This comparison is in the form of ratio and is termed as the specific gravity.

Specific gravity also known as relative density is the ratio of density of a substance to the density of a standard substance at the same temperature and pressure. For solids and liquids the standard substance is water while for gasses it is air. The specific gravity of food material play an important role in design of storage bins, conveying equipments, separation equipments, etc. The specific gravity is frequently used to solve the various phase relationships, including void ratio, porosity and degree of saturation together with the moisture content and unit weight. It is also used in calculations associated with the grain size analysis, consolidation and compaction. Pycnometer method, which is a standardized vessel for determining specific gravity. It involves a standard container of accurately defined volume used to determine the specific gravity of solids and liquids.

Determination of Specific Gravity

The palatability of the citrus fruit is associated with its internal quality. Measuring specific gravity provide clues to internal quality of the fruit. The ratio of the weight of the fruit to its submerged weight indicates specific gravity. Specific gravity also indicates relationship between fruit weight and volume.

Steps: To determining specific gravity

Record the weight of fruit in air on top loading balance.

Materials Required

Specific gravity bottles, weighing balance, sample, pipettes, thermometer, distilled water 

Procedure

    1.    Take the sample and distilled water record their temperatures (they should be same for correct estimation). If the temperatures are not same bring both to 20ºC or the room temperature.
    2.    Clean and dry the specific gravity bottles properly before use.
    3.    Record the tare weight of the specific gravity bottle (T) alongwith the stopper.
    4.    Carefully fill the specific gravity bottle with distilled water up to the brim and then place the stopper tightly so as to exclude extra water. 
    5.    Dry the exteriors of the bottle with a tissue paper
    6.    Record the weight of (bottle + water) say A
    7.    Similarly, fill the same specific gravity bottle with test sample and record the weight (bottle + sample) say B
    8.    Calculate the specific gravity of solution using the following formula

Theory

Specific heat is defined as the amount of heat needed to raise the temperature of one kg of the material by one degree at constant pressure without a change in state. With food materials, specific heat is a function of different components of a food, moisture content, temperature, and pressure. Specific heat is an important parameter in thermal analysis of food processing and equipment used in heating or cooling of foods. The numerical values of specific heat of the food materials are used in designing food processes and processing equipment. It can be used to calculate the heat load imposed on the refrigeration equipment by the freezing of foods. An apparent specific heat is used in freezing or thawing and in most food processing applications.

The specific heat of a food increases as the product moisture content increases. In unfrozen foods, specific heat capacity is relatively constant with respect to temperature. However, for frozen foods, there is a large decrease in specific heat capacity as the temperature decreases. The specific heat of a material can be determined experimentally by static (adiabatic) calorimetry or differential scanning calorimetry. It can be also predicted quite accurately with the help of a number of empirical equations. The equations are empirical expressions, obtained by fitting experimental data into mathematical models. The mathematical models are based on one or more constituents of the food. Since water is a major component of many foods, a number of models are expressed as a function of water content.

Stomata are pores formed between two specialized epidermal cells, the guard cells, which are found on the surface of aerial parts of higher plants (leaves). They control the gas exchange between the leaf and the ambient air. Their main functions is to facilitate the entry of CO² into the leaf for photosynthesis and loss of water vapour for evaporative cooling of the leaves.

The guard cells are formed by the transverse division of stomatal mother cells that differ morphologically and biochemically from other epidermal cells. The middle lamella disintegrates and the two walls separate leaving an opening between them the stomatal pore. The walls bordering the pore become considerably thickened. They are generally cutinised. This radial thickening of the inner wall plays an important role in stomatal movement. Stomata control 95 per cent or more of CO² and water vapour exchange between the leaf and atmosphere.

Location and occurrence of stomata: Stomata are found in all higher plants down to the evolutionary level of the phylum Bryophyta. Stomata are not found in liverworts. In the leaves of most herbaceous plants the stomata are found in both upper (Adaxial) and the lower (Abaxial) surfaces. Such leaves are known as Amphistomatus. Usually in tree species, stomata are found only on the lower surface of the leaves. Such leaves are called Hypostomatus. Aquatic plants with floating leaves such as water lilies have stomata only on the upper surface and such leaves are called Epistomatus. Most of submerged aquatic plants do not posses stomata and diffusion of gases occurs between water and plant tissues across a thin cuticle. In many cases subsidiary cells are present adjacent to the guard cells. These are specialized epidermal cells which are usually different in size and shape from other epidermal cells. They are usually smaller than the epidermal cells and have more dense cytoplasmic contents and a greater frequency of cell organelles. In grasses and some other species, subsidiary cells are intimately involved in stomatal mechanism.

Diffusion of gases through stomata is governed by the two important parameters associated with the stomata, viz., the stomatal frequency and the extent of opening of the stomata. Stomatal frequency is defined as the number of stomata per unit leaf area (no.mm ^-2 ). Apart from the stomatal frequency, yet another parameter that reflects the stomatal distribution is the stomatal Index. It is the ratio of the number of stomata per unit leaf area to the total number of epidermal cells and is generally expressed in percentage.

(Method of Bryant et al., 1959)

Strontium and barium are adsorbed on a cation-resin column, and  after a suitable gowth period, yttrium-90, the daughter of strontium-90, is selectively eluted. The yttrium-90 is then adsorbed on and eluted from a second cation-resin column and counted. The radiochemical yield is greater than 97% gravimetric measurement of the recovery of strontium and yttrium carriers is not required. The method, with modifications, is applicable to samples that contain 100 mg of iron or uranium.

Principle: The copper in the Fehling solution is reduced to red, insoluble cuprous oxide in the presence of invert sugars like glucose and fructose. The quantity of sugars solution liquids for complete neutralization of a known quantity of Fehling mixture is determined by titration using methylene blue as the indicator.

(Lane and Eynon Method)

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

Aim

To determine the tapped bulk density and flowability of the food powder.

Materials required

Raw material (powder)

Measuring cylinder

Electronic weighing balance

Tapped bulk density of the powder

Tapped bulk density is measured for powdery samples. The volume is measured after tapping the powdery material so that the product is adjusted to inter-granular spaces, but it cannot eliminate all the inter-granular spaces. The tapped density is an increased bulk density attained after mechanically tapping a container containing the powder sample.

Theory

Texture testing is a well-established technique for evaluating the mechanical properties of raw materials, food structure, pre- and post- harvest quality control checks. The texture testing has many applications across wide range of foods including cereals, fruits, vegetables, meat, poultry, fish, baked foods, confectionaries, snacks, dairy, gelatins, pasta, etc. Food texture analysis of foods can highlight the quality improvement throughout the supply chain. At research and development stage, new or alternative ingredients can be compared with existing ingredients.

Texture profile parameters

The texture profile parameters including hardness, firmness, crispiness, rupture strength springiness, chewiness, cohesiveness, gumminess, gel strength, adhesiveness, consistency, crunchiness, elasticity, extensibility, fracturability, stiffness, stringiness, toughness, work to cut, penetrate and shear can be measured by the mechanical food texture analyzer.

Aim

To determine the reduced level of various points

To find out difference of elevations of two points

Materials required

Dumpy level

Levelling staff

Field capacity may be defined as the amount of water held in soil after excess water has drained away and the rate of downward movement of water was materially decreased which usually taken place within two to three days after heavy rain or irrigation. In this condition water was moved out of the macropores and air has moved into take its place. 

Infiltration is the entry of a fluid from one medium into another. Soil water plays an important role in nature. The entry of rain or irrigation water into soil is called infiltration. Infiltration rate is defined as the amount of water entering into a soil per unit area per unit time under one atmospheric pressure. The infiltration rate depends upon initial moisture content of soil, condition of soil surface and permeability of various layers.

The maximum water holding capacity is the amount of water, expressed as percentage on oven dry basis held by a soil when it is completely saturated with water. At this moisture content, all the pore spaces are completely filled with water.

Object: Determination of the viability of seeds by tetrazolium test. 

Material required: Healthy seeds, tetrazolium solutions, beakers, water and petridishes. 

Theory: Viability of seeds shows their potential towards germination.

Aim

To determine the thermal conductivity of the agricultural materials probe method.

Materials required

Thermal conductivity probe

Stainless-steel sample container

Grain or powder sample

Graph sheet

Power source

Materials Required

Sample, 0.1 N NaOH solution, 1% phenolphthalein indicator solution, beakers, conical flasks, pestle and mortar, funnels, volumetric flasks, pipettes, pH meter, shaker.

Reagents

    1.    0.1 N NaOH: Dissolve 4 g NaOH (AR) crystals in sufficient water to make up the final vol. 1 litre.

    2.    1 % Phenolphthalein: Dissolve 1 g in 100 ml water. Readymade indicator solutions are more commonly used.  

Procedure

Preparation of sample: For fresh fruits and vegetables, prepare a well blended, uniform pulp in a pestle and mortar and for processed products, take out a representative sample and blend it well so that it becomes uniformly soluble when added to water. Filter through a fine muslin cloth or coarse filter paper if felt necessary. The liquid samples can be taken directly for estimation.
    1.    Take out a known weight / vol. (say 20 g) of sample in a beaker (in 3 replicates).
    2.    Make it to a known vol. (say 100 ml) with added distilled water.
    3.    Take 10 ml of this aliquot in a conical flask.
    4.    Add 1-2 drops of phenolphthalein indicator into the aliquot.
    5.    Titrate against 0.1 N solution of NaOH till light pink coloured end point is obtained.
    6.    For coloured products, i.e. tomato products where judgment of light pink colour is difficult, titration may be done over a shaker, and digital pH meter may be used for continuously measuring pH of aliquot which would show a steady increase with more and more addition of 0.1 N NaOH. Titre value may be recorded when pH of 8.3 is obtained.
    7.    Record the titre value and calculate the acidity of the sample as per the given formula.

Introduction

Demonstration of biological nitrogen fixation is usually accomplished by estimating the total nitrogen through use of some modification of the Kjeldahl procedure. This technique is accurate and reliable whenever a medium initially low in nitrogen is employed.

Principle

The dried organism is digested with sulphuric acid. Organic and nitrate nitrogen is converted to ammonium sulphate and the ammonia gas is distilled into boric acid forming ammonium borate making weak boric acid alkaline. The ammonium borate is back titrated with standard sulphuric acid until the reaction mixture changes its colour from green to pink due to formation of ammonium sulphate. The standard acid used in back titration of ammonium borate is a measure of nitrogen content of the organism.

Principle: A weighed sample is dried in an oven at 60-70⁰C up to a period of time when weighing made at 2 hour intervals do not differ by more than 2-3 mg.

Materials Required

Fruit/ sample, refractometers, thermometers, standard solutions of known TSS, temperature correction Tables.

Procedure

    1.    Calibrate refractometers of all ranges to be used.
    2.    The TSS of the juices/pulps/fresh fruits can be measured with the one having lowest range i.e. 0-32º Brix while, for measuring the TSS of concentrates, syrups, squashes, jams, jellies etc. refractometers of the higher ranges are made use of.
    3.    Put a drop of material on to the glass surface of refrectrometer’s prism, cover gently with the lid and observe the TSS against light.
    4.    Measure the temperature of the liquid which is being evaluated and apply temperature corrections (Annexure III).
    5.    Record the corrected value as the TSS of the sample.

23.1. Aim

To determine the total soluble solids in fresh and processed fruit and vegetable products.

23.2. Theory and principle

Total soluble solids (TSS) are determined by means of brix which measures the specific gravity or by using hand refractometer which measures refractive index. Brix is a measure of soluble solids only in case of pure sucrose solutions. Generally, the fruit juices contain more sugar than any other soluble constituent. Hence, brix provides a useful guide of soluble solids or sugar contents. Brix hydrometer is used to determine the percentage of brix in syrup while Abbe refractometer gives refractive index as well as degree brix. The Refractive index of the test solution is measured at 20°C using a refractometer and the soluble solid content is determined by the use of tables correlating refractive index with soluble solids. Generally, hand refractometer is used for determining TSS in food samples in fruit processing units (Table 3.1).

Purpose: The estimation of total soluble constituents gives the approximate amount of water soluble substance present in the sample. Among the various soluble substances the amount of sugar is 80-85 percent. The TSS value is roughly considered to be the amount of sugar and soluble minerals present in fruits and vegetables. The total soluble solid content can be determined by hand refrectometer.

Material and equipments: Hand refrectometer, juice, muslin cloth or tissue paper and glass rod.

Introduction

Urea is one of the common fertilizers used as a source of nitrogen. It has nitrogen in amide form (-NH₂) and this form of nitrogen is not utilized by plants. So it has to be hydrolysed. The enzyme, urease hydrolyses the urea to ammonia and ammoniacal nitrogen is taken up by plants or further nitrified.

The measurement of activity of this enzyme in soil is important from soil fertility point of view. There are two methods for determination of the urease activity of soils.

• First involves the measurement of end products of urea hydrolysis. i.e., NH₃ or CO₂.

Theory

The volume of food material play an important role in design of storage bins, conveying equipments, separation equipments, etc. Volume of larger products such as fruits and root crops can be determined using the platform scale or water displacement method.

Objective
After performing this experiment you will be able to learn:
The water absorption of packaging material by Cobb method.
Introduction

The water absorptive capacity, or Cobb value, is defined as the mass of water absorbed in a specific time by 1 square meter of paper, board, or corrugated f iberboard under a head of 1 cm of water. To determine this value, conditioned test pieces are weighed before and after being exposed to water under specified conditions. The excess water is removed using blotting paper and a brass roller of defined dimensions and weight. The test piece is weighed immediately before and after exposure to water for a specified duration on one surface. The increase in mass due to water absorption is expressed in grams per square meter (g/m²).

The Cobb value defines the water absorbency of paper as the amount of water absorbed by one side of a unit area of paper or board placed horizontally under a 1 cm water head within a given time.

Principle

This method, developed by Cobb, measures the amount of water absorbed by paper, paperboard, or corrugated fiberboard over a specified period under standardized conditions. It is applicable to paper or board that is not fully penetrated by water during the selected absorption time.

Purpose

The determination of water holding capacity in soils is important as it gives an idea of the capacity of soil to hold water for the use by crops. The light soils which do not hold such water require more frequent irrigations than heavy clay soils; well decomposed organic matter increases the water holding capacity. Exchangeable sodium and type of clay mineral also have a marked effect on water holding capacity. Water holding capacity of soils is useful for selection of soils for irrigability classification. It also helps for comparing other properties of soils.

Water holding capacity of soil is the amount of water that a given soil can hold against the force of gravity. Soil texture and organic matter are the key components that determine the water holding capacity of a soil. Soils with smaller particle sizes (silt and clay) have larger surface area and can hold more water as compared to sand which has large particle sizes and smaller surface area. It is an important hydro-physical characteristic of a soil because it provides simple means of determining useful moisture levels to maintain soil for good plant growth. Detail procedure for determination of water holding capacity of soil using Keen box method (Piper, 1966) is described here.

Over most of the wetness range in which plant roots normally function, all the water in soil is controlled by matric forces. A relationship between water content and matric or soil water suction is referred to as water characteristics of the soil. Increase in matric suction is associated with decrease in water content in the soil and the process is known as desorption. On the other hand with decrease in suction soil moisture content increases and this process is known as sorption. The curves showing relationship during progressive intake and withdrawal of water are known as sorption and desorption curves respectively.

Tensiometer is suitable for finding out soil water characteristic at tension values below 2.0 kPa, whereas gas phase pressure is required for higher tensions. A pressure chamber containing a porous membrane or a ceramic plate with all its accessories to provide regulated compressed air is most convenient. A detailed methodology to determine water characteristic curve by pressure plate apparatus is described below.

Aim

To determine the water solubility and absorption index of given food powder.

Materials required

Digital centrifuge machine

Graduated measuring cylinder

Beaker and distilled water

Soil aggregates refers to a group of two or more primary particles which cohere to each other more strongly than surrounding particles. Under field condition, all such units also cohere to some degree with neighboring aggregates and form larger aggregates. Such 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 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.

The soil aggregates which can stand the disruptive forces of wetting by water are known as water stable aggregates. Therefore, the process of wetting, disruption of dry aggregates and screening of aggregates of different sizes should reasonably compare to the disruptive action of water and mechanical forces of tillage under wet conditions. The vacuum wetting of dry soil largely simulates the process of soil wetting in the surface layers but the immersion wetting is more comparable to the wetting of surface soil by irrigation. Similarly, sieving under water compares more closely with the disruptive action of water and other mechanical forces. The mean weight diameter (MWD) of water stable aggregates is a useful index to characterize the change in structure due to tillage. The mean weight diameter can be calculated using following equation.

Primary particles tend to adhere under natural condition so as to form secondary unit called aggregates. It is very difficult to determine the diameter of the individual aggregated particle. So the mean weight diameter is calculated as a single value constant, which indicate the wetted average value for a group of aggregates.