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H.K. Rai, A.K. Upadhyay
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Soil Physical Analysis is written for Bachelor and Master Students to learn and perform soil physical analysis in how-to-do-it manner. It will also be useful for teachers, researchers and extension functionaries working in the field of soil science. This handbook includes 20 chapters containing standard procedures for the soil sampling, determination/ measurement of particle size distribution (texture), density (bulk & particle), compatibility, component phases, aggregate analysis, crust strength, consistency, porosity, water retention and characteristics, water holding capacity, hydraulic conductivity, infiltration rate, aeration and temperature of soils, glossary of soil physical terminologies and basics of soil profile and soil orders.

0 Start Pages

Preface Soil health is one of the key concerns in today’s agriculture system to achieve sustainable productivity and knowledge of the physical properties of soil is essential for defining and improving the soil health. There is a strong growing realization that productivity of crops is limited by the physical conditions rather than plant nutrient status in the soil. Proper understanding of the dynamics of soil solid, water, air and temperature under field conditions is crucial for developing a suitable management technology for improving soil health. However, soil physical changes and their impact on crop production are less easily apprehended because of their exceedingly complex and dynamic nature. A need for simple and easy-to-use procedures for soil physical analysis has long been felt by the students and scientists. This book is based on teaching and research experience of the authors for more than fifteen years. The methodologies presented in this book are user-oriented to help teachers / researchers and students for self-study. The suggested methods and illustrations may also serve as guide for experimenters to device techniques of measurement and instructions suited to their conditions. The authors are highly grateful to Hon’ble Vice Chancellor Prof. P. K. Bisen, Dr. P. K. Mishra, Dean Faculty of Agriculture, Dr. Dhirendra Khare, Director of Research Services, Dr. (Smt.) Om Gupta, Director of Extension Services, Dr. S.D. Upadhyay, Director Instruction, JNKVV, Jabalpur, Dr. R.M. Sahu, Dean, Dr. B. L. Sharma Professor and Head Department of Soil Science & Agricultural Chemistry, College of Agriculture, Jabalpur for encouragement, suggestions and provision of necessary facilities to complete this task. The authors are also thankful to all the faculty members and technical staff of Department of Soil Science & Agricultural Chemistry, JNKVV, Jabalpur.

1 Collection and Processing of Soil Sample

Precise collection and processing of soil sample is the first and foremost step for carrying out any soil study, therefore it is essential to collect the coil sample very accurately keeping in view the variations in slope, colour and texture of the soil management aspects. Soil sampling should be avoided from non-representative sites like recently fertilized plots, bunds, channels, marshy tracts and spots near trees, wells, compost piles etc. It should be kept in mind that erroneous samples will never yield accurate result in spite of very improved method of analysis. The depth of sampling may vary according to the need of soil study and extent of rhizosphere. However, in general the sampling depth for field crops is the plough layer (0 - 15 cm) but it could be up to one meter or as per the purpose of the study. Soil samples in dry farming areas should be taken depth wise as the roots of the crops in this conditions reaches to deeper layers in search of water and nutrients. Mainly two types of soil samples (disturbed and undisturbed) are used for determining the physical properties of soil. The disturbed soil samples are used for determination of particle size distribution, particle density, aggregation, column studies for diffusion-dispersion coefficients and soil-water diffusivity. Whereas, undisturbed (core) soil samples are used for the estimation of bulk density, porosity, retention and transmission characteristics of air and water.

1 - 4 (4 Pages)
2 Determination of Particle Size Distribution

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:

5 - 14 (10 Pages)
3 Determination of Bulk Density of Soil

Bulk density is the ratio of oven-dry mass of soil to its total volume before drying. Knowledge of bulk density is useful in calculating moisture content by volume, porosity using particle density, volume of a known weight of soil, and weight of a furrow-slice. Bulk density is a direct measure of soil compaction. Bulk density is highly dependent on any kind of physical manipulation, degree of wetness, and kind and arrangement of soil particles. For this reason, preservation of original structure in the sample is most essential aspect of the methodology for determination of bulk density. Common methods of determining soil bulk density use soil cores or clods in their natural state.

15 - 18 (4 Pages)
4 Determination of Particle Density of Soil

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 (ρp) 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:

19 - 20 (2 Pages)
5 Determination of Soil Compactibility

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

21 - 22 (2 Pages)
6 Determination of Soil’s Component Phases

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.

23 - 26 (4 Pages)
7 Determination of Soil Aggregates

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.

27 - 32 (6 Pages)
8 Determination of Crust Strength of Soil

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.

33 - 34 (2 Pages)
9 Determination of Soil Moisture

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.

35 - 40 (6 Pages)
10 Determination of Soil Consistency

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.

41 - 46 (6 Pages)
11 Determination of Soil Porosity and Pore Size Distribution

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:

47 - 50 (4 Pages)
12 Measurement of Soil Moisture Tension by Tensiometer

Tensiometer is a device to measure soil moisture tension or equivalent negative pressure of water in the soil. A tensiometer consists of a tube sealed at one end by a porous ceramic cup which is in contact with the soil and a vacuum gauge or a manometer connected at the other end above the soil surface. When the tensiometer is filled with water, the manometer or gauge will read zero tension. When such a tensiometer is placed in an unsaturated soil, the soil in contact with the porous ceramic cup will immediately start sucking water through the porous cup wall. The water in the soil pores immediately establishes hydraulic equilibrium with the water in the cup through saturated pores of the cup wall. Water and solutes can f low through the pores of the cup wall but not the air within the working range of the tensiometer. Water could be sucked from the tensiometer by the soil until vacuum created inside the tensiometer is just sufficient to overcome the suction of the soil. At equilibrium, the tensiometer reads directly the amount of soil suction. The outf low of water from the cup is very little. The effective range of the tensiometer is 0 to 0.85 bars depending on quality of the porous cup.

51 - 54 (4 Pages)
13 Determination of Soil Water Retention

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.

55 - 58 (4 Pages)
14 Determination of Water Retention Characteristics of Soil

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.

59 - 60 (2 Pages)
15 Determination of Water Holding Capacity of Soil

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.

61 - 64 (4 Pages)
16 Determination of Hydraulic Conductivity of Soil

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:

65 - 68 (4 Pages)
17 Determination of Infiltration Rate of Soil by Double-ring Infiltrometer Method

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.

69 - 72 (4 Pages)
18 Determination of Soil Aeration (Oxygen Diffusion Rate)

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.

73 - 76 (4 Pages)
19 Measurement of Permeability of Soil to Air

The ratio of soil air-to-soil water permeability indicates the magnitude of breakdown of structure as a result of wetting. A value of one indicates no change in structure and a value greater than one reflects a deterioration of soil structure. Principle The permeability of soil to air can be obtained essentially in the same way as the permeability of soil to water. The measurement of permeability of soil to air involves the preparation and placement of the soil in a suitable container with provision for monitoring pressure differential across the sample and the rate of flow of air through it. For steady state case, this involves measurement of inlet and outlet pressures and the volume flux of air. The suitable form of the Darcy’s equation for calculating permeability of soil to air is:

77 - 78 (2 Pages)
20 Measurement of Soil Temperature

Soil temperature plays an important role in plant growth. Solar radiation received on the earth surface raises the temperature of the soil surface from morning to solar noon followed by a decline until the next morning. Soil temperature gradients established on the surface lead to flow of heat downwards causing a rise in temperature of lower layers. These variations in temperature affect germination, emergence and further growth of plants. Several types of thermometers such as mercury or liquid in glass, bimetallic, bourdon gauge, thermocouples and electrical resistance have been used in soil research but the use of thermocouples and thermistors has been found most useful. A brief description of different types of thermometers used in measurement of soil temperature is given below: Liquids in Glass Thermometer A liquid-in-glass thermometer (Figure 20.1) consists of a capillary glass tube with a bulb at one end filled with a thermometric liquid (mercury), vacuumed and sealed. By reading the position of the liquid level on a scale the temperature value can be obtained.

79 - 82 (4 Pages)
21 End Pages

Annexure I Description of Soil Profile The soil profile is defined as a vertical section of the soil that is exposed by a soil pit. A soil pit is a hole that is dug from the surface of the soil to the underlying bedrock. Because of the way soils develop, most soil profiles are composed of a series of horizons or layers of soil stacked on top of one another like layers of a cake. These horizons can tell us a lot about how the soil formed and what was going on around the soil in the past, much like a diary of the landscape. Process of Soil Profile Development Imagine a brand-new landscape surface that could be a bed of some rocks and minerals exposed by a retreating glacier, debris laid down by a f looding river or many other things. We describe that brand-new landscape surface as parent material and at that point of time it is new and not technically soil. This parent material will be altered or weathered in some way by the five soil forming factors viz. climate, organisms, relief, parent material and time which may inf luence a parent material differently in different locations. The climate in which a soil is developed determines many things, most importantly, the amount of water that will flow or leach through the profile. It might seem odd that water dissolves rocks or minerals, but the more water a parent material is exposed to, the quicker it will be weathered. Freeze / thaw cycles and other climatic factors also weather parent material. Organisms, especially plants and soil microorganisms, do a lot to weather parent material by producing acids and other organic matter. Different groups of organisms have different effects on parent materials. Relief or topography inf luences where water and other materials accumulate on or leave the landscape. For example, the bottom of a hill will receive more water than the top because water runs down the hill. So, the parent material at the bottom will have more water leaching through it than the parent material on the top, so the soils will eventually look different. Parent material is what being altered into soil, while it will change over time because of weathering, a soil with a parent material like basalt lava will be different than a soil with a parent material like beach sand, because the parent materials are so different chemically and physically. Time is the last soil forming factor. Soils take a very long time to develop; new soils do not have distinguishing profiles or horizons. But given enough time and the other four soil-forming factors, soils develop interesting and storytelling horizons.


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