eChapters

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.