eChapters

Vacuole: A fluid filled space within the cytoplasm surrounded by membrane called as tonoplast, that occupies most of the volume of cell. Aside from storage, the main role of the central vacuole is to maintain turgor pressure against the cell wall.

Valinomycin: An antibiotic that uncouples electron transport and oxidative phosphorylation only in presence of some cations such as K⁺ or Na⁺

Van der Waals interaction: Weak intermolecular forces between molecules as a result of each inducing polarization in the other.

Wall pressure: The inward pressure exerted by the cell wall against expanding protoplast; equal but opposite to turgor pressure.

Warburg effect: Warburg effect states that the competition between CO₂ and O₂ for enzyme RuBisCO with CO₂ promoting photosynthesis and inhibiting glycolate formation and inhibiting photorespiration and O₂ promoting glycolate formation and inhibiting photosynthesis.

Water deficit: Water available to the plant is less than optimum requirement.

Water potential (ψ_w): Water potential is defined as free energy status of water in a system compared to the free energy status of pure water. Here system refers to water in plant cell or soil and pure water refers to the water which does not have any solute in it. Water potential is measured in terms of bars, Pascals or Megapascals and denoted by ψ_w.

Xanthophylls (Carotenoids): These are tetraterpenes with general formula C₄₀H₅₆O₂. Xanthophylls are a class of oxygen-containing carotenoid pigments, responsible for the color of many of the yellow, orange, and red hues of flowers, fruits, vegetables (corn, pepper, etc.), egg yolks, and feathers, shells, or flesh of many animal species (flamingo, canary, shrimp, lobster, chicken, or salmonids). In plants, they are involved in photosynthesis with chlorophyll and are responsible for the red, yellow, and/or brown colors of autumn foliage as the chlorophyll levels decline.

Xeromorphy: Morphological characteristics, such as small, tough leaves, a heavy cuticle, etc., that are generally important for adaptation to dry environmental conditions.

Yamogenin: A triterpene plant steroid belonging to a group of sapongenins that is obtained from Dioscorea and is used in making oral contraceptives.

Z scheme: Refer non-cyclic electron transport.

Z-DNA: A duplex DNA structure in which helical coiling of the two polynucleotide chains is left handed and phosphate backbone of the polynucleotides follows a zig-zag course.

Zeatin: Most abundant natural cytokinin present in plant.

LAND VERSUS SOIL

Land is a part of Earth’s surface that is not covered by water. It is used for agriculture, building houses, of?ce buildings, schools, hospitals, roads, railroads and even water canals. Land is measured by area [length (l) × width (w)] and the units are square meters (m²), square centimeters (cm²) etc. On the other hand, soil is Earth’s crust, which could vary from a few centimeters to a few meters in thickness (depth), needs a measure of volume [length (l) × width (w) × depth (d)] and the units are cubic meters (m³), cubic centimeters (cm³) etc. Soil is thus a three-dimensional natural body on Earth’s surface that provides anchorage to growing plants/ trees and shelter to soil biota and is responsible for the storage of water and plant nutrients, which are essential for crop production. In addition, it helps in the biodegradation of soil pollutants and puri?cation of ground water, the major source of drinking water in the world. Soil feeds all living beings on Earth and its degradation is one of the reasons for inadequate human nutrition

INTRODUCTION

In biology, fertility is de?ned as capacity to produce off-springs; however, in Soil Science a ?ne difference is made between a fertile and a productive soil. Soil Productivity is the ability of a soil to produce a crop using the entire spectrum of its physical, chemical and biological attributes, while Soil Fertility refers to the capacity of a soil to supply all essential plant nutrients in the desired amounts. Thus, soil fertility management is basically the nutrient management. To put it in simple words, a productive soil is always fertile, while a fertile soil may not be productive for a variety of factors, such as lack of adequate moisture in drier regions or lack of adequate air due to water-logging for most upland crops. Of late, a term ‘ Soil Health’ has been introduced. Soil Health is an assemblage of chemical, physical and biological parameters that closely relate to native or acquired production capacity and sustenance of ecologically important regulatory role (Katyal et al., 2016). It is measured against prede?ned parameters of soil’s status of fertility, compact ability, erodibility and biology. This again tends to relate soil fertility to nutrient content, availability and management.

German chemist Justus Freiherr von Liebig (1803–1873), Professor at the University of Geissen and the French chemist Jean-Baptiste Joseph Dieudonne’ Boussingault (1801–1887), Professor at the University of Lyon and later Chair, Conservatoires des Artes et Matier in Paris are to be credited for analyzing the plants and coming with the idea that plants absorbed nitrogen and other mineral nutrients from the soil for their growth. D.I. Arnon and P.R. Stout, Plant Physiologists and Plant nutritionists at the University of California, Berkeley, USA, grew plants by hydroponics using pure chemical compounds and established following 3 criteria for the essentiality of nutrients for plant growth

All life on planet Earth is carbon centered; hereditary characters are controlled by DNA and RNA, growth by proteins and activities by energy from carbohydrates and fats – all these are carbon compounds. Not only this, but the life also ends as carbon compounds CO₂ or CH₄, depending on aerobic or anaerobic decomposition. Incidentally soil fertility also depends very much on soil organic carbon (SOC). In ancient days, farmers used to apply manure to maintain soil fertility and even now, urea, the nitrogen fertilizer used in the largest amounts, is a carbon compound. All living forms on the Earth including all plant and animal species and their activities are a part of global carbon cycle. The carbon cycle is the biogeochemical cycle by which carbon is exchanged among the biosphere, pedosphere, geosphere, hydrosphere, and atmosphere of the earth.

SOIL WATER

Soil water and oxygen share the pore space in soil and generally the oxygen content is determined by the water content, which is added by irrigation or rains and then keeps on changing due to evapotranspiration. Water is the vehicle by which most nutrients move from the soil to root surface. But for the lowland rice, nutrients are most available to plants at ?eld capacity moisture (about 0.3 atmosphere soil water tension) and the availability decreases as the moisture content decreases to wilting point (about 15 atmosphere tension). That is why rainwater or irrigation increases nutrient availability, while extremely dry condition decreases it. There is thus a signi?cant interaction between water and nutrient availability in soil (Singh et al., 2014). In a study in India (Singh et al., 1980) the yield of wheat was signi?cantly affected by irrigation water and nitrogen treatments; maximum yield was obtained with irrigation at 50% available soil water in 60 cm soil depth and 120 kg ha ^–1 nitrogen addition. A study on wheat at Delhi (Kumar et al., 2015) very well shows the effect of irrigation water on yield and uptake of nutrients (NPK) (Table 5.1).

Introduction

Nitrogen constitutes 78% of the atmospheric gases and there is about 75,000 Mg N on each hectare of land. However, 99% of the atmospheric N is as N₂ and is inert and non-reactive, while the rest 1% or even less is the reactive-N. Reactive-N is linked with H, O and C and is responsible for the creation and sustenance of all life on the planet Earth. Nitrogen is the key nutrient that separates proteins from carbohydrates and fats, the three major macronutrients for humans. As a component of proteins, N is present in DNA and RNA, which carry the hereditary characters and then as the major constitute of the body muscles of humans (and animals). Plant growth and crop production are also very much dependent on the adequate availability of N in soil.

Fertilizer N has been the key input in increasing cereal production throughout the world. In India, it made possible to achieve the yield potential of high-yielding dwarf varieties of wheat that led to ‘Green Revolution in 1968’. Nitrogen fertilization also increases protein content in wheat grains (Rajeswara Rao and Prasad, 1987) and leads to increased production of proteins in cereals (Prasad and Shivay, 2015). Cereal production in India increased from 42.4 million tonnes (Mt) in 1950–51 to 259.6 Mt in 2017–18, while fertilizer N application increased from 0.05 Mt to 16.96 Mt. However, the average agronomic ef?ciency of N (AEn) in India for cereals is only 9.5 kg grain kg–1 N (Sharma and Batra, 2011) as compared to global average of 24 kg grain kg–1 N for maize, 22 kg grain kg–1 N for rice and 18 kg grain kg–1 N for wheat (Ladha et al., 2005). Besides this, more disturbing is the fact that in irrigated areas the AEn declined from 13.4 to 3.4 kg grain kg–1 N during 1970–2005 (Biswas and Sharma, 2006). This is due to poor N management. Bulk of fertilizer N is applied broadcast on surface in most crops.

As compared to total N soil total P is present in much smaller amounts in soil; it is roughly about 10% of total N. Further, while N in soil is in the largest amount in surface 0 to 15 cm layer, soil P may be the same or in larger amounts in the sub-soil. Again, organic P constitutes only a small portion of total P. Thus, the Chemistry of soil P is generally centered on inorganic P. Total and organic P in some Indian soils is given in Table 8.1; hill soils had the highest and the desert soils the least total as well as organic P.

FRACTIONS OF INORGANIC P

All P in soils has originated from apatites [3 (Ca₃ (PO₄)₂.Ca×₂, where ×₂ could be F, Cl, (OH) or CO₃). Large deposits of apatites form phosphate rocks, which are the major materials for making phosphate fertilizers. As the soil weathering proceeds, P in apatite reacts with Al, Fe and other cations and forms compounds of different solubility. A knowledge of these can help in understanding the P-supplying capacity of soils. A number of chemical extractants and resins are used for determining different fractions. In general, 7 fractions of inorganic soil P are mentioned. These are: saloid-P (soluble and loosely bound P extractable by NH₄Cl), labile P (determined by using anion-exchange resins), Al-P (NH₄F extractable), Fe-P (NaOH extractable), Ca-P (H₂SO₄ or HCl extractable), reductant-soluble- P (CDB or sodium citrate-sodium dithionite-sodium bicarbonate extractable) and occluded P (CTB followed by NaOH extractable)

Out of the three primary plant nutrients (NPK), potassium (K) is taken up by crop plants in the largest amount, yet as compared to N and P, only a tiny fraction of it goes in the grain or seed. For example, Shivay et al. (2015) reported from a study on rice that about 20 kg N, 6 kg P (13.7 kg P2O5) and 46 kg K (55.2 kg K2O) was taken up by the crop to produce each Mg or tonne (1,000 kg) of rice kernel; however, only 4.3% of K taken up by the crop was found in the rice kernel as compared to 44% of N and 66% of P taken up by the crop. Some information on N, P and K removed per tonne of grain/ seed for some crops is given in Table 9.1.

Secondary nutrients is a triad of three essential plant nutrients namely, Calcium (Ca), Magnesium (Mg) and Sulphur (S). Of these three, Ca and Mg are taken up by plants as cations, while S is taken up as anion SO4 2–. Among the secondary plant nutrients, sulphur is emerging as a major plant-nutrient de?ciency in crops grown in the Indo-Gangetic Plains, spread over 13 million ha in Pakistan, India, Nepal, and Bangladesh (Khurana et al., 2008). From the Indian point of view, S de?ciency has emerged in a large part of the country and needs to be attended. Uptake of Mg and S by some crops is given in Table 10.1.

There are nine essential micronutrients, namely, iron (Fe), manganese (Mn), zinc (Zn), copper (Cu), boron (B), molybdenum (Mo), chlorine (Cl), vanadium (V) and nickel (Ni). Out of these, Fe, Mn, Zn, Cu and Ni (Dominic et al., 1978) are taken up by plants in their ionic form X²⁺, Mo as MoO^2-, chlorine as Cl^– form and boron as boric acid (H₃BO₃) or as its ionic form B (OH)₄ ^–. The absorption of vanadium by plants has received only limited attention (Lepp et al., 1983); it may be absorbed as cation V or vanadyl (VO²⁺) and anion vanadate (VO₄ ^3–). Bulk of the information is available on Fe, Mn, Zn and Cu, because of the acceptability of the single DTPA (Diethylene triamine pentaacetic acid)–CaCl₂ extraction test of Lindsay and Norvell (1978) for availability of all these four nutrients followed by their estimation on Atomic Absorption Spectrometer (AAS). Available B is estimated by hot 0.15 M CaCl₂ extraction method (Saha and Singh, 1997). Molybdenum data base is poor, because its determination is dif?cult and Mo de?ciency in crops has not been much noticed in India. Chloride is available in plenty in irrigation water and soil in India and has not received much attention in India.

Internationally maize (Zea mays) and citrus (Citrus sp.) are reported to largely suffer from Zn de?ciency, for which these are considered as marker plants. Zinc de?ciency in maize results in pale or whitish top young leaves and the de?ciency is referred to as 'white bud' (Camberato and Maloney, 2012). In citrus, Zn de?ciency results in ‘rosette’ (cluster of stiff leaves at the end of young shoots) formation (Brennan, 1993). Zinc de?ciency in India was ?rst reported in rice by Nene (1966) at the Govind Ballabh Pant University of Agriculture and Technology, Pantnagar, Uttarakhand, and given the name ‘khaira’ disease due to dark-brown-coloured spots resembling kattha (Acacia catechu extract used in paan – the betel leaves) on the leaves of zinc de?cient plants. Soon All India Coordinated Project on Micronutrients was launched by the Indian Council of Agricultural Research (ICAR) with headquarters at the Punjab Agricultural University, Ludhiana, which was later shifted to the Indian Institute of Soil Science, Bhopal, Madhya Pradesh. About half of the cultivated soils in the India are de?cient in available zinc. Zinc de?ciency is most in Maharashtra and Karnataka and rice-growing states of north and northeast (Table 12.1). A good response of cereals to Zn fertilization has been reported (Rattan et al., 2008).

Boron (B) de?ciency has been realized as the second most important micronutrient constraint in crop production after that of Zn in the world (Ahmed et al., 2012). However, in Nepal, B de?ciency is more wide-spread than Zn de?ciency, as 80 to 90% of soil samples were de?cient in B, 20 to 50% in Zn and 10 to 15% in Mo (Anderson, 2007). In India, B de?ciency has increased from 2% of the soil samples de?cient in B in 1980s (Katyal and Vlek, 1995) to 52% soil samples in 2000s (Singh, 2012). Total B content in Indian soils varies from 7 to 630 mg kg^–1 soil (Kanwar, 1976); being the highest in arid regions of the country and the lowest in wet humid regions of the country (Behera et al., 2009; Prasad et al., 2014). Boron de?ciency is most in the heavy rainfall areas of Western Ghats of Maharashtra, Karnataka and the eastern states of Bihar, Odisha, Jharkhand and Chhattisgarh states of India (Fig.13.1). The wide-spread de?ciency of B in India can be seen from the fact that B fertilization recorded the highest bene?t : cost (B : C) ratio in rice–wheat cropping system at Palampur (Himachal Pradesh), Ludhiana (Punjab), Faizabad, Modipuram and Varanasi (Uttar Pradesh), Pantnagar (Uttarakhand) and in rice–rice cropping system at Maruteru (Andhra Pradesh), Karjat (Maharashtra) and Jorhat (Asom) (Singh and Goswami, 2014). Response of wheat to B fertilization is also reported from Bangladesh (Halder et al, 2007).

Before the discovery of chemical fertilizer in early 20^th century, organic manures and growing of legumes were the ways of replenishing soil fertility. The only other way was leaving land fallow for a few years before its re-cultivation, a practice followed in shifting cultivation (jhuming) (Garbyal, 1999). This was also true for India, where although the ?rst single superphosphate plant was set up in 1906 in Ranipet (Tamil Nadu), the real use of fertilizer started with the manufacture of urea, the ?rst plant for which was set up in 1959 at Sindri (Bihar) (Tandon, 2014). Organic manures could meet crop-nutrient needs in early days because the average grain yield of most cereals was about 1.5–2.0 t ha^–1, while the average yield of pulses was 0.5–0.75 t ha^–1. However, the crop production scenario changed rapidly after the introduction of dwarf, high-yielding varieties of wheat from Mexico, which led to the “Green Revolution” (Swaminathan, 2013). This was followed by the development of semi-dwarf, high-yielding varieties of rice and hybrids of maize, sorghum and pearl millet. These varieties/ hybrids of cereals needed 100–150 kg N ha^–1 in one growing season, and this could be met only by urea or other N fertilizers; organic manures had no chance and their use became almost nil over a period of next 25 years or so. Well-off young farmers cannot even think of making manure heaps or compost pits, the job is too hard and not a clean one. However, what is ignored is the fact, that organic manures not only supply plant nutrients but also build up soil organic matter, which improves soil physical properties, helps in storing moisture and is the elixir of life for the soil biota (Lal, 2014). A large numbers of reports are available on the effect of adding organic manures and crop residues on improving soil physical (Hati et al., 2007; Brar et al., 2015) and chemical and biological properties of soil (Prasad et al., 1999; Prasad and Mishra, 2001;Sharma et al., 2009; Prasad et al. 2014). Thanks for the organic farming (Prasad, 2005), the interest in organic manures and composts has increased in India and it is now a sought after material. However, it has not entered the main stream of crop production.

Soil-rejuvenation power of legumes in crop rotations has been known in several parts of the world since ages. It is mentioned in the Indian agriculture of the Chalcolithic period (ca 1000 BC) (Raychaudhuri and Mira, 1993), in Chinese agriculture in the Han dynasty (ca 2nd century) (Hsu, 1980) and in Roman agriculture (White, 1970). Legumes can be ?tted in cropping systems in India in a number of ways, such as, a crop in a cropping system, a catch crop in an intensive cropping system, an intercrop and as a green manure. A brief discussion on these follows.

Fertilizer recommendations are generally made on the basis of soil test. Recommendations of fertilizer for top-dressing are best made on tissue testing.

SOIL TEST

A number of approaches have been adopted to increase the precision of soil-test values in making fertilizer recommendations. Four most important ones are discussed.

Low-Medium-High Approach

Making fertilizer recommendations based on soil test for a nutrient involves conducting a large number of ?eld experiments on soils similar to those which are intended for making recommendations. For an individual nutrient (say P for example) experiments are conducted with graded doses of P on a crop for which the fertilizer recommendations are to be made. Thus, agronomic experiments are to be conducted for each crop on each kind of soil. The response curve can then be divided into three sections: a, low soil-test value and high response to fertilizer zone; b, medium soil-test value and medium response to fertilizer zone; and c, high soil-test values and no response to fertilizer zone. In India, this was done

The introduction of high-yielding varieties of wheat and rice, the development of intensive cereal–cereal rotations (rice–wheat, rice–rice, hybrid maize–wheat), application of high doses of N (above 100 kg N ha^–1 to each crop) without adequate P and K application, increased use of high-analysis fertilizer (urea and diammonium phosphate in place of ammonium sulphate and superphosphate) and ignoring the application of organic manures led to heavy depletion of P, K, S, Zn and other plant nutrients, which led to wide-spread de?ciencies of P, S and Zn in Indian soils. These de?ciencies of nutrients led to focus on integrated nutrient management.

Plant nutrients are important for increased production of food for the ever-increasing world population. Of the plant nutrients, primary nutrients N, P and K are applied to ?elds each season in fairly large amounts. In addition to increasing crop yields, they also impact the environment. This is briefly discussed in this chapter; only main points are made here.

NITROGEN

Nitrogen is blamed the most for environmental degradation. It contributes to eutrophication of inland and sea-waters, enrichment of air with NH₃ and N₂O and even with global warming. However, it is almost impossible to grow more food without additional nitrogen. It is estimated that by 2050 about 250 million tonnes (MT) of fertilizer nitrogen may be needed to meet global increased food demands as against an estimated current consumption of 116 MT

Manychemicalreactions innature,particularlyinlivingsystems,occur inaqueous solutions. Reactions are often carried out in the laboratory in aqueoussolution becausethese aregenerallymore rapid in solutionthanwhen puresolid reactions are mixed. Analysisof substancesmaybe qualitative or quantitative in nature. A quantitative analysisis one inwhich the amountor concentrationof a particular species in a sample is determined accurately and precisely.

1.1 Volumetric Analysis

Volumetric analysis is a quantitative analysis. It involves the measurement of the volume of a known solution required to bring about the completion of the reaction witha measuredvolume of the unknown solution.

The reaction used in volumetric analysis must fulfil the followingconditions;

Advances in analyticalchemistry and instrumentalanalytical chemistry in the pastfew decades have significantlyimproved, makingelemental determinations in soil extracts and water samples easier, faster and less challenging for the analyst. For most of the elemental analysis procedures, the more traditional wet chemistryprocedures havebeenreplaced byvarious analytical instrumental procedures that employ either emission or absorption spectrophotometry. The elemental concentration in prepared soil extracts and water samples can be determined by a number of instrumentalanalyticalprocedures.

The trend today is toward greater analytical sophistication, to multielement computer controlled analytical instrumentation, resulting in a less ended understandingof the analytical principlesinvolved, and “black box” concepts of instrument calibration, maintenance, and operation. Preparation of the analyte and suitable adaptation for a method of analysis requires an understanding, by the analyst, of the principle of the method as well as its requirements and limitations. Adequate testing is usually required before putting the method to use, following procedures such as those that have been adapted by the Association of Official Analytical Chemists (AOAC).

It is not possible to cover in adequatedetail every method of analysissuitedfor the elemental concentration determination in soil extracts and water samples. Therefore, the objective here is to provide sufficient background information about the instrumentalprinciples to guide the reader.

3.1 Concepts of Soil Sampling

Samplinginvolvesthe selection fromthe total populationof a subset ofindividuals upon which measurements will be made. The measurements made on this subset (or sample) will then be used to estimate the properties (or parameters) of the total population.Samplingisinherent toanyfield research programinsoilscience because the measurement of the total population is impossible for any realistic study.

Samplingdesign involves theselection of the mostefficient method forchoosing the samples thatwill be used to estimatethe properties of the population. The definition of the population to be sampled is central to the initial formulation of the research study. The sampling design defines how specificelementswill be selected from the population, and these sampled elements form the sample population.

Pedogenetic and soilgeomorphic studies focus on determining the processes thatformed the soilproperties or landscapes under study and the environments that controlled the rates of these processes. Pedonscale studies are closely associated with thedevelopmentof soil taxonomic systems andfocus onvertical, intrapedon processes. Soil geomorphic studies are the interface between quaternary geologyand soil science and soil geomorphologistsfocus on lateral transferprocesses and the historical landscape evolution. Hence sampling is difficult for a particular population and is differentfor differentstudy.

The influence of water quality on the soil and plant growth is related to the chemical and physicalproperties of the soil, the salt tolerance of the crop, the climaticregime of the area and the method,frequencyand the amount of irrigation water applied. Irrigation water quality refers to its suitability and sustainability for use in agricultural crops. The problems thatresultfrom the use of a particular irrigation water, vary both in kind and degree, and are modified by soil, climate and crop as well as by the skill and knowledge of the water user. As a result there is no set limiton water quality, rather itssuitability for use is determined by the conditions of use which affects the accumulation of water constituents andwhichmayrestrictcrop yield.The soilproblems mostcommonlyencountered and used as a basisto evaluate water quality are those related to salinity, water infiltrationrate,toxicityanda group of other miscellaneous problems. Numerous guidelines are available for evaluation of water quality. Time to time some of theseguidelines has been updated with modification of some criteria like SAR to Adj. SAR and the Adj. R_Na and development of new concepts like water qualityIndex (WQI) whichhave increasedour predictability. Thenewguidelines are relied heavily on old ones.The modified guideline by Ayers and Westcot (1985) was foundto be themost reliabletopredict thewaterqualityforirrigation.

Irrigated agriculture is playing a major role in enhancing food and livelihood security of a country. Supply of fresh water which is an importantinput for the sustainable and economic development of agriculture are not enough to meet the requirement of all sectors of economy. Reduced water availability to agriculture from the present share of 85% of country’s water resources to about 70-75% by 2020AD wouldaffect the capacity of the country to meet the food production target, unless supplies are augmented from unconventional sources. For future agriculture there are no alternatives but to rely on non conventional sources for partial alleviation of the forecasted water scarcity. Amongthese sources extraction of marginal quality groundwater and the waste water generatedfromurban and industrial activities have thepotential to augment the water supplies. Many associatedsoil, plant and environmental problems come up when these sources of water are directly used for irrigation. For addressing theseproblems and to get more benefit from thesesourcesof water, the first step is to understand how an irrigation water source affects the soil plantsystem.Thereforeknowledge of irrigationwaterqualityiscriticaltodevelop appropriate site and crop specific comprehensive short term and long term management programs.

Soil is a mass of large variability and thus its sampling is the most challenging problem for any soil analyst. Practically, soil sampling is a two step process which includes (i) collection of soil from homogeneous unit and (ii) processing which in turn includes drying, grinding, sieving, mixing, partitioning and storing.

2.1 Particle Size Distribution Analysis

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

3.1 Determination of Bulk Density of Soil

3.1.1 Determination of Bulk Density of an undisturbed Soil

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

4.1 Determination of Specific Surface Area of Soil

Specific surface area of a soil is defined as surface area per unit weight of soil and is expressed as m² g^-1. The specific surface of soil greatly influences the physical and chemical properties such as retention of water at high suction, swelling, plasticity, soil strength, cation exchange capacity and availability of nutrients. The basic principle of determination of specific surface is based on the amount of gas or polar liquid required to form a monomolecular layer on the soild. Knowledge of molecular size (diameter) and the mass of the adsorbate enables one to calculate specific surface of the adsorbent.

Principle

Dyal and Hendricks (1950) introduced a method for estimating specific area based upon the adsorption of ethylene glycol (EG) to form a monomolecular layer over the entire soil surface. The method involves adding EG to pretreated soil or clay sample and evaporating the excess EG in an evacuated system. The quantity of EG retained at the moment the evaporating rate decreases is proportional to the surface area. Later on Carter et al. (1965) suggested the use of ethylene glycol monoethyl ether (EGME) having high vapour pressure at room temperature than EG for determining surface area of the layer silicate minerals and soils.

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

6.1 Determination of Soil Moisture

6.1.1 Determination of Soil Moisture Content by Gravimetric Method (Direct method)

Traditionally, water content in soil is expressed as the ratio of the weight of water present in the soil to the weight of dry soil. When this ratio is multiplied by 100, it becomes the percentage of water in the soil sample on dry weight basis.

Equipments and Materials

Soil auger, aluminium moisture box with air tight lid, balance, oven and desiccator

7.1 Determination of Hydraulic Conductivity of Saturated Soil Laboratory Methods

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

Soil consistency is a term used to designate the manifestation of different physical forces acting within the soil at various moisture levels. The consistency limits, also known as ‘Atterberg Limits’ (Atterberg, 1911; 12) are defined by the water content of the soil to create specified degree of consistency. These limits are – (i) Liquid limit or upper plastic limit, (ii) Plastic limit or lower plastic limit and (iii) Sticky limit.

8.1 Determination of Liquid Limit or Upper Plastic Limit

Equipments and Materials

Liquid limit device (Casagrande, 1932) and accessories, spatula, dish, oven, moisture box, balance, desiccator

9.1 Determination of Oxygen Diffusion rate (ODR) of Soil

The rate of movement of oxygen from atmosphere to respiring plant root cells is important for root respiration and thereby the growth and development of plant. While diffusion through the air-filled pores maintains the exchange of gases between the atmosphere and soil, diffusion through water films maintain the supply of oxygen and disposal of carbon dioxide from active root tissues which are typically hydrated.

Principle

Lemon and Erickson (1952) introduced a method for measuring oxygen diffusion in soil with the help of platinum electrode. The process measures the extent of chemical reduction of oxygen by the platinum electrode maintaining a constant potential of 0.65V with a reference electrode.

Temperature is a fundamental property of usually used to characterize the thermal condition of a system. The growth of any biological system is optimal within certain ranges of temperature and inhibited or ceased beyond this boundary. In addition to plant growth, the significance of temperature on agriculture can be ascertained by its role on physical, chemical and biological processes occurring in soil. Therefore, to study these soil properties intensively measurement of soil temperature is essential.

10.1 Measurement of Soil Temperature

Temperature of any system cannot be directly measured. It can only be estimated by its influence on some properties (thermometric properties) which respond to the variation in the intensity of heat of that system. These thermometric properties include volume, pressure, length, electrical resistance, thermal emf of the matter. Instruments built to take the advantage of any of these thermometric properties of the matter are called thermometer.

11.1 Determination of Penetrability of Soil

Penetrability of a soil is usually assessed by the penetration resistance, which a measure of soil strength and thereby the soil compaction. Penetration resistance is the capacity of the soil in its confined state to resist penetration by a rigid object. Besides soil strength, the penetration resistance also depends on the size, shape and orientation of the axis of the penetrating probe. The penetrating probe may be a finger, pencil, stick, root or any specially designed object having a specified geometric shape and a device to measure the resistance as it is pushed into the soil.

Soil penetrability is intimately related to agriculture because of its adverse impact on draught requirement during ploughing, root development and crop yield, to civil engineering due to its relation to settlement, stability and ground water flow.

Soil penetrability is a measure of the ease with which an object can be pushed or driven into the soil. Any device designed to measure the resistance to penetration may be called penetrometer. There are several types of penetrometers: (i) pocket penetrometer, (ii) proctor penetrometer, (iii) cone penetrometer, and (iv) split-spoon penetrometer. Here, we will discuss about two most commonly used penetrometers viz. pocket penetrometer and cone penetrometer used for agricultural purposes.

Thermal Analysis

Physical and chemical properties of any soil are governed largely by the mineralogical make up of the soil, and more specifically by the clay fraction. Identification, characterization and an understanding of properties of different minerals are important in evaluating soils in relation to classification, agronomic practices and engineering purposes.

Clay fractions of soils are usually composed of mixture of one or more secondary phyllosilicate minerals along with primary minerals inherited from the parent materials. Identification of minerals species as well as their quantitative estimation in such polycomponent system require application of several qualitative and quantitative analysis.

The methods used for identification and quantification have steadily been improved over the last 50 years. Among the several methods often used for this purpose three are very common, e.g. (i) differential thermal analysis (DTA), (ii) x-ray diffraction analysis (XRD), and (iii) infrared spectroscopy (IR).

Thermal analysis is a term covering a group of analyses that determine the changes of some physical parameters, such as weight, energy, dimension and evolved volatile substances with change in temperature. Many soil constituents undergo various thermal reactions upon heating, which can be utilized as a diagnostic tool for qualitative identification and quantitative estimation of the substances. These reactions may be exothermic or endothermic, and phase change or crystal change may take place during reactions. Whatever be the reactions, if properly measured, they give some valuable information to identify the substance under consideration.

Among the several methods available for thermal analysis, three methods are frequently used in soil research: differential thermal analysis (DTA), differential scan calorimetry (DSC), and thermogravimetry (TG). Among these, DTA and TG will be discussed here.

The total energy of a molecule consists of translation, vibrational, rotational and electronic energies. Transition among different types of energy levels occurs in different regions of electromagnetic spectrum. Transitions between rotational and vibrational energy levels of the ground state molecules give rise to absorption bands throughout the infrared region of the electromagnetic spectrum. Organic molecules are characterized by possessing bonds and groups that vibrate independently of each other. Thus, initially infrared spectrometry was used for identification of functional groups of organic molecules. There is lot of ambiguity in the interpretation of infrared spectra of organic matter preparations due to complexity and diversity of the components of soil organic matter. But if used carefully it can provide information on: (i) nature, reactivity and structural arrangement of oxygen-containing functional groups, (ii) presence of protein and carbohydrate constituents, (iii) presence or absence of inorganic impurities (metallic ions, clays, etc in humic fractions), (iv) the amount of different soil organic matter components, and (v) interaction of organic components with pesticides, herbicides, fertilizers, etc.

On the contrary to organic molecules, minerals tend to have few isolated vibrating groups. It can be used for mineralogical analysis when used in conjunction with x-ray diffraction and other similar techniques.

Infrared spectrometry is used in the identification of inorganic compounds including minerals which have well-defined absorption bands in determining- (i) whetherlayer silicate isdioctahedral or trioctahedral in composition, (ii) in studying isomorphous substitution, (iii) hydration of minerals, and (iv) in quantitative analysis. Infrared spectrometer provides information about nature and identify of inorganic compounds that are amorphous in x-ray diffraction analysis.