eChapters

Introduction

Solar refrigeration technology engages a system where solar power is used for cooling purposes. It offers a wide variety of cooling techniques powered by solar collector-based thermally driven cycles and photovoltaic (PV)-based electrical cooling systems. Cooling can be achieved through four basic methods: solar PV cooling, solar thermal cooling, solar thermo-electrical cooling and solar thermo-mechanical cooling. The first is a PV-based solar energy system, where solar energy is converted into electrical energy and used for refrigeration much like conventional methods. The second method utilizes a solar thermal refrigeration system, where a solar collector directly heats the refrigerant through collector tubes instead of using solar electric power. The third one produce cool by thermoelectric processes. The fourth method converts the thermal energy to mechanical energy, which is utilized to produce the refrigeration effect.

The performance of solar refrigeration systems is determined based on energy indicators of these systems. The COP (coefficient of performance) can be calculated as follows:

Introduction

Solar air heating is the conversion of solar radiation to thermal heat. The thermal heat is absorbed and carried by air which is delivered to a living or working space. The transparent property of air means that it does not directly absorb effective amounts of solar radiation, so an intermediate process is required to make this energy transfer possible and deliver the heated air into a living space (Fig.10.1). The technologies designed to facilitate this process are known as solar air heaters. Solar air heating technologies use only free, renewable and clean energy and can help defray the rising cost of conventional energy.

Introduction

The sun is an abundant source of solar energy (average value on horizontal surface 6 kW m^-2 day^-1), which is freely available. This huge source of energy is non-polluting and inexhaustible in nature. It has the potential of supplementing the conventional energy sources to a great extent. There is acute shortage of conventional energy sources, which affects the overall development. Work on the utilization of such a huge, non-polluting and everlasting energy source has been carried out at CAZRI, Jodhpur for various domestic, industrial and agricultural applications in order to supplement the energy demand. This includes the development of a lot of solar thermal devices, such as, solar still, solar dryer, solar cooker, animal feed solar cooker, solar candle device and cool chamber. The details of some of these devices are given as following.

Introduction

A solar collector, the special energy exchanger, converts solar irradiation energy either to the thermal energy of the working fluid in solar thermal applications, or to the electric energy directly in Photovoltaic (PV) applications. For solar thermal applications, solar irradiation is absorbed by a solar collector as heat which is then transferred to its working fluid (air, water or oil). The heat carried by the working fluid can be used to either provide domestic hot water/heating, or to charge a thermal energy storage tank from which the heat can be drawn for use later (at night or cloudy days). Solar collectors are usually classified into two categories according to concentration ratios: non-concentrating collectors and concentrating collectors. A non-concentrating collector has the same intercepting area as its absorbing area, whilst a sun-tracking concentrating solar collector usually has concave reflecting surfaces to intercept and focus the solar irradiation to a much smaller receiving area, resulting in an increased heat flux so that the thermodynamic cycle can achieve higher Carnot efficiency when working under higher temperatures.

Introduction

Water is the primary source of life for mankind and one of the most basic necessities for crop production. The demand for water to irrigate the crops is increasing. For sustainable production from agricultural farms, irrigating the crops at right stages is highly important. Even in rainfed situation, lifesaving irrigation during long dry spell has also been found beneficial for crop survival and to obtain the targeted yield. Considering the depletion of groundwater below the critical zone in most part of the country, energy intensive pumping for irrigation is not a viable option. Therefore utilization of available runoff water through surface storage systems followed by pumping may be a potential solution to achieve the set goal of ‘crop per drop’ mission. In this connection, micro-irrigation system including drippers and sprinklers is of great importance. However, ensured power supply is essential to operate the micro-irrigation system even in remote areas. Reliable solar photovoltaic (PV) pumps are now emerging in the market and are rapidly becoming more attractive than the traditional power sources. These technologies, powered by renewable energy sources, are especially useful in remote locations where a steady fuel supply or electricity supply is problematic. About 16 million electric pumps and 7 million diesel pumps are in operations in the country for irrigation purpose; however they are highly energy intensive. Moreover, diesel operated and electrified pumps directly or indirectly emit large amount of CO₂ gas in atmosphere and hence are not environment friendly. To meet the energy demand for irrigation, solar PV pumps have been introduced under the off-grid power generation category of National Solar Mission (NSM) with a target of 1000 MW by the end of phase II (2013-2017). The target has been revised in 2015 to a total grid connected solar power generation of 1,00,000 MW comprising 40,000 MW roof top generation and 60,000 MW grid connected solar power plants (Resolution of MNRE, Govt of India, No. 30/80/2014-15/ NSM dated 1^st July 2015).

PV clad structures and hybrid devices for rural and agricultural applications have been developed after considering different design aspects, climatic parameters, solar radiation availability on different planes, heat transfer coefficients, performance of PV in arid region and inter phasing of different components while keeping in view, practicality, application and ease of operation. Performance of the developed systems was studied, mathematical model developed and energy savings worked out for techno-economic considerations. The chronology and details of the developed PV structures and hybrid devices are as follows:

Green structure for environmental control

First of all a Quonset type environment control structure having a length of 5.3 m and width 4 m was fabricated with angle iron and iron rods and it was covered by agro-net (75%) having surface area of about 55 m². The structure having a volume of 33 m³ was fixed with long side along east west direction and had provisions for incorporation of misting unit on west side and a door on east.

Introduction

Agriculture has been the back bone of Indian economy and culture and it will be continued to remain as such for a long time in future. Parallel to this, energy security of a country is also very important and efforts have been given on renewable energy utilization since the fossil fuel based energy is depleting at a very fast rate. In agricultural fields, considerable amount of energy is used to do different field activities e.g. ploughing, irrigation through pumps, intercultural operations, spraying of agricultural chemicals for plant protection, harvesting, post-harvest processing etc. Therefore, there is also need to replace the conventional energy source with renewable sources to operate above mentioned agricultural activities.

Approximately, 35% of the crop production is damaged if pest and diseases are not controlled at right time. Uniform spraying of liquid formulations throughout the crop field is very important for effective control of pest and diseases. Using sprayer, liquid pesticide formulations are generally broken down to minute droplets of effective size for uniform distribution over a large surface area. Different types of sprayers are used in agricultural field based on different requirements.

Introduction

For optimum use of harvested rain water in surface reservoir like farm ponds or tankas, small sized solar PV pumping systems with 1 hp AC and DC motor were experimentally tested at research farms of ICAR-Central Arid Zone Research Institute, Jodhpur (Fig. 16.1). Total suction head in both pumps was about 5 m. Among two installed pumps, the system with AC pump consisted of 1400 Wp (200 Wp × 7) whereas the DC pump consisted of PV array of 920 Wp (230Wp × 4). Each PV panel of AC solar pump was connected in series, which was further connected with an inverter to generate AC output of about 220-240 volt and 4-4.2 ampere. In case of DC solar pump, panels are connected in parallel to generate DC output of 40-50 volt and 15-20 ampere.

Introduction

Agricultural produce are being sun dried since ages as the first step to preserve the product for future consumption. It is still practised being most inexpensive over mechanical and other advanced drying systems. The most prevailing disadvantage of open sun drying is the quality losses due to insect infestation, enzymatic reactions, microorganism growth, and mycotoxin development. On the other side, sun drying is also a highly labor intensive and time consuming process prone to theft and damage by birds. It also suffers with the lack of process control and treatment uniformity (Bansal and Garg 1987, Bansal, 1987).

To overcome these disadvantages several types of solar dryers have been developed over the years (Ekechukwu and Norton 1999). These dryer may be identified into three group namely active, passive and hybrid. As regards their structural arrangement three generic subclasses were also identified: direct-modes (the solar-energy collection unit is an integral part of the entire drying system), indirect-modes (the solar collector and the drying chamber are separate units) and mixed-modes solar dryers. Natural convection solar dryers have become more suitable for rural sector and remote areas, as they work on single energy option and do not require any other external energy source (Pangavhane et al. 2002).

Introduction

Cultivation of crops in greenhouse or protected structure has been increased tremendously in recent times and also has been spread from temperate regions to the warmer regions of tropics and subtropics. Plentiful technologies have been developed for heating the inside environment of greenhouse for optimum plant growth (Sethi et al. 2013). However, in hot arid areas, cultivation in greenhouse is very difficult as the temperature reaches to a very high level to sustain plant growth (Pek and Hayles 2004, Saran et al. 2010). Under such situation requirement for cooling the greenhouse microclimate environment is the necessity, but is very energy and water intensive. Hence, in such regions, reduction of air temperature inside the greenhouse or the regulation of temperature closer to the ambient temperature during summer is necessary for successful crop production. A typical diurnal variation of temperature and global solar radiation during late winter months inside a shade net structure has been plotted in Fig. 18.1. It has been found that global solar radiation has been decreased to almost 1/3 of the radiation received outside by using 75% shade net cover. The temperature inside structure during morning time has also been found higher than outside temperature and in summer such increase is expected to be much higher.

Therefore, cooling is considered as the basic necessity for greenhouse crop production in tropical and subtropical regions to overcome the problems of high temperatures during summer months. Development of suitable cooling system that provides congenial microclimate for crop growth is a difficult task as the design is closely related to the local environmental conditions. Different cooling techniques for greenhouse crop cultivation may be found in Kumar et al. (2009). Fan pad cooling system has been widely used for greenhouse cultivation however, requires large amount of water during evaporation based cooling. Since water resources are becoming scarce in future such fan-pad cooling system may not be viable in future specifically in hot arid region.

Life cycle cost analysis

Economics of solar energy devices can be calculated through life cycle cost (LCC) analysis. Total life cycle cost of a device is comprised of capital cost, maintenance cost, replacement costs for damagaed components and operational cost. Before adding these above costs, all future costs (C) are converted to present worth considering the relative rate of inflation (i) and discount rate (d).

Where, PW is the present worth of any future cost, i is the relative rate of inflation and d is the discount rate per year and n is the time period in years. Relative rate of inflation accounts for the escalted increase or decrease in prices of a commodity in comparison to general inflation rate. For any commodity, if the price escalation is expected as per the general inflation rate then relative rate of inflation is considered zero. In general case, relative rate of inflation is considered zero. Discount rate accounts the real value of money in future and in most of the economies of the world it is about 8-12%, and therefore 10% is considered in most analyis. The discount rate also refers to the interest rate used in discounted cash flow (DCF) analysis to determine the present value of future cash flows. The discount rate in DCF analysis takes into account not just the time value of money, but also the risk or uncertainty of future cash flows; the greater the uncertainty of future cash flows, the higher the discount rate. A discount rate of 10 % per year would mean that in real terms it makes no difference to a farmer whether he has ? 100 now or ? 110 in one year’s time. Conversely, a cost of ? 110 one year from now has a present worth of ? 100. For a future single cost in n^th year, the present worth of that cost is calculated using Eq (1). However, for future multiple payments, costs are to be converted to present worth for each year and then needs to be cumulated. For calculation of annualized life cycle cost (ALCC) of solar devices, annuity factor (AF) needs to be calculated for a period of life cycle of the device as follows:

Introduction

Solar energy could be converted to electricity both through thermal route and photovoltaic cells but the latter is more advantageous as it has no moving part, it doesn’t require water, a rare commodity specifically in arid regions. Moreover, the maintenance of PV systems is easy, it can be installed right at the place of utility, the system is modular and these are reliable having life more than twenty years and of course eco-friendly.

In photovoltaic cells, generally known as solar cells, charge carriers are generated by the incoming irradiance (energy greater than band gap) and these are separated and swept away by an internal electric field, which is created by either making metal semiconductors contact (Schottky barrier) or p-n junction or MIS (Metal Insulator Semiconductor) or SIS (Semiconductor Insulator Semiconductor) devices. The height of potential barrier at the junction determines the voltage and the flowing charge carriers contribute to the current when connected externally. The choice of material depends on its band gap, absorption coefficient, diffusion length, mobility of charge carriers, ease in contact formation, type of junction with thermal and electronic compatibility, availability of material, toxicity and several other fabricational opto-electronic and practical aspects. PV cells based on single crystal and polycrystalline silicon, amorphous silicon, CdS, CdTe, CuInGaSe, InP, GaAs etc. have been much studied. PV modules of efficiency above 16% on silicon and 10% on CdTe, CuInGaSe and 8-9 % on amorphous silicon are commercially available. Dye sensitized TiO₂ devices have also been receiving more attention with coloured panels for windows. Organic semiconductors are explored and more recently perovskite solar cell has been studied. Flexible solar cell panels based on thin film devices are in use. The applications have gone from solar lighting to operation of pumps, small and medium size equipment, stand alone and grid connected solar power plants, roof top and building integrated PV and now even energizing cars and aero planes. With technological advancement and enhanced production (233 GW in 2015) the cost of commercially available PV modules, which used to be high at one time has been brought down from 30 $/Wp in seventies to about $0.5/Wp at present.

Introduction

As the most of the conventional resources are depleting and can last only for about a few 30 to 50 years. Scientists worldwide are looking for alternative energy resources and they have come to an agreement that solar energy can provide much of our energy needs in the future. The major challenge before them is to store this energy. The radiation falling on the earth surface can be tapped and stored in no of ways. However, the most efficient way is done through the latent heat storage through PCM. The PCM has many added advantages i.e. high energy density, isothermal behavior, non-polluting and low cost. Nowadays, it gaining popularity worldwide and now seen as the most efficient way of thermal energy storage. There is a large number of PCM at use i.e. organic, inorganic, eutectic and salt hydrates. Their utilization depends on the user’s requirement technological constraints. They also serve the purpose of energy availability at night beside the uninterrupted supply during the day. They are seen as the materials for ensuring the environmental sustainability

Introduction

The thermal performance of a building refers to the process of modeling the energy transfer between a building and its surroundings. For a conditioned building, it estimates the heating and cooling load and hence, the sizing and selection of HVAC equipment can be correctly made. For a non-conditioned building, it calculates temperature variation inside the building over a specified time and helps one to estimate the duration of uncomfortable periods. These quantifications enable one to determine the effectiveness of the design of a building and help in evolving improved designs for realizing energy efficient buildings with comfortable indoor conditions. The lack of proper quantification is one of the reasons why passive solar architecture is not popular among architects. Clients would like to know how much energy might be saved, or the temperature reduced to justify any additional expense or design change. Architects too need to know the relative performance of buildings to choose a suitable alternative. Thus, knowledge of the methods of estimating the performance of buildings is essential to the design of passive solar buildings.

Introduction

Considering the accountable post-harvest losses which is approximately 30-35 percent, food processing becomes an extremely important sector for preventing the losses and feeding the growing population. The Indian food industry is poised for huge growth, as currently valued at US$ 1.3 billion with compound annual growth rate of 20 per cent per year, increasing its contribution to world food trade every year. In India, the food sector has emerged as a high-growth and high-profit sector due to its immense potential for value addition, particularly within the food processing industry. Increasing population of the country and high cost of fuels have also created a demand for new alternate sources of energies for post-harvest processing of foods. Solar food processing technologies may become a viable option that provides good quality foods at low or no additional fuel costs. The energy policy of the developing countries now focuses on the clean renewable energy to create a further reduction of the petroleum import and to alter the utilization of petroleum energy toward the utilization of solar energy. It is an interesting fact that most of the developing countries fall in the climatic zone where the insolation is considerably higher than the world average of 3.82 kWh m-2 (Majumdar 2015). The daily horizontal insolation of India is approximately 5.80 kWh m-2 (Visavale 2009, Visavale et al. 2011). Solar dyers are an alternative to traditional drying technique and a contribution towards the solution of the problems of the open sun drying from perspective of renewable and clean energy. The International Energy Agency (IEA) declared in its 2011 solar energy perspectives executive summary (SEP 2011): “Solar energy offers a clean, climate-friendly, very abundant and inexhaustible energy resource to mankind, relatively wellspread over the globe. Its availability is greater in warm and sunny countries that will experience most of the world’s population and economic growth over the next decades.

Analytical chemistry is a branch of chemistry dealing with the determination of constituents of a substance or mixture of substances.

1.1 Qualitative Analysis

Qualitative analysis deals with the identification of the constituents of a substance, mixture of substances, or solutions and the way in which component element or a group of elements are constituted.

1.2 Quantitative Analysis

Quantitative analysis is concerned with the determination of quantitative contents of individual element or group of elements or compound present in the substances. This branch of analytical chemistry is of enormous scope and importance in science, industry and agriculture. The quantitative analysis can be divided into several categories based on the different methods employed for their quantitative measurements such as:

2.1 Beakers

Beakers are the one of most important vessels used in laboratory which is made up of heat resistant glass.

They are available in various capacities ranging from 10 to 3000 mL.

For convenience of pouring solution, spout is provided in the beaker.

They are used to perform titrations & for boiling solutions, etc.,

Beakers are constructed in tall and low forms.

Instrumental methods of chemical analysis are the quantitative methods of analysis that use, as principal measuring devices, instruments other than those employed in conventional gravimetric and volumetric determinations.

The main basis for this method of analysis is the physical property of any particular element or compound being analyzed. These properties include:

Practice to calculate the weight / volume of reagents required to prepare the following standard solutions.

Preparation of Standard Solution - (1 M, 0.1 M, 1 N, 0.1 N, 0.5 N, 5%, 10%, 20%, 10 ppm, 20 ppm, 100 ppm)

5.1 Volumetric Techniques or Titrimetric Analysis

Acid Base Neutralization Reactions-Acidimetry & Alkalimetry

Oxidation –Reduction Reactions-Oxidometry (Permanganometry) & Dichrometry

Principle

Standard solutions are the solutions of accurately known concentrations. There are two types of standard solutions viz., primary and secondary.

A known quantity of the substances is dissolved in a volumetric flask and the volume is made upto the mark. Standard solutions prepared in this way are known as primary standard. This method can be used for preparation of standard solutions of substances which satisfy the following conditions.

More often, if the reagent is not a primary standard, a secondary standard solution must be prepared. The chemical is dissolved and made up or diluted to the desired concentration and then it is standardized against a primary standard or against another reliably known secondary standard solution. Such solutions are called as secondary standard solutions.

Principle

Equal volumes of all acids / bases containing their gram equivalent weights per litre of the solution react completely with each other.

Soil is analyzed to know the nature of soil, to classify and advocate to farmers about the peculiarities of the soils and how much of fertilizers should be applied for better crop production.

A composite soil, of course, can not be moved to into a laboratory. The value of the laboratory work depends upon care in sampling. Each soil sample needs to be a fair representative of the specific area or horizon worth sampling.

If the sample is to be a representative of an area, it is necessary to take large number of samples spread over the area, pool them and sub-sample it so as to get a sample of desired size. For soil survey work, samples are collected from a profile which is typical of the soil of the surrounding area.

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.

Bulk density is the mass per unit volume of soil including pore space. The normal range of bulk density in most soils is ranging from 1.02 to 1.80 g / cm³. The bulk density of coarse textured soils will be on the higher side while organic soils will have lower bulk density.

Particle density is the mass of soil per unit volume excluding pore space. Pore space is that fraction of soil volume not occupied by solid particles.

Principle

The bulk density and percent pore space are determined from the apparent and true volumes of the soil measures by adding a known quantity of water to a measuring cylinder contain a weighed quantity of soil.

(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.

Principle

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

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

(Keen-Rackzowski/Hilgard Apparatus Method)

Principle

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

The pH is defined as negative logarithm of hydrogen ion concentration or the log of the reciprocal of the hydrogen ion concentration (Sorenson, 1906).

Principle

A glass electrode in contact with hydrogen ions of the solution acquires an electric potential which depends on the concentration of hydrogen ions. This is measured potentiometrically against some reference electrode which is usually a calomel electrode. The potential difference between glass electrode and calomel electrode is expressed in pH units.

Two electrodes are used in the determination of pH. One is reference electrode which provides a standard voltage and is usually a saturated calomel electrode which has two layers of saturated solution of KCl and mixture of solid HgCl₂ and Hg. The outer tube is usually 5-15 cm long, 0.5-1 cm in diameter. The mixture of solid HgCl₂ + Hg paste is contained in an inner tube that is connected to the saturated KCl solution in the outer tube by means of small opening. The resistance developed by this type of electrode is 2000-3000 Ohms.

The outer electrode is glass electrode that consists of a tube enclosing a lead wire made up of Ag coated with AgCl. This wire is again enclosed in a wax insulation. To the tube at the bottom is attached a glass bulb made up of a special kind of glass which is sensitive to hydrogen ions. The thickness of the glass membrane varies from 0.03 to 0.1 mm and has a resistance of 50 to 500 mega Ohms.

When these electrodes are dipped in solution, the saturated solution of KCl comes out of reference through small holes and forms an invisible ionic bridge between electrodes through which current passes. The hydrogen ions are absorbed by glass electrode and depending upon the amount of hydrogen ions present in the solution, an electric potential develops between electrodes. This potential difference is measured in terms of pH units by suitable Galvanometer.

The Electrical Conductivity (EC) of the soil is a measure of soluble salts present in the soil and is expressed as millimhos/cm or dSm^-1.

Principle

As the amount of the soluble salts in a solution increases, the electrical conductivity also increases. This electrical conductivity is measured in terms of resistance offered to the flow of current using a conductivity bridge.

It is known that solutions offer some resistance to the passage of current through them, depending upon the concentration of the soluble salts present. Hence, EC is measured in terms of electrical Resistance between parallel electrodes immersed in the soil water suspension.

In such a system, the solution between the electrodes becomes the electrical conductor to which the physical laws relating to resistance are applicable. The electrical resistance ‘R’ is directly proportional to the distance ‘L’ between the electrodes and inversely proportional to the cross sectional area ‘A’ of the conductor.

[Titrimetric/ Walkley and Black (1934) Method]

Introduction

The role of soil organic matter, in relation to soil fertility and physical conditions, are widely recognized. The organic matter is the source of plant nutrients which are released inassimilable forms during microbial degradation. A major proportion of N (95 – 99 % of the total), P (33 – 67 % of the total) and S (75 % of the total) in soils occur in organic combinations, which mineralize to release the nutrients in inorganic forms to be used by plants. Nevertheless, it serves as a reservoir of plant nutrients, in promoting water storage, and in regulating microbial activity.

The organic matter content of a soil varies from 0.344 % in very sandy arid soils to more than 86 % in Peats and Mucks. Soil organic matter contains 5 % N, and 0.5 % each of P and S, thereby, having a N:P:S ratio of 10:1:1. The organic matter content of a soil is estimated by the amount of organic carbon (C) present, as this element represents, on an average, 52 – 58 % of the organic matter, and the C:N ratio is 10 – 15. In the sub-soil, the average organic carbon amount is 36 – 44 % (40 %), and the C:N ratio is < 8. In order to find out the amount of humified organic material, the organic carbon content is multiplied by 1.724, which represents the ratio between the humus and the organic carbon (100/58) and is known as the ‘von Bemmlen factor ’.

The organic matter content of soils is estimated from the organic carbon, determined by using the ‘Titrimetric method / Walkley and Black method. The Titrimetric determination, also known as the ‘wet-digestion method’ involves a rapid titration procedure for the estimation of organic carbon content of soils.

[Alkaline Permanganate / Subbiah and Asija (1956) Method]

Introduction

Nitrogen (N) is found in the arable horizon of the soil, mostly in organic material. In the soil solution, organic N is gradually transformed into ammoniacal (NH₄ ⁺), nitrite (NO₂ ^–) and nitrate (NO₃ ^–) nitrogen by microbial processes. Organic-N is, in itself, of very little use to plants, as it can not be absorbed as such. It is, therefore, necessary to estimate the different forms of mineralized or available N. The NO₃ ^–_N and NO₂ ^–-N together, hardly, exceed 1 % of the total N in normal soil.

The available N in soil refers to a fraction of the total N which is converted into forms accessible to the plants. This constitutes, on an average, only 0.5-2.5 % (rarely 5 %) of the total N in a soil at any given time. Nitrogen is, generally, absorbed by plants as NO₃ ^–, under oxidized environment (upland condition), and as NH₄ ⁺, under reduced condition. NO₂ ^– is sometimes, detectable, but the amount is, generally, very small vis-a-vis NH₄ ⁺ or NO₃ ^– and, hence does not warrant its determination. The sum total of NH₄ ⁺ and NO₃ ^–- N is smaller than the total N, which becomes available to plants and the difference being attributed to the mineralization of the organic N, during the crop growth cycle.

[Bray’s Method (Bray and Kurtz, 1945) – Acid Soils]

Introduction

The term available phosphorus (P) refers to the inorganic form, occurring in the soil solution, which is almost exclusively ‘orthophosphate’. This orthophosphate occurs in several forms and combinations, and only a small fraction of the total amount present may be available to plants, which is of direct relevance in assessing the P fertility level.

The phosphate concentration in solution is governed by heterogeneous equilibria in which it takes part. This situation can be represented as follows:

P adsorbed in solid phase ↔ P in the soil solution ↔ P precipitated

The phosphorus absorbed by plants from soil solution comes from the soil solution in which it exists as inorganic orthophosphate ions viz., H₂PO₄ ^–, HPO₄ ^2– and PO₄ ^3–. The most accessible ion is H₂PO₄ ^–, with the greatest activity coefficient, followed by HPO₄ ^2–. The quantity of P accessible to the plants is influenced by a series of soil properties. The relative abundance of these ions is, however, relatively dependent on the soil pH. For soils having a pH between 4.5 and 7.5, ions of H₂PO₄ ^– as well as HPO₄ ^2– exist in soil solution. At a pH of 7.2, H₂PO₄ ^–and HPO₄ ^2– ions have an equal activity, and, when the pH is strongly alkaline (>8.3), ions of HPO₄ ^2– predominate in solution. Above pH of 9.0, the trivalent ion (PO₄ ^3–) becomes more important than H₂PO₄ ^–, but even at a pH of 12, the HPO₄ ^2– concentration is still greater than that of PO₄ ^3–. Soluble phosphorus may be adsorptively retained at the surface of colloidal particles. This retention is more marked when the higher amounts of clay and sesquioxides are present. Relatively, the available inorganic P tends to accumulate in its most stable state under prevailing conditions, thus, in calcareous soils, the available inorganic P would be acid-soluble, whereas, in acid soils the adsorbed P would be more available. In flooded soils, with low oxidation potential, certain forms of P, normally considered as unavailable, for example, iron and phosphate, can be regarded as available. In most soils, the main source of orthophosphate is organic matter unless, of course, direct fertilization, with soluble phosphate, has been made.

(Neutral Normal Ammonium Acetate Method)

Introduction

The total potassium (K) content of a soil varies from 0.05 to 2.5 %. The total K is distributed in mineral form (lattice – K, 90-98 %), fixed non-exchangeable or temporarily retrograded K (1-10 %) and exchangeable plus water soluble K (1-2 %). Exchangeable K represents an average of 2 – 15 % of the sum of exchangeable bases and 1 – 3 % of the total capacity for cation exchange. The K concentration in the soil solution is on an average 0.08 – 3 meq/litre. Both water soluble and exchangeable K are most accessible to the plant. Available K can, thus be separated into that, immediately available, which is water soluble and exchangeable, and that, potentially available or fixed. The neutral normal ammonium acetate extract contains both water soluble and exchangeable K. K extraction by this extractant is considered as a suitable index of K availability in most soils, based on crop response correlation study. Thus, the K extracted by this method is equated as the available K.

[Williams and Steinbergs (1959) Method]

Introduction

Sulphur (S) occurs in numerous forms in soil, viz., sulphites, sulphates, sulphides and in organic compounds. It is considered that the most accessible form is ‘sulphate’ (SO₄ ^2–). The organic forms of S compounds become assimilable, especially following, microbiological transformation into SO₄ ^2–. The mineralization of organic S in a soil depends primarily upon the N:S ratio, and any SO₄ ^2– formed may be fixed against extraction, particularly if much Fe or Ba is present or if the soil is very acid. Despite the fact that the plants absorb S almost exclusively as SO₄ ^2–, mobility of SO₄ ^2– in soil may not always yield satisfactory results in accordance with the time of sampling, while assessing SO₄ ^2– availability.

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

Introduction

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

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

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

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

[Schoonover (1952) Method]

Introduction

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

Principle

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

For the intelligent appreciation and understanding of various reactions and processes associated with soils a little basic knowledge of chemistry is helpful. If the reader follows this chapter carefully, the working knowledge of chemistry thus acquired will enable him to follow all the complicated chemical reactions taking place in the soil.

What is Chemistry? Chemistry is the science that deals with the chemical changes. What is a chemical change? When two or more substances combine to give a substance entirely different in properties to the individual components, the change is called a chemical action to distinguish it from the purely physical action in which the properties of the product are intermediate between the components. The latter naturally can be separated easily by mechanical means. For instance a mixture of sand and water can be separated by passing through a filter, and a mixture of salt and water can be separated by evaporating the water when the salt is left as residue. But water itself is a compound of two gases: oxygen and hydrogen. Its properties are totally different from its components. We can freeze it, evaporate it and perform a host of other operations on it without changing it in the least.

Atoms and molecules: The whole science of chemistry rests on atoms and molecules If these are understood clearly, basic chemistry is reduced to simplicity. Take a familiar object like gold or silver and go on subdividing it into smaller and smaller bits. You will reach a stage that further subdivision will become impossible. You will have reached the primary units, the elementary bricks of which the substance is made up, namely the “Atoms”. An atom in chemistry is the smallest brick from which all material substances in the world are made up. Not that you can see an atom. It is so small that even the most powerful microscope will nut reveal its presence. You could perhaps just see a collection of a million atoms as a tiny spot of light if it is illuminated with a powerful beam of light. Even then you could not say if it was round, square, or oblong. However, you need not look for any such tangible proof to believe in the existence of atoms. Even pure logic would compel you to stop somewhere if you started with an imaginary division and sub-division of a substance. You could not go on ad infinitum. Even the ancients, who knew no modern chemistry, believed in their existence purely from philosophical reasoning.

If the reader has endeavoured to follow the basic chemistry in the first chapter, he would be in a positions to follow up the various reactions that take place in soils and which hitherto had baffled him. Soils contain particles of various sizes ; some so fine that they would remain suspended in water for a long time; others so coarse that they would be sieved out by even a coarse sieve. Soils contain some salts that can be washed out by leaching with water; they contain some water which can be readily evaporated away by exposing to hot sun or by heating slightly. They also con-tain a little organic matter which can be burnt away or dissolved in alkali. When all the extraneous substances have been completely removed we are left with what we might call soil framework. This framework gives mechanical support to the roots of the plants. Without this framework the plant would not stand up even if it could be supplied with all the food material.

Let us suppose a soil freed from all the extraneous matter by continuous leaching with water or a dilute acid. In the former case the removal of all the extraneous matter is extremely slow, whereas by leaching with dilute acid everything except the soil framework is removed in a short time.

When soils from different localities are subjected to leaching with dilute acid we find that the soil framework in every case, behaves in a similar manner. No matter from what part of the world the soil has been obtained, the properties of its mineral framework only differ in quantity but not in kind.

Next to the mineral frame work the salts in the soil deserve consideration. In fact these salts constitute the main food material of plants besides what the root hairs can get out of the mineral framework in the form of basic materials already dealt with. We are, however, not concerned with the plant food materials, but salts that are in excess and which alter the properties of the soil to such an extent as to influence its behaviour profoundly.

Soil texture and mechanical analysis: The chemistry of the mineral framework was explained in Chapter II. Its physical properties are quite as important- When all the extraneous matter is removed from the soil we are left with a collection of particles of various sizes. These particles are not spherical in shape, but most of them are nearly so. At any rate the discussion is very much simplified if we suppose that they are spherical and the arguments advanced are equally applicable if the shape deviates from the spherical.

Moisture plays by far the most important role in soils as regards vegetation. It is through the medium of water that the plants obtain their quota of mineral food vital for their growth. If water is gradually withheld from the soil, at a certain stage, the plant will begin to wilt and soon dry up. The soil may still contain some moisture but the plant is unable to make use of it. The minimum moisture in the soil below which wilting will take place is called the Wilting Coefficient and at one time great importance was attached to it. The determination of this Coefficient, however, is tedius. One must grow a number of plants and slowly watch them wilt and determine the moisture content at that point. Moisture content of the soil is determined by heating a weighed quantity of the sample in an oven kept at 100-110 ^oC. The moisture is expressed as percentage on the oven-dry soil.

Soil mechanics as a distinct branch of soil science is comparatively of recent growth. Its need arose from the fact that the requirements of the engineer are very often quite different from those of the agriculturist. Whereas the former would as a rule require hard, compact, impervious soil, the latter would have no use for it for the growth of his plants which require soft, friable and porous soil. The agriculturist generally has to look up to nature to help him out of his difficulties, the engineer has to depend on his scientific skill to modify the soil to suit his requirements. In this chapter an attempt will be made to explain in a simple language the requirements of the building, road, and irrigation engineer for particular types of soils and how they are met by modifying the soil if the required properties already do not exist.

Soil fertility is the resultant of several factors, not the least important of which is the soil itself. Theoretically, the soil only plays the part of a support and could be very easily dispensed with and replaced by sand and even that can be replaced by a wire framework to support the plant mechanically and it will grow quite well if the right amount of nutrient materials have been added to water. ‘Where does the soil come in?” you will ask. The importance of the soil is two-fold: (1) It provides vast reserves of plant food which is slowly made available. (2) It provides limiting factors which restrict plant growth; these limiting factors must be neutralized. We have already discussed these reserve food materials and limiting factors in Chapter II.

It has been shown that if we treat a soil with dilute acid exhaustively we can remove everything from it except the mineral framework. If we try to grow anything in this mineral framework nothing will grow. Yet it is not like sand, for whereas in sand by adding appropriate amounts of nutrient materials in water solution you could grow a crop, nothing will grow if you added the same solution of nutrient materials to the soil. This mineral framework, therefore, is not inert like sand, but it is active and can react with the nutrient salts in such a way that either they are rendered unavailable or something else which is toxic is produced by the interaction between the soil and the nutrient salts. This mineral framework is not acted upon by dilute acids, but it reacts with alkalies in such a way that the alkalies are neutralized. In this respect, therefore, the mineral frame work behaves like an acid. With alkalies it forms salts and different alkalies are required in equivalent amounts for the formation of these salts. Thus we can have salts of a number of bases and if we use sodium hydroxide for neutralizing we shall have sodium soil, similarly we can have a calcium, magnesium or potassium soil. Now in nature, all the various bases principally, calcium, magnesium, sodium, and potassium, are associated with it in different proportions. The nature and amount of these bases determines the physico-chemical properties of the soil affecting crop yield and leading to soil infertility. We have already indicated the method of bringing the soiil to the normal state when it suffers from any of the defects associated with the soil framework.

1.1 Forest Resources

Forest resources usually provide the raw materials for homes, workplaces, the books and newspapers, and the packaging that contains food and other products of our labor. Forest ecosystems supply our water, maintain our climate, and help purify the air, protect soils, and provide for wilderness experiences. Forests provide habitat for wildlife, and serve as preserves of biological diversity. Forests are sources of food, fuel and medicine for people all over the world. Forests shape the recreational landscape, help stabilize our farms, and enhance our cities.

About 30 per cent of the world’s total land areas are covered by forest with the area of 3.9 billion hectares. Nearly 47 percent of the world’s forests are in the tropics, 33 percent in the boreal zone, 11 percent in the temperate zones, and 9 percent in sub-tropical areas. The 94 million hectares of forest lost over the ten-year period, represented about 2 per cent of the world’s total forest cover, or an area larger than Venezuela. Two thirds of the world’s forests are located in ten countries: the Russian Federation, Brazil, Canada, the United States, China, Australia, the Democratic Republic of the Congo, Indonesia, Angola and Peru. Most of the deforestation work was performed in natural tropical forests, which lost 14.2 million hectares a year over the last decade. Africa and South America have suffered the most deforestation.

Africa was the region with the highest deforestation in the world, which lost 5.3 million hectares of forest per year in the 1990s. Forests are a major factor in the climate change issue. Forest ecosystems contain more than half of all terrestrial carbon, and account for about 80 per cent of the exchange of carbon between terrestrial ecosystems and the atmosphere. Deforestation in the 1980s may have accounted for a quarter of all human-induced carbon emissions, the second greatest emitter after fossil fuels. Forest plantations comprise 5 percent of the world’s forests. Asia has the largest area of plantations, accounting for 62 per cent of the world total. China accounts for 24 percent of that total and India, 18 per cent. Plantations supply about 35 percent of the world’s round wood. The area of forest plantations increased by an average of 3 to 4 million hectares per year during the 1990s. Half of this increase was the result of afforestation on land previously under non-forest land use, whereas the other half resulted from conversion of natural forest. There are reports that 12 percent of the world’s forests, or about 480 million hectares, are in protected areas.

2.1 Ecosystem

An ecosystem can be defined as a dynamic entity composed of a biological community and its associated abiotic environment. Often the dynamic interactions that occur within an ecosystem are numerous and complex. Ecosystems are also always undergoing changes or alterations to their biotic and abiotic components. Some of these alterations begin first with a change in the state of one component of the ecosystem which then cascades and sometimes amplifies into other components because of relationships.

The human activities of various kinds have resulted in caused a number of dramatic changes to a variety of ecosystems found on the Earth. Humans use and modify natural ecosystems through agriculture, forestry, recreation, urbanization, and industry. The most obvious impact of humans on ecosystems is the loss of biodiversity. The number of extinctions caused by human domination of ecosystems has been steadily increasing since the start of the Industrial revolution. The frequency of species extinctions is correlated to the size of human population on the Earth which is directly related to resource consumption, land-use change, and environmental degradation. Other human impacts to ecosystems include species invasions to new habitats, changes to the abundance and dominance of species in communities, modification of biogeochemical, and modification of hydrologic cycling, pollution, and climatic change.

3.1 Air Pollution

Air pollution is a phenomenon by which particles either a solid or liquid and gases contaminate the environment. Such contamination can result in health effects on the population, which might be either chronic arising especially from long-term exposure or acute due to accidents. Other effects of pollution may be damage to materials, agricultural damage namely reduced crop yields and tree growth, impairment of visibility namely tiny particles scatter light very efficiently, and even climate change global warming.

3.1.1 Classification of Air Pollutants

Air Pollutants are classified as Natural as well as Anthropogenic. Natural pollutants are those that are found in nature or are emitted from natural sources. For example, volcanic activity produces sulfur dioxide, and particulate pollution may derive from forest fires or windblown dust. Anthropogenic pollutants are those that are produced by humans or controlled processes. For example, sulfur dioxide is produced by fossil fuel combustion and particulate matter comes from diesel engines.

Air pollutants are further classified as primary or secondary. Primary pollutants are those that are emitted directly into the atmosphere from an identifiable source. Examples include carbon monoxide and sulfur dioxide. Secondary pollutants are those that are produced in the atmosphere by chemical and physical processes from primary pollutants and natural constituents. For example, ozone is produced by hydrocarbons and oxides of nitrogen (both of which may be produced by car emissions) and sunlight.