eChapters

Introduction

In Indian Thar desert regions, excessive heat is the major problem that causes human and animal thermal discomfort. Space cooling is, therefore, the most desirable factor for the inhabitants. Various examples of dwellings responsive to climatic constraints are found in vernacular architecture throughout the world. Compact cellular layout with minimum external surface exposure to the sun, whitewashed surfaces to reduce absorptivity, blind external facades, courtyards, vegetation to provide humidity and shade, and heavy buildings constructed from high thermal capacity materials are common passive features in most of the arid regions. Wind towers for cooling ventilation are well known in Iranian and Middle East architecture, which along with cooling of the air by earth and water evaporation keep the building comfortable in hot periods. Building underground to take advantage of the large thermal storage capacity of the earth is also used in Tunisia and central Turkey.

Introduction

The age-old necessities of life are food, clothing and shelter. The 20^th century had dramatized a fourth-energy. Energy starvation of the technological complex that maintains modern society may soon be as crucial a problem as feeding the hungry. Indeed, energy starvation could well precipitate more wide spread food starvation. Solutions for the energy crisis are strongly dependent on the technology of how energy is produced and used. To make a physical change in the world it is necessary to use four resources: energy, matter, space and time. How well a task has been performed can be measured in terms of amount of fuel consumed, the mass of material used, the space occupied, the hours of labour to accomplish it, and the ingenuity with which these resources are utilized. Squandering of irreplaceable energy resources, waste of materials, or large expenditure of space and time cannot be tolerated if the necessities of life are to be provided for all. Energy is intricately linked to development of a country. Growing demand on account of industrialization, urbanization, transportation and also increase consumption in rural areas is putting added pressure on energy supply network. Electricity generation has been increased from 1,400 MW in the year 1947 to 303,118 MW up to August 31, 2016 in India but still there is 10-15% shortage of power in peak hours and all villages are not electrified. Even many electrified villages receive 6 to 8 hours of electricity per day in India. To overcome this shortage there is a need to utilise new and renewable sources of energy. Fortunately, India is blessed with abundant solar energy. The arid parts of India receives much more radiation as compared to the rest of country i.e. maximum solar radiation i.e. 7600-8000 MJm^-2 (2111-2222 kWhm^-2) per annum, followed by semi-arid parts, 7200-7600 MJm^-2 (2000-2111 kWhm^-2), per annum and least on hilly areas where solar radiation is still appreciable i.e. 6000 MJm^-2 (1667 kWhm^-2) per annum (IMD 1985). In this chapter review of different solar thermal technologies in Indian perspective has been carried out.

For electromagnetic radiation, there are four “laws” that describe the type and amount of energy being emitted by an object. In the following sections, these four laws of radiation e.g. Planck’s law, Wien’s displacement law, Steffan-Boltzman law and Kirchhoff’s law are described briefly.

Planck’s Law

Planck’s law describes the spectral density of electromagnetic radiation emitted by a black body in thermal equilibrium at a given temperature T. The spectral radiance of a body, B_í, describes the amount of energy it gives off as radiation of different frequencies. It is measured in terms of the power emitted per unit area of the body, per unit solid angle that the radiation is measured over, per unit frequency. Planck showed that the spectral radiance of a body at absolute temperature T is given by

Introduction

Cooking accounts for a major share of energy consumption in developing countries. Fifty per cent of the total energy consumed in India is for cooking (Fritz 1981). Most of the cooking energy requirement is met by non-commercial fuels such as firewood (75%), agricultural waste and cow dung cake (25%) in rural areas. The fuel wood requirement is 0.4 tons per person per year in India. In rural areas firewood crisis is far graver than that caused by a rise in oil prices. Marginal villagers have to forage 8 to 10 hours a day in search of firewood as compared to 1 to 2 hours ten years ago. One third of India’s fertilizer consumption can be met if cow dung is not burnt for cooking and instead is used as manure. The cutting of firewood causes deforestation that leads to desertification. Fortunately, India is blessed with abundant solar radiation (IMD, 1985). The arid parts of India receive maximum radiation i.e. 7600-8000 MJ m^-2 per annum, followed by semi-arid parts, 7200-7600 MJm^-2, per annum and least on hilly areas where solar radiation is still appreciable i.e. 6000 MJm^-2 per annum. Therefore, solar cookers seem to be a good substitute for cooking with firewood.

Principle of cooking

The different methods of cooking of food are boiling, frying, roasting, and baking. For boiling of rice, lentils etc. the temperature of food being cooked is about 100° C while for other methods, high temperatures are required. Heat is supplied at the bottom of the vessel for frying and boiling purposes in conventional cooking. Roasting and baking is generally performed on open fire or in ovens, wherein food is surrounded by hot surfaces and heat is transferred to the food by radiation and convection.

Introduction

Flat plate collector is the heart of a solar heat collection system designed for delivery of heated fluid in the low to medium temperature range (5o70oC above ambient temperature) for applications, such as water heating, space heating, drying and similar industrial applications. The flat plate collectors absorb both beam and diffuse radiation. The absorbed radiation is converted into heat which is transferred to water or air flowing through the collector tubes or duct, respectively. Such collectors do not require tracking of the sun and little maintenance is required.

The conventional flat plate solar air heater, shown in Fig. 7.1, consists of a flat blackened absorber plate, a transparent cover (such as a glass cover) at the top and insulation at the bottom and on the sides. The air to be heated flows through the rectangular duct below the absorber plate. The glass cover transmits a major part of solar radiation incident upon it to the absorber plate where it is converted into heat. The glass is, however, opaque to longwavelength radiation and thus it does not allow the infrared radiation from the heated absorber plate to escape.

Karwa et al. (2002) have presented the following deductions of heat collection rate and pumping power equations for the flat plate solar air heater.

Introduction

We are blessed with solar energy in abundance at no cost. The solar radiation incident on the surface of the earth can be conveniently utilized for the benefit of human society. Solar energy can be used as thermal energy for water heating, cooking, drying, distillation, space heating, cooling and power generation or it can be converted to electricity through photovoltaic cells, commonly known as solar cells. One of the popular devices that harness the solar energy is solar hot water system (SHWS). Flat plate collectors are used for low temperature applications (below 100^oC) while concentrators are preferred for higher temperatures. The most efficient application of flat plate collector is for getting hot water. Hot water in winter is an essential requirement for domestic uses such as bathing, cleaning of utensils and washing of clothes. In rural areas hot water is also required for softening of animal feed. Generally it is obtained by using firewood and cow dung cake in rural areas or using kerosene, liquid petroleum gas, coal or electricity in urban areas. Therefore, the use of solar water heaters will conserve lot of commercial and non-commercial fuels which are being wasted in merely getting hot water. A solar water heating system (SWHS) is made of several important elements: one or more solar collectors, a pump, a heat exchanger, a storage tank (or multiple tanks) and a back-up storage tank. The Solar heating system can be classified as passive or active. For water heating purposes, the general practice is to use flat plate solar energy collectors (FPC). The evacuated tube collectors (ETC) and evacuated tube heat pipe collectors (ETHP) are more efficient, though, the initial cost is comparatively higher. There are three type of water heaters extensively studied by various workers viz. natural circulation, collector-cum-storage and forced circulation in all over the world.

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.

Trees are the largest plant components of gardens and landscapes. However much architectural features dwarf them, for human beings, they are the largest living entities in the environment. Profoundly influencing our habits, styles of habitation, culture and daily life, trees have been revered, sanctified and given symbolic importance and above all, depended upon as means of livelihood by entire communities through the ages. Through folklore, traditional knowledge and rituals, the usefulness of trees has been handed down from generation to generation-in societies all over the world. In India, cave paintings and carvings in addition to ancient literary works document the aesthetic and medicinal knowledge of our forefathers about trees found in the sub-continent.

Trees differ in stature, branching habit, girth of trunk and branches, canopy structure, outline and crown shape. The colour of the foliage, its texture, glaze, shape and size, foliage colour, floral features and season of flowering also differ among trees. These characteristics in addition to giving distinctiveness, also determine their ornamental use in gardens and landscapes. In tropical gardens evergreen dicotyledons and palms dominate among the trees, whereas in temperate situations evergreen conifers are selected over the deciduous dicots for tree planting. Palms among trees are universally grown as avenue trees or as clumps in areas where they can be easily grown while the other trees are grown as such or pruned and trained into various forms or as bonsai in gardens.

Shrubs are the most diverse groups of ornamentals grown in tropical gardens. In habit they are hardy perennials having low spreading woody branches. They vary widely in stature and include dwarf bushes to those attaining the stature of small trees.

IMPORTANCE AND USES OF SHRUBS

Shrubs are quick growing and many of them flower continuously in the humid tropics. Their variability in flower colours and foliage characteristics and their easiness in propagation by vegetative means make them garden favourites. The various uses to which shrubs are put to in gardens are detailed below: -

In A Shrubbery : Grouping of shrubs in a shrubbery is done to display the attractiveness of their flowers and foliage. In temperate situations shrubs grown in shrubberies include Rhododendron, Peony, Fuchsia, Buddleia, and Camellia while in the tropics and subtropics, the range of shrubs available for planting are very wide.

As Focal Point Plants : Shrubs that are ever flowering like Hamelia and Hibiscus or those that have spectacular flowers such as Mussaenda, Neerium and Ixora are grown as focal point plants in small gardens. They are also grouped for mass effect in large gardens.

H erbs include non-woody or succulent-stemmed plants that are relatively shorter in stature when compared to shrubs. Based on their ornamental nature they can be broadly grouped as Flowering Herbs and Foliage Herbs. Flowering Herbs can be further classified based on their duration into Annual Flowering Herbs and Perennial Flowering Herbs. Foliage herbs of a perennial nature are further differentiated into monocotyledonous and dicotyledonous foliage herbs.

ANNUAL FLOWERING HERBS

Annual herbs are grown in gardens for their spectacular beauty when they are in flower. They are seasonal in habit and have duration of up to four months. Most of them have a short vegetative phase, which is followed by continuous flowering for several weeks. They are used in gardens for obtaining various colour effects. Varieties of annuals having analogous or similar flower colours can be grouped in flowerbeds and flower borders for monochromatic effects. For contrasting colour effects, varieties having complementary or contrasting colours like violet and yellow or blue and orange as well as green and red can be used. For colour blending effects, flowers having different shades of the same colour can be grown.

Pants that tend to climb vertically on to supports are called climbers. reepers are plants that tend to creep or ramble over the ground or other surfaces spreading horizontally. These have showy flowers or decorative foliage. Climbers are conventionally used in gardens to cover fences, bare walls and tree trunks. They are used to provide ornamentation in a vertical aspect. Creepers on the other hand are used as ground covers and function as decorative elements in a horizontal aspect.

CLASSIFICATION OF CLIMBERS AND CREEPERS

Depending upon their habit, vigour, duration, nature of flowering, adaptations for climbing and shade tolerance, they can be classified as follows:

Among garden plants, herbaceous perennials that produce perennating structures like bulbs, corms, tubers, rhizomes or fascicled tuberous roots are called bulbous plants. They belong to various plant families such as Amaryllidaceae, Cannaceae, Heliconiaceae, Iridaceae, Liliaeceae, Musaceae, Ranunculaeceae and Zingiberaceae. The possession of perennating structures that are modifications of plant parts- mostly stems- is common to these plants. These modifications aid in their vegetative propagation as the dormant buds associated with them regenerate when conditions are favourable to result in new plants. In both tropical and temperate climate zones, bulbous plants have evolved and naturalized in several ecological ‘niches’. Many of these have been domesticated centuries ago and are familiar garden plants. Generally, bulbous plats are adapted to grow vegetatively putting forth shoots carrying a limited number of leaves. When further vegetative growth has ceased these shoots produce terminal or basal inflorescences. Bulbous plants are grown and displayed in clumps.

Based on the requirement of relatively low or high temperatures for sprouting and flowering they are classified into warm- season bulbous plants and cool-season bulbous plants. These groupings are not exclusive and many plants have evolved varieties that thrive in both seasonal conditions.

These are a broad group of plants that have evolved in situations characterized by very high day temperatures and low night temperatures with very low available moisture in the growing medium. Their plant body is modified to conserve moisture with physiological adaptations that help to reduce moisture loss considerably and to survive in hot desert situations or drought conditions. Stems, leaves and roots of these plants are fleshy and rich in watery mucus. The unusual morphological features that these plants have developed to enable them to survive in their harsh native habitats, set them apart from other plants and have given them rarity value when grown as ornamentals. They are hence displayed individually or grown in a rockery where a landscape of rocks and stones highlight their features and at the same time provide desirable contrast with other garden plants. Many members of Cactaceae, Crassulaceae, Aizoaceae, Euphorbiaceae, Asphodelaceae and Agavaceae belong to this group and are commonly referred to as rock garden plants.

Terrestrial or epiphytic members of the family Bromeliaceae that are grown in gardens for their unusually coloured and patterned leaves or showy inflorescences and fruits are referred to as ornamental ‘Bromeliads’. Being low growing, they are ideally displayed in containers or as epiphytes. They are sun loving, suckering in habit and have xerophytic adaptations. The leaves are tough and fibrous often with spiny margins and sheathing leaf bases that hold water. A grayish-white waxy or powdery coating often covers the lamina. Certain bromeliads have a water collecting depression at the center. Inflorescences are sunken, erect or reclining, with condensed or long and much branched stalks carrying colourful bracts and usually, less showy flowers. In many, the terminal leaves become brightly coloured at flowering. These and the floral bracts remain showy even after the flowers are shed.

Ornamental bromeliads popularly grown in gardens include the following:

Aechmea blanchetiana

Copper leaf bromeliad

Plants have strap like leaves that develop streaks of coppery red colour on the upper surface in bright sunlight.

Aechmea burle-marxii

Burle-Marx bromeliad

Pteridophytes such as ferns belonging to several families, selaginellas popularly called ‘club mosses’ belonging to Selaginellaceae and the Lycopods which resemble both these, belonging to Lycopodiaceae are shade loving plants adapted to cool and moist or hot and humid ecosystems of tropical to temperate climates. They are mostly epiphytic or lithophytic and are grown as foliage ornamentals in a fernery or in shaded areas in gardens. Many are favoured indoor plants.

Popularly grown ferns, lycopods and selaginellas of gardens are listed below:

Adiantum capillus-veneris

Adiantaceae

Common Maidenhair fern

Plants are shade loving bushy lithophytes and have delicate looking thin black stems carrying light green oblique-ovate fronds. They are pot grown as ornamental foliage plants.

Adiantum decorum

Syn. Adiantum raddianum

Adiantaceae

Maidenhair fern

Grasses, bamboos, sedges and rushes are plants that are being used increasingly in xeri-scaping and in marsh gardens for getting the benefits of their unusual foliage textures and colours. Their hardiness, stature and canopy frame make them more suited for specimen planting in gardens and landscapes.

GRASSES

Grasses are the most successful of monocots belonging to the family Poaceae that are found the world over. Low growing perennial grasses are used to make attractive lawns in gardens and to provide a desirable turf cover for various situations. A number of species belonging to various genera differing in leaf colour, shape and growth pattern are used for this purpose. Mixtures of different varieties of grasses are also used for making turf having desirable blends of textures and hardiness. Tall bushy grasses having a mound-like or fountain–like habit and fine to velvety surfaces are grown for their textural and architectural features in landscaping. Many are grown as specimen plants for their display value at flowering. In selecting grasses for gardens, their potentially invasive nature is to be taken into account and seed dispersal controlled by timely cutting.

Grass species that thrive in the hot and humid tropics are called warm-season grasses and include Bermuda grass, Carpet grass and St. Augustine grass. Cool season grasses that are adapted to temperate regions and thrive at temperatures of 10 to 20^oC include Blue grass, Fescues and Zoysia.

Ornamental plants that are adapted to an aquatic environment are commonly grown in water gardens and pools. They include those that are entirely submerged or floaters that are free living in the surface water or having roots anchored firmly in the ground beneath and marginal plants that thrive along the edges of water bodies and in shallow water. Spectacular flowering plants like the lotuses and water lilies to plants like the tape grass that are entirely submerged and grass like are grown in natural or structured fresh water habitats throughout the world. Many of these are also inhabitants of aquaria.

Some of the popularly grown water garden plants are listed below:

Acorus calamus

Araceae

Sweet Flag, Vayambu

Plants are rhizomatous herbs adapted to low lying and wet situations. The leaves are closely spaced, distichously arranged and resembling a fan, leathery, dark green, smooth and shiny. The inflorescence is a spadix that emerges from the middle of the terminal leaf. Plants are valued medicinally for their aromatic rhizomes and are also commonly grown as marginals of water gardens.

Precision farming is also known as precision agriculture. A farmer is not only expected to produce more for the increasing population and his family but save and protect the fragile environment while practicing it. Advanced technologies with appropriate and exact inputs such as precision farming can take care better of environment. Precision farming is right farming operations at right time leading to maximize profit. Every farming activity performed with precision by doing the right thing, in the right place, in the right way, at the right time and with right technology leading to maximize profit is known as precision farming. It is an efficient, eco-friendly and accurate management of agriculture inputs such as water, seed, fertilizers, pesticides and harvesting tools and machinery to maximize profit and reduce waste. The basic concept of precision farming is to match agricultural inputs and operations as per crop (variety/hybrid/cultivar) and agroclimatic conditions with accuracy to their better use efficiency. Perfection (nothing requisite is wanting) in farming is the key to precision farming.

Precision farming is based on global positioning system (GPS). The unique character of GPS is precision in time and space. Its use is spreading to all farm operations. Precision farming is an integrated information and production based farming system that is designed to increase long term, site specific and whole farm production efficiency, productivity and profitability while minimizing, waste and unintended impacts on the environment. This concept of precision agriculture is based on observing, measuring and responding to inter and intra field variability in crops.

Farm management strategy is the key to precision agriculture that uses integrated smart technologies such as sensors, GIS, GPS, satellite radars, drones, IoT (Internet of things) and data analytics to develop informed crop production and management decisions. The field variations are precisely measured with the help of technologies to optimize applications of inputs such as manures, fertilizers, pesticides, bio-agents and water use to increase crop yield at minimum cost. The aim of precision farming is to increase productivity, decrease production costs and minimize the environmental impact of farming.

It is a common knowledge that most of the farmers apply irrigation water until all the points in the field are fully wetted and covered with a thin sheet of water. Studies have indicated that a significant (20-25%) amount of irrigation water is lost during its application at the farm due to poor farm designing and unevenness of the fields. This problem is more pronounced in the case of rice fields. Unevenness of fields leads to inefficient use of irrigation water and also delays tillage and crop establishment options. Fields that are not levelled have uneven crop stands, increased weed burdens and uneven maturing of crops. All these factors tend to contribute to reduced yield and produce quality which reduce the potential farm income. Effective land levelling is meant to optimize wateruse efficiency, improve crop establishment, reduce the irrigation time and effort required to manage crop.

Reasonably levelled agriculture fields has advantage over uneven and undulating crop fields as unevenness of the soil surface has a significant impact on the seed germination, crop stand and its yield. Traditionally land levelling was being done throughout India by bullock drawn plough or tractors and applying different devices for levelling the field by visual assessment e.g. on small plots with hoes, or with draft animals and equipment such as ploughs and bars or scrapers or grading. Equipment such as grading blades and hydraulically operated levellers mounted on wheels are used with tractors. More advanced and sophisticated levelling equipment is operated with a laser emitter, a laser sensor or receiver, and a scraper pulled by a tractor. After the desired level or slope of the field and/or the difference of the high and low spots have been surveyed, the emitter is set to send a rotating laser beam creating a plane of laser light above the field surface. The laser light is used as the levelling reference. It directs the hydraulic system of the moving tractor and scraper, and thereby controls the levelling. Hence, laser levelling or laser land levelling is levelling the crop field within certain degree of desired slope using a guided laser beam throughout the field.

Sowing of Seed: It is a process of planting seeds of crops in well prepared or no till field or nursery beds/pots for raising crops in following methods:

Methods of Sowing: The sowing method is determined by the crop to be sown and resources on the disposal of farmer. There are 6 sowing methods which differ in their merits, demerits and adoption. Those are:

1.Broad casting

2.Broad or Line sowing

3.Dibbling

4.Transplanting

5.Planting

6.Putting seeds behind the plough.

Seedling:A seedling is a very young plant that grows from a seed. When the moisture, light, and temperature conditions are correct, the seedling’s development begins with seed germination and the formation of three main parts:

1.Radicle - Embryonic root

2.Hypocotyl - Embryonic shoot

3.Cotyledons - Seed leaves.

In vegetables and annual ornamental crops where generally seed size is small they are germinated in nursery beds or in plug trays indoor (greenhouse) or outdoor(field), hardened before transplanting.

Sapling: A sapling is a young tree, say more than 1 year and less than 3 years’ old. In the life cycle of trees, a seedling converts into a sapling as the next stage of development. It is nearly one-meter tall and generally remains to be known as a sampling until its stem reaches 7 centimetre in diameter. The time period of this stage depends upon the particular species of a tree. In vegetable and annual ornamental crops seedlings generally are one month to 40 days old. Older than this, seedlings do not give better yields on transplanting. Very young seedlings exhibit larger mortality on transplanting.

Transplanting: Transplanting or replanting is the technique of moving a plant by uplifting from one location to another. From nursery beds seedlings or saplings are shifted and planted either in the field or greenhouses for commercial growing. Seedlings are uprooted if grown in soil medium. They are transplanted with growing medium. Uprooted seedlings take time to establish at a new place and face transplanting shock- time taken by roots to establish contact with the soil or growing medium.

In soil culture, also known as in-ground culture, crops are raised on level ground as well as in raised beds. Soil culture is more often used in growing vegetables (e.g., tomatoes, lettuce, cucumbers, and peppers) rather than ornamental crops such as herbs, roses, freesia, and foliage plants as well as popular with smaller commercial and organic growers. The soil should be well drained, fertile, have a medium to light texture and be high in organic matter. The soil in a greenhouse should be worked as deeply as possible to break any existing hardpans or those that may have developed because of repeated traffic over the area. Fumigation or pasteurization of the soil is a must before each crop or at least once a year. This will help to destroy disease, nematodes and weeds. If soil is steamed, hold it at 180 degrees F (82°C) for 4 hours. Production techniques used in soil culture are similar to intensive crop production methods used in the field. Although soil is readily available at most places where a greenhouse is located most commercial growers, no longer rely on soil culture in the greenhouse as they did ealier. Some of the reasons include insect and disease pressures and costly fumigation.

Crops are grown in soil. There are different types of soil. The soil type helps farmers decide what crops will grow best because some crops do better with particular types of soil. Testing the soil helps determine what nutrients are available or unavailable for growing certain crops. Farmers usually have local agricultural experts test the soil on their land for information on how to proceed in growing crops. The best soil has a balance of nutrients, such as potassium, magnesium, and others. Certain crops may need more of one nutrient, so farmers put back those ingredients into the soil for better crops.

Six different types of soil are listed below along with the crops which do well in these soils.

1.Loam: This type of soil helps grow the best possible crops because it provides the necessary elements for plant growth and development. Loamy sand contains mostly sand plus clay and silt. Such soils are good for root vegetables, such as carrots, radish, turnip, beets, leafy vegetables, potatoes and tomatoes.

2.Clay: Cabbage and broccoli grow well in clay soil, but it is not good for root vegetables because of its dense texture which does not allow free flow of water.

3.Sand: Root vegetables, such as turnips, parsnips, and carrots, grow well in sandy soil.

4.Silt: Silty soil is fertile and offers nutrients for plant development. Lettuce, cabbage, carrots, turnips, and many other vegetables flourish in silt. Strawberries, raspberries, blackberries do well.

5.Peat: Peaty soil provides plant growth when mixed with rich organic matter, lime, and compost that reduces its acidity. Legumes, root crops, cabbage, and spinach are easily grown in soil that’s well-drained.

6.Chalk: Sweet corn, beets, spinach, and cabbage are among the vegetables that benefit from chalkier soil. It is recommended that farmers use fertilizers and balance the pH levels to deal with the alkaline that can sometimes stunt growth.

Soil map is a map showing diversity of soil types and its properties such as pH, texture, organic matter and depth of horizons in the area of interest obtained as the end of soil survey. Generally, soil maps are used to simply identify soils and their properties, but are sometimes required for more specific purposes, such as the suitability of soils for a particular crop, suitability for irrigation, erosion risk and many other specific needs or environmental threats.

Soils fulfil many functions important for agriculture, forestry and the management of soil resources and natural hazards. The functionality of soils depends on their properties; hence, accurate and spatially highly resolved maps of basic soil properties such as texture, organic carbon content and pH for specific soil depth are needed for the sustainable management of soils. The surface soil organic carbon (SOC) concentration is a useful soil property to map soils, interpret soil properties, and guide fertilizer and agricultural chemical applications.

Traditionally farmers apply fertilizers, chemicals and other crop production inputs to optimize the production of the field as a whole. This results in application of over application of inputs and low inputs in certain locations of the field because of variation in field characteristics including soil organic carbon, soil texture, soil nutrients, field topography, and other properties. In addition, uniform applications may increase the chances of pollution of the environment due to excess application in some field area. Precision farming technology has been shown to optimize application rates if the variation of field characteristics can be used to guide the application rate of crop-production inputs. The organic carbon concentrations of surface soil have been used to spatially vary the application rate of some crop-production inputs. The surface soil organic carbon concentration affects the activity of many herbicides, influences plant-available nitrogen and also affects the soil’s ability to adsorb plant nutrients. Knowing its concentration may therefore be useful, especially if its spatial distribution could be determined accurately and for low cost.

In crop production mostly basic inputs are seeds or seedlings/planting material, irrigation water, fertilizers, weedicides and pesticides. Site specific input application means providing plant input requirement at right time, in right amount and at right place. Site specific is a term that refers to treating the smallest possible area as a single unit. A site is simply the smallest unit the farmer can manage with the tools available. The treatment of each site is specific to the needs of each site as determined by soil test data and crop scouting reports. There are technology tools for using site-specific management more effectively. In precision agriculture site specific application tools and technologies are as under

In crop production mostly basic inputs are seeds or seedlings/planting material, irrigation water, fertilizers, weedicides and pesticides. Site specific input application means providing plant input requirement at right time, in right amount and at right place. Site specific is a term that refers to treating the smallest possible area as a single unit. A site is simply the smallest unit the farmer can manage with the tools available. The treatment of each site is specific to the needs of each site as determined by soil test data and crop scouting reports. There are technology tools for using site-specific management more effectively. In precision agriculture site specific application tools and technologies are as under:

A. Precision irrigation

The prevalent flood irrigation in crops would be the thing of past because of limited water resources, waste of water in leaching and evaporation and cumbersome management resulting in considerable loss of water. The micro irrigation (sprinkler and drip- irrigation) are preferred depending on the field topography. Undulated fields are better irrigated by sprinkler and laser levelled field by drip irrigation. There are precise tools to deliver only required amount of water at different stages of growth of crops with the help of sensors. Irrigation water volume release is mechanised and automated. Efforts of using microirrigation methods such as sprinkler and drip irrigation have been made in last three decades in many parts of the world. There is no ideal irrigation method available which may be suitable for all weather conditions, soil structure and variety of crops cultures.

Precision irrigation is worldwide accepted concept in irrigation. Precision irrigation involves the accurate and precise application of water to meet the specific requirements of individual plants or management units and minimize adverse environmental impact. Commonly accepted definition of precision irrigation is sustainable management of water resources which involves application of water to the crop at the right time, right amount, right place and right manner thereby helping to manage the field variability of water in turn increasing the crop productivity and water use efficiency along with reduction in energy cost on irrigation. It utilizes a systems approach to achieve ’differential irrigation’ treatment of field variation (spatial and temporal) as opposed to the ’uniform irrigation’ treatment that underlies traditional management systems. Precise and regulated application of irrigation water and plant nutrients at low pressure and frequent intervals through drippers/emitters directly into the root zone of plant with the help of close network of pipes is known as drip irrigation system.

Sensing of heterogeneities in the field is the prerequisite for timely and spatially adjusted insect pests and disease management. Recent developments in sensor technologies have led to a broadening of their capacity to detect pests and thereby improve application in crop protection practice. These sensors can be space-borne (satellite), air-borne (airplane, unmanned aerial vehicle) or groundbased (handheld, vehicle-mounted) and provide spatial information that has added value management. Insects are usually unevenly distributed throughout an area in nature. Despite this knowledge, analytical methods and technology have limited insect pest management to uniform management decisions in the field. The methods and technology to map and analyse insect spatial distribution have been developed to a level where the uneven nature of insect populations can be considered for application of insect-pest management practices. Site-specific pest management utilizes spatial information about pest distribution to apply control method only where pest density is economically high within a field. This results saving in pesticide and labour cost. In conventional farming pesticides are applied uniformly to the field resulting in heavy use of pesticide and labour beside polluting the environment. Site-specific application of pesticides in precision farming helps to overcome this problem

Near-range and remote sensing techniques have demonstrated a high potential in detecting diseases and in monitoring crop stands for sub-areas with infected plants. The occurrence of plant diseases depends on specific environmental and epidemiological factors; diseases, therefore, often have a patchy distribution in the field. The use of non-invasive optical sensors for the detection, identification and quantification of plant diseases on different scales is made of in precision farming. Most promising sensor types are thermography, chlorophyll fluorescence and hyperspectral sensors. For the detection and monitoring of plant disease, imaging systems are preferable to non-imaging systems. To utilise the full potential of these highly sophisticated, innovative technologies and high dimensional, complex data for precision crop protection, a multi-disciplinary approach—including plant pathology, engineering, and informatics—is required.

Yield mapping or yield monitoring is a technique in agriculture of using GPS data to analyse variables such as crop yield and moisture content in a given field. This data produces a yield map that can be used to compare yield distribution within the field from year to year. This allows farmers to determine areas of the field that, for example, may need to be more heavily irrigated or are not yielding any crop at all. It also allows farmers to show the effects of a change in field-management techniques, to develop nutrient strategies for their fields, and as a record of crop yield.

Yield is ultimate indicator of variation of different agronomic parameters in different parts within the field. So mapping of yield and interpretation and correlation of that map with the spatial and temporal variability of different agronomic parameters helps in development of next season’s crop management strategy. Yield monitors measure the volume or mass flow rate to generate time periodic record of quantity of harvested crop for that period. Time periodic yield data is then synchronized with location address obtained from on-board GPS system to create most common colour coded thematic map. Yield mapping can be carried out easily in mechanized crops. By using either loading cells that weighed the crop passing on a conveying belt or an array of sonic beam mounted over the grape discharge chute to estimate the volume, and the tonnage, of fruit harvested. Tree crops seem to have more stable yields. In handpicked fruits yield mapping is more difficult. Yield maps are one of the most valuable sources of spatial data for precision agriculture. In developing these maps, it is essential to remove the data points that do not accurately represent the yield at a corresponding location. Map averaging or smoothing is usually done to aid data interpretation. A long yield history is essential to avoid drawing conclusions that are affected by the weather or other unpredictable factors during a particular year. Processed yield maps can be used to investigate factors affecting the yield or to prescribe variable rate applications of agricultural inputs according to spatially variable yield goals (yield potential).

Precision farming involves the collection of data by observation and measurement, and the subsequent response. Sensing is key to the collection of data; whether this is remote from space by satellites, by aircraft or drones, or by proximal sensors used on the ground, mounted on vehicles or otherwise. Responses are increasingly automated and data are often integrated into wholefarm management systems. Early commercial applications were directed towards improving crop nutrition, however, utilising precision technologies to control weeds, pests and diseases is now a reality.

Precision crop protection requires detailed data on the occurrence and distribution of problems at the earliest possible stage of detection. An early detection is the key for successful management of pests in precision farming. Some of the key aspects are:

Identification: Identifications of crops, targets and beneficial; temporal and spatial distribution: presence of the target before, during and after the crop growth period;

Sampling techniques: statistically valid sampling regimes must be used to permit an accurate view of any problem

Thresholds: under particular circumstances, the point at which action to ensure levels of infestation or damage are kept to those deemed to be acceptable;

Prediction: the speed and pattern of development of a problem must be capable of being modelled, e.g. spread and severity of insect pests and fungal diseases; weed emergence according to species, dormancy and environmental conditions.

Site-specific weed control technologies practiced in precision farming are defined as machinery or equipment embedded with technologies that detect weeds growing in a crop and take action to maximise the chances of successfully controlling them taking into account predefined factors such as economics.Some of the commercial hardware and software developed for precision agriculture practices comprise tools for weed mapping and control software that adapts spraying to local weed occurrence. However, few farmers have adopted sitespecific weed management, although several studies have shown that weed occurrence and density varies significantly within a farm or a field. Site-specific weed control techniques have gained interest in the precision farming community over the years. Managing weeds on a subfield level requires measuring the varying density of weeds within a field. Decision models aid in the selection and adjustment of the treatments, depending on the weed infestation. The weed control can be done either with herbicides or mechanically. A site-specific herbicide application technology can save large amounts of herbicides. Mechanical weed control techniques adapting to the weed situation in the field are applicable to a wide spectrum of crops.

A system for the discrimination of different weed species and crops from images is done which generates weed maps automatically. Models for the yield effect of weeds are developed and applied in on-farm-research experimental setups. Economic weed thresholds are derived and used for an herbicide application with a patch sprayer.Herbicide use has not eliminated weed problems in agriculture and the need for manual weed removal is still an issue within specialty crop systems, particularly during the first several weeks of crop emergence when competition for resources is greatest.

Precision agriculture is a powerful technology with potentially important applications to weed management. However, growers should remember good agronomic practices, including preventing weed infestations, managing herbicides wisely to prevent resistance, and minimizing environmental impact associated with weed management. Weeds, and methods used to control weeds, can have negative economic and environmental impacts. With precision agriculture, growers can take advantage of the patchy nature of weeds by targeting management efforts only where they are needed instead of wasting expensive and potentially hazardous inputs where weeds are not present. Weeds are patchy because weed spread, survival, and reproduction are variable within a field and over time. Weed patches stay in about the same place from year to year, even though weed density within a patch may vary

Precision agriculture is yet to be popularized for taking its enormous advantages. In laser land levelling and micro irrigation/ fertigation it must have made its prominence but others it is with research institutes where research is focussed on the spatial variability of production environments, development of efficient and suitable data management systems, efficiency of various types of image analyses and optical sensing, efficiency of sensors and related technologies, designs of precision agriculture equipment, optimal inputs and service uses, and their spatial allocations, potentials of unmanned aerial vehicles (UAVs), nanotechnologies and several others.The adoptions of precision-agriculture technologies have, so far, mostly been limited to parts of developed countries. Major contributions to the profitability of precision agriculture is reduced costs of plant protection inputs and higher output revenues.

Plants require proper environment for its growth and development. If environment is not suitable crop would not do well. To ensure proper environment to commercial crops for their better growth and development covered structures are used which provide microclimate appropriate for their growth and development. Growing of crops under such protective structures, also known as greenhouses, is termed as greenhouse technology. The structures harness solar radiation for photosynthesis in plants or can be provided with artificial lights (LEDs) for the purpose. The technology embraces many aspects of farming such as modelling of structures, agronomic practices for the production of the crop and the commercial viability of the venture. The technology ensures crop production in a cover more or less independent of natural prevailing environment. The covering or cladding permits desirable aspect of sun and air and help in modifying crop production environment inside. The modification of environment is aimed of creating micro-climate inside the structure/greenhouse most appropriate for the growth and development of the crop.

Hence greenhouse can be defined as a framed or inflated structure with claddings like glass polyethylene sheets, poly carbonates, fibre reinforced plastics and others in which plants are grown by creating microclimate conducive to crop for its growth and development with or without automation and mechanization. The purpose of growing crops under greenhouse conditions is to extend their cropping season and to protect them from adverse environmental conditions, such as extreme temperatures, precipitation and pests. A large number of greenhouses/poly houses have come up in almost all states of India. Though in infancy but greenhouse cultivation particularly in horticulture crops and nursery production is visible.

Different types of greenhouses or protected structures are designed to meet the specific needs. They can be classified based on shape, utility, construction, cladding material, net houses and technology adopted.

A. Classification based on shape of greenhouse: Greenhouses can be classified based on their shape or style. For the purpose of classification, the uniqueness of the cross section of the greenhouses can be considered as a factor. As the longitudinal section tend to be approximately the same for all types, the longitudinal section of the greenhouse cannot be used for classification. The cross sections depict the width and height of the structure and the length is perpendicular to the plane of cross section. Also, the cross section provides information on the overall shape of the structural members, such as truss or hoop, which will be repeated on every day. The commonly followed types of greenhouse based on shape are lean-to, even span, uneven span, ridge and furrow, saw tooth and quonset.

Lean-to type greenhouse: A lean-to design is used when a greenhouse is placed against the side of an existing building. It is built against a building, using the existing structure for one or more of its sides. It is usually attached to a house, but may be attached to other buildings. The roof of the building is extended with appropriate greenhouse covering material and the area is properly enclosed. It is typically facing south side. The lean-to type greenhouse is limited to single or double-row plant benches with a total width of 7 to 12 feet. It can be as long as the building it is attached to. It should face the best direction for adequate sun exposure. The advantage of the lean-to type greenhouse is that, it usually is close to available electricity, water, and heat. It is a least expensive structure. This design makes the best use of sunlight and minimizes the requirement of roof supports. It has the following disadvantages:

Application of greenhouses in agriculture is mainly to modify microclimate for the production of crops with multiple objectives of raising crops in season offseason, round the year, early crops, late crops, breed crops, raise genetically engineered plants, seed production so and so forth. With hi-tech agriculture and modern techniques greenhouses are used for growing crops anywhere and everywhere, on soil under soil, water, under water and space. Greenhouses are mainly used for crop nursery production, crop production largely in vegetables, flowers and herbs. Among fruits mainly strawberry is grown under greehouses commercially. Other fruits can also be grown.

Besides crop production greenhouses are used for dehydration or drying of produce, seed production, for crop improvement using biotechnology and other sciences. Use of greenhouses in uncongenial environments for crop production is common in places like high altitudes, deserts, arctic climate and alike. One of the largest greenhouse complexes in the world is in Almería, Andalucía, Spain, where greenhouses cover almost 200 km2 (49,000 acres). One of the important use of greenhouses world over is raising tissue culture plants through a popularly known process hardening. For hardening of tissue culture plants different types of low and high technology greenhouses are must. Greenhouses in hot, dry climates used specifically to provide shade are sometimes called shade houses. Hydroponic crop production is generally done in greenhouses. Greenhouses are also used as retail market centres mainly for ornamental plants and seedlings and grafts of horticultural crops. Greenhouses are used for maintaining bee (honey bees and bumble bees) colonies as crop pollinators.

Many crops can be grown in greenhouse but the choice depends on number of factors.

1.Requirement of the farmer, what for she/he wants to grow the crop.

2.For personal use number of vegetable crops and herbs and fruits like strawberry and papaya can be grown.

3.In case of commercial crops, the market must be ascertained before the choice of the crop.

4.Greenhouse must be equipped to create micro-climate for the crop of choice.

5.The choice of crop is a crucial factor for the economic feasibility and success of a commercial greenhouse operation. The revenue of cultivated crop makes the venture to progress.

6.Crop varieties, suitable for greenhouse cultivation should differentiate themselves from open-field varieties to fetch better price on quality.