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The world’s drylands include hyper-arid, arid, semi-arid and dry sub-humid areas where rainfall is highly scanty and variable. Droughts are common in areas / zones where water is the principal limiting factor for agriculture (https:// www.fao.org/3/t0122e/t0122e03.htm) . Dryland soils are characterized by low levels of moisture, organic matter, and biological activity, consequently such soils are very poor in fertility devoid of plant nutrients. When, these soils suffer further degradation in fertility, resulting in erosion, desertification, and salinization when crops are grown on such soils inappropriately without any scientific basis. However, the major factor limiting agricultural production on drylands is the lack of adequate moisture for crop growth or drought.

Multipurpose trees and shrubs are defined as all woody perennials that are purposefully grown to provide more than one significant contribution to the production and/or service functions of a land-use system that they occupy. In agroforestry systems, different species of trees and shrubs can be planted with many types of crops in a variety of patterns. Thus, it is required to know about the methods of propagation and nursery raising, planting and other silvicultural management practices along with the diverse uses of multipurpose trees and shrubs of tropical and subtropical areas before their selection for forestry and agroforestry activities. It is important to select the most suitable tree species since it is not easy to replace them once they have been planted.

Trees have been used in cropping systems since the beginning of agriculture. Throughout the world, at one period or another in its history, it has been the practice to cultivate tree species and agricultural crops in intimate combination. In the tropics, human beings underwent a transition from hunting/gathering to the use of domesticated plants and livestock. As a part of the process, they cut down trees, cleared the debris by burning and sowed crops in the ashenriched soil. It was the ‘slash-and-burn’ agriculture, a primary forerunner of the present-day agroforestry and a practice that might have originated in the Neolithic period, around 7000 BC. There are innumerable examples of traditional land-use practices involving combined production of trees and agricultural species on the same piece of land in many parts of the world. Trees were an integral part of these farming systems and they were deliberately retained on farmlands to support agriculture. These practices are now known as agroforestry. However, the ultimate objective of these practices was not tree production but food production.

Farmers have been growing trees for different purposes for thousands of years. Tree species that are grown to provide more than one significant function are called multipurpose trees. These functions may be productive such as producing fuelwood, timber, fibre, fodder, food, medicine, etc. and/or protective such as soil conservation, shade, shelterbelt, microclimate amelioration, land sustainability, biodiversity preservation, etc. All trees are multipurpose; some, however, are more multipurpose than others. Tree species can be multipurpose in two ways. 1. A single tree can provide more than one function. For example, Gliricidia sepium is grown as living fences that provide fuel, fodder and green manure for agricultural crops - all at the same time. 2. Trees of the same species, when managed differently, can provide different functions. For example, Leucaena leucocephala is managed so that some trees will mainly yield wood while others mainly produce leaf fodder.

Management approaches to soil, including problems of soil degradation and low soil fertility, have recently undergone major changes. The earlier concept was to concentrate on achieving high levels of production from the more fertile areas, leaving the marginal lands for extensive use only. Steeply sloping and highly drought-prone areas were mostly left without any cultivation as production from these areas was hardly cost effective. Soil constraints were to be overcome by inputs like improved crop varieties, fertilizers, chemical control of pests and diseases, and the use of irrigation. The approach of use of newly developed high yielding crop varieties, improved agronomic management practices, use of chemical fertilizers, efficient water management and timely plant protection measures has been successful in achieving large increases in crop productivity in recent years. On the other hand, continuous application of fertilizers at higher rates leads to environmental problems. Yield responses to fertilizers have declined because of soil physical degradation and micronutrient deficiencies.

Agroforestry systems are not simply systems where trees and crops or animals give useful products to the farmers, rather systems where trees and crops and/ or animals interact. Interaction literally means influence or mutual or reciprocal action. So component interaction refers to the influence of one component of a system on the performance of other component as well as the system as a whole. In agroforestry systems, trees are grown in close proximity to crops and pasture. Their performance would largely depend on their ability to share various growth resources in a given environmental situation. Various interactions take place between the woody trees and herbaceous plants (crops or pastures) which is referred to as tree-crop interface. These interactions take place through the media of soil and microclimate and may exert favourable or adverse effects on the crop. Study of interaction helps to know how the components of agroforestry utilize and share the resources of the environment, and how the growth and development of any of the components will influence the others. Interaction occurs both above and below the ground and includes a complex set of interactions relating to radiation exchange, the water balance, nutrient budget and cycling, shelter and other microclimatic modifications.

In agroforestry systems the various components like tree, crop and pasture exist in different proportions and orientations. It is difficult to find out which agroforestry system is the best suited for a given land situation. Similarly, it is to be decided which technologies are required for refinement and improvement of the existing agroforestry practices. But without sufficient knowledge of the existing system in a particular land situation, it is very difficult to set the research priorities for modification and development of this system. Diagnosis and Design (D & D) is a systematic and objective methodology developed by International Centre for Research in Agroforestry (ICRAF) to initiate, monitor and evaluate agroforestry programmes. D & D is based on the philosophy that knowledge of the existing situation (diagnosis) is essential to plan and evaluate (design) meaningful and effective programmes in agroforestry research for development. The methodology plays a strategic role in all the phases of the agroforestry research process. D & D in agroforestry is unique and it has been specially developed for the following purposes.

Agroforestry is a land use system that involves two or more plant species at least one of which must be a woody perennial. When perennial woody and herbaceous components are grown together on the same piece of land their performance would largely depend on their ability to share various growth resources in a given environmental situation. Due to difference in growth pattern and resource requirement of the components in agroforestry situation, a close interactive relation is obvious. Thus, careful management practices for the components are required to establish a successful agroforestry system. The characteristics of the trees and crops, and their interactions, can be modified with good management practices in order to take advantage of the positive characteristics and minimize the effects of the negative ones. Effective and efficient agroforestry management may be divided into two groups; tree management and agricultural crop management.

The words ‘system’, ‘sub-system’ and ‘practice’ are commonly used in agroforestry literature. In a broad sense, a system is defined as a group of associated elements forming a unified whole and working together in a well defined regular relation for a common goal. An agroforestry system refers to a type of agroforestry land-use that extends over a locality to the extent of forming a land utilization type of the locality. Sub-system and practice are lower-order terms in the hierarchy with lesser magnitudes of role, content and complexity. However, these terms are used loosely, and almost synonymously. Classification of agroforestry systems is necessary in order to understand and provide a practical framework for evaluating systems and developing action plans for their improvement. Any classification scheme should satisfy the following criteria (Nair, 2008).

A forest is a large area dominated by trees. Many definitions of forest are used throughout the world, incorporating factors such as tree density, tree height, land use, legal standing and ecological function. Although a forest is usually defined by the presence of trees, under many definitions an area completely lacking trees may still be considered a forest if it grew trees in the past, will grow trees in the future, or was legally designated as a forest regardless of vegetation type.According to the Food andAgriculture Organization definition, forests covered 4 billion hectares or approximately 30% of the world’s land area in 2006. Forests are the dominant terrestrial ecosystem of earth, and are distributed around the globe. Forests account for 75% of the  gross primary production  of the earth’s  biosphere, and contain 80% of the earth’s plant biomass.

Agroforestry is a new name for a set of old practices. The farmers have been practicing agroforestry since ancient times. The general concept of agroforestry is to integrate trees and agriculture so as to create a more diversified landscape, while providing the producers with new environmental and economic benefits. In other words, agroforestry is a method of farming that allows trees and shrubs to grow along with crops and/or livestock, therefore blending agriculture and forestry in the same production system. In fact, man’s association with forest is much older than with agriculture. First man was a food gatherer and hunter in forests. Then he realized that the seeds of the fruits he collected germinated, grew into plants and bore the fruits again and thus man started to cultivate foods. Man’s desire to live in a community created settled agriculture.

Among the various natural resources upon which all living organisms depend, soil is the unique because of its non-renewable in nature, heterogeneity in space and time and susceptibility to degradation inflicted by biotic as well as abiotic agents. Whereas, escalating population demands, in one way, to bring more land under the plough for providing food, fodder, fuel and fibre, simultaneously, on the other way, these finite natural resources are dwindling by leaps and bound, through overexploitations and mismanagement, the combined effect of which leads to land degradation, a serious global threat and major environmental issues in the present times.

Micronutrients are those elements essential for green plants but required in very small quantity for their growth and development. There are altogether eight micronutrient elements in soil consisting of iron, manganese, zinc, copper, boron, molybdenum and chlorine with the recent addition of nickel. These together with large number of other elements usually present in plants with small concentrations are formed as trace elements. Another element, cobalt is known to be required for microorganisms concerned mainly with nitrogen fixation in legumes affects indirectly the growth of many higher plants, thus, may be considered to be included in the list of essential micronutrient element. Similarly, Vanadium and sodium although present as beneficiary elements for plant growth may also have the possibilities to be enlisted into this group of elements.

Nitrogen Content in Indian Soil Because of tropical and subtropical climates Indian soils have very low nitrogen content and in the majority of cultivated soil it varies from 0.02% to 0.13%. Low levels of N in Indian soil is primarily attributed to climate and secondarily to cropping. Climate plays the dominant role in determining the nitrogen content in soil through the effect of soil organic matter content. Generally heavy textured soils have higher N content than loamy or sandy soils. Other factors remaining the same, total N content in soil decreases with the increase in annual temperature at a given humidity level, on the other hand, it increases with increasing humidity level at a given annual temperature. Maximum N content in soil occurs in the surface layer of soils and decreases with depth in soil profile. The variation of distribution of N content in soil with depth is attributed by varying accumulation of organic matter in soil profile.

The living organisms in the terrestrial ecosystem have unique characteristics of taking up the substances from atmosphere and soil for the synthesis of their own cellular component or as a source of energy. The supply and absorption of chemical compounds required for the growth and metabolism of the organisms may be defined as nutrition and the compounds required by them are called nutrients. The process by which the nutrients are converted to cellular material is known as the metabolism. Out of 118 chemical elements so far detected in the modern periodic table, 92 elements from hydrogen to uranium are naturally occurring, a large portion (more than sixty) of such have been detected in the plant tissues. But, the elements which take part in the nutritional requirement and metabolic processes of the plant are said to be essential elements, others which do not participate in such processes are considered non-essential.

Soil acidity is most conveniently measured by pH of soil suspension with water. Pure water undergoes dissociation into hydrogen and hydroxyl ions and are in equilibrium with undissiociated water molecules Thus, H2 O ??? ??? H+ + OH- (1) and in presence of another H2 O molecule, H+ converts to H3 O+ H+ + H2 O ??? ??? H3 O+ (2) Since water is a weak electrolyte and dissociates as per Eqn. 1, the ionic product of water can be written as – KW = aH+ x aOH- = (CH+ x COH– ) x ( fH+ x fOH-) (3) where, a, C, and f represents the activities, concentrations and activity coefficients of respective ions. In pure water and in dilute solutions the activity coefficients are approximately unity thus the product of concentrations of H+ and OH-1 is the ionic product of water and at ordinary temperature (250 C) this has a constant value of 10-14 gm ions per litre so, Kw =(CH+ x COH–1) = 10-14 gm ion per litre (4) such that, CH+ = COH–1 = 107 gm ion per litre Therefore, if hydrogen ion concentration of an aqueous solution exceeds this value, (10-7gm ion per litre), the solution is said to be acidic then there will be a proportionate decrease of OH- concentration to maintain the constant value of ionic product.

Salt affected soils can be defined as the soils containing excess amount soluble salts in their profiles in a proportion that adversely affect to soil properties and ultimately on crop growth. The soluble salts present in these soils are composed primarily with calcium, magnesium and sodium as cationic and chlorides sulfates and carbonates as anionic constituents along with the presence of potassium, nitrate and others as minor quantities. All soils invariably contain some soluble salts, but they cause harmful effects on crop growth only when accumulates in excess amount in soil solution or increase their concentration in exchange complex of the soil. Based on the presence of sodium salts and the predominance of sodium in the exchange complex, salt affected soils are classified into main categories of 1) Saline soils – contain excess soluble sodium in soil solution with little or no sodium content in exchange complex, 2) Alkaline (Sodic) soils – contain appreciable quantity of sodium adsorbed in exchange complex of soil but may or may not contain soluble sodium salts in soil solution. These soils are also divided into following types –

Soil survey is essentially a systematic examination of morphological, a set of other variable soil characteristics, description, classification and mapping of soil in an area alongwith a report making an interpretation to predict their potentials for various uses particularly for crop production.

Activities of Soil Survey Soil survey is composed of the following activities –

1. Anticipation of soil variability - The initial activity of soil survey is obtained by classifying the surveyed data according to different land forms and existing land uses either through field traversing, or aerial photo or by satellite images of the concerned area.

2. Characterisation and classification – This is done on the basis of soil profile description supported with soil analytical data.

3. Soil mapping - This is performed by showing the distribution of types of soil or soil mapping units based on already classified soils with their landforms, and land uses or their combinations.

4. Interpretation - It is the process of making prediction about the potentials of soils for various uses like choice of farm practices – agriculture, horticulture, forestry, pastures etc. and their management practices.

Classification means the process of ordering or grouping of objects having large number of population and wide variations into shorter one in a systematic manner with selected characteristics. Soils being the natural objects have no exception with other objects to classify them in a best possible way. Thus, soil classification refers to the systematic categorization of soils into various groups, classes or any simpler form based on the similar properties and other distinguishing characteristics. The main purposes of soil classification are to organize knowledge, easy remembering of the soil properties for each group and better understanding about the components and the relationships among individuals and various groups of soils. Various soil classification systems have been evolved during the past for categorizing the soils into various groups but the Soil Taxonomy has been now accepted as most scientific system of classification.

The solid phase of a soil system makes up a mixture of inorganic and organic materials. The inorganic fraction of soil originated from the weathering of rocks and minerals consists of rock fragments and minerals of varying sizes and composition. Hence, this inorganic soil fraction composed by soil minerals also referred as mineral fraction of soil and is caused due to alteration of mineralogical composition of soil by bioclimatic factors. A great deal of attention is being paid to the composition of minerals in soils, their formation and distribution over the earth’s surface. Soil genesis or pedogenosis (a more useful term as soil belongs to pedosphere) deals with the processes of formation and development of soil. The branch of soil science that integrates the studies of genesis, morphology and classification of soils as natural body is called Pedology. The process of weathering of rocks, pedogenesis and development of soil profile are described successively in this chapter.