eChapters

Introduction

Intensive soil tillage, burning of crop residues and over use of fertilizer and irrigation water under current agricultural practices has accelerated the pace of degradation in Indian agriculture. Intensive soil tillage increases soil erosion and nutrient runoff into nearby waterways. Crop residue burning resulting in loss of plant nutrients and release of greenhouse gases (GHGs) in the atmosphere. Imbalanced use of chemical fertilizer leads to soil compaction, slows down the fertilizer utilization rate and contaminates the local environment. Increasing demand of irrigation water causes water shortage and harms the environment in several ways including increased salinity, nutrient pollution, and the degradation of flood plains and wetlands. In the face of these management and environmental challenges, there is a need to formulate such agricultural practices which improve the productivity of natural resources as well as of external inputs and help to prevent soil degradation. Conservation agriculture (CA) practices such as reduced tillage, residue retention and proper crop rotations offer such solutions. CA also helps in making agricultural systems more resilient to climate change and safeguard ecosystem services.

Introduction

Conservation of biodiversity and mitigation of global warming are two major environmental challenges today. In the context of climate change and the global carbon cycle, the relationship between plant biodiversity and soil organic carbon (SOC) sequestration has become a subject of considerable scientific interest. The Earth’s terrestrial vegetation plays a pivotal role in the global carbon cycle. Not only are tremendous amounts of carbon stored in the terrestrial vegetation, but large amounts are also actively exchanged between vegetation and the atmosphere. Anthropogenic perturbations exacerbate the emission of CO₂ from soil caused by decomposition of soil organic matter (SOM) or soil respiration (Schlesinger, 2000). The emissions are accentuated by agricultural activities including tropical deforestation and biomass burning, plowing, drainage of wetlands and low-input farming or shifting cultivation. In addition to its impact on decomposition of SOM, macroclimate has a large impact on a fraction of the SOC pool which is active. Conversion of natural to agricultural ecosystems increases maximum soil temperature and decreases soil moisture storage in the root zone, especially in drained agricultural soils (Lal, 1996). Thus, land use history has a strong impact on the SOC pool (Pulleman et al., 2000). The dynamic relationship between plant biodiversity and SOC depicts that any land use practices that increase vegetative cover, or reduce its removal, could have an influence on the global carbon budget by increasing the terrestrial carbon sink.

Introduction

The inevitability of detrimental effects of climate change is looming large not only on the sustenance of agriculture, but also on the survival and progress of mankind. Estimates by the Organization for Economic Cooperation and Development (OECD) reveal that the atmospheric concentration of GHG would exceed 680 parts per million (ppm) CO₂-equivalents by 2050, which is about 50% higher than baseline year 2010 (OECD, 2012). Additionally, global temperature is predicted to increase by 2°C within this period, resulting in meansea level rise and erraticweather conditions. Consequently, the catastrophic effects on food consumption pattern by the end of 2050 will create malnutrition in at least 290 million people. Agricultural crops provide 63% of the human protein, and additionally supply almost 100% protein to the livestock, which are the second major sources of protein in human diet. Altogether, the detrimental effect of climate change on agriculture would make about 300 million additional people malnourished. Add to that a 2.7 billion increase in human population by 2050, coupled with shortage of water availability that would affect 3.9 billion people by 2050, and one can easily perceive the grim scenario for our kind in not so distant future.

Introduction

The basic needs of life are food, clothing and shelter. In the 21^st century a fourth need added to mankind is energy. According to the laws of physics, energy is defined as capacity to do work. Energy, economics and the environment are mutually dependent. Also, there is very close relationship between agriculture and energy. The level and pattern of energy use in agricultural production system depends on variety of agronomic and socio-economic factors. Energy use in agriculture encouraged maximization of yields from limited available arable land, minimization of labour intensive operations and improving standard of living. Efficient and effective energy use in agriculture is one of the conditions for sustainable agricultural production, since it provides financial savings.

Introduction

India agriculture is now at turning point as we have travelled a long way after the advent of green revolution. Over the past four to five decades our strategies, policies and actions were guided by goals of ‘self-sufficiency’ in foodgrains production via green revolution. Indian agriculture has been successful in achieving increased food grains production albeit at a low level of satisfaction. While the mission of increasing foodgrains production stands somehow achieved, these gains were accompanied by widespread problems of resource degradation and high factor productivity, which now pose a serious challenge to the continued ability to meet the demand of an increasing population and lifting our people above the poverty line (Tan et al., 2010). Indian agriculture has reached a point where it must seek new directions – those by way of strategies, policies and actions which must be adopted to move forward addressing sustainable intensifications. The past strategies to increase food grains production, however, have resulted in massive exploitation of natural resources, contributing to unsustainable growth; there is need to change this approach in the future. This will call for strategies, which are different than the ones we adopted in the ‘green revolution’ era. High levels of fertilizer use and decreasing resource use-efficiency are increasingly contributing to groundwater pollution and increased emissions of green-house gases (GHGs) (Gupta, 2004). High level uses of pesticides in many areas have become a major health hazard. Thus, with continuously deteriorating resources, widespread problem of soil and water contamination and eroding ecological foundation, sustainability of agriculture is becoming highly questionable (Conway and Barbier, 1990). There is need to seriously debate on the strategies required to ensure that agriculture continues to play a critical role in the overall development.

Introduction

Conservation tillage is a model of sustainable agriculture as it leads to profitable food production while protecting and even restoring natural resources. Conservation agriculture benefits farmers because it reduces production costs and increases yields, but it also has positive impacts on the whole society: enhancement of food security thanks to a better soil fertility, improvement of water quality, reduction of erosion and mitigation of climate change by increasing carbon sequestration. Conservation agriculture (CA) systems are also less sensitive to extreme climatic events and therefore contribute to the adaption of climate change and the resilience of agricultural systems. Hence, CA becomes a fundamental element of sustainable production intensification, combining high production with the provision of environmental security.

CA is based on healthy functioning of the whole agro-ecosystem with a maximum attention and focus on the soil. The soil is the entry point and it has to be considered not only as simple physical support to roots and plants, but as a living entity with its physical, chemical and biological characteristics. The focus of CA embraces not only the nutrient contents of the soil but also its biological and structural status, which are determinants of sustained productivity.

The paradigm of CA is that an undisturbed soil has the opportunity to develop and produce healthier plants. Soil life can develop in a stable habitat in quantity and quality better than on tilled soils, the structural integrity of soil is maintained, so continuous vertical macro-pores are not destroyed and remain as drainage channels for rainwater into the soils. Seeding under conditions of minimum soil disturbance is achieved by direct seeding through the mulch cover without tillage.

Introduction

The main aim of conservation agriculture is to sustain and improve the crop productivity and to provide protection against biotic and abiotic stress, while at the same time protecting and enhancing the biological activities of the soil. The essential principles of conservation agriculture are no-tillage (and direct seeding) or reduced tillage, the maintenance of a cover of live or dead vegetative mulch on the soil surface and the wise use of crop rotations. The crop sequences are planned in such a way that which will discourage the buildup of pests or diseases and will help to optimize plant nutrient use by synergy between different crops of the rotation. While deciding the crop rotation, the locally important crops should be considered and there should be deep rooted and shallow rooted crops in the sequence so that the utilization of soil nutrients is maximized. At least one leguminous crop should be there in the crop sequence. The use of plant residues influences the soil physical, chemical and biological properties by reducing surface temperature, rate of evaporation and maintaining water content, nutrient load and rate of organic matter decay. The soils under conservation agriculture are happened to be highly active and diverse biologically and can supply plant nutrients for a longer period of time because of their higher nutrient loading capacities.

Soil biodiversity comprises of all organisms whether single-cell organisms or multi-cell animals or plants that live in the soil. Soil microbial diversity includes “genetic diversity, that is the amount and distribution of genetic information within microbial species, diversity of bacterial and fungal species in microbial communities, and ecological diversity, that is variation in community structure, complexity of interactions, number of trophic levels and number of guilds (Nannipieri et al., 2003).

Introduction

There is a widespread degradation of natural resource, e.g., soil erosion, nutrient loss, water-logging, salinity, alkalinity, acidity, decline in soil organic carbon (SOC) content, ground water depletion and micronutrient depletion. The major causes are washing away of topsoil and organic matter due to water erosion, intensive deep tillage and inversion tillage, dismally low levels of fertilizer application, mining and other commercial activities, faulty agricultural practices, no or low use of organics, indiscriminate use of herbicides, pesticides, fungicides etc. Other cause is loss of organic matter with higher temperatures, which increase rate of microbial decomposition of organic matter due to adverse effect of climate change. Climate change due to global warming as a result of increased concentration of GHGs in the atmosphere is a well-established fact. Impacts of climate change are experienced throughout the world. Increases in global mean temperature, rise in sea-level, and occurrence of extreme events are the consequences of climate change. These changes may deplete the SOC pool and soil structural stability, increase soil’s susceptibility to water runoff and erosion, disrupt cycles of water, carbon, nitrogen, phosphorus, sulphur and other elements, and cause adverse impacts on biomass productivity, biodiversity and the environment. Climate change is likely to have a variety of impacts on soil quality. In order to restore soil quality, enhance productivity, it is of utmost importance to focus on conservation agriculture practices on long-term basis. Conservation Agriculture (CA) is gaining importance as an alternative to conventional agriculture.

Introduction

Increase in population in our country with decrease in cultivable land is putting the challenges to the scientists and policy planners for increased food supply. There is very little scope for horizontal increase in cultivable land. Hence, the way to meet the need for providing food to ever increasing population is to increase the production of food per unit area of cultivable land.

Farmers, generally apply fertilizer to soil on blanket recommendation. Sometimes, the addition of nutrients is inconsistent. Sometimes, the application of fertilizer is much higher than the recommended dose. Imbalanced application of fertilizer to soil results in low nutrient use efficiency and low crop yield. There is need of site specific nutrient management for increased crop yield and for achieving higher nutrient use efficiency. Proper care should be taken for selection of fertilizer, method of application of fertilizer, rate of fertilizer application and also time of nutrient supply to the soil. Excess application of fertilizer has an adverse effect on soil health.

Traditional tillage may affect physico-chemical as well as biological properties of soil. Change in tillage practice from conventional tillage to conservation agriculture method may have an impact on soil properties and nutrient dynamics. Conservation agriculture affects soil health, proper fertilizer application, both rate and time, along with efficient use of soil moisture improves nutrient use efficiency. For obtaining a good crop yield nutrient should be available for plants in adequate quantity and in right proportion. Nutrients removed by the crop should be replenished by application of inorganic fertilizers alongwith incorporation of organic manures. Soil test is an important tool for determining the quantity of nutrients available in the soil. Based on soil test values and nutrient requirement of the crop rate of fertilizer to be applied in the soil for the plant is determined. Ample quantity of farm yard manure is not available nowadays. Hence, other organic manure such as town waste, green manure etc should be used.

Introduction

The green revolution is exemplified by too much adoption of high soil disturbance, modern varieties, energy exhaustive technologies, high amount of capital investment and simplification of farming systems (Meeus, 1993), resulting in uniform and featureless landscapes (Nassauer and Westmacott, 1987) in the most intensively farmed arable areas. In post-green revolution epoch, we had experienced a period of remarkable escalation in food productivity, despite rising land values and increasing land scarcity. Farmers from both developing and developed countries get exposed to several production constraints for achieving a sustainable, cost-effective and assured return from their field. Depending upon the circumstances and the scale of farm activities, farmers face varying degrees of climate risk, biotic invasion, and economic uncertainty. Thus, from the context of economical ambiguities, volatile food prices, social pressure to buffer ups and downs of agricultural production and achieving food security for ever growing population and curtailing poverty, farmers must be equipped with novel cultural practices to achieve a sustainable and assured cost-effective grain production. Research experiments throughout the globe found that the principal indicators for unsustainability of agricultural systems are: (1) intensive tillage induced decrement of soil organic matter (OM), structural degradation of soil, accelerated water and wind erosion, reduced water infiltration rates, surface sealing and crusting, soil compaction, (2) insufficient return of organic materials into soil, and (3) monocropping.

Introduction

Since the dawn of the civilization when mankind started agriculture, soil erosion has been the single largest environmental problem and has remained so till date (Sullivan, 2004). This is so because removal of the topsoil by any means has, through research and historical evidence, been severally shown to have many deleterious effects on the productive capacity of the soil as well as on ecological wellbeing. Doran and Parkin (1994) captioned the impact of soil erosion in their popular maxim that “the thin layer of soil covering the earth’s surface represents the difference between survival and extinction for most terrestrial life”. Soil degradation implies decline in its capacity to provide ecosystem services (ESs) of interest to humans and useful to nature’s functions. Principal processes of soil degradation are erosion, salinization, nutrient and carbon (C) depletion, drought, decline in soil structure, and tilth. Examples of ESs provided by soil include ecological/supporting (biomass production, nutrient cycling), regulating (water purification and flow, C sequestration, temperature fluctuations), provisional (food, fiber, fuel, and forages), and cultural (aesthetical, spiritual, and cultural). Erosion-induced degradation diminishes soil’s capacity to provide ESs, and support ecosystem functions.

Although fertile top soils could be lost when scraped by heavy machineries (Ngwu et al., 2005), the key avenues of topsoil loss include water erosion and wind erosion. Sometimes erosion can be such gradual for so long a time as to elude detection in one’s lifetime, thus making its adverse effects hard to detect. Eswaran et al., (2001) propose an annual loss of 75 billion tons of soil on a global basis which costs the world about US $400 billion per year. A review of the global agronomic impact of soil erosion identifies two severity groups of continents and reveals that Africa belongs to the more vulnerable group (Biggelaar et al., 2004). Soil erosion by water seems to be the greatest factor limiting soil productivity and impeding agricultural enterprise in the entire humid tropical region. This is evident in many regions of Africa (Dregne, 1990), mainly in the humid and subhumid zones of Sub-Saharan Africa (SSA) where population pressure and deforestation exacerbate the situation and the rains come as torrential downpours, with the annual soil loss put at over 50 t ha⁻¹ (FAO, 1995).

Introduction

The 21st Century agriculture needs knowledge integration and maximum input efficiency. Crop simulation models offer possibilities to evaluate and target agricultural information towards sustainability (Elliott et al., 2015; Van Oort and Zwart, 2018). All the modern day’s crop growth models not only predict the growth and yield, but also they are strong Decision Support System (DSS). Due to the impact of climate change, land degradation and biodiversity loss, the agricultural production system becomes the most vulnerable one (Araya et al., 2017). Crop intensification is inevitable to meet the ever-increasing food demand for the billions of populations. It leads to deterioration of soil physical health, reduction of the organic carbon percentage and micro-environmental pollution. Conservation agriculture (CA) has a great potentiality to enhance the inherent resilience against all sorts of degradation and possess the potentiality to regenerate the degraded lands (Haggblade and Tembo, 2003). CA is a farming system that maintains a permanent soil cover to assure its protection, avoids soil tillage, and cultivates a diverse range of plant-species to improve soil conditions, reduce land degradation and increase water and nutrient use efficiency (Friedrich et al., 2012; Sapkota et al., 2015; Bell et al., 2019; Steward et al., 2019). For last few decades, climate change becomes a serious threat to agriculture in context to influence crop and livestock production, hydrological balances, input supplies and other components of agricultural system (Kabir, 2015; Kumar and Kumar, 2016). Agriculture is inherently sensitive to climatic conditions and conservation agriculture may be the only answer to combat the negative impact of climate change.

Introduction

The resource conservation technologies (RCTs) primarily focus on resource-savings management systems for agricultural production through minimal tillage operation, maintaining soil health for available plant nutrients and conserving soil moisture through crop residues and/or cover crops, and following cropping sequence in spatial and temporal scale. The integration of resource-conserving technologies working in synergy is commonly referred to as conservation agriculture (CA). The CA is “a concept for resource-saving agricultural crop production that strives to achieve acceptable profits together with high and sustained production levels while concurrently conserving the environment” (FAO 2007). The CA envisages four principles: (i) minimizing mechanical soil disturbance and seeding directly into untilled soil, (ii) maximizing crop residue retention and/or growing cover crops, (iii) optimizing diversification of cropping sequences and rotations, and (iv) minimizing mechanical soil compaction through controlled traffic. The CA technologies involving no- or minimum-tillage with direct seeding and bed planting, residue management and crop diversification have potential for improving productivity and soil quality, mainly by soil organic matter build-up. Conservation agriculture systems appear to be appealing options to achieve sustainable and intensive crop production under different agroecological environments because they use available resources efficiently and maintain soil fertility. However, there is a need for wider scale testing of these new technologies under diverse production systems, as the CA technologies are site specific and therefore appraisal of CA is important to have significant adoption (Ramesh et al., 2016)

Introduction

Attaining food and nutritional security for a growing population and reducing poverty while sustaining agricultural production under the current scenario of depleting natural resources, increasing cost of inputs and impending negative impacts of climatic change are the major challenges the country is currently facing (FAO, 2009). In addition to these challenges, decline in soil organic matter, consequent deterioration of soil health, large scale erosion of fertile top-soil are the other bottle-necks for increasing agricultural productivity from a finite land resource base. These constraints are evolving mainly due to: (1) intensive tillage induced soil organic matter decline, soil structural degradation, water and wind erosion, reduced water infiltration, surface sealing and crusting, soil compaction, (2) insufficient return of organic material to soil owing to prevalent crop residue removal and burning (Prasad et al., 1999) and (3) mono-cropping. Conservation agricultural (CA) is a panacea to address these challenges the farming community of this country is presently facing. The CA system incorporates a holistic approach to agriculture that results in improved soil health and water quality, lower water and nutrient consumption, reduced soil loss, strategic land use and healthy biodiversity. It involves minimum soil disturbance, permanent soil cover through crop residues or cover crops, and crop rotations for achieving higher productivity (Hobbs et al., 2008).

Introduction

Environmental friendly Insect Pest Management (IPM) relies on preventive rather than reactive strategies. The cropping program should focus primarily on preventive practices above and below ground such as crop management and soil management to build the farm’s natural defenses. Reactive management viz., reactive inputs for pest management and reactive inputs to reduce plant stress are reserved for problems not solved by the preventive or planned strategiesfor supplemental pest management practices and planned supplemental soil practices to reduce crop stress and/or optimize yield and crop quality. Earlier IPM models are designed from the scientific perspective with a focus on ecological, environmental friendly and evolutionary aspects of pest management to reduce or prevent economic losses.There are four major components in the new environmentally pest management model that address various pest management options, the knowledge, and resources the grower has to address the pest issue, planning and organization of information to take appropriate management actions, and maintaining good communication to acquire and disseminate knowledge about pests and their management. These models mainly address the human, environmental, social and economic factors that influence food production.

Introduction

Organic farming is basically a holistic management system, which promotes and improves the health of the agro-ecosystem related to biodiversity, nutrient biocycles, soil microbial and biochemical activities. Organic and bio-dynamic farming emphasises management practices involving substantial use of organic manures, green manuring, organic pest management pratices and so on. It has also come to mean that it is a system of farming that prohibits the use of artificial fertilisers and synthetic pesticides.

Biodynamic agriculture is an advanced form of organic farming system that is gaining increasing attention because of its added advantage of soil health and food quality. It is an alternative variant where the chemical fertilisers are totally replaced by microbial (biological) nutrient givers such as bacteria, algae, fungi, mycorhiza, actinomycetes. Biological Pest management of crops is undertaken by employing predators, parasites and other plethora of natural enemies of pests, in addition to all the rest of option that help to avoid resorting to chemical pesticides. These agents could be augmented into farms or promoted through such activities that favour their flourished activities. Composting, Green manuring, crop rotations. Intercropping, mixed cropping etc. as well as bird perches, trap crops promote such biological activities.

Introduction

Modern intensive agriculture systems based on the principles of yield maximization have been plagued with multiple constraints such as declining factor productivity, decreasing resource use efficiency (fertilizers, water, labour etc.), soil organic matter decline, salinization, soil structure degradation, water and wind erosion, reduced water infiltration rates, soil compaction due to surface sealing and crusting, declining ground water table, pest and disease outbreak, weed resistance and weed shifts (Paustian et al., 2016). To address these overwhelming issue, conservation agriculture (CA) has been defined by FAO as a concept of resource saving agricultural crop production which is based on enriching the natural and biological processes above and below ground. The major purpose for its advancement is to reduce the production cost, save resources, enhance yields and utilize resources like nutrient and water efficiently. Conservation tillage denotes soil management system that follows the principle of at least 30% of the soil surface coverage with crop residues after seeding of the crop (Jarecki and Lal, 2003). CA, a resource-efficient agriculture helps to conserve, improve and utilize natural resources efficiently through integrated management of available soil, water and biological resourwces combined with external inputs and thus contributes to environmental conservation as well as enhanced and sustained agricultural production in the long term. The practice and wider extension of conservation agriculture thus requires a deeper understanding of its ecological underpinnings for managing its various elements for sustainable intensification.

Introduction

Natural resources, especially those of soil, water, plant and animal diversity, vegetation cover, renewable energy resources, climate, and ecosystem services are fundamental for the structure and function of agricultural systems and for social and environmental sustainability, in support of life on earth. Historically the path of global agricultural development has been narrowly focused on increased productivity rather than on a more holistic integration of natural resource management with food and nutritional security. A holistic, or system-oriented approach, is preferable because it can address the issues associated with the complexity of food and other production systems in different ecologies, locations and cultures. Modern agriculture largely depends on the use of fossil fuel-based inputs, such as chemical fertilizers, pesticides, herbicides and energy intensive farm machinery. While the applications of such high input technologies have undoubtedly increased production and labour efficiency. There is a growing concern over their adverse effect on soil productivity and environmental quality which is emerging to recognize that the farmer has a great social responsibility as a land owner than merely agribusiness considerations. Therefore, there is need to develop agricultural techniques that are ecologically sound, economically viable, and socially responsible. Sustainable agriculture in the context of development helps to achieve production efficiency, protect ecosystem functions, enhance resilience to climate change, ensure healthy communities, satisfies basic needs and ensures optimum use/conservation of natural resources. Organic/biodynamic farming is among the various types of sustainable agriculture. Traditionally these methods has been playing very vital role in the conservation of natural resources in terms of soil health sustenance, environmental protection, food safety, waste recycling, biodiversity conservation with an economically viable approach. The chapter discusses the role of organic/biodynamic agriculture in natural resource conservation.

Introduction

Generally speaking, Conservation Agriculture (CA) includes minimum soil disturbance, permanent soil cover, and crop rotation (Hobbs, 2007). CA has been recognized as an efficient way for sustainably increasing crop yields in different countries (Hobbs et al., 2008; Pittelkow et al., 2015). Farmers following CA will face several managerial problems, and weed management is the most challenging (Wall, 2007; Giller et al., 2009; Farooq et al., 2011). Weed growth in an agroecological system and crop yields are inversely related; so, knowledge on weed dynamics and weed management are vital for achieving the possible yield gains in CA systems. There are few studies that examined both the direct and interactive effects of the 3 basic CA principles on weed dynamics (Chauhan et al., 2012; Giller et al., 2009; Farooq et al., 2011).

For comprehensive understanding the current topic, a diagrammatic representation of an annual weed’s life cycle (Fig. 1) will be useful and the same corresponds to four likely areas managing the weeds:

Introduction

Resource Conserving Technology refers to any management approach or technology that increases factor productivity including land, labour, capital and inputs. The resource conserving technologies (RCTs) involving no or minimum tillage, direct seeding, bed planting and crop diversification with innovations in residues management are the possible alternatives to the conventional energy and input-intensive agriculture. The RCTs with innovations in residue management avoid straw burning, improve soil organic C, enhance input efficiency and have the potential to reduce GHGs emissions (Pathak et al., 2011). Conservation agriculture is one the effective Resource Conservation Technologies, which is based on three principles which are minimum soil disturbance, maintenance of permanent soil covers by the use of crop residues and cover crops and diverse plant associations which included crop rotations. Globally, Conservation Agriculture is being practiced in about 125 M ha area. USA with 26.5 M ha contributes to the major CA-practicing countries followed by Brazil (25.5 M ha), Argentina (25.5 M ha), Canada (13.5 M ha) and Australia (17.0 M ha). However, in the context of climate change to increase the production and productivity, India’s CA adoption is still in preliminary phases in area of 1.5 million ha with adoption of Zero Tillage (Jat et al., 2012, www.fao.org/ag/ca/6c.html). The major CA-based technologies being adopted is zero-tillage (ZT) wheat in the rice-wheat system of Indo-Gangetic Plains.

Introduction

Conservation Agriculture (CA) is a set of soil management practices that minimize the disruption of the soil’s structure, composition and natural biodiversity. These include: i) maintenance of permanent or semi-permanent soil cover (using either a previous crop residue or specifically growing a cover crop for this purpose); ii) minimum soil disturbance through tillage (just enough to get the seed into the ground); iii) regular crop rotations to help combat the various biotic constraints. CA also uses or promotes where possible or needed various management practices listed below: i) utilization of green manures/ cover crops (GMCC’s) to produce the residue cover; ii) no burning of crop residues; iii) integrated disease and pest management; iv) controlled/limited human and mechanical traffic over agricultural soils. When these CA practices are used by farmers one of the major environmental benefits is reduction in fossil fuel use and greenhouse gas (GHG) emissions.

In India, efforts to develop, refine and disseminate conservation-based agricultural technologies have been underway for nearly two decades and made significant progress since then even though there are several constraints that affect adoption of CA. Particularly, tremendous efforts have been made on no-till in wheat under a rice-wheat rotation in the Indo-Gangetic plains. The technologies of CA provide opportunities to reduce the cost of production, save water and nutrients, increase yields, increase crop diversification, improve efficient use of resources, and benefit the environment. However, there are still constraints for promotion of CA technologies, such as lack of appropriate seeders especially for small and medium scale farmers, competition of crop residues between CA use and livestock feeding, burning of crop residues, availability of skilled and scientific manpower and overcoming the bias or mindset about tillage. The need to develop the policy frame and strategies is urgent to promote CA in the region. (Bhan and Behera, 2014).

Introduction

Climate change refers to change of weather variables over a long period of time. The Inter-Governmental Panel on Climate Change (IPCC) defined climate change as a “change in the state of the climate that can be identified by changes in the mean and/or the variability of its properties and that persists for an extended period, typically decades or longer”. This may occur due to natural variability or as a result of human activity. The change may be in the form of magnitude or variability of a single or multiple weather variables or weather phenomena. Global shift in temperature and precipitation pattern as well as increasing frequency of extreme weather events are consequence of climate change which is perceived over several decades. Climate change is indeed increasingly recognized as a considerable risk to agriculture particularly with respect to direct impact on crop production and yield stability (Barzman, 2015).

Effect of Climate Change in Global Warming and Greenhouse Gases

Global warming as the resultant of climate change has become an issue of serious concern worldwide for existence of life on the earth. Over past hundred years, the global temperature has increased by 0.8⁰C and is expected to reach 1.1-5.4⁰C by the end of next century. Major factors affecting the global warming and the climate change is the concentration of greenhouse gases like carbon dioxide, methane and nitrous oxide. On the other hand, CO₂ concentration in the atmosphere has increased drastically from 280 ppm to 370 ppm and is likely to be doubled in 2100 (IPCC, 2007).

Introduction

The Indo-Gangetic Plain is one of the world’s major foodgrain producing regions. The Indian states falling under this region, viz. Punjab, Haryana, Uttar Pradesh, Himachal Pradesh, Bihar and West Bengal, are also the major rice-wheat growing states spread over 10.5 million hectares in the country. During the past 30 years, agricultural production growth in this region has been able to keep pace with population demand for food in the country mainly due to adoption of green revolution technologies inducing yield growth, followed by area expansion. But this opportunity is ceasing very fast due to limited scope for increasing the availability of arable land and natural resources. The other issue is the conservation of the basic resources of land and water for sustainability of agriculture in the Indo-Gangetic Plain. It is generally believed that the rice-wheat system has strained the natural resources in this region and more inputs are required to attain the same yield levels (Swarup and Singh, 1989; Kumar and Yadav, 1993; Lal et al., 2004). It is, therefore, imperative now to promote alternative technologies that would help conserve the much needed but gradually depleting natural resources while boosting productivity growth in the long-run by maintaining soil health and production environment. As a part of this strategy, conservation agriculture and resource conserving technologies play a major role in sustaining and enhancing the productivity of the rice-wheat system at a lower cost of production.

Introduction

The Indian agriculture is very complex and carrying out multi-functionalities of providing food, nutrition and ecological security besides employment and livelihood for over 700 million people. Indeed, India has made a marvelous achievement in attaining self-sufficiency in food grain production after the induction of Green Revolution which eventually resulted in maintaining all-time high buffer stock in warehouses of our country. Such rosy picture in production trends turned to be bleak in the past few years. There are various reasons behind this. Shrinking resources of prime lands, deforestation and accelerated erosion, deterioration of soil physical environment, increasing waterlogging and salinity in canal irrigated areas, declining water table in well-irrigated areas, poor management of rainwater, lower efficiency of inputs such as water, fertilizers and agrochemicals, rapid industrialization coupled with pollution and environmental degradation and hazards have aggravated the problem. At the same time, increase in atmospheric concentrations of greenhouse gasses (GHG), of which the most common is carbon dioxide (CO₂), is the primary cause of global warming and the Intergovernmental Panel on Climate change (IPCC) estimates that the current greenhouse gases (GHGs) concentrations are 30% more than the pre-industrial level. C is accumulating in the atmosphere at a rate of 3.5 Pg (Pg = 1015 g or billion tons) per annum, the largest proportion of which resulting from the burning of fossil fuels and the conversion of tropical forests to agricultural production. Under such circumstance, there is an imperative need to produce more from less arable land and water through meticulous management of basic agricultural resources such as soil, water and biological inputs.

Introduction

Persistent use of conventional farming practices based on extensive tillage, especially when combined with removal or in-situ burning of crop residues, has magnified soil erosion losses and swiftly degraded soil resource base. One of the glaring examples for the aforementioned statement is the ‘Dust Bowl’ in the U.S during 1930s in Great plains, where 91 M ha of land was degraded by severe soil erosion (Hobbs, 2007).

One of the primary challenges of our time is to feed growing and more demanding world population with reduced external inputs and minimal environmental impacts (nature paper). Conservation Agriculture (CA) is a set of management practices for sustainable agricultural production without excessively disturbing the soils, while protecting it from the processes of soil degradation like erosion, compaction, aggregate breakdown, loss of organic matter, leaching of nutrients, and processes that are accentuating due to anthropogenic interactions in the presence of extremes of weather and management practices. The organic materials conserved through this practice are decomposed slowly, and much of materials are incorporated into the surface soil layer, thus reducing the liberation rate of carbon as CO₂ into the atmosphere. In the total balance, carbon is sequestered in the soil, and turns the soil into a net sink of carbon. So, CA enhance soil health by improving soil aggregation, reducing compaction through promotion of biological tillage, increasing surface soil organic matter and carbon content, and moderating soil temperature and weed suppression. CA reduces cost of cultivation, saves time, increases yield through timelier seeding/planting, reduces pest and diseases through stimulation of biological diversity, and reduces green house gas emissions.

Machinery development, refinement and adoption for a range of soil and cropping situations will be fundamental in any success to promote conservation agriculture practices. Agricultural machinery or tools, which support conservation agriculture generally refer to the cultivation systems with minimum or zero tillage and in-situ management of crop residues. Minimum tillage is aimed at reducing tillage to the minimum necessary that would facilitate favorable seedbed condition for satisfactory establishment of crop. Zero tillage is however an extreme form of minimum tillage. With the development of direct drilling machines, almost all research work was attempted to define the responses of direct-drilled seeds in relation to soil micro-environments. Different designs of direct drilling machines viz. zero till drill, no till plant drill, strip till drill, and rotary slit no till drill have been developed with controlled traffic measures for energy efficient and cost-effective seeding of crops without tillage. In this paper various issues, challenges and prospects of conservation agriculture in India has been discussed.

Introduction

Climate change is a statistically significant variation in either the mean state of the climate or in its variability, persisting for an extended period (typically decades or longer). Warming of the climate system is unequivocal, as is now evident from observations of increases in global average air and ocean temperatures. Eleven years from 1995-2006 rank among the twelve warmest years in the instrumental record of global surface temperature (since 1850). The 100-year linear trend (1906-2005) of 0.74 [0.56 to 0.92] °C is larger than the corresponding trend of 0.6 [0.4 to 0.8] °C (1901-2000) and over the 21st century average temperature of earth surface is likely to go up by an additional of 1.8-4°C (IPCC, 2007). This temperature increase can be attributed to the altered energy balance of the climate system resulting from changes in atmospheric concentrations of the green house gases (GHGs). Among the principal components of radiative forcing of climate change, CO₂ has the highest positive forcing leading to warming of climate. CO₂ has the least global warming potential among the major green house gases but due to its much higher concentration in the atmosphere; it is the major contributor towards global warming and climate change. Agriculture sector in India contributes 28% of the total GHG emissions (NATCOM, 2004). The global average from agriculture is only 13.5% (IPCC, 2007). In future, the percentage emissions from agriculture in India are likely to be smaller due to relatively much higher growth in emissions in energy-use transport and industrial sectors. The emissions from agriculture are primarily due to methane emissions from rice fields, enteric fermentation in ruminant animals and nitrous oxides from application of manures and fertilizers to agricultural soils (NATCOM, 2004).

Introduction

Climate smart agriculture (CSA) refers to the incorporation of adaptation and mitigation practices in agriculture that enhances the system-resistance and recovery-mechanism against climatic hazards. Climatic hazards / disturbances include drought, flood, heat / cold wave, erratic precipitation, sudden dry spell etc. In nutshell, CSA is the ability of the system to bounce back. It can also be explained as an approach to build-up resilience of agricultural systems, to increase adaptive capacities of farming communities to climate change and variability, and to ensure food security (Prasad et al., 2015).

Climate change can be defined as a statistically significant variation in either the mean state of the climate or in its variability, persisting of an extended period, typically three decades or longer (Pathak et al., 2014; NAAS, 2013, Bhattacharyya et al., 2016). The surface air temperature, sea level rise, occurrence of extreme events (heat wave, cyclone, flash flood) is the direct evidences of climate change. The annual average surface temperature has increased by 0.87^°C in 100 years (IPCC, 2018). Recent report revealed that more than 90% of Northern Hemisphere land areas outside the tropics showed at least 1^°C above average, whereas temperatures were less extreme in Southern Hemisphere. Importantly, temperatures were above normal over most of the ocean areas. Global sea level rose to about 15 mm, in between November (2014) and February (2016), as a result of ElNino, which was well above the post-1993 trend of 3.0-3.5 mm year^-1. However, Indian summer monsoon rainfall since 1901 to 2012 showed no long-term trends, with some regional changes. Importantly, night temperatures have increased sharply during recent years, which couple with erratic precipitation could significantly affect agricultural production. The fifth IPCC (IPCC, 2014) reports clearly brought out global and regional impacts of climate change on Agriculture, Forestry and Other Land Use (AFOLU). South Asia has been characterized as one of the most vulnerable regions. Impacts of climate change witnessed all over the world; however, countries like India is more vulnerable because of its huge population primary dependent on agriculture for livelihood, predominance of small and marginal farmers, and fragmented land holding.

Introduction

Water and food security are the key challenges under climate change as both are highly vulnerable to continuously changing climatic patterns. Climate change has resulted in increases in globally averaged mean annual air temperature and variations in regional precipitation and these changes are expected to continue and intensify in the future. The projected changes in climate patterns over India include increase in surface temperature, variations in rainfall, increasing occurrence of extreme weather events like floods and droughts, rise in sea levels and impact on the Himalayan glaciers. Sectors of the Indian economy are most likely to be impacted by such changes in climate are those dependent on natural resources, namely agriculture, water and forestry. The likely impacts such as reduction in food production, water scarcity, loss of forest biomass Climate change is expected to bring more intense and more frequent extreme weather events including droughts and floods in India. The global atmospheric concentration of carbon di-oxide CO₂ for increased from preindustrial value of about 280 ppm to about 410 ppm at the present (2021). The global increase in CO₂ concentration is primarily due to fossil fuel use and land use change. Atmospheric methane was 1803 ppb in 2011, this is 150% greater than before 1750. Atmospheric nitrous oxide (N₂O) was 324 ppb in 2011, this is 150% greater than before 1750 (Table 1). These increase in GHGs have resulted in warming of the climate system by 0.74°C between 1906 and 2005.

Salt affected soils occupy 962.2 m ha globally which accounts for 33% of the arable land. In India 8.373 m ha is affected with salts. Of which 2.359 m ha are alkali soils, 3.829 m ha are saline soils and 2.185 m ha coastal saline soils. Reclamation of these soils provides additional area for food grains production and employment.

SOIL MANAGEMENT PRACTICES

Good soil management is the basic for deriving greater benefits from the fertilizers used in crop production. Some of the good soil management practices are

Good tilth is the first feature of good soil management. It means a suitable physical condition of the soil and implies in addition a satisfactory regulation of soil moisture and air.

The maintenance of soil organic matter which encourages granulation is an important consideration of good tilth.

Tillage operations and timings should be adjusted as to cause the minimum destruction of soil aggregates. Good tilth minimizes erosion hazards.

The choice and sequence of adaptable crops or crop rotation are other very important considerations. These are related to climate, particularly rainfall and its pattern of distribution and the characteristics of the soil profile, including drainage and extent and duration of available soil moisture. A proper sequence of crop varieties greatly influences soil conditions. It is more realistic to evolve cropping patterns and land management practices according to land capability. Cropping patterns chosen and management practices adopted should aim at soil and moisture conservation for efficient nutrient and moisture utilization.

Integrated plant nutrition system conceptualized by food and agriculture organization is the maintenance or adjustment of soil fertility and of plant nutrient supply to an optimum level for sustaining the desired productivity through optimization of benefits from all possible sources of plant nutrients viz. organic manures, fertilizers, crop residues, compost or nitrogen fixing crops etc. in an integrated manner.

Biofertilizers act as complementary and supplementary sources of plant nutrients. Biofertilizers help in increasing the biologically fixed atmospheric nitrogen and enhancing phosphorus availability to the crops. Among the biofertilizers for increasing nitrogen supply, nitrogen fixing bacteria (Rhizobium, Azotobacter and Azospirillum), blue green algae (BGA) and Azolla are important. The availability of phosphorus is improved by phosphorus solublizing microorganisms (PSM) and mycorrhizae (VAM).

Rhizobium is of vital importance to various pulses and some of the oilseeds and fodder legumes as 80–90 per cent of their nitrogen requirement is met through biological nitrogen fixation (BNF). Rhizobium fixes atmospheric nitrogen ranging from 50–60 kg per ha in groundnut, 100–300 kg per ha in alfalfa. In pulses it is estimated at 50–110 kg/per ha. Field studies indicated a yield increase of 14–53 per cent due to Rhizobium inoculation. Nitrogen left over the demand of legumes in combination with the nitrogen present in roots of the legumes will enhance the soil fertility. Consequently the nutrient needs of the subsequent crop from applications will be reduced.

Green manure is defined as fresh organic matter/undecomposed plant material/ real organic matter, natural plant food added to the soil for the purpose of supplying plant nutrients.

CLASSIFICATION OF GREEN MANURES

Nonlegumes: Rape seed, buck wheat, niger, rye, oats and maize

Legumes:

Grain legumes: cowpea, mungbean, pigeon pea, soybean, khesari (Lathyrus sativus)

Dual purpose legumes: cluster bean (Cyamopsis tetragonoloba). pillipesara, Sesbania speciosa.

Non grain legumes: sunnhemp, dhaincha, pillipesera (Phaseolus trilobus), stylosanthes and desmodium.

Farmyard Manure is the manure prepared by using the mixture of dung and urine soaked litter by microorganisms.

Good farmyard manure is one

ZINC (Zn)

Chandler et al., (1932) deduced the requirement of zinc by plants while working solution for little leaf or “rosette” on peach. Zinc plays significant role in assimilation of NO₃ and for synthesis of tryptophan. Involved in biosynthesis of indole acetic acid which is responsible for f lowering and fruiting, it is an activator of several enzymes (triphosphate dehydrogenase and tryptophan synthetase), involved in photosynthesis and nitrogen metabolism.

Deficiency results in poor growth of the terminal bud. Decrease in stem length resulting in resetting and whorling of leaves. Zinc deficiency disorder in rice is called khaira. Common when zinc availability is < 0.7 ppm. White bud in maize and malformation of fruits, reduction in water uptake. Interveinal chlorosis often with necrosis and pigmentation and reduced leaf size in broad leaved crops known as little leaf and malformation of leaves.

The crops which are known to respond for zinc application are maize, rice, soyabean, castor, onion, barley, wheat, potato, sorghum and cotton

CALCIUM

Calcium is important in imparting structure and permeability to cell membrane. Deficiency produces general breakdown of membrane structure. Calcium enhances the uptake of NO₃ ^– N and interrelated with NO₃ ^– metabolism. It regulates cation uptake, required for cell elongation and cell division. Deficiency causes failure of terminal buds and apical tips of roots to develop. Blossom end rot of tomato, bitter pit of apples are due to calcium deficiency. Calcium is beneficial in imparting good soil structure.

It is immobile in plant. Very little translocation of calcium in the phloem is observed. Hence there is poor supply of calcium to fruits and storage organs. Downward translocation of calcium is also limited. Hence roots fail to enter the low calcium soils.

Glauber J. R. (1604–1670) in Netherlands first proposed that the salt peter (KNO₃) was the principle of vegetation. He obtained large increases in crop yield by addition of salt peter.

Mineral soils contain 0.04–3.0% K. Total potassium content of soils range from 3000 to 100000 kg per ha in the upper 0.2 m of soil profile. Of this total potassium approximately 98 percent is bound in mineral form whereas 2% in soil solution and exchangeable phases.

ROLE OF POTASSIUM

It is Involved in activating the enzymes (as many as 80) responsible for carbon dioxide reduction and required for synthesis of adenosine triphosphate. Hence deficiency of potassium reduces crop growth and yields.

Liebig (1840) on the basis of soil and plant analysis deduced the phosphorus requirement of plants. J.B. Lawes conducted series of field experiments to improve the fertilizer value of common sources of phosphorus such as bone meal, apatite, rock phosphate. Lawes (1843) produced super phosphate by treating apatite mineral containing phosphorus with sulphuric acid. He also stated that the phosphorus present in bones of higher animals can be made available by treating with sulphuric acid.

Phosphorus is the second most deficient nutrient in the world. Annual application of phosphorus fertilizers to crops is 36 million tons of which

1.32 m tones are consumed in India. Most of the arable soils in tropical and subtropical regions are deficient in phoshphorus, but the response of crops to phosphorus is uncertain. Phosphorus concentration in plants ranges from 0.1 to 0.4 per cent.

Greek scientist Theophrastus (300 BC) advocated legume rotation. As early as 1750 AD it was known that plants require Nitre. Louis Pasteure in 1860 AD stated the possibility of atmospheric nitrogen fixation by microorganisms. Hellriegel and Willfarth (1886) strengthened the views of Louis Pasteure by his observations on the irratic behaviour of legumes to nitrate nutrition. Beizerink (1890) isolated the organism involved in nitrogen fixation. The systematic field experiments conducted at Rothemstead experimental station showed that the plants respond to inorganic nitrogen.

ROLES OF NITROGEN IN CROPS

Plants contain 1-5 per cent nitrogen by weight. It is a structural component of chlorophyll, aminoacids and proteins. Plant proteins contain 16 per cent nitrogen (6.25 × %N = protein). Plants are dependent on protein for their propagation. Cereals possess a threshold protein level below which grain will not form.

Plants are the generators or the primary source of organic matter while animals are the transformers- secondary source of organic matter. In agriculture, plants add 10–30 percent of organic matter through their roots stubbles and leaf fall and exudations. The excreta of livestock, the organic wastes of urban and rural residential areas include the garbage and sewage and sludge. The by products of agro based industries viz. bagasse, vinasse, pressmud from sugar industries, the coir dust from the coir industries, dust of cotton from ginning industries, the fruit and food processing industries wastes.

Concept of nutrient availability: The ions in the soil solution are available to plant provided they can reach an active root or it can reach them. The concentration of ions in the soil solution at any given moment is however; usually low, except immediately after the application of water soluble fertilizers.

Most of the nutrients are taken up by the plant in the form of cations which are positively charged. Available cations either in the soil solution or held against leaching by negatively charged clay or humus particles are easily taken up by the plants. Other plant nutrients notably phosphorus, boron and molybdenum are taken up by the plant as anions which are negatively charged. The concentration of anions in the soil solution is usually low because they become converted to new less soluble forms e.g., soluble phosphate ions forms less soluble iron or aluminium phosphates. Minerals upon weathering and organic matter upon mineralization release nutrient ions.

Physical factors

However, much readily available nutrients a soil may contain various physical features of the soil make it impossible for the plant roots to absorb it. It then becomes positionally unavailable. These features include hard pans, undesirable soil structures, presence of hard clods, dry soil horizons and water logged situation.

Low soil cation exchange capacity leads to less retention of calcium, magnesium, potassium, copper and zinc.

ESSENTIAL ELEMENTS FOR PLANT GROWTH

Arnon (1954) set three criteria to state an element is essential to plants. They are:

1. The plant must be unable to grow normally or complete its life cycle in the absence of the element,
2. The element is specific and cannot be replaced by another and,
3. The element plays a direct role in metabolism.

Accordingly there are 16 elements essential for plant growth. They are carbon, hydrogen and oxygen called primary nutrients, calcium, magnesium and sulphur called secondary nutrients; iron, zinc, manganese, copper, boron, chlorine and molybdenum are known as micronutrients.

[Schoonover (1952) Method]

Introduction

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

Principle

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

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

Introduction

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

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

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

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

[Williams and Steinbergs (1959) Method]

Introduction

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

(Neutral Normal Ammonium Acetate Method)

Introduction

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

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

Introduction

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

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

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

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

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

Introduction

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

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