Preface The book on Acid Soils: Their Chemistry and Management is the result of a novel exercise to highlight the technological, management and policy issues involved in increasing the agricultural out put on low productive acid soils. During the last 10 to 15 years, excellent work has been done in this field after taking note of the pioneering research on liming acid soils by eminent scientists. It was thought proper to collate and consolidate the work for effective use by scientists, research scholars, post-graduate students, and extension personnels. Our knowledge on soil acidity and its effect on soil health and crop productivity has expanded in recent years. Management of knowledge and its fast dissemination for better life is of great value. Though basic chemistry of acid soils has remained similar, but the land use, cropping patterns, methodology and sources of lime application have changed over the years. This has made the planners to realize that the untapped potential of acid soil regions must be taken care of by a technology based development. The chapters for the book have been written by scientists, who are engaged in this work for the past two to three decades. I express sincere appreciation and gratitude to the authors for their hard work, cooperation and adherence to a time frame.

Soil acidity is a major constraint for crop production. The production of acid in soils is a natural process, especially in the high rainfall areas. The removal of basic cations especially Ca and Mg, by leaching and erosion results in their replacement by acidic cations, H, Al and Fe on cation exchange sites and in the soil solution, which are in dynamic equilibrium with each other. Soils, that develop from weathered granite are likely to be more acidic than those developed from shale or limestone. Organic matter decay produces H+ in soils, which also contribute to soil acidity. Intensive cropping and use of hybrid & high yielding crop varieties remove large quantities of basic elements essential for their growth i.e. Ca, Mg and K & deplete the soils. Rate of acidification is also accelerated by  faulty fertilizer (especially Nitrogen) management practices.

Soil pH Danish scientist Sorensen (1909) first used the term pH. The H+ concentration is usually small and is expressed in terms of pH, defined as the logarithm of the reciprocal of the H+ concentration, in moles per litre. The pH scale ranges from 0 to 14, with 7 being neutral. Soil acidity is the concentration of H+ in the soil solution. This is active acidity. Besides, there is a potential acidity, that includes the H+ adsorbed on the colloidal fraction of soil. The pH scale is derived from the dissociation constant of water, Kw:

Soil  is an interface of inorganic and organic componentsof the planet earth, which combines carbon and nitrogen from the atmosphere with different mineral elements of the lithosphere through millions of microorganisms, intercepting energy from the sun and moisture from the hydrosphere and atmosphere. It is a source as well as sink to adsorb, desorb, fix or release mineral elements and gases, decompose organic residues, simultaneously and support the growth of plants. All these natural processes are accompanied by runoff, percolation and evapotranspiration processes of the hydrologic cycle. Thus, soil is a dynamic body that acquires properties in accordance with the forces, which act upon it. Many soil properties emanate from physical, chemical and biologically mediated reactions, where the factors like climate, vegetation, topography and time  influences the processes. Soil reaction is one of the most important chemical characteristics of the soil solution,which has both direct and indirect effects on plant nutrition.

Acid soils are found in India in about 93 million hectares ( Table 1) and they occur in the Himalayan region, the great Indo-Gangetic and Brahmaputra plains, peninsular plateau and uplands and the coastal plains including island eco-systems under varying environmental conditions of landscape, geology, climate and vegetation. Conditions which are conducive for the formation of acid soils, such as high rainfall coupled with high temperature encouraging intense weathering and loss of alkali and alkaline earth metals with a simultaneous accumulation of iron and aluminium hydroxy compounds or the formation of free acidic substances in the humid forest foot hill regions, exist in some region or other in almost all the states in India.  A soil with pH less than 7.0 is deemed as an acid soil (Soil Survey Staff,1951). However, for characterization and classification, acid soils may be defined as those having pH less than 5.5 in 1:1 water extract (Soil Survey Staff, 1999).

Many farmer participatory field  experiments were designed and conducted in different states of India having large area under acid soils, with locally available soil ameliorants to study their effect on soil health and crop performance. This work was done during 2000 & 2005 under an ICAR network project on “Soil characterization and resource management of acid soil regions for increasing productivity”. The details of the field locations are presented in table 1.

The north eastern hill region covers a large share of acid soils of India occupying 14.95 m ha which is a major constraint for sustainable crop production. Acid soil limits the availability of calcium and phosphorus in soil, lowers base saturation and reduces microbial activities in soil adversely affecting the crop growth and yield. About 95 % of the soils of this region, except Nagaland (about 70 %) are acidic in reaction. About 84, 77, 76, 60, 57 and 47 per cent soils of Arunachal Pradesh, Manipur, Meghalaya, Mizoram, Sikkim and Tripura, respectively have soil pH below 5.0 and are considered strongly acidic (Panda, 1998). The soils are highly degraded because of heavy rainfall and removal of top soils. Land slides are also contributing towards land degradation. About 16 % of the area is affected by water erosion (www.india stat.com). About 53.8, 28.2, 42.6, 53.9, 89.2, 60, 33 and 59.9 per cent  area are degraded in Arunachal Pradesh, Assam, Manipur, Meghalaya, Mizoram, Nagaland, Sikkim and Tripura, respectively. Including Assam, north-east constitutes 80 % of the total area under acid soils of the country. The land use pattern of north-east region is presented in table 1 and area, production, yield of foodgrains and fertilizer consumption are presented in Table 2.

Jharkhand state lies between 21º58’2" to 25º8’32" North latitude and 83º19’05"to 87º55’03" East longitude covering an area of about 7.97 m.ha. and accounts for  2.4 per cent of  the total geographical area (TGA) of the country. It is bound on the east by West Bengal, on the west by Chhattisgarh, on the north by Bihar and on the south by Odisha. Reliable information on the location, extent and quality of soil and land resources is the first requirement in planning for the sustainable management of land resources. The components of land i.e., soils, climate, water, nutrient and biota are organised into an eco-system, which provides a variety of services that are essential for the maintenance of the life support system and the productive capacity of the environment.

The state of Odisha located in the Coromondal coast of India has a tropical monsoon climate characterized by high humidity, high rainfall (1497 mm) and short and medium winter. It is the 10th largest and 11th most populous state accounting about 5% of the geographical area and 4% of the population of the country. The geographical area of the state is 15.57 m ha, out of which 0.62 m ha is arable land. Agriculture contributes 28% to  state domestic production and employs 64% of the work force.

Acid soils occupy one fourth of the land in India. The formation and distribution of acid soils depend upon several factors like temperature, vegetation, parent material and hydrological conditions etc. For this reason, these soils have low pH, poor base saturation, high sexquioxide content, which affects the transformation and availability of plant nutrients.  Secondary and micronutrient deficiency as well as toxicity are major problems in cultivation of crops in acidic soils. These soils are generally deficient in Ca, Mg and S and adequate in Fe, Mn and Cu. Apart from the secondary nutrients, Zn, B and Mo deficiencies affect crop production. In lowlands, rice crop suffers from toxicity of Fe. Aluminum toxicity, common in highly acidic soils, also pose major problems in crop production. Management of secondary and micronutrients in acid soils for agriculture is a national priority to achieve food sufficiency and sustainability.

Soils become acidic due to natural  and artificial (anthropogenic) reasons. The natural reasons include their origin from acidic parent materials namely granite, rhyolite, pegmatite and dacite etc. and areas under high rainfall  (rainfall  exceeding evapotranspiration) removing  basic cations through leaching and erosion particularly in coarse textured soils with undulated, hilly and sloping topography. Under natural condition, accumulation of organic matter and its decomposition also releases organic acids into the soil system, turning soils  acidic. The anthropogenic factors    responsible for turning soils  acidic,  are (i) removal of basic cations through crop cultivation, (ii) continuous use of acid forming fertilizers and (iii) acid rain in industrial areas.

The adjustment and maintenance of soil pH at a value suitable for optimal crop production is the first step in soil management, because soil pH affects soil properties, nutrient availability and plant nutrition. In India, about 30M ha cultivated  land  suffer from high soil acidity (pH < 5.5). These soils are critically degraded and are of poor health producing crop yields, which are often less than 50% of that in normal soils. Due to this, farmers use much less fertilizers for crops (50 to 80 kg NPK ha-1), since this is not profitable.