The book, ‘fundamentals of Soil Chemistry’, is meant for the students of Soil Science in particular and for those who are studying agriculture in general. The book is written in a simple language to understand the basics of Soil Chemistry and its applications. This book will be useful both for the undergraduate and post graduate students. The book comprises of ten chapters on fundamental aspects of soil chemistry. The chapter 1 deals with the basic principles of chemistry while chapter 2 is intendent to know the composition of soil. The elements in plant nutrition and soil organic matter are discussed in chapter 3 and 4 respectively. The soil colloidal properties, the structure of silicates and non-silicate minerals, identification of clay minerals as well as charge characteristics of clay are presented in chapter 5. The concept of ion exchange phenomena and its significance in plant nutrition are addressed in chapter 6. The function of acid and alkaline soils and their reclamation processes are discussed in chapter 7. The effect of soil reaction on plant growth has also been discussed in this chapter. The adsorption and desorption of nutrients in soil and its implication towards fertilizer application are dealt in chapter 8. The chemistry and implication of submerged soils are discussed in chapter 9. The organo-metal complexation in soil and the nature of stability of the complexes are discussed in chapter 10.

The knowledge on fundamental principle of chemistry is important to understand and examine the mechanisms of reactions to form products. The chemistry of soil processes will also help understanding the correlation between the basic and applied sciences for achieving the sustainable ecosystem in the environment. The formation and stability of bonds, thermodynamic principles, radioactivity, coordination number etc. are essential to solve the on-going interactions in nature. The structure of atomic models, valence and equivalent weight, creating elementary particles, quarks, neutrinos, bosons are some of the important arena where fundamental study and research can be explored for the basic and strategic ideas. Hence, it is very much essential to apprehend the concept of physical chemistry, soil chemistry and geochemistry to correlate with the definitions of chemistry for understanding the source/sink properties of soils for nutrient adsorption and release characteristics.

Soil is one of the major platforms on which life sustains. Soil, the product of weathering, covers the earth’s surface. Soil is the medium of plant growth and thus it requires proper attention for better crop yield. The land is the constituent of soil, vegetation, physico-chemical and biological attributes affecting its usages. The soil may act as a sink or source for the elements and other materials. Hence, it requires to understand the various components to facilitate management options for improving the fertility status of soil. 2.0 Soil The soil is a natural dynamic body on which plant grows. It is a complex mixtures of minerals, water, air, organic matter and microorganisms. Besides, there exist an unconsolidated mineral or organic material on the surface of the earth and thus becomes a natural medium for the growth and nutrition of the plants.

The plant can absorb more than sixty elements as revealed from the plant tissue analysis, still a few elements is considered essential for the growth and nutrition of the plants. The deficiency of any one of the essential elements can stop completing the life cycle of the plants which is reflected on the yield of the crop. The growth of the plant which is expressed in terms of dry weight, length ,diameter etc. are interlinked with the plant available nutrients of soils. 3.1 Essential Nutrients The essential elements are those elements which are required for growth and nutrition of the plants. There are 17 elements known to be essential nutrients for plants. The essential elements are classified as micro and macronutrients depending upon the requirement of the crop. Among the naturally occurring elements, eighteen elements are considered essential to plants. These are: C, H, O, N, P, K, Ca, Mg, S, Fe, Mn, Zn, Cu, B, Mo, Cl, Ni, Co. The elements which are found in different sources are required to be in available forms (Table 3.1) for growth and nutrition of the plants.

4.0 Soil organic Matter The soil organic matter (SOM) is a storehouse of multi-nutrient elements, composed mainly of carbon, hydrogen and oxygen and small amounts of other elements like nitrogen, phosphorous, sulphur, potassium, calcium , magnesium and organic residues coming out from the decomposed plant or animal products (Figure 4.1). The SOM may have the recently incorporated materials or age-old decomposed materials or with living flora or fauna in the organic matrix. Soil organic matter is one of the important components determining soil quality indices and also for enriching the fertility status of soil for production maximization. The nutrient retention capacity , water holding capacity, degradation of pollutant elements, carbon sequestration, soil structure and aggregate stability are affected by the presence of SOM.  

Soil colloidal substances are composed of very fine particles (< 0.002 mm diameter) that undergoes two phase system in a heterogenous manner, where, one phase (disperse phase) is disperse in a fine state of subdivision in another medium called dispersion medium. The soil colloidal properties are affected by the physico-chemical and biological parameters of soils. The availability of plant nutrients are governed mostly by the surface charge characteristics of clay colloids. 5.0 Soil Colloids Soil colloids being one of the active constituents determine the physical and chemical properties of the soil with the diameter ranging from 1 μm to 2 μm. The origin of the particles may be organic and inorganic constituents.

The exchange of ions (cations or anions) are important phenomena in soil for the growth and nutrition of the crops. The oppositely charged ions are being adsorbed by the clay colloids which are subjected to be released or exchanged reversibly from the soil solution. The participating ions in the exchange process are called exchangeable or counter ions. The activity of ions depends on the pH of the soil. Hence, factors like, soil texture, organic matter, concentration of ions could govern the ion exchange phenomena in soil. 6.0 Ion Exchange Ion exchange is a reversible process where exchange of ions are possible between the two electrolytes or between an electrolyte solution and a complex. The ion exchange processes help in purification, separation, and decontamination of aqueous and other ion-containing solutions. The ion-exchange resins (functionalized porous or gel polymer), zeolites, montmorillonite, clay and soil humus are considered as typical ion exchangers which are either cation exchangers [exchanging positively charged ions (cations)], or anion exchangers [which can exchange negatively charged ions (anions)].

The soil acidity and alkalinity are major driving forces in regard to the mobility of plant nutrients from soil to plants. The extreme acidity or alkalinity cause damage to the crops and thus yield is affected. Correction of soil acidity or alkalinity depend upon the pH of the soil and its previous management options. Application of fertilizer in soil may be one of the factors affecting soil conditions. Exchangeable and soluble iron and aluminium ions and their hydroxy ion species may contribute hydrolysis to release H+ ions to the soil solution. The soluble salts of carbonate and bicarbonates also develop salinity in soils. 7.0 Soil reactions Soil reaction is an indication of the acidity or alkalinity of soil and is measured in pH . The pH scale ranges from 0 to 14 with pH 7 as the neutral point. When the amount of active hydrogen ions in the soil solution increases the soil pH decreases and thus becoming more acidic in reaction. One of the outstanding physiological characteristics of the soil solution is its reaction. Soil reaction can influence the physico-chemical properties of soil which can regulate the growth and activity of plant and soil organisms associated with it.  

Nutrient adsorption in soil is one of the important phenomena in terms of nutrient availability to plants. Due to the nutrient fixation of the added nutrients supplied through fertilizers or manures may result the increase in cost of production due to the non-availability of the added nutrients at the optimum level to the plants. Hence, management options are required to release the sorbed nutrients from soil for better availability to the plants. The ion-fixation is driven by physico-chemical and biological processes and varies under agro ecological situations. The chemistry of ion fixation in the form of specific or non-specific adsorption on soil colloids will help understanding the bonding and adsorption reactions of ions in the soil. 8.0 What is fixation? Fixation of plant nutrients in soil may be defined as the process, where readily soluble plant nutrients are transformed into less soluble forms by physical or chemical reaction with organic or inorganic compounds and thus restricting the mobility of ions in soil that leads to the decrease in availability to the plants. There are two kinds of fixation in general, viz; cation (N, K, Fe, Mn, Cu, Zn, Ca,Mg etc.) and anion (P, B, Mo etc.) in soils. The fixation of ions in soil is governed by important physical and chemical parameters of soil based on which the available pools of nutrients for plants may be determined.

Understanding the chemistry of submerged soil in the field of Geology, Pedology, Agriculture and other allied sectors draws attention to the research on characteristic changes in soil under submergence. It has been reported that, about 72% of the earth’s surface is covered by submerged soils or sediments . Rice plants have unique adaptation capability to grow under submerged soil and in any type of soil (acid/alkaline) condition or land situation (upland/lowland). The knowledge on changes of important physical, chemical and biological properties under submergence have important role to play in understanding the mechanisms of availability of nutrients to the plants and sustainability of the crop during its growing period. 9.0 Submerged Soil The soils which are saturated with water for a longer period of time in a year to make the soil a distinctive gley horizons resulting from oxidation-reduction processes are called submerged soil. About 72% of the earth’s surface is covered with soil –submergence or sediments.

The native and applied organic matter play an important role in terms of nutrient adsorption , nutrient mobility and microbial habitat on soil. The dominant ions present in soil under a particular agro-ecological situation may or may not be available to the growing plants due to the presence of organic matter. The binding energy and the stability of the organo-metal complexation would play an important role in availability of nutrients to the plants. The organo-metal complexation reaction depends on the nature of the ionic species (ionic/non ionic) present in soil. The organic fractions (Humic acid/Fulvic acid) react with metal ions present in soil to form complexes which may be stable or unstable depending upon the molecular arrangement and degree of coiling of humic/fulvic substances. The surface charge density on clay and soil pH also play an important role to determine the order of stability of different nutrients (macro/micro) in soil.