Khan Towhid Osman

Physical Properties of Soil

Physical properties of soil include color, texture, structure, porosity, density, consistence, temperature, and air. Colors of soils vary widely and indicate such important properties as organic matter, water, and redox conditions. Soil texture, structure, porosity, density, and consistence are related with types of soil particles and their arrangement. There are two types of soil particles—primary and secondary. Primary particles include sand, silt, and clay, categorized on the basis of their effective diameter. There are important differences in physical, chemical, and mineralogical properties of these fractions. Their relative proportion in a soil is called soil texture. It is a fundamental property of soil. It is not easily altered. There are 12 textural classes ranging from sand to clay. Soil structure is the arrangement of soil particles into different geometric patterns. It is classified into different types on shape, classes on size, and grades on stability. Soil structure is amenable. Soil texture and structure together regulate porosity, density, compactness, retention, and movement of water and air in soil. Soil temperature is slightly higher than air temperature in a place. It influences life processes of soil biota including plants. Soil air is retained in soil pores; its composition is variable, and it contains higher carbon dioxide and moisture and lower oxygen concentration than atmospheric air. Soil air has a great role in respiration of plant roots and microorganisms and transformation of mineral and organic matter.

Soil Erosion by Water

Erosion is a natural process of detachment of soil particles and their transport and deposition at distant places by natural agents such as water, wind, glacier, and gravity. Detachment of soil particles from aggregates primarily by raindrops and flowing water and their transport by runoff water are involved in soil erosion by water. Natural erosion is considered as normal erosion and is usually of little concern from soil quality point of view because its rate is low and soil loss can be naturally compensated by soil formation. Human actions such as deforestation, overgrazing, over tilling, and shifting cultivation have accelerated soil erosion beyond the tolerance limit. A tolerance range of 2–11 t ha−1 year−1 depending on soil types is accepted in the USA. There are places and situations where erosion rates are much higher than this limit, even as high as 100 t ha−1 year−1. The principal types of soil erosion by water are splash erosion, sheet erosion, interrill erosion, rill erosion, gully erosion, landslides, and stream erosion. Soil erosion has on-site and off-site effects. The on-site effects include loss of soil, loss of organic matter and nutrients, damage to growing crops, exposure of plant roots, and decline in soil fertility and productivity. The off-site effects are burrowing of crops and installations, siltation of reservoirs, eutrophication of ponds and lakes, pollution of water, etc. Several agronomic and engineering practices are employed for the control of water erosion. These are no-tillage, minimum tillage, mulching, strip cropping, contour cropping, contour strip cropping, and terracing, but several methods are needed to be integrated for an efficient soil erosion control.

Plant Nutrients and Soil Fertility Management

Seventeen chemical elements such as C, H, O, N, P, S, K, Ca, Mg, Fe, Mn, Cu, Mo, B, Zn, Cl, and Ni have so far been recognized as essential for plants. Plants cannot complete their life cycles and accomplish normal physiological functions in the absence of these nutrients. Growth and yield of crops are reduced by their deficiencies. There are some other elements, namely, sodium (Na), silicon (Si), vanadium (V), iodine (I), and cobalt (Co), reckoned to be beneficial for growth of certain plants and microorganisms. Plants often suffer from inadequate supply of nutrients by the soil. These inadequacies are met by the application of fertilizers. Fertilizers are any materials added to soils or plant leaves to supply nutrients. There are various natural and synthesized materials used as fertilizers. Composts, farmyard manures, poultry manure, oilcakes, guano, etc. are very good organic fertilizers obtained from natural materials. These substances contain low concentrations of nutrients; so they are needed in huge amounts. Moreover, their composition is not fixed; and estimating their required amounts is difficult. If added in adequate amounts and well ahead of time, they give good results. Industrial fertilizers are soluble, fast acting, and high analysis materials. They contain nutrients in available forms, and therefore, they are very efficient in correcting current deficiencies. Nutrients may be lost from applied fertilizers, especially nitrogenous fertilizers. Some nitrates and phosphates are transported from agricultural lands to surface and groundwater reservoirs. These contaminants have tremendous environmental impacts. Slow-release N fertilizers are being used to minimize loss of nitrogen from crop fields. There are some methods of fertilizer application that might reduce nutrient losses.

Chemical Properties of Soil

The soil is a chemical entity. All the materials there are chemical substances. Soils are composed of solid, liquid, and gas; soluble and insoluble; and organic as well as inorganic substances. There are ions and compounds, salts, acids, bases, minerals, and rock fragments. There are also colloids which are very active chemically. They are very fine soil particles, including humus, fine silicate clays, and oxides and hydroxides of iron and aluminum. Colloids carry electrochemical charges, both positive and negative, and these charges develop by substitution in the lattice structure and dissociation of active groups. These charges hold oppositely charged ions which are replaceable by ions of similar charges in the soil solution. Along with ion exchange properties, two other important indices of soil chemical environment are pH and Eh. Soil pH is the negative logarithm of hydrogen ion concentration in soil suspensions. It denotes the degree of acidity and alkalinity and influences solubility of chemical substances, availability and uptake of nutrients, and growth and activity of soil microorganisms. Some nutrients become unavailable if the soil pH remains at extremely acid or extremely alkaline conditions. The Eh represents the redox potential which indicates the tendency of a soil to be reduced or oxidized. Redox reactions are very important in soil genesis. There are a variety of other chemical reactions occurring all the time in the soil.

Chemical Properties of Forest Soils

In soil, there are inorganic and organic solids, solutes, liquids, and gases. There are larger and smaller particles, including sand, silt, and clay, and colloids—fine crystalline minerals and amorphous humus. Fine silicate clays and oxides and hydroxides of iron and aluminum, lime, gypsum, and phosphates are there along with hundreds of many other compounds, and nutrient ions. These materials are variably active and reactive; some are almost inert such as the sand grains, and some undergo continuous dynamic reactions such as the colloidal and charged clay particles. Insoluble materials are made soluble, and soluble materials are insolubilized by diverse chemical and biochemical reactions. Important indices of the chemical behavior of all soils, including forest soils, are pH, cation-exchange capacity (CEC), anion-exchange capacity (AEC), base saturation (BS) percentage, exchangeable sodium percentage (ESP), electrical conductivity, and redox potential. These indices characterize the forest soils and affect the growth and distribution of forest tree species.

Phosphate sorption in some representative soils of Bangladesh

An experiment was conducted to observe the phosphate sorption potential of some soils of Bangladesh. Three soil series of calcareous origin, namely Sara (Aquic Eutrochrept), Gopalpur (Aquic Eutrochrept) and Ishurdi (Aeric Haplaquept), and two soil series of non-calcareous origin, namely Tejgaon (Rhodic Paleustult) and Ghatail (Aeric Haplaquept), were selected. The soils were equilibrated with dilute solution of calcium chloride containing graded concentrations of phosphate (0, 1, 2, 5, 10, 25 and 50 μg P mL−1), and the amount of phosphate sorbed or desorbed was determined. Although all the soils showed potential for sorbing phosphate from applied phosphorus, their ability to sorb phosphorus differed. Increasing rates of phosphate application increased the amount of P sorption but reduced phosphate sorption percentage in all soils except Tejgaon. Phosphate was sorbed by the soils in the order: Tejgaon > Ghatail > Ishurdi > Gopalpur > Sara at 50 μg P mL−1 application. Soils possessing higher amounts of free iron oxide and clay sorbed more phosphate from applied phosphorus.

Chemical Soil Degradation

Estimates in 1991 suggest that about 240 M ha land is chemically degraded. Nutrient depletion has affected 136 M ha, salinization damaged 77 M ha, and acidification degraded 6 M ha. Another 11 M ha is affected by soil pollution. Agricultural mismanagement (58 %) and deforestation (28 %) are the main causes of chemical degradation of soil. Nutrient depletion is the most prevalent in Africa (65 M ha) and South America (68 M ha), while salinization is the major chemical degradation in Asia (53 M ha). Nutrient depletion has caused serious nutrient imbalances in soils under low-input agriculture in marginal lands. In a study, all the African agricultural soils exhibited negative NPK balances. Nutrient depletion is caused by leaching, residue harvest and burning, erosion, and crop removal. Salinization occurs naturally by pedogenic processes in different climatic conditions, but human-induced salinization has compelled to abandon many soils, which were productive earlier. The principal cause of human-induced salinization is inappropriate irrigation system in arid and semiarid regions. Leaching of salts by extra irrigation and growing salt-tolerant crops are the strategies for salty soil management. Soils are acidified by acid rains, base leaching, and by the use of acidifying fertilizers. Liming is an ancient method of reclaiming acid soils. Growing crops suitable for the current soil pH may be profitable in low to medium acid soils.

Soil as a Part of the Lithosphere

The lithosphere is the upper part of the earth. It includes the crust and the solid portion of the mantle. Lithosphere interacts with atmosphere, hydrosphere, and biosphere and produces the pedosphere (the soil with its biotic and abiotic components). The lithosphere contains rocks, minerals, and soils. It is made up with more than 100 chemical elements, but most of them are rare. Only eight elements—oxygen (O), silicon (Si), aluminum (Al), iron (Fe), calcium (Ca), sodium (Na), potassium (K), and magnesium (Mg)—constitute more than 99 % of its volume. In the earth’s crust, these elements generally form crystalline solid compounds of definite chemical composition which are known as minerals. Chemically, minerals may be sulfides, sulfosalts, oxides and hydroxides, halides, carbonates, nitrates, borates, sulfates, phosphates, and silicates. Most rock-forming minerals are, however, aluminosilicates of Ca, Mg, Na, and K because these elements are most abundant. Minerals are aggregated into rocks. Rocks may be igneous, sedimentary, and metamorphic. Igneous rocks are formed by solidification of magma or lava, sedimentary rocks are formed by lithification of sediments or by precipitation from solution and consolidation of remnants of plants and animals, and metamorphic rocks are formed from preexisting rocks by the change temperature and pressure in the solid state. By the action of natural forces over geological time, rocks and minerals are disintegrated and decomposed into new minerals and new compounds such as salts, acids, bases, and soluble substances. The processes are collectively known as weathering. However, the effects of rocks and minerals on mature soils are usually temporary. Their effects are profound in young and immature soils. Eventually, similar soils may develop from dissimilar rocks depending on other soil-forming factors.

Heavy metal pollution of soil from industrial and municipal wastes in Chittagong, Bangladesh

Heavy metal pollution was assessed in soils collected from 0–15, 15–30 and 30–45-cm depths of three industrial (FMC, PMC and CMC), and two municipal (BSD and MLF) waste disposal sites around Chittagong city in 2008. Soils were analysed for pH, organic carbon, total nitrogen, available P, exchangeable Ca, Mg, K and Na, and total Cd, Pb, Cu, Mn and Zn. The pH, organic C, total N, available P, total Cd, Pb, Cu and Mn, and contamination indices for Cd and Pb varied significantly among sites. Mean Cd, Pb, Cu, Mn and Zn were in the range 0.5–1.9, 54–86, 25–50, 261–624 and 204–330 mg kg−1, respectively. Contamination indinces for Cd, Pb, Zn and Cu were estimated by comparison with respective threshold values. Contamination indices showed that the sites MLF and FMC had low Cu contamination. Other sites were not contaminated with thisheavy metal. All sites except PMC were highly contaminated with Cd, FMC was moderately contaminated and the others had low Pb contamination. FMC was highly contaminated, but the others were moderately contaminated with Zn. The integrated contamination index revealed that PMC had low contamination and the other sites were highly contaminated with heavy metals.

Soil Water, Irrigation, and Drainage

Water is a precious natural resource, and freshwater is scarce too. So, water—soil water or irrigation water—need cautious management. In soil, water is held in pores and on particles in various forms and under different forces. All of it is not available to plants. The amount of water held between field capacity (FC) and permanent wilting point (PWP) is the amount of plant’s available water in soil. The FC and PWP correspond to −10 and −1,500 kPa soil water potential, respectively. Soil water potential is the amount of free energy that soil water possesses, and it is the driving force of water in soil. Water moves to the direction of the gradient of soil water potential, and the rate of movement is proportional to the potential difference (∆ψ) between the two points and the hydraulic conductivity of the soil. Water moves along soil-root-stem-leaf-air pathway because there is a water potential gradient along the soil–plant–atmosphere continuum (SPAC). Water is needed by plants for many physiological functions. Their growth is hampered if adequate water is not available. Yields of many crops are reduced significantly due to water stress. Therefore, irrigation should be applied well ahead of developing water stress and at growth stages when water is urgently needed. Sometimes, over irrigation adversely affects crop yield. Excess water, applied or natural, must be removed by artificial drainage for growing most crops.