Soil Physics By Ak Saha Pdf
Zareen Zapata
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Soil physics is the study of the solid, liquid and gaseous phases of soils, and their interactions. Soil texture, structure and bulk density reflect how soil mineral and organic particles combine to form the soil matrix and pore spaces. Pore spaces hold gases and water. Understanding soil water retention and soil water movement is crucial in determining water availability for plants and soil organisms, infiltration and drainage, runoff and erosion. Many soil nutrients are transported in the soil as solutes in soil solution. Soil aeration and gas exchange govern CO2 emissions from the soil and O2 availability for plants. Soil thermal properties regulate soil temperature with depth and determine how quickly (or slowly) a soil warms up in the spring. Soil strength is influenced by soil texture and water content, and determines the susceptibility of a soil to slope failure and compaction.
Understanding the impacts of land management on soil physical properties can help us develop alternative practices for managing soils in a variety of ecosystems (forest, agriculture, urban). For example, in forest operations, soils may be compacted on landings, where logs are stored and loaded onto trucks by heavy machinery. But not all soils are equally susceptible to compaction, nor will all soil types require mechanical loosening of the soil prior to tree replanting. Forest practitioners need to consider soil physical properties, such as texture and water content, as part of their assessment of the need for costly rehabilitation measures. Agriculture accounts for 85% of ammonia (NH3) emissions in Canada (Bittman et al., 2017), in large part associated with the application of animal manure to fields. In regions with a large number of dairy and poultry farms, such as the Lower Fraser Valley in British Columbia, farmers are faced with excessive amounts of manure that are often applied to cropped fields. By understanding gas exchange in the soil-plant-atmosphere system, researchers from Agriculture and Agri-Food Canada in Agassiz were able to develop a manure spreader that injects the dairy manure into the soil between corn rows. This type of machinery reduced both air pollution (through reduction of nitrous oxide N2O emissions) and groundwater pollution (through reduction of leaching losses of nitrates) from agricultural fields. In urban environments, the construction of houses commonly involves the removal of vegetation and topsoil from the site, and purposefully compacting the soil to reduce settling. Once construction is finished, a thin layer of topsoil is spread and seeded with grass. But the water holding capacity of this thin layer of soil is insufficient to support live grass in the summer. In the driest parts of Canada, cities such as West Kelowna in British Columbia have introduced bylaws, which require a minimum 30 cm topsoil thickness, aimed at reducing outdoor water consumption by increasing the volume of plant available water held in the soil. These are just a few examples of how soil physics can help us solve a range of soil-related environmental challenges.
The solid phase of a soil system can be comprised of soil particles that are either mineral (i.e., rocks, stones, cobbles, gravel, sand, silt, clay) or organic (i.e., soil organic matter) in nature. Ideally, the solid phase occupies approximately 50% of the soil by volume (Figure 4.1). However, this may vary between 30 and 60% of the total soil volume based on physical conditions as well as management impacts. For example, in a sandy soil, larger mineral particles may occupy a greater volume of soil. Similarly, compaction may increase the volume of solid particles within a specified volume. While the solid mineral content is generally stable over time, the soil organic matter content may change relatively quickly. The soil mineral content consists of primary particles such as crystalline minerals (e.g., quartz, aluminosilicates) and amorphous gels (e.g., oxides, hydroxides and hydrous oxides of iron, aluminum, and manganese). These primary soil particles interact with each other. For example, amorphous gels may coat crystalline particles and form secondary particles or aggregates (as described in Chapter 2: Formation of Soil Structure). Thus, the mineral content is comprised of particles of different shapes, sizes, and chemical composition. Organic materials also contribute to the total solid phase of a soil system, but generally occupy a smaller proportion (3-5%) in mineral soils. Soil organic matter is generally comprised of live organisms, and plant and animal residues in various stages of decomposition. These organic materials can bind other mineral and organic materials together, and form secondary units (called aggregates), which also contribute to the solid phase of a soil system. The composition and surface characteristics of solids dictate the behaviour of soil and determine the interaction of solid phase with liquid (e.g., soil water, plant nutrients) and gaseous (e.g., soil air) phases.
The liquid phase of a soil system consists of soil water and various nutrients, chemicals, and gases dissolved in soil water (sometimes referred to as soil solutes), forming a soil solution. Both the amount of water in soil (water quantity) and the chemical composition (water quality) contribute to the liquid phase of a soil system. Ideally, the liquid phase consists of 25% of the total volume of soil. However, it is more dynamic than the solid phase as it may vary between
The gaseous phase of a soil system consists of soil air, which is a mixture of gases commonly including nitrogen (N2), oxygen (O2), water vapour and carbon dioxide (CO2). Ideally, soil gases comprise about 25% of the total soil volume, but it is highly dynamic in nature. Generally, the pore spaces that are not occupied by the liquid phase (i.e., soil water), will be occupied by the gaseous phase (i.e., soil air). The amount, composition and mobility of gaseous vary with time and position in the soil profile. Soil air also varies in composition from atmospheric air. For example, soil air contains higher amount of carbon dioxide and lower amount of oxygen than atmospheric air.
The solid phase of a soil system is comprised of various primary and secondary particles that are mineral or organic in nature, and occur in various amounts, shapes, sizes, and chemical and mineralogical compositions. For example, some of the particles are coarse enough to be seen with the naked eye, while others are small enough that they can only be seen with a microscope and exhibit colloidal properties.
The mineral particles in soils are divided into size classes. Coarse fragments, >2 mm in size are separated from the fine earth fraction (25 cm in diameter is are named as stones or boulders. These coarse fragments can affect the selection of land management practices (e.g., selection of tillage implements), but they contribute little to basic soil functions such as water retention and the capacity to store and release plant nutrients.
In determining the soil textural class using the Canadian soil textural triangle (Figure 4.2), mark the value of sand content on the bottom axis reading from left to right and then take the clay content and mark the value on the vertical axis reading from bottom to the top. Draw a line from the bottom axis identified point parallel to the vertical axis and draw another line from the vertical axis identified point in parallel to the bottom axis. Mark the point where two lines cross and identify the soil textural class. If the point falls on the line of two soil textural classes, the class with finer particles or high clay content will get the designation (customary). Though the third axis presents the silt content, often it is enough to identify textural class with sand and clay data.
Specific surface area of a soil is the total surface area of soil particles per unit mass of soil or per unit volume of soil particles. It is commonly expressed as square meters per gram of soil (mass) or per cubic centimetre (volume) of soil particles. The specific surface area depends on the size as well as shape of the particles. Smaller sized particles contribute to large surface area per unit mass or volume. Similarly, soil particles with flattened or elongated shapes can expose greater surface area per mass or volume than the soil particles of cubic or spherical shape. Clay particles, in addition to their small size, are generally of plate like shape, and they contribute a large surface area per unit mass or volume of soil. While sand particles can have specific surface area of about 1 m2 g-1, clay particles can have specific surface area of as high as several hundred square meters per gram of soil. Specific surface area of any soil material is a fundamental property and is correlated with other important properties such as cation exchange capacity, water retention, nutrient availability, and mechanical properties including plasticity, cohesion and strength.