Physical & Chemical Properties of Soil

Physical & Chemical Properties of Soil

Soil environment is studied under physical, chemical and biological environments. Thorough understanding of soil properties is essential for its management for higher productivity.

Physical Properties of Soil

The physical properties of soil with respect to crop production, are texture, structure, bulk density, porosity, consistency, temperature, colour and resistivity. Soil physical properties regulate the movement, retention and availability of water and nutrients to plants, and determine the flow of heat and air into, within and out of the soil. Water movement through a root zone is hampered by any changes to soil physical properties including changes in texture, compaction, porosity or organic matter content. Physical properties of soil vary through the depth of a soil profile, i.e., through soil horizons.

• Soil texture is determined by the relative proportion of soil separates sand, silt, and clay. Soil texture and structure (the arrangement of particles) both have a direct impact on soil porosity. In general, finer-textured clayey soils and soils with larger organic matter content will have greater total porosity than coarser-textured sandy soils or soils with less organic matter. The larger particles affect soil packing, porosity, bulk density, nutrient status, and root growth.
• Bulk density is the dry soil weight per volume of soil and is a good measure of porosity and compaction. Generally, uncompacted soils containing 50% pore space with a bulk density of 1.1-1.6 g cm³ are ideal for agricultural soils. Soil minerals have an average particle density of ~ 2.65 g cm³. Pore space is included in the volume of a soil but the air content does not weigh much, thus as porosity increases, the dry weight decreases for a given volume, resulting in a lower bulk density. Compacted soils have large bulk densities (1.5–1.8 g cm³), few macropores, and tend to restrict root growth, water and air flow. Texture and organic matter both affect bulk density as well. Sandy soils have a larger bulk density than clayey soils because of smaller total porosity. Organic matter weighs very little, takes up a large volume, and so creates lower bulk density. Soils with a smaller bulk density contain more plant available water than soils with a large bulk density, but they tend to drain quickly and become droughty.
• Soil porosity consists of the void part of the soil volume and is occupied by gases or water. It is a property that reflects the number and size of pores in the soil. Porosity results from the arrangement of soil particles into aggregates with inter- and intra-aggregate pores, as well as from root growth and faunal activity.
• Soil consistency is the ability of soil materials to stick together.
• Soil temperature is one of the most important properties that affect crop growth. The major source of heat is sun and specific heat of soil is 0.2 Cal/g. The specific heat of soil increases with increase in moisture content. Soil temperature has considerable influence on several aspects of growth, especially in early stages of crop. Germination entirely depends on soil temperature as too high or too low temperature kills the seed. Mineralisation, nitrogen fixation in legumes and other microbial activity are all temperature dependent.
• Soil colour, besides being useful for classifying soils, is indirectly helpful in indicating many properties of soil. Like, dark brown soils indicate high organic matter and fertility, red colour shows good aeration, white colour accumulation of salts etc.
• Resistivity refers to the resistance to conduction of electric currents and affects the rate of corrosion of metal and concrete structures which are buried in soil.

Chemical Properties of Soil

The soil’s chemical properties are inherited from the processes of soil formation, during weathering and transport of the parent material from which the soil has formed. Thus, the chemical nature of the rocks and minerals and the intensity of the weathering processes are fundamental in determining the chemical properties of the soil.

Soil chemical properties including pH, electrical conductivity (EC), cation exchange capacity (CEC), exchangeable Ca, Mg and K, exchangeable Al and hydrogen (H), C/N ratio of added amendment, and organic matter (OM) content can impact soil N supply by influencing the activity of microorganisms and the concentrations of NH4+ and NO3− in the soil solution.

Many soil microorganisms function optimally in the soil micro environment with a pH ranging from 6 to 7 since most soil nutrients are available in this range, however it depends on microbial group as pH 5.5–6.5 is ideal for fungi.

The nutrient transformation and its availability in soils depends on pH, clay minerals, cation and anion exchange capacity.

• The pH of a soil indicates its acidity or alkalinity. pH influences nutrient availability, soil physical condition and plant growth. Soil with <7 pH is acidic whereas, >8.5 pH is alkaline. Optimum pH range for availability of different nutrients:

• Electrical conductivity is reciprocal of the electrical resistance of the extract of the soil which is one cm long and a cross sectional area of 1cm². EC is expressed as dSm at 25⁰C and is used to express the salinity of the soil. Exchangeable sodium percentage is defined as the degree of saturation of the soil exchange complex with sodium.
• Clay minerals are hydrated aluminosilicate secondary minerals with particle size less than 0.002mm in dm. The clay minerals together with organic matter constitute the colloidal fraction of the soil which is the active seat of physical and chemical properties of soil. Most of the clay minerals (<0.0002mm in dm) and humus exhibit colloidal properties.
• CEC is very important property of the soil from plant nutrition point of view. Due to the presence of negative charges cations are absorbed on surface of the micelles (colloids) and are capable of exchanging with those in solution. This process of exchange of cation between solid and liquid phases is called cation exchange. CEC of soil influences the capacity of the soil to hold nutrients such as, Ca, Mg, NH⁴⁺, and the quantity of a nutrient required to change its relative level in soils.
• Soil solution- the solubility of a materials determines its ability to move in soils. Soluble salts are chlorides, sulphates, nitrates and some carbonates and bicarbonates of Na, K, Mg, Ca and NH⁴⁺. When a material dissolved in water, they dissociate into cations and anions. The concentration of soil solution and its pH are changing continuously. Soil solution and clay minerals exchange cations depending on addition or depletion from soil solution. When nutrients are absorbed by plants, cations from clay complex move to the soil solution. The pH of the soil solution varies, when plants absorb cations, root release H⁺ ions in exchange resulting in decrease in soil pH. Similarly, if anions are absorbed, OH^– ions are released into the soil solution increasing the pH.
• Nutrients- Carbon, hydrogen and Oxygen are mainly obtained from water and air. Rest of plant nutrients are obtained from mostly organic matter and mineral matter. Plants nutrients present in minerals and organic matter are in non-available form. Weathering of minerals and mineralisation of organic matter make the nutrients available to plants. The nutrients present in the soil are subjected to physical, chemical and biological changes or transformation. Several substances are produced during this transformation which are not directly connected to plant nutrition. Some may be toxic and others are harmless. This transformation may either release or fix nutrients. Several factors mainly soil pH, organic matter, calcium carbonate content, amount of soil phosphorus and submergence influence the transformation or say availability of micronutrients.

Source of Plant Nutrients in Soil