Physical-Chemical Properties of the Soil, Biology tutorial

Physical Properties of Soil Color:

Soils display the wide range of colors like red, yellow and even green. Few soils are almost black, white, bright and dull grays. Soils usually change with depth through the different layers inside a soil profile. Soil colors have little effects on behavior and use of soils. Though, they do give valuable dues to nature of other soil properties and conditions.

The standard system for accurate color description has been developed using munsell color charts. In this system, the small piece of soil is compared to standard color chips in a soil color book. Each color chip is explained by the three components of color i.e. the hue (in soils, generally redness or yellowness), the chroma (intensity or brightness) and gray (lightness or darkness). Three main factors have the greatest influence on the color of soil:

  • organic matter content
  • water content
  • Presence and oxidation states of iron and manageress oxides.

Soil Texture:

Soil texture explains size of the soil particles three broad groups of textural classes are recognized sandy soils, clayey soils. Inside each group, particular textural class names convey the idea of size distribution of particles and signify general native of soil physical properties.

Soil Structure:

Soil structure explains manner in which soil particles are aggregated, in other words, this property states the native of system of pores and channels in the soil. Term structure associates to arrangement of primary soil particles in grouping called aggregates or peds. Pattern of pores and peds stated by soil structure really influences heat transfer, water movement, aeration, and porosity in soils. Soil structure is characterized in terms of shape (or type), size, and distinctness (or grade) of peds. Four principal shapes of soil structure are spheroidal, platy, prim like, and block like.

i) Spheroidal Soil Structure:

Granular, generally separated from each other in the loosely packed arrangement. They usually range from less than 1 to greater than 10mm in diameter. Granular structures are features of several surface soils, particularity those high in organic matter. They are prominent in grassland soils and soils which have been worked by earthworms.

ii) Plate Like Soil Structure:

It is characterized by thin horizontal peds or plates and may be found in both surface and sub surface horizons. Platy structure may be inherited from soil parent materials, and in few cases compaction of clayey soils by heavy machinery can make plate like soil structure.

iii) Prism Like Soil Structure:

Columnar and prism like structure are characterized by vertically oriented prism or pillar like peds that vary in height among different soils and may have diameter of 150mm or more. They usually take place in subsurface horizons in arid and semiarid regions.

iv) Block Like Soil Structure:

Blocky peds are irregular, approximately cubelike and range from approx 5 to 50mm across. Individual blocks are not shaped separately, but are molded by shapes of the surrounding blocks. Blocks like soil structures encourage good drainage, aeration and root penetration.

Soil Density:

Soil particle density (Dp) is stated as mass per unit volume of soil solides. Particle density is really the same as specific gravity of the solid substance. Chemical composition and crystal structures of the mineral determines the particle density. Dp is not affected by pore space, and thus is not related to particle size or to arrangement of particles. The second significant mass measurement of soils is bulk density (Db) that is stated as mass of the unit volume of dry soil. This volume comprises both solids and pores.

Soil Pore Sizes:

One of the main reasons for measuring soil bulk density is that this value can be used to calculate the pore space. For soils with the same particle density the lower the density, the higher the percent pore space (total porosity).

Percentage pore space = 100 - [(Db x 100)]/Dp

Soil pores take place in the wide variety of sizes and shapes which largely determine what role pore can play in soil. Pores can be grouped by size in macropores, mesopores and miropores. Continuous cropping of soils initially high in organic matter, frequently results in the reduction of macropore spaces.

Chemical properties of soil:

Chemical Nature of Soils:

Soil is the entity which has its chemical nature which arises from parent materials from which it is made. Rocks are the main soil forming materials. Every kind of rock has its composition which ultimately forms chemical content of any soil which is formed after weathering processes. Approx 92 chemical elements are known to exist in earth's crust. When one considers number of possible combinations of large number of these elements, it is not surprising that 2000 minerals have been recognized. Though, a few of these elements dominate and of real significance. These are hydrogen (H), carbon (C), oxygen (O), nitrogen (N), phosphorus (P), silicon (Si) and alkali and alkaline earth metals. In addition, approximately 98% of the crust of the earth is made up of 8 chemical elements. O-46%, Si-27.7%, Al-8.1%, Fe-5%, Ca-3.6%, Na-2.8%, K-2.6%, and Mg=2.1%. Silicon and oxygen compose approx 75% of all these elements.

Mineral Matrix (Inorganic/Organic Soils):

Soil is predominantly minerals or inorganic in composition. Even in the surface layer organic matter contents of mineral soils are moderately low, usually ranging from 1-10%. On the contrary, soils in swamps, bogs and marshes usually have 80-90% organic matter. These organic soils when drained and cleaned are most productive, particularly for high value crops like fresh market vegetables. Minerals are very variable in size-some are large as smaller rock fragments, other like colloidal clay particles. Quartz and some other minerals have persisted with little change in composition. Other minerals like the silicate clays and iron oxides have been formed by weathering of less resistant minerals.

Soil Reaction:

The degree of soil acidity or alkalinity, generally stated as soil pH, is the master variable which affects the wide range of soil properties- biological, chemical, and, indirectly, even physical. This chemical variable really influences availability for root uptake of several elements comprising both nutrients and toxins. Activity of soil microorganisms is also affected. Mix of plant species which dominate the landscape under natural conditions frequently reflects pH of the soil.

Soil pH:

pH is the measure of active hydrogen ion (H+) concentration. It demonstrates acidity or alkalinity of a soil, and also known as soil reaction. pH scale ranges from 0-14, that value below seven 7 acidic and above 7 is alkaline. pH 7 (neutral) that is H+ and OH- are equal (10-7moles/liter). pH of 4.0 is ten (10) times more acidic than pH of 5.0. Significant effect pH in soil is on ion solubility that in turn affects microbial and plant growth. The pH range of 6.0-6.8 is perfect for most crops as it coincides with optimum solubility of the most significant plant nutrients. Few minor elements like iron and most heavy metals are more soluble at lower pH that is at acidic conditions. This makes pH management significant in controlling movement of heavy metals (and potential ground water contamination) in soil. In the acid soil, H+ and Al+ are dominant exchangeable cations. Al+ is soluble under acid conditions, and its reactivity with water hydrolysis makes H+.

Cation Exchange Capacity:

Soil is a store house of plant nutrients that exist in solution as positively charged cations and negatively charged anions. Both clay and organic component have the net negative charge, though, clay particles are known to have excess negative charge at edges and surfaces of their crystals. These charged sites attract cations that would otherwise be moving in solution and hold them to crystals surface which depends upon cation and kind of clay involved. In soil, attracted nutrient cations are generally not enduringly fixed to particle, they bind loosely to negatively charged site until they are absorbed by plant roots or exchanged for other nutrient cations in soil solution. This replacement is known as cation adsorption or more generally called as cation exchange. Ability of the soil to hold cations in willingly exchangeable positions is considered good for plant nutrition.

Some plants nutrients and metal exist as positively charged ions or cations in soil environment. Among the cations found in soils are hydrogen (H+), Aluminum (Mg+) and Potassium (K+), Calcium (Ca++), Sodium (Na+) and Ammonium (NH4+). Many heavy metals also exist as cations in soil environment. Clay and organic matter particles are mainly negatively charged (anions) and have capability to hold cations from being leached or washed away. Adsorbed cations are in the rapid reversible process known as cation exchange (CE).

During cation exchange, hydrogen ions are released from root hairs. These root hairs in turn exchange with nutrients cations adsorbed on the surfaces of the clay particles forcing them in solution where they can be incorporated by plants. The uptake of these nutrient cations relies not only on solubility of the nutrient cations and their being in the exchangeable position but also on their being in close proximity of root surfaces. Procedure of diffusion can be attained. Action site of the cation exchange is the interface where soil colloid meets root surfaces in the bath of soil solution.

The moment a plant rootlet comes in contact with the root surface holding adsorbed cation, the root provides hydrogen ions in exchange for the nutrient cations. Few cations then migrate from the swarm of cations adsorbed unto the clay surface and move from the root hairs. Places of these cations are then taken by hydrogenions released from root hairs. Nutrient cations are then translocated in the water transporting through xylem tissue of the plant to the stem and leaves for plant nutrition therefore, this is why the cation exchange capacity quantifies the skill of the soil to give nutrient reserve for plant uptake. CE is the significant mechanism in soils for retaining and supplying plant nutrients, and for absorbing contaminants. CE plays the significant role in waste water treatment in soils. Indeed, adsorption of cations and their exchange is perhaps second only to photosynthesis, as it is primary mechanism in plant nutrition.

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