User:4Yz12/Soil Chemistry

Key Concepts

Soil Structure

Soil structure refers to the manner in which these individual soil particles; are grouped together to form clusters of particles called aggregates.

Formation of Aggregates:

• Aggregates can form under varying conditions and differ from each other in soil horizon and structure
• Natural aggregates results in what are called peds, whereas artificial aggregates are called clods.
• Clods are formed due to disturbance of the field by ploughing or digging.
• Microbial activity also influences the formation of aggregates.

Types of Soil structure: The classification of soil structural forms is based largely on shape.

I.) Spheroidal Structure-

Its characteristics are being Sphere-like or rounded in shape. All the axes are approximately of the same dimensions, with curved and irregular faces. These are found commonly in cultivated fields.

a.)Crumb structure being small and are like crumbs of bread due to them being porous

b.)Granular structure is less porous than crumb structure aggregates and are more durable than crumb structure aggregates

II.) Platy Structure-

its characteristics are being mainly horizontally aligned along plant based areas, with thin units being laminar and the thick units of the aggregates are classified as platy. Platy structures are usually found in the surface and sometimes in the lower sub-soils.

Spheroidal structured soil particles

Platy structured soil particle

III.) Block-like Structure-

It is characterized by particles that are arranged around a central point are enclosed by surfaces that may be either flat or somewhat rounded. These types are generally found in subsoil.

a.)Angular blocky- have angular corners present

b.)Sub angular blocky- corners are more rounded than the angular blocky aggregates

IV.) Prism-like Structure-

It is characterized by particles that are longer than they are wide with the vertical axis being greater than horizontal axis. They are commonly found in subsoil horizon of arid and semi-arid region soils. They may be further divided into two sub-groups.

a.)Prismatic- more angular and hexagonal at the top of the aggregate

b.)Columnar- particles that are rounded at the top of the aggregate

Block-like structured soil particles

Prism-like structured soil particles

Minerals

• The mineral components of the soil are derived from the parental rocks or regolith. The minerals present about 90% of the total weight of the soil. Some important elements, which are found in compound state, are O, Fe, Si, Al, N, P, K, Ca, Mg, C, H, etc.
• The formation of primary and secondary minerals can better define what minerals are in the rock composition

Soil Pores

The interactions of the soil's micropores and macropores are important to soil chemistry as they allow for the provision of water and gaseous elements to the soil and the surrounding atmosphere. Macropores help transport molecules and substances into the micropores. Micropores are comprised within the aggregates themselves.

Soil Water

• Water is essential for organisms within the soil profile and it partially fills up the macropores in an ideal soil.
• Leaching of the soil occurs as water carries along with it ions deeper into the lower soil horizons causing the soil to become more Oxidized in other soil horizons.
• Water also will go from a higher water potential to a lower water potential, this can result in capillarity activity and gravitational force occurring with the water duet to Adhesion of the water to the soil surface and Cohesion amongst the water molecules.

Air/Atmosphere

The atmosphere contains three main gases, namely oxygen, carbon dioxide and nitrogen. In the atmosphere, oxygen is 20%, Nitrogen is 79% and CO₂ is 0.15 to 0.65% by volume. CO₂ increases with the increase in the depth of soil due to decomposition of accumulated organic matter and abundance of plant roots. The presence of oxygen in the soil is important in the since that it helps in the process of breaking down of insoluble rocky mass into soluble minerals and organic humification. The atmosphere of the soil can provide ideas about its aeration capabilities. Air in the soil is comprised of gases that are present to those found in the atmosphere but they are not at the same levels. These gases provide microorganisms to proceed with chemical reactions. The accumulation of soluble nutrients in the soil makes it more productive. If the soil is deficient in oxygen, the rates of microbial activities are slowed down and may be eliminated. The important factors, which bring about changes in the soil atmosphere are temperature, atmospheric pressure, wind/aeration and rainfall.

Soil Texture:

Soil Texture triangle

Soil Texture influences the soil chemistry pertaining to the soil's ability to maintain its structure, the restriction of water flow and the contents of the particles in the soil. Soil texture considers all particle types and a soil texture triangle is a chart that can be used to calculate the percentages of each particle type adding up to total 100% for the soil profile. These soil separates differ not only in their sizes but also in their bearing on some of the important factors affecting plant growth such as soil aeration, work ability, movement and availability of water and nutrients.

Sand

Sand particles range in size ( about 0.05mm-2mm).Sand is the most coarse of the particle groups. Sand has the largest pores and soil particles of the particle groups It also drains the most easily. These particles become more involved in chemical reactions when coated with clay

Silt

Silt particles range in size (about 0.002mm-0.5mm).Silt pores are considered a medium in size compared with the other particle groups. Silt has a texture consistency of flour. Silt particles allow water and air to pass readily, yet retain moisture for crop growth. Silty soil contains sufficient quantities of nutrients both organic and inorganic.

Clay

Clay has particles smallest in size ( about <0.002mm) of the particle groups. Clay also has the smallest pores which give it a greater porosity and it does not drain well. Clay has a sticky texture when wet. Some kinds can grow and dissipate or in other words shrink and swell.

Loam

A combination of sand, silt and clay that encompasses soils. It can be named based on the primary particles in the soil composition, ex. sandy loam , clay loam , silt loam, etc.

Biota:

These organisms along with organic matter help comprise the biological system of the soil.

Soil organisms

• Detritivores
• Decomposers
• Protozoans

(SOM) Soil Organic Matter

• Humus
• The chief elements found in humus are carbon, hydrogen, oxygen, Sulphur and nitrogen. The important compound found in humus are carbohydrates, phosphoric acid, some organic acids, resins, urea etc.
• Humus is a dynamic product and is constantly changing because of its oxidation, reduction and hydrolysis. Hence it has got much carbon content and less nitrogen.

Other associated concepts:

• Anion and cation exchange capacity
• Soil pH
• Mineral formation and transformation processes and pedogenesis
• Clay mineralogy
• Sorption and precipitation reactions in soil
• Chemistry of problem soils
• C/N Ratio
• Erosion and Soil degradation