Tag: STUDY OF SOIL

CLASSIFICATION OF SOIL MOISTURE OR WATER




soil water is water available for consumption by plants .
Soil water has been classified from a physical and biological point of view as Physical classification of soil water, and biological classification of soil water.

classification of soil water

 Physical classification of soil water

Gravitational water




Gravitational water occupies the larger soil pores (macro pores) and moves down readily under the force of gravity. Water in excess of the field capacity is termed gravitational water.

Gravitational water is of no use to plants because it occupies the larger pores. It reduces aeration in the soil. Thus, its removal from soil is a requisite for optimum plant growth. Soil moisture tension at gravitational state is zero or less than 1/3 atmosphere.

Factors affecting gravitational water




  • Texture: Plays a great role in controlling the rate of movement of gravitational water. The flow of water is proportional to the size of particles. The bigger the particle, the more rapid is the flow or movement. Because of the larger size of pore, water percolates more easily and rapidly in sandy soils than in clay soils.
  • Structure: It also affects gravitational water. In platy structure movement of gravitational water is slow and water stagnates in the soil. Granular and crumby structure helps to improve gravitational water movement. In clay soils having single grain structure, the gravitational water, percolates more slowly. If clay soils form aggregates (granular structure), the movement of gravitational water improves.




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Capillary water

Capillary water is held in the capillary pores (micro pores). Capillary water is retained on the soil particles by surface forces. It is held so strongly that gravity cannot remove it from the soil particles.

The molecules of capillary water are free and mobile and are present in a liquid state. Due to this reason, it evaporates easily at ordinary temperature though it is held firmly by the soil particle; plant roots are able to absorb it. Capillary water is, therefore, known as available water. The capillary water is held between 1/3 and 31 atmosphere pressure.

Factors affecting capillary water:

The amount of capillary water that a soil is able to hold varies considerably. The following factors are responsible for variation in the amount of capillary water.




  • Surface tension: An increase in surface tension increases the amount of capillary water.
  • Soil texture: The finer the texture of a soil, greater is the amount of capillary water holds. This is mainly due to the greater surface area and a greater number of micro pores.
  • Soil structure: Platy structure contains more water than granular structure.
  • Organic matter: The presence of organic matter helps to increase the capillary capacity of a soil. Organic matter itself has a great capillary capacity. Undecomposed organic matter is generally porous having a large surface area, which helps to hold more capillary water. The humus that is formed on decomposition has a great capacity for absorbing and holding water. Hence the presence of organic matter in soil increases the amount of capillary water in soil.




Hygroscopic water:

The water that is held tightly on the surface of soil colloidal particle is known as hygroscopic water. It is essentially non-liquid and moves primarily in the vapour form. Hygroscopic water held so tenaciously (31 to 10000 atmospheres) by soil particles that plants can not absorb it.

Some microorganisms may utilize hygroscopic water. As hygroscopic water is held tenaciously by surface forces its removal from the soil requires a certain amount of energy. Unlike capillary water which evaporates easily at atmospheric temperature, hygroscopic water cannot be separated from the soil unless it is heated.




Factors affecting hygroscopic water:

Hygroscopic water is held on the surface of colloidal particles by the dipole orientation of water molecules. The amount of hygroscopic water varies inversely with the size of soil particles. The smaller the particle, the greater is the amount of hygroscopic water it adsorbs.

Fine-textured soils like clay contain more hygroscopic water than coarse-textured soils. The amount of clay and also its nature influences the amount of hygroscopic water. Clay minerals of the montmoril1onite type with their large surface area adsorb more water than those of the kaolinite type, while illite minerals are intermediate.




BIOLOGICAL CLASSIFICATION OF SOIL WATER

Biological classification is based on the availability of water to plant. in this classification soil water is classified as available water, unavailable water and super available or superfluous water

available soil water

available soil water is that water that is between wilting coefficient and field capacity. it is obtained by subtracting wilting coefficient from the moisture equivalent. the soil moisture between field capacity (1/3 atmosphere) and wilting point (15 atmospheres) is readily available moisture.




what is field capacity?

field capacity is the maximum quantity of water which a soil can retain against the force of gravity. value of field capacity os -1/3 bar.

what is wilting coefficient?

the wilting coefficient is the level of soil moisture at which water becomes unavailable to plant and permanent wilting ensues.

unavailable soil water

the unavailable water includes the whole of the hygroscopic water plus a part of capillary water below the wilting point.

what is wilting point?




wilting point also known as the permanent wilting point is defined as a minimal point of soil moisture the plant requires not to wilt. if moisture decrease to this or any lower point a plant wilts and can no longer recover its turgidity when placed in a saturated atmosphere for 12 hours (wikipedia)

the wilting point depends on plant variety but is usually around 1500 Kpa (15 bars). at this stage, the soil still contains some water but it is difficult for the roots to extract from the soil. nearly 15 bars of tension is required to extract water by plants. at this limit, if no additional water is supplied to the soil, most of the plants die




moreover the moisture content at the wilting point varies with soil texture. fine-textured soil retains a higher amount of water (26%-32% v/v) than coarse-textured soil (10%-15% v/v) at the wilting point.

types of water that are not available to the plants are:

  • hygroscopic water
  • fraction of inner capillary
  • water vapour

super available or superfluous soil water

this is soil water beyond the field capacity stage. it includes gravitational water plus part of the capillary water removed from large interstice. this water is unavailable for the use of plants because it is lost due to deep percolation. the preference for superfluous water in the soil for a longer period is harmful to plant growth.




5 FACTORS INFLUENCING THE DEVELOPMENT OF SOIL PROFILE




The nature of the soil profile may be such that it is fully developed or partially developed implying that soils may be deep or skeletal or soils of medium depth. 

The nature of the soil profile may be such that it is fully developed or partially developed implying that soils may be deep or skeletal or soils of medium depth.

Soil profile development is influenced by a number of factors namely:

Nature of the parent rock

The parent material is the nature of rock upon which weathering and other soil-forming processes operate to create to create soil.




In the first place, the parent material provides the basis for soil profile development. It influences soil profile development in the following ways;

  • Hard or resistant rocks lead to the development of thin soils i.e. with poorly developed profiles. On the other hand softer rocks are easily weathered and acted upon by other soil forming processes leading to the formation of deep soils with a well developed profile.




Nature of the parent rock The parent material is the nature of rock upon which weathering and other soil forming processes operate to create to create soil. In the first place the parent material provides the basis for soil profile development. It influences soil profile development in the following ways;
  • The rocks with lines of weakness or joints have facilitated weathering and other soil forming processes leading to the formation of fairly deep soils. Such soils normally lead to a well-developed soil profile.
  • Young parent material has led to poorly developed soil or poor soil profile while older rocks have had enough time to be weathered and to develop into well developed soil profiles.
  • Permeable or porous rocks have enabled the easy infiltration of agents of weathering resulting into deep weathering and consequently deep soils with a well developed profile unlike impervious rocks.




Climate

Rainfall and temperature determine the nature and rate of weathering and soil-forming processes.

Climate also determines the growth of plants and animals that contribute to the soil profile through weathering and through the addition of humus.

Therefore different climatic conditions influence soil profile developments differently.

In areas where the climate enhances weathering and other soil forming processes there is a well-developed soil profile.  




Climate Rainfall and temperature determine the nature and rate of weathering and soil forming processes. Climate also determines the growth of plants and animals that contribute to the soil profile through weathering and through the addition of humus. Therefore different climatic conditions influence soil profile developments differently. In areas where the climate enhances weathering and other soil forming processes there is a well-developed soil profile.  

Living organisms

Vegetation provides the needed organic matter for the soils therefore a well-vegetated area has a better-developed soil profile.

In addition, animals influence the mechanical breakdown of rocks, therefore, contributing to soil profile development.

Man’s activities such as cultivation, mining, construction tend to physically weather rocks thereby contributing to the development of soil profile therefore areas of abundant biodiversity have a well-developed soil profile as compared to those with limited biodiversity  




Topography/Relief

The nature or shape of the earth’s surface influences soil profile development.

Highly or steeply sloping areas tend to have less developed soil profiles unlike areas of gentle slopes.

This is because the rate of erosion is greater on the steep slopes and this removes the topsoil resulting in shallow or skeletal soils.

On the other hand, in the gently sloping lands and generally flatlands, soil profile tends to be more developed i.e. there are deep soils.   




Time

It takes time for the soil profile to be fully developed.

A typical or well-developed profile of soil must have undergone adequate time, therefore the longer the time to which the rocks are exposed to weathering and other soil-forming processes, the more the developed profile.

Young rocks normally yield skeletal soils i.e. with a poorly developed profile

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THE 4 HORIZONS OF SOIL PROFILE




Soil profile refers to the vertical arrangement of the various soil layers from the top layer down to the parent rock or bottom layer.

It is a vertical section through the soil horizons extending into the parent material or the bedrock.

It describes the sections downwards through the soil which comprises differing characteristics in terms of texture, color, mineral composition, the ratio of the combination of organic and inorganic matter, hardness, and rate of weathering.




The different layers are referred to as horizons.

A soil horizon is a well-defined layer within the soil profile parallel to the local round surface. There are four main horizons namely:

A horizon, B horizon, C horizon, and D horizon. Each horizon has different physical and chemical properties, which result from various soil-forming processes such as weathering, the introduction of humus, and the movement of minerals.

O-HORIZON

This is the topmost/ surface layer of the soil comprising organic matter.




The constituents of this layer include un-decomposed litter, decomposing organic matter, and humus.  

A-HORIZON

This is also known as the topsoil and it is rich in organic matter which organic matter accounts for dark colour.

Leaching and Eluviation may at times impoverish the topsoil.  




B-HORIZON

This is known as subsoil. Nutrients removed from the A horizon through leaching and Eluviation accumulate or are deposited in this horizon.

The process of plant nutrients precipitating or accumulating in this horizon is known as illuviation.

This horizon may also be characterized by hardpans due to the accumulation of large quantities of clay and other nutrients.  

C-HORIZON




This consists of partially weathered rock, this is because weathering and other soil-forming processes may not effectively operate at this depth.

D-HORIZON

This consists of solid parent rock or unweathered rock or fresh parent material. It is also known as the bedrock.

It has no soil particles but has potential for future soil formation




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10 PRINCIPLES OF SOIL BIOLOGICAL FERTILITY




Biological fertility refers to the activities of soil organisms that improve plant growth. 

The biological fertility of soil provides us with great opportunities for land management and monitoring because of its dynamic nature.

It is thought the biological state of soils may be able to provide early warning of land degradation, therefore enabling us to employ more sustainable land management practices.




The biological components of soil perform a number of important processes, soil biological fertility :

  • Soil organisms are most abundant in the surface layers of soil,
  • Soil organic matter is necessary for nutrient cycling and soil aggregation,
  • Maximum soil biological diversity depends on the diversity of organic matter and habitats,




  • Nitrogen fixing bacteria form specific associations with legumes under specific conditions ,
  • Nitrogen is released during organic matter breakdown, either into soil or into the soil microbial biomass,
  • Arbuscular mycorrhizal fungi can increase phosphate uptake into plants in P-deficient soils,
  • Soil amendments can alter the physical and chemical environment of soil organisms,
  • Some crop rotations and tillage practices decrease the suitability of soil for plant pathogens,
  • Production systems based on soil biological fertility can be profitable,
  • Soil biological processes develop slowly, and the time required will differ for different soils, environments and land management practices.




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4 MAIN TYPES OF SOIL STRUCTURE




Soil structure is a measure of the arrangement of these soil particles and the spaces between them. Soil structure is somewhat dependent on soil texture.

There are two main types of structureless or non-structured soil:

  • Single grain – like sands, and
  • Massive – like compacted clays.

There are four main types of soil structure:

Crumb structure

which has small rounded aggregates of soil particles loosely adjoining other aggregates. This soil is therefore porous and permeable, yet retains moisture. It is the most ideal soil structure.




Prismatic structure

which forms aggregates in columns with flat tops and separated by deep cracks. Aggregates usually form larger units called clods. Permeability is variable – better around the deep cracks and poorer inside columns.

This type of soil structure is typically found in soils that have a high percentage of clay particles and a moderate percentage of silt particles. The clay particles, which are the smallest particles, tend to stick together and form long, thin, vertical structures.

These structures can be of varying thickness and length, and they can be oriented in different directions. This type of soil structure can be considered desirable for plant growth because it allows for good water infiltration and storage.

However, it can be difficult to till and maintain, and it can be prone to erosion. Additionally, when the soil is wet it can be hard to work with.




Blocky structure

aggregates are blocky and soil is moderately permeable. This type of soil structure is typically found in well-drained soils that have a high percentage of clay particles and a moderate percentage of silt and sand particles.

The clay particles, which are smaller than the sand and silt particles, adhere to each other and form large blocks or aggregates.

This type of soil structure can be considered desirable for plant growth because the large blocks or aggregates allow for good aeration and water infiltration. However, blocky soil can be difficult to till and maintain, and it can be prone to erosion. Additionally, when the soil is wet it may be hard to work with.

Platey structure

Platy structure refers to the way in which soil particles are arranged in flat, plate-like structures, giving the soil a layered texture.

This type of soil structure is typically found in soils that have a high percentage of clay particles and a moderate percentage of silt particles.

The clay particles, which are the smallest particles, tend to stick together and form thin, flat layers.

These layers can be oriented horizontally or at an angle, and they can be of varying thickness.

This type of soil structure can be considered desirable for plant growth because it allows for good water infiltration and storage. However, it can be difficult to till and maintain, and it can be prone to erosion. Additionally, when the soil is wet it can be hard to work with.

WHAT IS SOIL PHYSICAL FERTILITY?




Physical properties and processes of soil affect soil fertility by altering water movement through soil, root penetration of soil and waterlogging. Important physical properties that affect fertility include soil structure and texture.

The structure is the amount of aggregation and pores in soil and texture is the proportion of clay and sand particles in the soil.

Both affect soil fertility by affecting water movement through soil, root penetration, and waterlogging.




Erosion is an important physical process that decreases soil fertility. When soil structure and texture are unfavorable for water movement through soil water erosion and waterlogging may be increased.

Soil salinity is a chemical property but can affect soil physical fertility by decreasing the movement of water through the soil.




Physical soil characteristics important to soil physical fertility include:

  • soil structure,
  • soil texture,
  • water repellence.

Physical processes related to soil physical fertility include:

  • aggregation,
  • water infiltration,
  • waterlogging,
  • soil erosion.




8 ROLES OF SOIL ORGANISM IN BOOSTING SOIL FERTILITY




Soils are alive! A variety of soil organisms live in the soil.

These include bacteria, fungi, microarthropods, nematodes, earthworms and insects.

These organisms live on soil organic matter or other soil organisms and perform a number of vital processes in soil.

Other organisms are involved in the transformation of inorganic molecules. Very few soil organisms are pests.




The role of soil organisms in soil fertility may involve the following:

  • helping soil to form from original parent rock material,
  • contributing to the aggregation of soil particles,
  • enhancing cycling of nutrients,
  • transforming nutrients from one form to another,
  • assisting plants to obtain nutrients from soil,
  • degrading toxic substances in soil,
  • causing disease in plants,
  • minimizing disease in plants,
  • assisting or hindering water penetration into soil




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