TOPIC 3: SOIL
Definitions of Soil
– Soil is the upper layer of earth in which plants grow, a black or dark brown material typically consisting of a mixture of organic remains, clay, and rock particles.
– Soil is the thin upper layer of the earth’s crust which has been weathered from the parent material and decomposed animals and plants. soil support plants growth and animal life.
– Soil is a mixture of organic matter, minerals, gases, liquids, and organisms that together support life.
– Soil is the portion of the earth’s surface consisting of disintegrated rock and humus.
– Soil is the material on the surface of the ground in which plants grow:
– Soil is the upper layer of earth that may be dug or plowed and in which plants grow
– Soil is the top layer of the earth in which plants grow
– Soil is the substance on the surface of the earth in which plants grow.
Pedology is the scientific study of soil, especially its formation, nature, classification and uses.
Pedologist is a person who studies origins, composition and distribution of soils and the materials from which soils are formed.
Soil formation is sometimes called Pedogenesis. The formation of soil is mainly initiated by weathering process. But this type of soil and rate of soil formation depend on a number of interacting factors in a particular environment.
Factors Influencing Soil Formation
Soils are a product of their own environment, as their formation tends to vary from place to place depending on various obtaining factors. There are several factors which influence soil formation these are as follows
- Parent Rock Material
This is one of the Achief factors of soil formation. It determines soil type, color, depth, rate of soil formation, structure, texture, porosity and soil fertility. Parent rock influence soil maturity, therefore hard rocks take a long time to mature while soft rocks take a short time to mature. Shallow and poorly productive
The most variable elements under climate are temperature, precipitation (rainfall) and wind. Temperature affects decomposition of organic matter hence it influences the development of soil profile Rainfall and wind encourage the formation of soil due to their role in the erosion process On the other hand rainfall adds moisture which encourages chemical and physical weathering
- Living Organism
Some plants have nodules with bacteria which add nitrogen into the soil hence improve aeration of soil. Microorganisms are active in the decomposition of the organic matter to form humus on the other hand barrowing of animals and plant roots facilitate the state of both physical and chemical weathering hence lead to the formation of soil easily.
- Relief [Topography]
The role of relief in soil formation is mostly in indirect way. Relief influences climate and vegetation. The most important aspect of topography in soil formation, steep slopes areas soils are shallow due to erosion while on a gentle slopes and low land areas soils are deep due to deposition of materials.
This involves the duration that has been taken in the process of soil formation. Time determines the maturity of soil, when soil formation has taken a long time, soil tends to be mature i.e. they are deep and well developed.
Importance of Soil
Soil is virtual life support to both flora and fauna organism ,because all the organisms depend on the soil as their source of food. Soil is therefore important to both plants and animals life in various ways including the following:
- Animal life support
Soil acts as plant habitat in which animals uses plants as food for their survival.
- Building materials
Soil is used directly in making of bricks, tiles and white wash, The materials are the used in building of houses ,bridges and other structures.
- Source of minerals
Some soils contains minerals which can be extracted for commercial purposes. For example: Titanum is obtained from soil deposit of Kwale near Mombasa in Kenya, Bauxite which is mined in Guinea in West Africa
- Cultural and medicinal values
Some soils are cultural value in some communities e.g red ochre and clay are used for body decorations by Maasai communities and clay mixed with herbals and being used for medicines
- Farming and settlement
Fertile soil influence cultivation of crops .settlement distribution also depends on arable fertile soil where as people tend to dwell in areas with food availability.
- Habitat for organisms
Soil functions as a habitat for organisms such as burrowing rodent, earthworms and termite. These organisms perhaps are significant in the process of soil formation
THE COMPONENTS OF THE SOIL
Soil is made up of the following components; organic matter (humus), inorganic matter (minerals), soil water and soil air.
- ORGANIC MATTER
This forms 5% of the total volume of soil and is made up of plant and animal remains (humus). This forms humus as a result of decomposition of animals and plant remains.
Importance of Humus
- Improving the structure of the soil and its water retaining capacity limits the leaching process and improves the soil acceleration
- Storing and supplying nutrients to the plant like nitrogen, phosphorus, potassium, calcium and lead to high production.
- Humus regulates the temperature of the soil and soil pH
- The living micro-organisms help in decomposition.
- INORGANIC MATTER
This forms 45% of the total volume and is made up of minerals from the parent rock. Minerals constitute several nutrients which are needed by plants.Minerals include silicon, iron, calcium, magnesium, copper, lead, nitrogen, phosphorous, sulpher, manganese, zinc, potassium and vanadium.
- SOIL WATER
Forms 25% of the total volume and it is one of the most important soil components. It
is derived essentially from rainfall especially from infiltration and through flow.
Importance of Water
- It regulates temperature in the soil
- It helps in the solution and transfer of nutrients in the soil
- Too much water in the soil leads to the leaching of mineral nutrients in the soil
- It controls chemical processes like weathering as well as mechanical weathering.
- SOIL AIR
It forms 25% of the total volume. It consists of the soil atmosphere from which plants and soil organisms obtain oxygen for their metabolism and dispose of carbon dioxide and other gases.
The Components of Soil
Soil has both physical and chemical properties whereby the physical properties include soil profile, soil depth, soil colour, soil texture, porosity and structure while chemical properties include soil Ph and cation exchange
- A) PHYSICAL PROPERTIES
This include soil profile, soil depth, soil colour, soil texture, porosity and structure.
- SOIL PROFILE
This is the vertical section from the surface to the parent rock characterized by distinct layers usually of different texture and colors. The ideal soil profile has three horizons that include A, B and C horizons.
A – Horizon
Is the topmost layer and can include organic matter to form humus. Horizon ‘A’ varies in color from place to place for example dark, grey etc. this zone is also called the zone of Elleviation from which materials are washed down ward. It is in this place where leaching process takes place. LEACHING Is the washing down of nutrients in solution from the topmost layer to another layer
B – Horizon
This zone is also known as the zone of accumulation. In this layer the materials washed from ‘A’ horizon are deposited or accumulated.
C – Horizon
Is the partially weathered parent rock from which the soil develops, it is underlined the
D – Horizon
This is the fresh [unweathered] parent rock. D – horizon (Bedrock) It is the un weathered parent rock. it is the parent in sense that it is the sourceof the in organic content of the soil
Soil depth varies from place to place depending on maturity. Maturity is influenced by the nature of the rock as well as duration of the soil forming processes which have been operating. Soil depth is important for agricultural activities. Thus deep soil is important for agricultural activities while shallow soil is not good for cultivation.
- SOIL COLOUR
Soil color is determined by the materials and the mineralogical composition from which the soil is derived and organic matter content. It varies from one place to another. Soil color can be classified and described in terms of:
- a) Dark [black, grey, dark brown etc] and cinnamon
- b) Bright [yellow, orange, red, reddish brown and yellow brown]
- c) Light [white, whitish grey]
- SOIL TEXTURE
This refers to the degree of coarseness of soil (especially soil mineral particles). It can also be referred to as variations in the particle size, caliber or mechanical composition. According to the soil texture, soil can be classified as;
- a) Coarse sand (2 to 0.2mm)
- b) Fine sand
- c) Silt (0.02mm)
- d) Clay (less than 0.002mm) e) Loan soil is a mixture of sand, clay and silt.
NB; measuring of soil texture can be done through the use of finger testing
Importance of Soil Texture
- It influences soil porosity, permeability, structure and retention capacity
- It influences plant growth and root penetration
- It influences the cultivation during agricultural activities
- It influences soil resistance against erosion
- It influences soil fertility
- SOIL POROSITY
These are the total volume of the pores or empty spaces between particles of the soil materials especially in the soil. Soil porosity is mainly influenced by soil texture, organic matter, soil structure, individual undisturbed soil aggregate compounds referred to as peds.
- SOIL STRUCTURE
This refers to the arrangement of soil particles into aggregate compound particles. The aggregation of soil particles produces the peds of different shapes and sizes. When agregation is absent, as in loose sand, soil is described as structureless. Soil structure can be described as weak, moderate or strong.
Importance of Soil Structure
- It determines water retention capacity and aeration
- It is an indicator of soil fertility or suitability for agricultural activities, settlement locations and construction
- Good structure facilitates the activities of the micro organism
It influences the cultivation process
It influences the plant growth by influencing the root penetration and water retention
Therefore it is quite fundamental to note that the best soil is that which influences the water holding and aeration capacities of the soil compounds particles. Individual undistributed soil aggregate referred to as peds.
- SOIL TEMPERATURE
Soil has a certain degree of temperature and this tends to vary from one place to another due to the variation in the climatic condition. In polar latitudes the soils are cold because of small amount of insolation received there. In tropical areas soils are warm due to high intensity of insolation reaching the surface.
Importance of Soil Temperature
- It controls biochemical and chemical processes especially the decomposition of organic matter and plant growth. Thus plant growth and decomposition tend to be fast in warm areas and slow in cold areas, this is due to the fact that growth cells and micro organisms tend to be very active in the warm areas unlike in the cold areas where they tend to be inactive or less active.
- It also determines the existence of micro organisms in certain areas. In extremely hot areas and cold areas may not support the survival of animals and other micro organisms
- It controls the amount of moisture in the soil where there is high evaporation soil moisture is less or the soils are dry
- B) CHEMICAL PROPERTIES
These include soil properties like soil reaction (PH), reaction exchange and leaching.
- SOIL REACTION (SOIL PH)
This is the term used to describe the degree of acidity and alkalinity in the soil and it is related mainly to climate. This degree of acidity and alkalinity is expressed in the PH value which is the measure in terms of hydrogen ions concentration held by the soil colloid. Soil PH scale range from 1 to 14 where ph 7 is neutral, the condition below 7 is acidic while the condition above 7 is alkalinity which means it has more alkalis.
Importance of Soil pH
- It helps in determining the selection of crops and agricultural distribution
- It affects plant growth such that where there is too much acidity there will be poor plant growth. This is because the increase of acidity leads to the increase in leaching with affects soil structure
This is the process in which the nutrients are washed down in solution from the topsoil layer. The pH in leached soils is quite low, that is high acidity. Leaching is very effective in wet conditions like equatorial areas and less effective in dry areas where bases accumulate in the topsoil by the process of calcification characterized by the capillary action.
- CATION EXCHANGE
Cation exchange refers to the process in which the cations of Calcium (Ca), Magnesium (Mg), Potassium (K) and Sodium (Na) replace Hydrogen Ions (other weaker cations) in the soil. It takes place between soil particles and solution as well as soil particles and plant roots.
SIMPLE SOIL CLASSIFICATION
Soil classification refers to the grouping of soil according to specific characteristics, such as properties or factors like climate also soil can be classified according to the age, texture and color. One common classification is that based on texture. According to the soil texture triangle, there are three main texture namely sand, silt and clay. This is based on the size of their particles as discussed earlier. There percentage content of each one of these determines the type of soil according to texture. Note that sandy soil have sand content of over 45% clay soil have above 27% while silt soil have silt content of above 40%.
SOIL TEXTURE TRIANGLE
- Sand: This soil consists mainly of course and fine sand and contain very little among of clay such that it not sticky when wet and is lose when dry, percentage of sand is above 85, that of clay is up to 10 and silt is up to 10. When such soil is rubber, it does not leave any film on the figures.
- Loamy Sand: This consists most of sand but with sufficient clay such that it gives it a slight plastic quality when it is very moist. When it is rubbed between figures it leaves a slight film of fine material, sand particles account for 70% to 90% clay up to 15% and silt up to 30%.
- Sandy Loam: This soil has high percentage of sand between 43% and 85% with clay content of up to 20% and silt up to 50%. It moulds easily when it is sufficiently moist but does not stick easily to the figures.
- Loam: In this soil, sand and silt dominate an average of 40% each while clay account for about 20% on average. It moulds easily when it has sufficient moisture and does stick to the figures to some extent.
- Silt Loam: It has a high percentage of silt of between 50% and 87% sand between 13% and 50% and clay up to 27%. It is moderately plastic and not very sticky it has a smooth soapy feeling due to high content of silt.
- Sand Clay Loam: This has over 45% sand, up to 28% silt and clay between 20% and 35%. It can be a bit sticky because of the clay content but quit porous because of the sand.
- Clay Loam: Sand content between 20% and 54% silt between 15% and 53% clay between 27% and 40%. This one has sticky distinction when moist because of clay.
- Silt Clay Loam: The amount of sand is between 27% and 60%, silt between 40% and 78% and clay between 27% and 40%. The high silt content makes it smooth and has a soapy feeling. It is less sticky than clay loam or silt clay.
- Silt: This have over 80% silt particles, up to 20% sand and less than 12% clay. It is predominantly smooth and has a typical soapy feeling of silt.
- Sandy Clay: Sand between 45% and 65% silt up to 20% and clay between 35% and 55%. In the presence of sufficient moisture this soil is plastic and sticky clay and sand is dominated.
- Clay: The proportion of sand goes up to 45%, while that of silt is up to clay account for above 40%. The soil is sticky when moist has a plastic feel. It can be rolled into threads when moist and can be molded into different shapes. And can retain fingerprint.
- Silt Clay: Sand up to 20% silt between 40% and 60% and clay between 40% and 60%. This soil is composed of almost fine particles throughout. It is smooth and has to some extent the soapy feel of silt but has a degree of stickiness because of the high proportion of clay
Clay Soil Sandy Soil Loamy Soil
Soil erosion is the wearing away, detachment and removal of soil material from one place to another place through the agents like water, wind, ice etc
Agents of Soil Erosion
- Water. This is the most important agent of soil erosion Erosion by water involves:
- a) Splash erosion caused by rain drops
- b) Sheet erosion which involves the removal of the maximum cover of soil by surface water
- c) Sill erosion which leads to the formation of small channels called sills on the surface
- d) Gully erosion that leads to the formation of deep troughs called gullies due to severe under cutting River erosion that takes place in the specific channels called river valleys
- Wind. This is another agent of soil erosion. It takes place in arid and semi-arid areas or where the soil is loose
- Ice. It is also another agent of soil erosion. It takes place in cold areas where there is the formation of ice
- Gravity.This leads to the gradual movement of weathered material down the slope without involving transporting agents.
Types of Soil Erosion
- Normal geological erosion. It is the wide spread type of erosion that occurs wherever there is a natural flow of energy and matter on the earth’s surface without man’s influence. It is normally very slow and so infectious to the soil cover of the world.
- Accelerated soil erosion. Is the type of erosion associated with man’s activities [man included] it is spectacular in nature therefore it has attracted man’s attention.
Factors Affecting / Accelerating Soil Erosion
- a) Natural Factors
- Climate. Where there is heavy rainfall erosion tends to be severe while where there is low rainfall erosion is low
- Topography. On steep slopes soil erosion can be fast while on gentle slopes the rate of erosion tends to be low
- Nature of soil. This depends on the characteristics or features like texture, structure, permeability e.tc. Unstable soils with coarse textures are prone to severe erosion than fine textured stable soils
- Vegetation cover Where there is dense vegetation soil erosion is less or low rate unlike where there is scanty or no vegetation cover.
- b) Human Factors
- Good management of the soil Which involves the way human beings use soil wisely and skillfully and undertaking conservation measures to reduce or mitigate erosion through afforestation.
- Poor management. That involves unwise use of soil through over cultivation, over grazing and deforestation.
- The increase in pollution. This leads to the over exploitation of resources especially minerals, forests and over cultivation.
Effects of Soil Erosion
Soil erosion is a serious challenge which has many effects on social and economic activities. Some of these effects are explained below:
- Loss of productive soil. When the to productive soil is eroded it is lost forever only the unproductive stony soil is left behind the consequently is the lowering of agriculture productivity of land.
- Depict of vegetation cover. The eroded land cannot hold the plants including crops are washed away or are buried in the deposit soil way from their original location.
- Destruction of structures. when erosion takes place bridges can being taken away from their original areas . other structures like road ,railways, as well as buildings can collapse after erosion of soils.
- Pollution to environment. Eroded soil that is carried into rivers lake s and oceans may contain chemical pollutants collected by water from farm and dumping grounds, hence resulting to loss of aquatic organisms.
- Provides sand for building and construction activities. sand which is eroded from steep slope is deposited on river bed from where it is scoped when the water flows in the river has diminished or even stopped. The sand is then used for construction purpose this is common in Mpiji river at Bunju Dar es salaam and most parts of kisarawe II in coastal region.
Population Growth and the Rate of Soil Erosion on Quality of Life
As discussed above we can see that soil erosion can affect the quality of life of the people positively and negatively.
1.When the region is severely affected by the soil erosion, where crop production is impeded, when useful soils are carried away, the region experiences shortage of food. This causes famine and malnutrition. With inadequate nutrition child mortality rate goes up and population growth is impeded.
- When the foundation of existing buildings and roads are eroded. Accessibility to areas is made difficult. Such areas are isolated in terms of social services such as hospitals and education. The general health and welfare of the people become poor leading to increase in mortality and lowering of population.
- When life becomes unbearable in the rural areas because of severe soil erosion, able-bodied persons especially men migrate to urban areas to other better areas in search of employment.This reduce population in the affected areas as well as the required man power to develop the areas.
Effect of Soil Erosion
TECHNIQUES FOR SOIL CONSERVATION
When using the land it must at the same time be properly maintained by apply proper farming techniques to reduce erosion and then to improve it productive capacity. Therefore a number of ways of doing this for example:
- Crop Rotations
This practice offers protection to the land from soil erosion and good chance to cover its original fertility crop rotation makes it possible to have the land occupied with crops most of the year. In addition the loss of crop most of the year. In addition the loss of nutrient elements by leaching is minimized and losses from erosion are greatly reduced Erosion hazard are n important factor in determining the kind and sequence of crops to be grown in a rotation of a particular piece of land in area where erosion can easily occur due to either slope or soil characteristic, permanent crops such as trees or pasture should be planted rotation will not provide erosion protection on steep slopes.
2. Contour Farming
Contour farming is ploughing, planting and cultivating across the slope following the contours, generally on gently sloping land each contour row can be viewed as a small dam that checks the speed of non-off water and reduce erosion on well drained soil. Contour farming is simple and easier of all the supplemental soil conservation
- Strip Cropping
This is a system in which crops are grown in strips that are arranged across the general slope or at right angle to the path of the prevailing wind. The strip do not necessary have to follow contours.
A terraced is an embankment of earth or stone or other suitable materials or combination of these materials made across the slope for the purpose of controlling run-off. Terrace decrease the length of the slope thus reducing erosion and run-off . There are Two Types of Terraces namely:
- a) Level Terrace: Is a ridge built generally on sandy soil with little or no grade it is designed to hold water in the field until absorbed it adopted in areas where rainfall and soil characteristics are such that there is only slight danger of water accumulating on the soil and breaking the soil surface.
- b) Channel Terraces: Consist that are cut across the slope these channel carry the excess rain water from the fields but at a low speed thus minimizing erosion .they are commonly constructed in regions that receive heavy rainfall.
Planting of Trees and Grasses
Trees and grasses can act as wind breakers and can also control water erosion. In controlling erosion caused by wind trees or grasses may be planted in strips so that soil particles carried by wind may be deposited on or near the grass strip.
Planting Trees and Grasses
Overgrazing can be dangerous as most or all the vegetation can be removed with resultant exposure of the land to erosion rotational grazing with the optimum number of animals in one area can help to maintain the vegetation cover.
Mulching act as a huge sponge which absorbs the water that fall on it and releases it slowly and harmlessly to the underlying soil if there is no protective cover over a wide area erosion may occur rapidly.