GEOGRAPHY Form 3 Topic 3
SOIL
Soil Profile
Define soil profile
Soil profile is the vertical section of the soil from the surface to the parent rock characterized in distinct layers (horizons), usually of different textures and colours. Ideal soil profile has horizons A, B, C.
Soil Profile and its Characteristics
Illustrate soil profile and its characteristics
Physical properties of the soil include soil profile, soil depth, soil colour, soil texture and soil structure
Chemical properties of the soil includes soil P.H, carbon exchange and leaching.
- A – Horizon – Is the top most layer which include organic matter to form Ao. This horizon varies in colour from place to place example dark to grey. It is also called the zone of eluviation from which the materials are washed down through leaching.
- B – Horizon – Is also called the zone of accumulation or illuviation. In this layer materials washed from the A – horizon are deposited or accumulated. Accumulation of washed down materials lead to formation of another layer called Hard Pan.Horizon A and B are also referred to as Solum (soil) such that A – horizon is the top soil and B – horizon is the sub soil
- C – Horizon is the partially weathered parent rock from which the soil develops. It is underlain by D – Horizon which is a fresh non – weathered parent rock.
Soil According to Textural Groups
Classify soil according to textural groups
SOIL DEPTH
Soil depth is the variation of soil from place to place due to materials influenced by the nature of the rock as well as duration of the soil forming processes. Soil is shallow especially in steep slopes due to erosion and in areas where the underlying rock is hard. Other places have deep soil due to soft parent rocks.
SOIL COLOUR
Soil colour is determined by the mineralogy composition from which the soil is derived and organic matter content. It varies from one place to another. Soil colour can be used for classification and description of soil of a certain place. The colours are grouped into:
- Dark (Black, grey, dark grey and dark brown)
- Bright (yellow, orange, red, redish brown)
- Light (whitish – grey, white)
Some soil can have one colour throughout like red earths and yellow brown.
SOIL TEXTURE
This refers to the degree of coarseness or fineness of the soil materials especially soil mineral particles. It can also be referred to as variation in the particle size.According to soil texture, soil can be classified as
- Course sand 2 to 0.2 mm diameter
- Fine sand 0.2 to 0.02 mm diameter
- Silt 0.02 to 0.00 mm diameter
- Clay less than 0.002 mm
- Loan soil i.e. mixture of sand, clay and silt
IMPORTANCE OF SOIL TEXTURE
- It influences the soil porosity, permeability, compaction, and structure
- It influences plant growth
- It influence cultivation during the agricultural activities
- It influences soil resistance against erosion
- It influences soil fertility
SOIL TEMPERATURE
Soil has certain degree of temperature and this tend to vary from place to place due to the variation in the climatic conditions. In cold areas like Tundra regions soil is also cold this is due to the small amount of insolation received there. In tropical areas soil are warm due to high intensity of insolation heating the surface.
IMPORTANCE OF SOIL TEMPERATURE
It controls biochemical and chemical processes especially the decomposition of organic matter and plant growth.
- It determines the existence of micro- organisms in a certain area.
- It controls the amount of moisture in the soil
SOIL POROSITY
This refers to the total volume of pores or spaces between particles of the soil materials in undisturbed soil.
Soil porosity is influenced by
- Soil texture – The finer the particles the greater will be the total surface area. Hence the soil with fine particles like clay has greater porosity.
- Structure of the soil also influences permeability.
SOIL STRUCTURE
This refers to the arrangement of soil particles into aggregate compound particles. Individual undisturbed soil is called ped. The aggregation of soil particles produce peds of different shapes and sizes when aggregation is absent as in loose sand soil, the soil is described as structure less. Strongest of the soil is influenced by its organic matter.
IMPORTANCE OF SOIL STRUCTURE
- It determines water retention capacity and aeration
- It is an indicator of soil fertility or sustainability for agricultural activities
- It influences plant growth by influencing the root penetration and water retention
CHEMICAL PROPERTIES
SOIL REACTION (PH)
Soil reaction is the term used to describe the degree of acidity and alkalinity in the soil and it results mainly from climate. This degree of acidity and alkalinity in the soil is expressed in the PH.
PH is the value which is measured in terms of the hydrogen ion concentration.PH scale runs from 1 to 14.The PH 7 is neutral.Any condition below 7 is acidic and any condition above 7 is alkaline.
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 which affects soil structure.
LEACHING
This is another chemical property of soil referring to the process in which nutrients are washed down in solution from the top- soil layer. During leaching process the bases are washed down leading to concentration of hydrogen ions which in turn cause the increased acidity in the top soil.Leaching is very effective in wet conditions.
SIMPLE HYPOTHETICAL PROFILE FOR MATURE SOIL
The soil profile varies from one place to another depending on the variation in environment conditions.
For example under deciduous forest, soil with little organic matter can be produced (brown earth or brown forest soil) while in mid latitude grasslands deep black earth soil (chernozem) is formed.Chernozem has a lot of organic matter. In the desert area the soil profile usually lack the Ao horizon due to scarcity or absence of vegetation.
SIMPLE SOIL CLASSIFICATION
Soil classification refers to the grouping of soils according to specific characteristics, such as properties or factors like climate also soil can be classified according to the age, texture and colour. One common classification is that based on texture.
According to the soil texture triangle, there are three main textures namely sand, silt and clay. This is based on the size of their particles as discussed earlier. The 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 coarse and fine sand and contain very little amount of clay such that it is not sticky when wet and is loose when dry, percentage of sand is above 85, that of clay is up to 10 and silt is up to 15. When such soil is rubbed, it does not leave any film on the fingers.
- LOAMY SAND:This consist 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 fingers 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 fingers.
- LOAM:In this soil, sand and silt dominate with 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 fingers 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 quite porous because of the sand.
- CLAY LOAM:Sand content between 20% and 45% 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 are dominant.
- CLAY:The proportion of sand goes up to 45%, while that of silt is up to 40% clay account for above 40%. The soil is sticky when moist and has a plastic feel. It can be rolled into threads when moist and can be moulded into different shapes. And can retain fingerprints.
- 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.
- READ TOPIC 4: Elementary Survey And Map