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This can be defined as the genesis or evolution of the soil from the parent rock material.
o The process is continuous and is taking place through the action of weathering process on the parent rock materials being physical, chemical and biological activities in nature.
o As the process of weathering take place the parent rack material is broken down into smaller units which then become mixed up with organic matter water air and living organisms to make up the soil.
If a hole of about 4m deep is dug in the soil, it is possible to see several horizontal layers of soil which differ in depth colour as well as organic matter content.
1. O horizontal – Organic horizontal
2. A horizontal
3. B horizontal – Mineral horizontal
4. C horizontal

o A soil showing such layers is said to be STRATIFIED and each layer in called a STRATUM OR HORIZON.
o The whole vertical section of such layers is called soil profile.
o The SOIL PROFILE can be defined as a vertical section through the soil from the surface to be underlying unweathered materials (or bedrock) consisting of several horizontal layers differing in physical chemical and biological properties.
Characteristic of soil profile; Soil horizontal: The horizontal found in the soil profile are general divided into 4 main groups.(NB): The number of horizontal depends on the age of the soil i.e. the older the soil, the greater number of horizontal.
I. O horizons (Organic horizons)
o There are found on the surface of the soil above the mineral soil.
o These are farmed from remains of dead plants and animals.
o The uppermost part is called O1, sub horizon and below is O2 sub horizons.
o The original forms of animal and plant material
in O1 can be seen by naked eye, while 02 organic matter is more decomposed then the original forms of animal and plant
o Material cannot be easily seen.
II. A‖ horizons or horizon of eluviations
These are horizon that lie below O horizons and are generally leached (Eluviation) and donated as A1 A2 A3.
o A1 is darker in color due to organic matter.
o A2 is composed of day oxides of iron and iron have been washed away and deposited down the B HORIZONS.
III. B horizons or horizon of eluviations
o These are horizon which is below a horizon.
o They contain materials washed from A and O horizons and have accumulated ( i.e. illuviated) and ar e donated by B1 B2 B3
o In this horizons day oxide of iron and aluminum are deposited after being washed away from horizon above
IV. This is the zone of the soil where very little weathering or no weathering has taken place.
o It is unconsolidated bedrock weathered slowly to give rise to soil particles.
o Below this horizon it the consolidated bed rock.
o Good soil profile facilitates good plant growth.
o Facilitate drainage aeration and penetration of plant roots e.g. a deep and well structured profile hold more water than shallow one also deep rooted plant require a deep soil.
o Likewise hard Pans and impervious layers in the soil profile slow down the rate of water movement plant root growth, root distribution as well as air movement.
Meaning: This refers top physical characteristics in a given soil divided from physical force interaction in the development of the soil. The resulting effect renders characteristic properties to soil which can be described in physical terms as:-
I. Soil structure
II. Soil texture

III. Soil cloud
IV. Soil density
V. Soil porosity
VI. Soil water
VII. Soil temperature
This is the relative proportion of soil separate i.e. sand salt and day particles or relative proportion of soil particles of different diameter in the soil.
Therefore clay soil particulars with less than 0.002mm diameter
Silt – Soil particles ranging from 0.002x-0.02mm
Sand – Soil particles ranging from 0.002x-0.02mm
Gravel – Soil particles ranging from 0.002x-0.02mm
Importance of soil texture
I. Determine the rate of water movement: This varies with the finesse of the soil texture.
II. Soil fertility i.e. the finer the texture of the soil the greater is its fertility.
III. Root penetration where the surface of the soil is loops textured.
This is the arrangement of individual soil particles within the soil and subsequent arrangement of this aggregation in the soil profile.
Types of soil structure
I. Single grained
structures: In this type of structure, each soil particle in not cemented to any other soil particle example of which is sand.
II. Massive structure: In this the soil particle are cemented together to form aggregates or pad. These pads may be in different forms.
These pads (aggregates) are bound together by organic matter and substances such as mucilage and gums decomposition. Through this binding we get different shapes as:-
Plate like structure (platy) in this type the soil particles are formed into plate like aggregates that are arranged horizontally in the soil.
Such structure can reduce penetration of air water and roots. Is very common in horizoA.
Prism-like structure: In this type the pads are arranged vertical in some soils.

They have sharp pointed ends (tops)
Columnar-like structure: These are prim like structure which has rounded tops.

NB: Broth of them are found in horizon B
Blocky structure: These are made up of equal amount of flat and upright unit which easily fit together.
They can further divided into:-
Angular blocky: This is when the angles of the sides of the pads approach 900 (right angle).
Sub angular blocky: These are blocks whose angles of the pads are sharps than 900.
Sheet like structure: These are types usually loose and never more than 1-2 cm in diameter. These sphere like aggregated are characteristics of many surface soil in horizon A especially those high in organic matter.
They may have two types; depending on the porosity of the aggregation.
Granular like These are non-porous
Crumby like-These are porous
Importance of soil structure
Soil structure influence almost all plant growth factors e.g. water supply creation availability of plant nutrients microbial activities; seed germination root penetration. e.t.c.
Characteristics of a good structure of soil.
1. Stability: Soil stability is a power of resistance against disintegrating force of water and physical action such as wind.

2. Facilitate porosity: Assist creation downward water movement.
Improving soil structure
Soil structure is liable to change under different management practices such as tillage (pouching) liming fertilizer application maturing and drainage system improvement soil structure in the soil horizon.
SOIL COLOUR- Refers to various colours which may compact in the soil mass.
Usually all colours except pure blue and green occur in the soil.
Such colors include, white yellow, grey Brown red and black.
Color can be an indicator of climatic condition parental materials of the soil.
Importance of colour
Affect temperature hence regulation soil moisture retention.
Determines the productivity of the soil e.g. black colored soils have high productively compared to white or seed colored soil.
SOIL TEMPERATURE-Temperature is extremely important of the soil.
It affects plant growth directly and influence moisture, aeration, structure, microbial activities along with decomposition of plant/animal residues and availability of plant nutrient i.e.
Low temperature has the following effects:
i. Slow enzyme reaction thus decrees metabolic activities.
ii. Plant cell partiality decreases.
iii. Root elongation is retarded limiting the ability to search for water and nutrients.
iv. Affect used germination of different special.
Water plays an important role in the soil plant system. These include:
It is are vent and a carries of plant nutrients in the soil
It is essential for photosynthesis.
It is a constituent of protoplasm.
It maintains turgidity and body temperature of a plant cell.
Affects weathering of rocks will formation and soil.
The forces with which water is held in soils determine its availability to plants. Forces of adhesion hold water strongly to mineral and humus surfaces and less strongly to itself by cohesive forces. A plant’s root may penetrate a very small volume of water that is adhering to soil and be initially able to draw in water that is only lightly held by the cohesive forces. But as the droplet is drawn down, the forces of adhesion of the water for the soil particles make reducing the volume of water increasingly difficult until the plant cannot produce sufficient suction to use the remaining water. The remaining water is considered unavailable. The amount of available water depends upon the soil texture and humus amounts and the type of plant attempting to use the water. Cacti, for example, can produce greater suction than can agricultural crop plants.
The following description applies to a loam soil and agricultural crops. When a field is flooded, it is said to be saturated and all available air space is occu
pied by water. The suction required to draw water into a plant root is zero. As the field drains under the influence of gravity (drained water is called gravitational water or drain-able water), the suction a plant must produce to use such water increases to 1/3 bar. At that point, the soil is said to have reached field capacity, and plants that use the water must produce increasingly higher suction, finally up to 15 bar. At 15 bar suction, the soil water amount is called wilting percent. At that suction the plant cannot sustain its water needs as water is still being lost from the plant by transpiration; the plant’s turgidity is lost, and it wilts. The next level, called air-dry, occurs at 1000 bar suction. Finally the oven dry condition is reached at 10,000 bar suction. All water below wilting percentage is called unavailable water.
This refers to the relationship between weight and volume of a given soil mass.
Types of soil density.
i. Particle density (P.D)
This refers to the weight per unit volume of the solid portion of the soil mass. It is not affected by the fines and arrangement of soil particles. Hence
EcoleBooks | AGRICULTURE  O LEVEL(FORM TWO) NOTES - SOILS ITS AGRICULTURE UTILIZATIONGenerally for mineral soil the range of P.D may be from 2.40-2.75g/cc. But accepted volume for normal soil is 2.65g/cc.
b. Bulk density (B.D)
This refers to the weight per unit volume of the total volume of the soil occupied by both solids and pore spaces. It is given as:-
o EcoleBooks | AGRICULTURE  O LEVEL(FORM TWO) NOTES - SOILS ITS AGRICULTURE UTILIZATIONThe B.D of soil is always small then than its P.D
o Normally sand soil have B.D between 1.2.-1.80g/cc and peat soil 0.5g/cc
o The magnitude of B.D decreases with finess in tenure.
This term refers to the percentage of the soil volume that is occupied by water and air.
o Since water and air occupy the non-solid space of the soil; the arrangement of the soil particles in the soil, determines the total pone space to a great extent.
o Soil porosity (% pore space) is calculated as follows:-
% soil space +% pore space =100
% pore space =100-% solid space. But.
Soil porosity varies with texture, shape of individual particles structure, organic matter content and compactness of the soil.
o Sand soil has large pores but % porosity is small due to great P.D while fine textured soil have high % porosity due to high B.D
Types of soil commonly found in Tanzania are:-
Volcanic soil
Sand so clay soil
Clay soil
Sand soil
Loam soil clay loam soil
Stony soil
floury soil
Red plateau soils
Alluvial soil
Colluvial soil.
Sand soil: These are particles lying loosely to each other as such easily eroded by water or coined.
Well aerated
Easy to cultivate (light soil)
Have low water holding capacity.
Have low nutrient supply/status in supper layers due to high leaching.
Good for deep rooted crops.
Types of crops grown: Cashew nuts, coconuts, pineapples etc.
Clay soil (35% clay)
  • Have fine pores thus have moderate to poor aeration and drainage.
  • High moisture retention
  • Difficult to cultivate (heavy soil) Types of crops: Rice and sugar cane.
Sand clay/loam sand.
  • This soil contains clay and sand in almost equal proportion.
  • It is composed of approximately 40% sand 40% site and 20% clays
  • They are ergonomically the most important kind of soil as they are ideal for the majority of agriculture crops.
Volcanic soil
These types of soil are located in volcanic mountain such as Kilimanjaro, Meru, Monduli, and Rungwe. Types of crops: Bananas, maize, beans, coffee, tea and sugarcane.
Colluvial soil
These soils are located in the bottom of mountain ships such as Uzambara and Ulluguru mountains. Crops grown: Bananas, maize, beans, coffee, tea and soil. vii. Red platedux soil.
Types are located in arid and semi arid areas e.g. central zone of Tanzania –Dodoma, Singida and Shinyanga. Crops grown Sorghums, Millets Groundnut and grapes.
Alluvial soil.
These are located in big river bases such as Ruaha, Rufiji, Ruvu. They are very fertile.
Crops grown: rice maize banana cotton sugarcane tropical fruits etc.
Floury sand soils.
These are mainly located in the western part of the country e.g. Kigoma and Tabora. They are generally low in fertility is low soil fertility (or infertile)
Crops grown: Sorghum, Millets, Maize, Groundnuts, Cassava, Sweet potatoes and tobacco.

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