FULL LECTURE NOTES ON SOIL (GEOGRAPHY)

What is soil




meaning of soil

Soil is a naturally occurring substance forming the thin uppermost layer of the earth’s surface made by the accumulation of the weathered materials on which plants and animals live.

Also, the soil is defined as the thin uppermost part of the earth’s surface consisting of water, air, organic, and mineral matters made by the accumulation of the weathered materials on which plants and animals do live.

Other scholars have defined soil as a natural body resulting from the interrelationship between several; physical, biological, and chemical processes all of which vary according to the different natural environments




benefits or importance of soil

Medium for plant growth.

Soil is where most plants grow. Soil provides anchorage for the plants as well as protection of roots from damage.

It is where or a medium through which water, air and nutrients are made available to plants.

The well-aerated soil facilitates the absorption of water and nutrients from the soil by plants.

Soil support animal life.

As soil support plant life it also support animal life because plants are the source of foods to animals and this is most for herbivores.




Also some animals eat soil as food in form of salt licks for example pregnant women who lack some minerals in their bodies.

Soil provide habitat for living organisms

In the soil there are some animals living there example burrowing animals like rodents, earthworms and termites

Provide sites for agricultural activities

The fertile soil promotes agriculture activities, both animal husbandry and crop cultivation.

This is because soil supports the growth of pasture for animals.




Provide settlement

Soil influences distribution of settlement for example the areas with good fertile soil are densely populated compared to the areas with poor soil.

Soil provide building materials

Soil is used in making bricks, tiles and white wash. All these materials are used in building houses, bridges etc.

Also soil is used directly in road construction

Source of minerals

There are some minerals found in soil that can be extracted for commercial purposes.

Also it is used to manufacture fertilisers as it contain minerals for example the fertilisers that contain phosphate e.g. In Minjingu (Manyara) region.

It provides raw materials for pottery and ceramics




Soil is used in making pots and these help to provide income to those who engage in this activity.

COMPONENTS OR COMPOSITION OF SOIL

Soil composition is an important aspect of nutrient management. While soil minerals and organic matter hold and store nutrients, soil water is what readily provides nutrients for plant uptake.

Soil air, too, plays an integral role since many of the microorganisms that live in the soil need air to undergo the biological processes that release additional nutrients into the soil.

Soil is composed of both organic and inorganic matter, and it is essential for life on earth to exist

Soil is composed of :

Mineral Particles




Mineral particles are the largest ingredient and make up approx 45% of soils. They are the original rock that got broken down by weathering and erosion to form the basis of soil.

The type of rock that was broken down to form it is called the parent rock. The broken-down rock produces minerals such as calcium, phosphorus, and potassium in the soil on which the plants feed.

The parent material influences the soil color, depth, texture, and pH value.

Organic Matter

Organic matter is decayed vegetation that is broken down by microorganisms in the soil to form humus.




Humus is a dark jelly-like substance that binds the soil together and improves its texture. It increases the soil’s ability to retain moisture.

The color of the soil is an indication of the amount of organic material it contains with darker soils having more organic content.

importance of soil organic matters

  • help to provide nutrient to soil organisms
  • it act as a major reservoir of plant nutrients
  • it improves soil structure
  • it influence soil temperature
  • it reduces the risk of soil erosion
  • it increases water holding capacity of the soil
  • it make nutrients exchange between soil and the root of the plant easier




how to improve soil organic matters

  • by recycling crop residue back to field without wasting and burning
  • by application of compost
  • through mulching organic wastes
  • by using green manures and cover crops
  • by use of crop rotation
  • by reducing soil tillage
  • by avoiding or preventing soil erosion
  • by applying organic manures

components of soil organic matters

  • Living organisms (soil biomass) 
  • The remains of microorganisms that once inhabited the soil  
  • The remains of plants and animals  
  • Organic compounds that have been decomposed within the soil and, over thousands of years, reduced to complex and relatively stable substances commonly called humus

As organic matter decomposes in the soil, it may be lost through several avenues.




Since organic matter performs many functions in the soil, it is important to maintain soil organic matter by adding fresh sources of animal and plant residues, especially in the tropics where the decomposition of organic residues is continuous throughout the year.

Soil Air and Water

Air is vital for the survival of micro-organisms and without these, there would be a shortage of humus. Plants cannot survive without water present in the soil.

Mineral particles are soluble in water and the roots of plants can only absorb the nutrients after they have been dissolved.




In nutrient management, a proper balance between soil water and soil air is critical since both water and air are required by most processes that release nutrients into the soil.

Soil water is particularly important in nutrient management. In addition to sustaining all life on Earth, soil water provides a pool of dissolved nutrients that are readily available for plant uptake.

Therefore, it is important to maintain proper levels of soil moisture.

Soil water is important for three special reasons:

  • The presence of water is essential for all life on Earth, including the lives of plants and organisms in the soil. 
  • Water is a necessity for the weathering of soil. Areas with high rainfall typically have highly weathered soils. Since soils vary in their degree of weathering, it is expected that soils have been affected by different amounts of water.  
  • Soil water is the medium from which all plant nutrients are assimilated by plants. Soil water, sometimes referred to as the soil solution, contains dissolved organic and inorganic substances and transports dissolved nutrients, such as nitrogen, phosphorus, potassium, and calcium, to the plant roots for absorption




importance or Roles of soil organic matter

  • Organic Matter creates a granular condition of soil which maintains favorable condition of aeration and permeability. 
  • Water holding capacity of soil is increased and surface runoff, erosion etc., are reduced as there is good infiltration due to the addition of organic matter.
  • Surface mulching with coarse organic matter lowers wind erosion and lowers soil temperatures in the summer and keeps the soil warmer in winter.  
  • organic matter serves as a source of energy for the microbes and as a reservoir of nutrients that are essential for plant growth and also hormones, antibiotics.  
  • Fresh organic matter supplies food for earthworms, ants and rodents and makes soil PH readily available in acid soils.
  • Organic acids released from decomposing organic matter help to reduce alkalinity in soils; organic acids along with released CO2 dissolve minerals and make them more available.  
  • Humus (a highly decomposed organic matter) provides a storehouse for the exchangeable and available cations.  
  • It acts as a buffering agent which checks rapid chemical changes in pH and soil reaction.




factors affecting the formation of soil organic matters

  • Climate: Temperature and rainfall exert a dominant influence on the amounts of nutrients and Organic matter found in soils.
  • Temperature: The Organic Matter content of comparable soils tend to increase if one moves from warmer to cooler areas. The decomposition of Organic matter is accelerated in warm climates as compared to cooler climates. For each 10 C decline in mean annual temperature, the total Organic matter and nutrients increases by two to three times. 
  • Rainfall: There is an increase in Organic Matter with an increase in rainfall. Under comparable conditions, the Organic matter increase as the effective moisture becomes greater.
  • Nature of Vegetation: The total organic matters is higher in soils developed under grasslands than those under forests. 
  • Soil texture: Fine textured soils are generally higher in Organic matter than coarse textured soils
  • Drainage: Poorly drained soils because of their high moisture content and relatively poor aeration are much higher in Organic matter than well drained soils. 
  • Cropping and Tillage: The cropped lands have much low N and Organic matter than comparable virgin soils. Modern conservation tillage practices helps to maintain high Organic matter levels as compared to conventional tillage. 
  • Rotations, residues and plant nutrients: Crop rotations of cereals with legumes results in higher soil Organic matter. Higher Organic matter levels, preferably where a crop rotation is followed.




5 factors that influence soil formation

  • Climate has two major components for soil formation. The first is the temperature. As the mean annual soil temperature increases, the weathering of the rocks and minerals in the soil will be faster. Along with temperature is the climate factor of precipitation or rainfall. In general, areas with more rainfall will have greater weathering and greater leaching.
  • Organisms include animals living in the soil that contribute to soil development by their mixing activities. The mixing of the soil by organisms is called bioturbation. Humans also influence the soil with their activities of agriculture, urbanization, grazing, and forestry.  
  • Topography as a soil forming factor is related to the soil’s position on the landscape elevation, direction and depth to the water table. Topography will have a great deal to do with the soils character as different topographic locations vary in respect to water runoff, erosion, leaching and temperature.
  • Parent material refers to the primary material from which the soil is formed. The type of soil that forms depends on the type of rocks available, the minerals in rocks, and how minerals react to temperature, pressure, and erosive forces. Soil parent material could be bedrock, organic material, an old soil surface, or a deposit from water, wind, glaciers, volcanoes, or material moving down a slope.  
  • The length of time required for a soil to form depends on the intensity of the other active soil forming factors of climate and organisms, and how topography and parent material modify their affect.




Each of the world’s soils is assigned to one of twelve taxonomic soil orders, largely on the basis of soil properties that result from the five soil-forming factors acting on the parent material over time.

what is soil texture

Soil texture is one of the physical properties of soil along with soil structure, soil color, soil temperature, soil porosity, and others.

Soil texture can be defined as the coarseness or fineness of the soil determined by the relative proportion of soil particles of different diameters.

The size of soil particles can make the soil coarse-textured, medium-textured, and fine-textured.

The texture of the soil can be assessed by either sense of feel method or particle analysis method. 




Methods of assessing soil texture

Sense of feel method

this method is done in the field in which soil sample is rubbed, preferably in wet condition between fingers, and may give any of the following results;

  • gritty feel – this imply the soil is of coarse texture as its particle are large in size and is recognized as sand soil.
  • flour feel – the soil is slightly fine or medium texture as its particles are medium in size and is recognized as silt soil.
  • plastic feel – the soil is of fine texture as its particles are quite small and is known as clay soil.

Particle analysis method

this method is more accurate as it is done in the laboratory and involves recognizing the texture of the soil by measuring the size of soil particles in the soil sample.

Diameter of the particle (mm)Name of soil
Less than 0.002clay
0.002 – 0.02silt
0.02 – 0.2Fine sand
0.2 – 2Course sand
2 – 20Fine gravel
20 – 200gravel




importance of soil texture

  • It influences other physical soil properties of soil like; soil permeability, soil structure, soil porosity, soil water retention capacity, and so forth.
  • Soil texture influence soil resistance to erosion. Erosion is easier to the soil which is coarse-textured as their particles are loose.
  • It determines the relative penetration of plant roots in the soil. Where the soil particles are large, roots can penetrate more easily than they do in fine grained soil which are usually compact.
  • Soil texture determine the infiltration rate of water into soil. Infiltration rate is easier in coarse-textured soil compared to fine textured soil as its particles are compacted.
  • Lastly, soil texture influence soil fertility as it determine the ability of soil to hold nutrients and water for plant use 

what is fertigation




fertigation is the judicious application of fertilizer by combining it with irrigation water.

fertigation can be achieved through fertilizer tank, venturi system, injector pump, non electric proportional liquid dispenser and automated system

advantages of fertigation

  • reduction of soil compaction and mechanical demage to the crop
  • potential reduction of environmejtal contamination
  • ensure a regular flow of water resulting in increased growth rates for higher yields
  • offer greater versatility in the timing of the nutrients appkication to meet specific crop demand




  • improves the availability of nutrients and their uotake by the roots
  • it is safer application method which eliminates the danger of burning the plant root system

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