INTRODUCTION TO SOIL SCIENCE

 

INTRODUCTION TO SOIL SCIENCE

 

Soil is the mixture of weathered rocks, air water living organisms and organic matter.

It is a medium in which plant grow and derive their nutrients and anchorage.

Importance of Soil

• Provides anchorage to the plants by holding their roots firmly.
• Provides plants with mineral salts/ nutrients which are necessary for their growth.
• Provide the plants with water.
• Contains oxygen necessary for respiration of the plants and soil micro-organisms.

Soil Constituents

Soil is made up of five major components. These are water, organic matter, air, living organisms and inorganic matter or minerals

Soil water

 This is found in the spaces between the soil particles that are not filled with air. It constitutes 25% of the soil by volume. Sources of soil water are precipitation and irrigation.

Importance of water in the soil

• Dissolves mineral salts so that roots of the plant can take them in.
• Maintain turgidity of leaves and young stems of the plants.
• Used for germination of seeds
• It soften the soil particles, thus enabling plant roots to grow between the soil particles.
• It keeps soil temperature cool, thus enabling living organism to survive in the soil.
• It provides hydrogen which is useful in photosynthesis, a process used by green plants to make their food

Types of water in soil

Gravitational Water

 It is the one that moves down the soil under the influence of gravity. It is available to plant when it is still in the root zone.

Hygroscopic Water

This is found in thin films on the soil particles. It is held by strong adhesive forces between water and soil particles. It does not move and hence not available for plant use.

Capillary Water

It Occupies micro-pores in the soil particles. Held by cohesive forces between water molecules and moves through capillary action. It is available to plants for use.

 

Experiment to find out the amount of water in the soil.

Apparatus needed

• a sample of fresh soil
• evaporating dish
• source of heat e.g. Bunsen burner or oven
• thermometer
• desiccator
• weighing scale
• Stirring rod.

Procedure

• Weigh an empty evaporating dish.
• Put the soil sample onto the dish and weigh again. Find the weight of soil by subtraction
• Heat the evaporating dish and soil at about 100- 105⁰c for 30-50 minutes.
• Keep stirring the soil to allow the moisture to evaporate. Then cool the soil with the soil in the desiccator.
• After cooling, weigh the dish and its contents. This should be repeated two or three times until a constant weight is obtained.

Observation

The final weight of the dish and soil will be less than the original weight recorded before heating.

Explanation

The reduction in weight is due to loss of moisture from the soil. The difference of weight before and after heating is the weight of water lost during heating.

Calculating the water content in the soil.

Weigh of evaporating dish= x gm

Weight of fresh soil and dish before heating = (x+y) gm

Weight of fresh soil only =(x+y-x) =y gm

Weight of soil and dish after heating =(x+z) gm

Weight of soil alone after heating =(x+z-x) =z gm

Weight of water lost in the soil = (y-z) gm

 

The percentage of water in the soil =change in weight of the soil  x 100 =

                                                            Weight of fresh soil sample

 

Organic matter

 This is the mixture of plant and animal residues and their products at various stages of decay.

Importance of organic matter in the soil.

• It increase the water holding capacity of soil.
• Binds soil particles together, thereby improving soil structure.
• Provides food and shelter to the living organism found in the soil.
• Releases nutrients such as nitrogen, carbon, potassium into the soil, which are essential for plant growth.
• It improves on soil aeration so that soil organism and plant roots can respire well.
• Has a buffering characteristic on soil, which prevents rapid changes in the pH of the soil.
• Holds and retains nutrients that would otherwise be easily lost from the soil by leaching.
• Because of its black colour, it absorbs sun's heat, making the soil warm.

Experiment to find out the amount of organic matter in the soil.

Apparatus

• evaporating dish
• weighing scale
• stirring rod
• tripod stand
• sample of fresh soil
• source of heat
• desiccator

Procedure

• Place a sample of fresh soil from the garden in an evaporating dish of known weight.
• Heat the evaporating dish and soil at 100- 105⁰c for about 1 hour.
• Keep stirring the soil to allow the moisture to evaporate.
•  Cool the soil with the soil in the desiccator and weigh until a  constant weight is obtained
• Then heat the dish and dry soil strongly while stirring. Not the change in soil colour as heating goes on.
• Leave the dish and soil to cool in the desiccator and weigh.
• Repeat the process until a constant weight is obtained.

 

Observation

 During strong heating, the color of soil change as organic matter is burnt to gases which escapes into the air. The final weight is less than the recorded weight after evaporating water from soil

Explanation

The reduction in weight is due to loss of organic matter from dry soil. The difference of weight before strongly heating and after heating is the weight of the organic matter lost during strong heating.

Calculating the organic matter content of soil

Weight of the evaporating dish =B gm

Weight of dish and dry soil = (B+E) gm

Weight of dish and soil after strong heating= (B+F) gm

Weight of dry soil alone before strong heating = (B+E-B) = E gm

Weight of dry soil alone after strong heat = (B+F-B) =F gm

Weight of organic matter burnt = (E-F) gm

Percentage organic matter in the soil = weight of organic matter burnt X100 =

                                                                        Weight of dry soil

 

Air

 This component constitutes 25% of the soil by volume. It is found in the pore spaces of the soil.

Importance of soil air

• Used for root and organism respiration
• Used for germination of seeds.
• Helps in decomposition of organic matter.
• Regulates the movement of water through capillary action.
• It contains nitrogen that can be converted into nitrates for use by plants.

Living Organisms

These include worms, insects such as ants and termites, bacteria, fungi, nematodes and rodents.

They are divided into two categories namely soil fauna which belong to the animal kingdom, and soil flora which belong to the plant kingdom.

Importance of living organism in the soil

·         They decompose organic materials into organic matter (humus).

·         Burrowing animals help in the aeration, drainage and infiltration of water into the soil.

·         Living organism like Rhizobia bacteria convert nitrogen gas in the soil into nitrates.

·         They add humus to the soil when they die and decay.

·         Improvement of soil structure is aided by vegetation roots, hence better soil aggregation.

·         Some of them produce antibiotics that kill pathogens, such as harmful bacteria.

·         They may act as pest to crops. For example, nematodes destroy plant roots.

·         They transmit diseases from crop to crop.

·         They compete with crop for nitrogen and other nutrients.

Inorganic or Mineral Matter

This include the mineral particles which vary in size from large rock particles to fine particles like clay.

Importance of inorganic or mineral matter

·         Supplies nutrients to plants

·         Offers anchorage to plants

·         Makes the framework of soil

·         Offers a surface onto which water is held.

 

 

Soil Formation:

 Soil is formed through weathering process.

 Weathering is the breakdown (disintegration) of rocks into smaller particles when subjected to different agents of weather like rain, temperature and wind.

 

Types of Weathering

There are 3 types of weathering namely, physical weathering, chemical weathering and biological weathering

 

Physical /mechanical weathering.

Physical weathering is the breakdown of rocks into smaller particles without changing their chemical composition.

 

 Agents of physical weathering

These include wind, running water, ice, temperature changes, plant roots, human and animal activities.

• Wind carry materials which hit against each other to break into fragments.
• Running water. When rain water flows, it carries with it rock fragments which knock against each other and eventually breakdown into small particles.
• Frost action. Water trapped in the cracks within rocks expand when it freezes and the force that develops widens the cracks. Moving ice has grinding effects which tear off rock particles.
• When the sun heats up the rock during the day, the rocks expand and at night, they contract. The alternate expansion and contraction of the rocks expands the cracks or joints in the rock and eventually the rocks break into small particles.
• Exfoliation. Differential heating and cooling of the different rock layers makes the rock layers expand and contract at different rates. The outer layer are heated more and expand more than the inner layers and so they tend to pull away  from the cooler layers beneath them and peel off.
• Activities of living organisms. During the process of mining, road construction, building and cultivation, people break down rocks. Burrowing animal such as rodents breakdown rocks as they dig tunnels within the soil.  Roots of plant grow into cracks in rocks, and as they expand they widen the cracks.

 

Chemical Weathering

This involves chemical reactions that change the chemical composition of rocks.

It involves processes such as; hydrolysis, hydration, carbonation and oxidation.

Hydration; The process by which soluble minerals in the rocks absorb water and expand weakening the rock thus leading to disintegration.

Hydrolysis; This is where a chemical reaction occurs between the minerals and water, leading to the replacement of metallic cations in the minerals by hydrogen ions.

Oxidation; The reaction of rock minerals with oxygen to form oxides which break easily.

Carbonation; The process whereby carbonic acids formed when rain water dissolves carbon dioxide,

Carbonic acid reacts with calcium carbonates in limestone causing it to disintegrate.

 

Biological Weathering

This involves the action of living organisms, plants and animals on the rocks.

 

• Burrowing animals, for example, termites and moles bring soil particles to the surface exposing them to other agents of weathering.
• Big animals like, elephants, buffaloes, camels and cattle exert a lot of pressure on
• The rocks as they step on them due to their heavy weights causing the rocks to disintegrate.
• Earthworms take part in the decomposition of plant matter with the soil particles.
• Man's activities like, mining and quarrying expose rocks to the surface during excavation. These activities breakdown large rocks into smaller rock particles.
• Plant roots force their way through the cracks in the rocks thus widening and splitting them.

• Humic acids formed when plant tissues decompose react with the rocks weakening them further.
• Plant remains -decompose adding humus into the soil.

Factors influencing soil formation

1. Parent material. Some rocks disintegrate very fast while others, very slowly. Those that disintegrate very fast give rise to a lot of soil in a short time. Also, the type of soil formed depends on the type of the original parent material. Parent materials which are rich in quart give rise to sandy soils while volcanic rocks give rise to clay soils.

2. Climate. The main climatic factors include rainfall, temperature and wind.

 Rainfall.

 Rainfall causes both mechanical and chemical weathering of soil in the following ways:

 

• Rainwater dissolves carbon dioxide from the atmosphere to form carbonic acid. This acid attacks rocks and dissolves them.
• As rainwater carrying small rock particles they knock against each other and break into small pieces to form soil.
• In areas with high rainfall intensity the action of raindrops on bare ground dislodge the weathered material.

Temperature.  Temperature influence soil formation in the following ways:

• High temperature brings about physical weathering by exfoliation. Repeated heating of rocks during the day and rapid cooling at night causes cracking of rocks especially in hot deserts.
• High temperature speed up the rate of chemical weathering and activities of organisms that assist in soil formation.

Wind:

Wind action is more pronounced in deserts where it blows sand particles against rock surfaces causing abrasions. This is more effective when the speed of wind is high.

3. Living organisms

 These breakdown the rocks by physical and chemical action in the following ways.

• Burrowing animals e.g. rodents break down soft rocks
• Bigger animals with hooves break bigger rocks into smaller ones as they walk on them.
• When living organisms die they decompose and contribute to soil formation
• tips of plant roots secrete chemical substances which dissolves the mineral contents of the rock thus weakening them,
• The action of micro-organisms break down organic matter to form humus.
• Man activities such as digging, road construction etc. influence soil .formation.
• Soil living organisms feed on organic matter thereby decomposing it.

4. Topography

 This is the nature of landscape or appearance of landscape.

This affect soil depth because on steep slopes there is erosion and in valleys there is deposition.

The aspect of slope in temperate areas affect solar radiation and therefore the temperature of the soil.

As one move up the mountain the temperature decrease. This means the rate of oil formation is low.

5. Time

 The length of time over which soil forming processes have been in action affects the age of soil. Deep mature soils are seen where soil forming processes have taken place for a long time.