5.1A Soil system Notes

Soil is a Dynamic System Within the Larger Ecosystem

1. Soils contain five major components: mineral particles, organic matter, water, air, and living organisms.
2. The proportions of each component vary depending on climate, parent rock, vegetation, land use and time.
3. The interactions among these components make soil a self-regulating system that changes in response to environmental factors.

Components of Soil

Mineral Particles

1. Mineral particles come from the weathering of parent rock and determine soil texture (sand, silt, clay).
2. Sand particles are the largest and provide good drainage but low nutrient retention.
3. Silt particles are intermediate and help retain moisture.
4. Clay particles are the smallest and have high surface area, allowing them to hold nutrients and water effectively.
5. Mineral composition influences soil fertility, pH, structure and water-holding capacity.

Organic Matter

1. Organic matter includes living organisms, fresh plant and animal residues, and humus, which is well-decomposed material.
2. Humus improves soil structure by binding particles into stable aggregates.
3. Organic matter increases nutrient holding capacity, water retention and soil aeration.
4. It provides the main energy source for soil organisms and is essential for nutrient cycling.

Water

1. Soil water comes from precipitation, irrigation and capillary rise from groundwater.
2. Water fills pore spaces and enables chemical reactions, nutrient transport and weathering.
3. The balance between gravitational water (drains quickly), capillary water (available to plants) and hygroscopic water (unavailable) affects plant productivity.
4. Water movement depends on texture, structure and organic matter content.

Air

1. Air fills the pore spaces not occupied by water.
2. Well-aerated soils support healthy root growth, aerobic respiration and decomposer activity.
3. Poorly aerated soils (waterlogged) restrict oxygen and may encourage anaerobic processes like denitrification, reducing fertility.

Living Organisms

1. Soil organisms include bacteria, fungi, earthworms, nematodes, mites and plant roots.
2. They break down organic matter, cycle nutrients, improve soil structure and help form humus.
3. Earthworms increase infiltration and aeration through burrowing.
4. Mycorrhizal fungi enhance plant nutrient uptake, especially phosphorus.

Soil as a System

Inputs to the Soil System

1. Matter enters the soil system from both biotic and abiotic sources.
2. Key matter inputs include:
   1. Organic material such as leaf litter, dead plants, animal remains, and microbial biomass.
   2. Inorganic parent material from weathered bedrock or mineral particles.
   3. Water entering from precipitation, infiltration, and surface runoff.
   4. Gases such as oxygen, nitrogen, and carbon dioxide from the atmosphere.
   5. Nutrients carried by rivers, flooding, or wind deposition.
3. Inputs provide the raw material that enables soil formation and supports ecosystem productivity.

Outputs from the Soil System

1. Soil loses matter through several pathways:
   1. Leaching, where water transports dissolved nutrients downward through soil horizons.
   2. Gaseous losses, including carbon dioxide from respiration and nitrous oxide from denitrification.
   3. Erosion, which removes soil particles through wind or rainfall.
   4. Crop removal, where nutrients stored in plants are harvested and taken out of the system.
2. Excessive outputs reduce soil fertility and can destabilize the soil system.

Storages in the Soil System

1. Soil contains several major storage components:
   1. Organic matter, including humus, roots, and microorganisms.
   2. Mineral particles, such as sand, silt, and clay.
   3. Water, held between soil particles.
   4. Air, occupying pores and enabling root and microbial respiration.
   5. Nutrients, stored in organic and inorganic forms.
2. These storages determine soil properties such as texture, structure, fertility, porosity, and water-holding capacity.

Transfers in the Soil System

1. Transfers move materials within the soil system.
2. They include:
   1. Bioturbation, where organisms like earthworms mix soil layers.
   2. Translocation, where water moves minerals up or down soil horizons.
   3. Decomposition, breaking down organic matter into simpler compounds.
   4. Salinization, where salts accumulate due to evaporation.
   5. Compaction, reducing pore spaces and restricting movement of air and water.

Transformations in the Soil System

1. Transformations change materials chemically or biologically.
2. They include:
   1. Humification, forming stable humus from decomposed organic matter.
   2. Mineralization, releasing available nutrients from organic forms.
   3. Weathering, producing new minerals from parent rock.
   4. Nitrification and nitrogen fixation, converting nitrogen into usable forms.
   5. Microbial transformations, reshaping the chemical composition of soil compounds.

Soil Profiles and the Development of Horizons

1. Soils develop a layered, stable, vertical structure called a soil profile, which forms extremely slowly over hundreds to thousands of years.
2. A soil profile shows distinct horizons, each created by long-term interactions between organic matter inputs, mineral weathering, water movement, and biological activity.
3. Upper horizons contain more organic material, while deeper horizons contain progressively more inorganic minerals and less biological activity.
4. Horizons develop through the four main soil system processes: additions, losses, transfers, and transformations.

Major Horizons in a Soil Profile

How Horizons Form Over Time

1. Weathering breaks the parent rock into sand, silt and clay.
2. Organic material accumulates at the surface and mixes through the profile due to organisms such as earthworms, ants, termites and fungi.
3. Water movement redistributes minerals and organic compounds vertically.
4. Soil organisms decompose organic matter into humus.
5. Leaching removes soluble minerals from upper layers and deposits them lower down.
6. Continuous interaction produces the layered profile typical of mature soils.

Investigating Soil Properties

Collecting Subsoil Samples (B Horizon)

1. Collect one sample from a managed soil such as a garden or agricultural field.
2. Collect another sample from a natural ecosystem such as a forest, wetland, or grassland.
3. Ensure that sampling depth is consistent to compare horizons accurately.

Properties to Investigate

1. Texture: proportions of sand, silt and clay.
2. Organic matter content: using combustion methods.
3. NPK concentrations: using test strips or meters.
4. Aeration: using soil bulk density or oxygen diffusion rate.
5. Drainage rate: by measuring infiltration speeds.
6. Water retention: by comparing water held before and after drainage.
7. Carbon content: by burning dry soil and measuring mass loss.

Methods for Soil Texture Analysis

1. Finger Assessment (By Feel)

1. Moisten soil and work it between fingers.
   1. Sandy soils feel gritty
   2. Clay soils feel sticky
   3. Silty soils feel smooth.
2. Provides a rapid, low-cost estimate of soil texture.

2. Sieving Method

1. Soil is passed through sieves of decreasing mesh size.
   1. Larger particles remain on top
   2. Smaller particles fall through.
2. Percentages of particle sizes determine the soil’s texture.

3. Sedimentation Method

1. Soil is suspended in water and allowed to settle.
2. Larger particles settle first, smaller ones last.
3. Layer heights indicate relative proportions of sand, silt and clay.

Measuring Soil Moisture Content

1. Moisture content is determined by drying soil at 105°C and comparing masses before and after.
2. The difference represents evaporated water.
3. Multiple drying cycles ensure accuracy.

Measuring Organic Matter Content

1. Organic matter is burned off at 550°C or using a Bunsen burner.
2. Mass loss reflects the amount of organic carbon.
3. Higher organic matter indicates greater fertility, porosity, and biological activity.

Measuring Aeration (Soil Bulk Density)

1. Soil bulk density equals mass divided by volume.
2. High bulk density indicates compaction and low aeration.
3. Low bulk density indicates good pore space, high organic matter, and healthy structure.

Measuring Drainage (Infiltration Rate)

1. Use a bottomless cylinder inserted into the ground.
2. Fill with water and measure time for the water level to fall.
3. Faster infiltration indicates sandy or loose soils; slower infiltration indicates clay or compacted soils.

Interpretation of Results

1. Natural ecosystem soils (e.g., forest, wetland) tend to have higher organic matter, better aeration, and higher water retention.
2. Cultivated soils (e.g., farmland, gardens) may have lower organic matter due to continuous cropping and erosion but higher NPK if fertilizers are used.
3. Sandy soils drain quickly but hold fewer nutrients, while clayey soils retain water but may become compacted.

Soil System Inputs: Natural and Anthropogenic Sources

1. Soil is a dynamic system influenced by both natural processes and human activities.
2. The inputs into a soil system can be categorized into organic and inorganic sources, both of which play a crucial role in soil fertility, structure, and function.

Inputs of Dead Organic Matter

Natural Organic Inputs

1. Includes plant litter, fallen leaves, twigs, seeds, fruit, dead animals, feathers, fur and faeces.
2. Roots contribute organic exudates such as sugars and amino acids.
3. Organic inputs provide food for detritivores and decomposers.
4. Decomposition releases nutrients (nitrogen, phosphorus, potassium, carbon) into the soil.
5. Organic material improves water retention, soil structure, fertility and colour.

Anthropogenic Organic Inputs

1. Compost provides stable carbon, improves aeration and enhances water retention.
2. Manure adds nitrogen, phosphorus, potassium and organic matter.
3. Both improve soil structure and reduce soil bulk density.
4. Organic fertilizers release nutrients slowly and enhance microbial activity.

Inputs of Inorganic Matter

Natural Inorganic Inputs

1. Weathering: Releases minerals from parent rock, increasing depth and fertility.
2. Deposition: Wind and water transport sediments from other ecosystems.
3. Decomposition: Releases mineral ions and inorganic molecules.
4. Precipitation: Supplies dissolved minerals such as nitrates, sulfates and bicarbonates.
5. Diffusion: Air enters pore spaces, supplying oxygen, nitrogen and water vapour.
6. Solar Energy: Heats the soil, influencing decomposition, root growth and microbial activity.

Anthropogenic Inorganic Inputs

1. Irrigation: Provides water but can cause salinization in arid regions as salts accumulate.
2. Inorganic fertilizers: Supply concentrated nitrogen, phosphorus and potassium but may cause leaching, soil acidification and contamination.
3. Pesticides and herbicides: Can accumulate in soil, reduce biodiversity and affect soil microorganisms.
4. Industrial pollution: Heavy metals and waste can contaminate soils, reducing fertility and harming ecosystems.

Importance of Inputs in Soil Development

1. Inputs determine soil fertility, structure, hydrology, pH and biological activity.
2. Organic inputs contribute to humus formation and improve soil ecosystem functioning.
3. Inorganic inputs influence soil mineral composition and nutrient cycling.
4. Anthropogenic inputs may enhance productivity or degrade soil health depending on use.

Soil System Outputs

Losses of Dead Organic Matter

1. Dead organic matter is removed from the soil through microbial decomposition.
2. Aerobic decomposers such as bacteria and fungi convert organic carbon into carbon dioxide, releasing it into the atmosphere.
3. Anaerobic decomposers in waterlogged soils convert organic matter into methane, a more potent greenhouse gas.
4. The removal of organic matter reduces the carbon content, nutrient availability, and soil structure quality.
5. Decomposition rates increase with warm temperatures, high moisture, and good aeration.

Loss of Mineral Components

1. Minerals leave the soil when absorbed by plant roots and transported into plant tissues.
2. Leaching removes dissolved ions such as nitrate, potassium and magnesium as rainwater percolates downward.
3. Leached nutrients may accumulate in deeper horizons or be permanently lost into groundwater, streams, or lakes.
4. Minerals are also lost when soil particles erode because minerals are physically carried away.

Loss of Soil Particles Through Erosion

1. Soil particles are removed by water, wind, gravity, or ice.
2. Water erosion includes sheet erosion, rill erosion and gully erosion.
3. Wind erosion removes fine particles such as clay and silt, leaving behind coarse sand.
4. The topsoil layer, which contains the most organic matter and nutrients, is lost first.
5. Loss of topsoil causes declines in fertility, reduced crop yields, and weakened plant anchorage.

Loss of Energy From the Soil System

1. Soils lose heat to the atmosphere through long-wave radiation, especially at night.
2. Soil also loses heat when water evaporates because evaporation requires latent heat.
3. Vegetation reduces heat loss by providing insulation and limiting exposure.
4. Cooler soils slow down microbial activity and nutrient cycling.

Transfers Into, Out of, and Within Soil

Infiltration

1. Infiltration is the downward movement of water from the surface into the upper soil layers.
2. Infiltration rates are influenced by soil texture, soil structure, organic matter content, and compaction levels.
3. Sandy soils have high infiltration; clay soils have low infiltration.
4. Vegetated soils promote infiltration because roots create channels for water movement.

Percolation and Groundwater Flow

1. Percolation is the continued downward movement of infiltrated water through soil horizons.
2. Water eventually reaches the saturated zone and becomes part of an aquifer.
3. Rock permeability determines how easily water enters bedrock and recharges groundwater.
4. Percolation helps transport dissolved minerals from upper horizons to lower ones.

Leaching

1. Leaching transports dissolved nutrients through soil horizons.
2. Nitrate is highly mobile and easily leached because it does not bind strongly to soil particles.
3. Excessive leaching can create nutrient-depleted soil such as laterite soils in tropical regions.
4. Leaching is more intense in climates with high rainfall.

Biological Mixing

1. Soil organisms mix soil layers vertically and horizontally.
2. Earthworms, beetles and termites transport organic matter downward and bring mineral material upward.
3. Biological mixing increases aeration, nutrient distribution and root penetration.
4. This process reduces the sharp boundaries between horizons.

Aeration

1. Oxygen diffuses into soil pores while carbon dioxide diffuses out.
2. Good aeration promotes aerobic decomposition and rapid nutrient cycling.
3. Waterlogged soils lack air spaces, leading to anaerobic conditions and methane production.
4. Plant roots require oxygen; poor aeration can suffocate roots.

Erosion as a Transfer

1. Erosion transfers soil particles from one landscape to another.
2. Sediments deposited on floodplains enrich those soils with nutrients.
3. Wind can deposit mineral dust over long distances, such as Saharan dust enriching Amazon soils.

Transformations Within Soils

Decomposition

1. Organic matter breaks down into humus through the activity of microbes and detritivores.
2. Humus stores nutrients, improves soil structure and increases water-holding capacity.
3. Mineralization converts humus into inorganic ions that plants can absorb.
4. Decomposition rate depends on temperature, moisture and oxygen availability.

Weathering

1. Mechanical weathering breaks rocks into smaller particles through heating, cooling, frost action and abrasion.
2. Chemical weathering dissolves minerals through reactions with water, acids and oxygen.
3. Biological weathering occurs when roots, lichens, or microbes break down rock.
4. Weathering supplies the soil with minerals, clay particles and nutrient ions.

Nutrient Cycling

1. Nutrients move between the soil, vegetation, decomposers and atmosphere.
2. Bacteria perform nitrogen fixation, nitrification and denitrification.
3. Decomposers release ammonium, which can be converted into nitrate for plant use.
4. Nutrient cycling maintains productivity and resilience of ecosystems.