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.