Soil Structures & Properties (SL IB Environmental Systems & Societies (ESS))

• Soils vary greatly in their structure and properties, with distinct characteristics that influence their ability to promote primary productivity
• Sand, clay, and loam soils differ in their mineral and nutritional content, drainage, water holding capacity, air spaces, biota, and their potential to hold organic matter
• Understanding these soil properties is crucial for comprehending their impact on plant growth, nutrient cycling, and the overall health and functioning of ecosystems

Photo by Dylan de Jonge on Unsplash 

Soil is a fundamental component of terrestrial ecosystems, serving as a vital medium for plant growth and supporting the intricate web of life in the natural world

   Sand Soils

• • Mineral content: Dominated by large particles of sand, primarily composed of silica and other minerals
  • Nutritional content: Typically low in organic matter and nutrients, as sand particles do not have a high capacity for nutrient retention
  • Drainage: Excellent drainage due to the large pore spaces between sand particles, allowing water to move quickly through the soil
  • Water holding capacity: Low water holding capacity as sand particles have limited ability to retain water
  • Air spaces: Sand soils have ample air spaces due to the large particle size, facilitating oxygen availability for plant roots and soil organisms
  • Biota: Generally lower microbial activity and lower diversity of soil organisms compared to other soil types
  • Potential to hold organic matter: Low potential to hold organic matter, as sand particles do not have strong binding capacity

   Clay Soils

• • Mineral content: Dominated by small clay particles, composed of various minerals such as silicates and aluminum oxides
  • Nutritional content: Clay soils often have higher nutrient content and cation exchange capacity, allowing them to retain and supply nutrients to plants
  • Drainage: Poor drainage due to the small particle size and compactness, which limits water movement through the soil
  • Water holding capacity: High water holding capacity as clay particles have the ability to hold water tightly
  • Air spaces: Clay soils have fewer air spaces due to the compactness of the particles, leading to limited oxygen availability for roots and soil organisms
  • Biota: Clay soils can support a diverse range of soil organisms due to their ability to retain water and nutrients
  • Potential to hold organic matter: High potential to hold organic matter, as clay particles have strong binding capacity and can retain organic compounds.

   Loam Soils

• • Mineral content: Loam soils have a balanced mixture of sand, silt, and clay particles, providing a combination of different mineral compositions
  • Nutritional content: Loam soils generally have a moderate nutrient content and cation exchange capacity, allowing for adequate nutrient retention and availability
  • Drainage: Moderate drainage characteristics, providing a balance between water movement and retention
  • Water holding capacity: Loam soils have a moderate water holding capacity, retaining enough water for plant use while allowing excess water to drain
  • Air spaces: Loam soils have a balanced structure, with sufficient air spaces for root respiration and soil organism activity
  • Biota: Loam soils support diverse soil biota, including microorganisms, earthworms, and other soil organisms
  • Potential to hold organic matter: Loam soils have a moderate potential to hold organic matter, as they provide a suitable environment for organic material decomposition and nutrient cycling

Soil Properties Summary Table

Property	Sand	Loam	Clay
Nutrient status	Poor	Moderate	Good
Water infiltration rate	High	Medium	Low
Water holding capacity	Low	Medium	High
Aeration	Good	Moderate	Poor
Potential to hold organic matter	Low	Medium	High
Ease of working (ease with which soil can be manipulated)	Good	Moderate	Poor

Primary Productivity of Different Soils

• The structure and properties of soils directly influence their ability to promote primary productivity:
  • Nutrient content affects the availability of essential elements for plant growth
  • Water holding capacity and drainage characteristics determine the availability of water to plants (water being essential for photosynthesis), whilst preventing waterlogging (which can be damaging for some plants and therefore lower primary productivity)
  • Air spaces ensure oxygen availability for root respiration and soil organisms
  • Biota, including microorganisms, play crucial roles in nutrient cycling, organic matter decomposition, and symbiotic relationships with plants (e.g. nitrogen-fixing bacteria)
  • The potential to hold organic matter influences soil fertility, as organic matter provides essential nutrients and improves soil structure

• The combination of these factors in different soil types determines their suitability for supporting primary productivity, influencing the growth and health of plants and consequently the overall functioning of ecosystems

Soil Structure and Texture

• Soil structure refers to the arrangement or shape of soil particles and has a direct impact on primary productivity
• Soil texture, on the other hand, refers to the size of soil particles, with sand particles being less than 2 mm in diameter, silt particles less than 0.02 mm, and clay particles less than 0.002 mm
  • Soil texture is an important characteristic as it influences various soil properties, including moisture content, aeration, nutrient retention, and ease of cultivation and root penetration

• Clay soils have a high potential for nutrient exchange due to their large surface area relative to volume
  • However, they tend to become waterlogged and are often described as "cold" or "heavy"
  • In periods of drought, clay soils can shrink, leading to structural damage (e.g. cracking)

• Sandy soils have excellent drainage capabilities and are commonly referred to as "light" soils
• Silt soils are particularly prone to compaction if ploughed when wet, which can negatively affect soil structure and plant growth
• A loam soil, which is a balanced combination of sand, silt, and clay, is often considered the most favourable for cultivation
  • It is easy to work with, drains well, retains moisture and nutrients, and provides good aeration
  • As a result, loam soils have the highest potential for primary productivity (plant growth)

Soil Texture Triangles

 

Soil texture triangles are used to classify soil types based on their relative proportions of sand, silt, and clay

• A soil texture triangle is a graphical tool used to classify soil types based on their relative proportions of sand, silt, and clay
  • The triangle is divided into three sections representing the different soil particle sizes: sand, silt, and clay
  • The soil texture triangle allows for easy visualisation and classification of soil types based on their particle size distribution
  • By locating a soil sample's percentage of sand, silt, and clay on the triangle, it is possible to determine its textural class (e.g. sandy loam, silty clay, etc.)

• The soil texture triangle illustrates the differences in composition of soils by showing how the varying proportions of sand, silt, and clay affect soil properties such as moisture retention, drainage, nutrient holding capacity, and workability
  • Soils with higher percentages of sand have larger particles, resulting in good drainage but lower water and nutrient holding capacity
  • Soils with higher percentages of clay have smaller particles, which leads to higher water and nutrient retention but slower drainage
  • Soils with higher percentages of silt have intermediate properties, offering a balance between drainage and water-holding capacity

• The soil texture triangle provides a practical tool for understanding and classifying soils, aiding in agricultural and environmental management decisions, such as irrigation practices, fertiliser application, and crop selection