Class 1: Introduction to Soil Formation, Texture and Structure Chris Thoreau February 11, 2012
Soil is a dynamic composition of: Minerals Water and its solutions Organic matter (detritus) Air and other gas mixtures......which, through interacting with each other and with plant roots, allows for the growth of photosynthesizing terrestrial plants and acts as a habitat for micro- and macroorganisms which also interact with plants and plant roots Or
Physical Aspects: Minerals (from rocks) Sand Silt Clay and Colloids Organic Matter Plants and Roots Detritus (decaying organic matter) Animal waste (including microbes) Pore Space Air/Gases Water
Soil is the Mother of All Terrestrial Life
Conceptual Aspects: Habitat Micro-organisms Bacteria, Fungi both good and bad Viruses Macro-organisms Worms, Arthropods, Detrivores and Predators Plants Small Mammals Birds
Conceptual Aspects: Provider to plant life Rooting substrate Water holding and release Nutrient supply and reserve Heat sink and release Soil gases Symbionts Bacterial and fungal Insects
Habitat What happens when we disturb this habitat? At micro and macro level? What happens when we make additions to, or removals from, this habitat? Carbon:Nitrogen ratio? How do soil organisms and plants respond? Nutrient loss or gain? Providing for plant life What are the short-term and long-term results? Are we providing for the soil as well as the plants? What is the difference?
As a habitat we need to treat soil like a living organism, which requires: Food Organic Matter and Minerals Water Irrigation and Natural Air Shelter Cover crops, Cash Crops, Mulches General tender loving care
Questions?
Where does soil come from? Soil comes (mostly) from the weathering of rocks over long periods of time a process highly influenced by biological organisms, topography, aspect and human activity
1. Parent Material 2. Climate 3. Biota 4. Topography 5. Time
1. Parent Material Transported Gravity - Colluvial Water Alluvial, Marine, and Lacustrine Wind Eolian Ice Glacial Cumulose Due to plant life and anaerobic conditions High water table Peat and muck soils Residual In situ; long periods of weathering
2. Climate Temperature and rainfall are major factors Affect intensity of weathering Increased T and precipitation accelerate weathering 3. Biota Plants influence organic matter Arthropods and worms mix soil; add to OM Small mammals also mix soil
4. Topography Slope influences soil development Water infiltration rate Surface runoff Vegetation Aspect North and South slopes develop differently Elevation Climate changes with altitude
5. Time Often noted as most important soil formation factor Our soils in Lower Mainland are relatively young Since last ice age 10-12,000 years ago
Primary Minerals Sand and Silt Formed at high T and P (at depth); anaerobic conditions Physically and chemically formed Quartz, Feldspars, Micas, Secondary Minerals Clay Come from primary minerals Formed at low T and P (at surface) with Oxygen present Mostly chemically formed Silicate Clays
Weathering of Rocks Physical Chemical 1 Chemical 2
Additions Losses Transformations Translocations
Organic (O) Horizon High in organic residue from plant drop A Horizon Mineral component plus OM Most fertile part of soil; location of much root activity B Horizon Subsoil A horizon leaches here C Horizon Little influence by soil-forming processes
In a given area, over the period of soil formation, environmental conditions cause a certain set of soil processes to occur, which leads to a distinctive set of soil horizons at the time we observe the soil. These soil horizons are the basis for classifying the soil in the Canadian System of Soil Classification.
Podzol Gleysol Chernozem Brunisol Luvisol Organic Solonetzic Vertisol Regosol Cryosol
Video here
Any questions? Check follow up readings and websites for more detailed information UBC Virtual Soil Science Soil Orders of Canada Videos Canadian System of Soil Classification
Soil texture refers to the relative amount of sand, silt, and clay found in a soil The mixture of these components affects the feel of the soil as well as water, nutrient, and pore space interactions
Mineral Components Sand Silt Clay
Mineral ratios determine soil texture
Sand Largest soil mineral particles (.02 2 mm) Formed greatly from physical processes Spherical/erratic in shape Sand = little rocks Larger pore spaces Good drainage Does not hold a charge Difficult to compact
Silt Size between sand and clay (.002 -.02 mm) Usually physically formed out of sand Hold and releases water well Flat or round in shape Holds very little charge Feels soapy Carried in moving water
Clay Smallest soil mineral particle (<.002 mm) Holds water very well Holds strong negative charge for mineral adsorption Susceptible to compaction Platy-/flat-shaped particles Various lattice structures
Clay Understanding structure of clay is important for: Compaction Water holding Cation adsorption Soil cultivation Clays are categorized by their layer structure Relationship of Si-tetrahedral and Al-octahedral sheets 2:1; 1:1; 4:1; 5:2
1:1 Clay No change 2:1 Clay Shrink and swell
Shrink and Swell of Clay Interlayer space expands with increasing water content in soil Space contracts as water is removed Clay can crack when it shrinks
Why is Texture Important? Water Infiltration Water Storage Fertility Aeration Trafficability Soil texture knowledge is the key to developing an overall soil maintenance and improvement plan
Notes: We do not change the texture of soils We can change the characteristics of certain textured soils We change soil characteristics through: Additions of organic matter In soil and on top of soil Cultivation practices Raised beds
Attributes of Different Soil Textures Property Sand Silt Clay Water Holding Nutrient Holding Poor Poor Medium to high Medium to High High High Aeration Good Medium Poor
We can determine the texture of the soil by feeling it Ribbon test Ball test Jar test Laboratory tests give more accurate results
Texture Questions?
Soil Structure: How the soil fits together Primary particles are arranged into secondary particles called aggregates (or peds)
Why is Structure Important? Pore space Air and water movement Rooting space Nutrient storage and release Contributes to soil resilience Cultivation Erosion resistance
How does aggregate formation occur? Flocculation + Cementation = Aggregation Flocculation: Primary pulled close together (into flocs) by attractive forces (electrostatic forces, H bonding) Cementation Primary particles held together by cementing agents Carbonates; clays; OM; Oxides
Soil Aggregates are classified by their shape
Ideal structure: Spheroidal Typical in A Horizon Rounded; loose Granular (porous) or Crumb (very porous) Greatly affected by soil management OR mismanagement Improved with OM additions and microbial activity
Soil structure is particularly important in providing adequate pore space for: Root growth Water movement Gas exchange Microbial activity Macrobial activity
Structure can be easily observed in the soil and structural stability, or aggregate stability, can be measured in the lab Structure can be improved, to a point, by soil cultivation. Soil cultivation is also a great way to destroy structure
Related to texture Very important when considering soil cultivation Dependent on: Texture/clay content Clay type Soil water content
Cultivating soil when too dry Breaks aggregates into small pieces De-aggregates Can result in dust Very damaging to soil structure The drier the soil the more it acts like powder
Cultivating soil when too wet Where to start?! Compaction Risk and depth of compaction increases in wet soil
Cultivating soil when too wet The wetter the soil - the more it acts like water
Soil consistency, determined greatly by water content and percentage of clay, plays a major role in when soil can be cultivated! Not as crucial when hand digging
We promote good structure in soil by: Minimizing cultivation (especially in sandy soils) and using appropriate cultivation methods Avoiding compaction (especially in clay soils) No tractor in wet soil! Especially careful with clays Cultivating at proper soil consistency Adding various types of organic matter regularly Maintaining a proper ph Promoting microbial life especially fungi Always keeping the soil covered Preferably by crops
Soil texture influences soil s ability to aggregate Clay soils aggregate more readily Sandy soils have les stable aggregates Organic matter, plant growth, and microbial activity all contribute to aggregate stability