Soil Texture and Structure Chris Thoreau February 24, 2012
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
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
Mineral ratios determine soil texture
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