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 ( 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

2:1 Clay Shrink and swell 1:1 Clay No change

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 PropertySandSiltClay Water Holding PoorMedium to high High Nutrient Holding PoorMedium to High High AerationGoodMediumPoor

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 soils 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