UBC Farm Soil Workshop Series

Kwantlen Farm School Soil Texture and Structure

Chris Thoreau February 24, 2012Soil 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 interactionsSoil TextureMineral ComponentsSandSilt Clay

Soil Texture

SandLargest soil mineral particles (.02 2 mm)Formed greatly from physical processesSpherical/erratic in shapeSand = little rocksLarger pore spacesGood drainageDoes not hold a chargeDifficult to compact

Soil Texture

SiltSize between sand and clay (.002 - .02 mm)Usually physically formed out of sandHold and releases water wellFlat or round in shapeHolds very little chargeFeels soapyCarried in moving water

Soil Texture

ClaySmallest soil mineral particle (< .002 mm)Holds water very wellHolds strong negative charge for mineral adsorptionSusceptible to compactionPlaty-/flat-shaped particlesVarious lattice structures

Soil Texture

Mineral ratios determine soil textureSoil TextureClayUnderstanding structure of clay is important for:CompactionWater holdingCation adsorptionSoil cultivation

Clays are categorized by their layer structureRelationship of Si-tetrahedral and Al-octahedral sheets2:1; 1:1; 4:1; 5:2

Soil Texture2:1 ClayShrink and swell

1:1 ClayNo changeSoil Texture

Shrink and Swell of ClayInterlayer space expandswith increasing watercontent in soilSpace contracts as wateris removedClay can crack when it shrinks

Soil TextureWhy is Texture Important?Water InfiltrationWater StorageFertility AerationTrafficability

Soil texture knowledge is the key to developing an overall soil maintenance and improvement plan

Soil TextureNotes:We do not change the texture of soilsWe can change the characteristics of certain textured soils

We change soil characteristics through:Additions of organic matterIn soil and on top of soilCultivation practicesRaised bedsSoil TextureAttributes of Different Soil TexturesSoil TexturePropertySand SiltClayWater HoldingPoorMedium to highHighNutrient HoldingPoorMedium to HighHighAerationGoodMediumPoorWe can determine the texture of the soil by feeling itRibbon testBall testJar testLaboratory tests give more accurate results

Soil TextureTexture Questions?

Soil Texture

Soil Structure: How the soil fits togetherPrimary particles are arranged into secondary particles called aggregates (or peds)Soil StructureWhy is Structure Important?Pore spaceAir and water movementRooting spaceNutrient storage and releaseContributes to soil resilienceCultivationErosion resistance

Soil StructureHow does aggregate formation occur?

Flocculation + Cementation = Aggregation

Flocculation: Primary pulled close together (into flocs) by attractive forces (electrostatic forces, H bonding)CementationPrimary particles held together by cementing agentsCarbonates; clays; OM; OxidesSoil Structure

Soil StructureSoil Aggregates are classified by their shape

Soil Structure

Ideal structure: SpheroidalTypical in A HorizonRounded; loose Granular (porous) or Crumb (very porous)Greatly affected by soil management OR mismanagementImproved with OM additions and microbial activity

Soil StructureSoil structure is particularly important in providing adequate pore space for:Root growthWater movementGas exchangeMicrobial activityMacrobial activitySoil StructureStructure 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 structureSoil StructureRelated to textureVery important when considering soil cultivation

Dependent on: Texture/clay contentClay typeSoil water content

Soil Consistency Soil Consistency

Cultivating soil when too dryBreaks aggregates into small piecesDe-aggregatesCan result in dust

Very damaging to soil structure

The drier the soil the more it acts like powder

Soil Consistency

Cultivating soil when too wetWhere to start?!

CompactionRisk and depth of compaction increases in wet soilSoil Consistency

Cultivating soil when too wet

The wetter the soil - the more it acts like waterSoil Consistency

Soil Consistency

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

Soil Consistency We promote good structure in soil by:Minimizing cultivation (especially in sandy soils) and using appropriate cultivation methodsAvoiding compaction (especially in clay soils)No tractor in wet soil!Especially careful with claysCultivating at proper soil consistencyAdding various types of organic matter regularlyMaintaining a proper pHPromoting microbial life especially fungiAlways keeping the soil coveredPreferably by crops

Good Soil StructureSoil texture influences soils ability to aggregateClay soils aggregate more readilySandy soils have les stable aggregates

Organic matter, plant growth, and microbial activity all contribute to aggregate stability

Soil Structure and Texture