Larry Zibilske, Ph.D.Larry Zibilske, Ph.D.

Texas Plant & Soil Texas Plant & Soil Lab Edinburg, TXLab Edinburg, TX

Soil Science Soil Science FundamentalsFundamentals

SoilSoilA Living, Breathing Dynamo A Living, Breathing Dynamo Filled with Complex, Filled with Complex, Interacting Populations of Interacting Populations of OrganismsOrganisms

DirtDirtWhat’s left when What’s left when nutrients have been nutrients have been depleted; unable to depleted; unable to support large support large populations of populations of organismsorganisms

Soil ConstituentsSoil Constituents

1.1.Mineral MatterMineral Matter

2.2.AirAir

3.3.WaterWater

4.4.Organic MatterOrganic Matter

1. Soil 1. Soil MineralsMinerals

MineralogyMineralogy

RocksRocks (a noun, not a verb) (a noun, not a verb)

1.1.Contain elements in Contain elements in crystal formcrystal form

2.2. Practically inert forms of Practically inert forms of oxygen, silicon, aluminum, oxygen, silicon, aluminum, iron, calcium, magnesium, iron, calcium, magnesium, copper, zinc…many copper, zinc…many elements except nitrogen.elements except nitrogen.

MineralogyMineralogy

Soil comes from rocks Soil comes from rocks

by by weatheringweathering

Over a long time, rain, Over a long time, rain, ice, atmospheric acids, ice, atmospheric acids, break rocks into smaller break rocks into smaller and smaller and smaller particlesparticles

HorizonsHorizonsSoils develop Soils develop from bottom upfrom bottom up

OO=Organic =Organic LayerLayer

AA=“Plow Layer”=“Plow Layer”

BB=Subsoil=Subsoil

CC=Parent =Parent MaterialMaterial

Soil Forming ModifiersSoil Forming Modifiers

““ResidualResidual” soils are ” soils are rarerare

Most have been Most have been transported from other transported from other placesplaces

•WaterWater

•WindWind

Soil ParticlesSoil Particles

Categorized by sizeCategorized by size

SandSand: 2 mm down to 0.05 mm: 2 mm down to 0.05 mm

•SiltSilt: 0.05 mm down to 0.002 : 0.05 mm down to 0.002 mmmm

•ClayClay: < 0.002 mm: < 0.002 mm

Categorized by sizeCategorized by size

--as if they were bank --as if they were bank accounts--accounts--

SandSand: $1,000,000 down to : $1,000,000 down to $25,000$25,000

•SiltSilt: $25,000 down to $1000: $25,000 down to $1000

•ClayClay: < $1,000: < $1,000

Soil Soil TextureTexture

How much of each How much of each particle size are in the particle size are in the soilsoil•% Sand, silt, clay% Sand, silt, clay

•ClayClay dominates texture issues dominates texture issues

•Loam not dominated by any one sizeLoam not dominated by any one size

Clay soils can be Clay soils can be tightly packed. What tightly packed. What does this suggest?does this suggest?

Sandy soils are Sandy soils are difficult to pack. difficult to pack. What does this What does this suggest?suggest?

Soil Soil PropertiesProperties

Cation Exchange Cation Exchange CapacityCapacity

““CEC”CEC”

CECCEC is the capacity of a is the capacity of a soil to hold and exchange soil to hold and exchange cations with soil watercations with soil water

CationsCations are chemical elements are chemical elements that have been ionized and now that have been ionized and now carry a positive (carry a positive (++) charge.) charge.

Examples: CaExamples: Ca2+2+, Mg, Mg2+2+, NH, NH44++

AnionsAnions are chemical elements that are chemical elements that have been ionized and now carry have been ionized and now carry a negative (a negative (--) charge.) charge.Examples: SOExamples: SO44

2-2-, NO, NO33--, PO, PO44

3-3-

•Clay minerals are often layered Clay minerals are often layered crystalscrystals

•Weather to very small flat, platy Weather to very small flat, platy particles with lots of (-) charges on particles with lots of (-) charges on their edgestheir edges

•Imperfections inside the crystals also Imperfections inside the crystals also cause (-) charges.cause (-) charges.

•Organic matter greatly affects CEC…Organic matter greatly affects CEC…more on this later.more on this later.

The origins of The origins of CECCEC

Soil Particle edges Soil Particle edges have (-) chargeshave (-) charges

Negative charges Negative charges attract positively attract positively charged ions charged ions ((cationscations))

CEC is a CEC is a “balance”“balance”

Percent Base Percent Base SaturationSaturation

•Shows up on soil test reportsShows up on soil test reports

•Proportion of bases Proportion of bases occupying CECoccupying CEC

•Ca, Mg, K, Na (not H or Al)Ca, Mg, K, Na (not H or Al)

•Index of fertility and Index of fertility and managementmanagement

•Calcium should dominate:Calcium should dominate:

•Ca 80%, Mg 15%, K 5%, Na 1%Ca 80%, Mg 15%, K 5%, Na 1%

Soil differences in Soil differences in CECCEC

Sandy Soils: 0-3Sandy Soils: 0-3

LS to SL Soils: 3-10LS to SL Soils: 3-10

Loam Soils: 10-15Loam Soils: 10-15

Clay Loams: 15-30Clay Loams: 15-30

Clay Soils: >30 Clay Soils: >30

Organic Matter: Organic Matter: 200200!!

Relative importance of Relative importance of mineral particles and organic mineral particles and organic matter:matter:

CEC=(% OM x 200) + (% Clay x CEC=(% OM x 200) + (% Clay x 50)50)

Example: Soil with 2% OM and 10% Example: Soil with 2% OM and 10% ClayClay

(0.02 x 200) + (0.1 x 50)= 9(0.02 x 200) + (0.1 x 50)= 9

Soil pHSoil pH(Soil (Soil

Reaction)Reaction)The balance between acidity and The balance between acidity and alkalinity in soilalkalinity in soil

Technically, the negative log of Technically, the negative log of the hydrogen ion concentration the hydrogen ion concentration in the soil.in the soil.

pH scale: 0-14pH scale: 0-14

7.0 is neutral7.0 is neutral

< 7.0 is acidic< 7.0 is acidic

> 7.0 is alkaline> 7.0 is alkaline

Affects Affects solubilitysolubility of soil of soil nutrientsnutrients

Direct affects on plantsDirect affects on plants

Where does all this acid come Where does all this acid come from?from?

1.1.Lack of basic elements (Ca, Mg,) in Lack of basic elements (Ca, Mg,) in soil minerals from which the soil is soil minerals from which the soil is generated.generated.

2.2.Leaching of basic elements from the Leaching of basic elements from the soilsoil

3.3.Plant and microbe activity produces Plant and microbe activity produces acidic end productsacidic end products

Why do other soils have little acid Why do other soils have little acid (alkaline soils)?(alkaline soils)?

1.1.Formed from minerals with high Formed from minerals with high amounts of basic elementsamounts of basic elements

2.2.Leaching losses are less due to arid Leaching losses are less due to arid climateclimate

3.3.Hostile growing conditions for plants Hostile growing conditions for plants and microbes means little biological and microbes means little biological acid formedacid formed

Effects of low soil pH (high Effects of low soil pH (high acidity)acidity)

Increased levels of several Increased levels of several nutrients:nutrients:

Copper, Iron, Manganese, ZincCopper, Iron, Manganese, ZincLeaching of bases from Leaching of bases from soilsoilSoil structure Soil structure problemsproblems

Effects of high soil pH (low Effects of high soil pH (low acidity)acidity)

Low solubility of metallic Low solubility of metallic nutrients:nutrients:

Copper, Iron, Zinc, ManganeseCopper, Iron, Zinc, ManganeseMarked reduction in P solubility (Ca, Marked reduction in P solubility (Ca, Mg)Mg)

Soil structural Soil structural problemsproblems

2. Soil Air2. Soil Air

Soil Particles mean Soil Particles mean Pore Pore SpaceSpace•Spaces between soil particles Spaces between soil particles allow air in and waste gases outallow air in and waste gases out

•Pore space size determines soil Pore space size determines soil aeration capacityaeration capacity

•Air shares pore space with Air shares pore space with waterwater

•More water, less airMore water, less air

Pore sizePore size is very important is very important

•Sandy soil: Higher Sandy soil: Higher proportion of larger pores proportion of larger pores – easier air movement– easier air movement

•Clay soil: Higher Clay soil: Higher proportion smaller pores – proportion smaller pores – slows air movementslows air movement

Why is Why is aerationaeration important?important?•Oxygen is needed for plant rootsOxygen is needed for plant roots

•Oxygen is needed for many soil Oxygen is needed for many soil microbesmicrobes

•Affects chemical reactions:Affects chemical reactions:

•Oxidation-reduction reactions Oxidation-reduction reactions affect many soil nutrients (N, S, affect many soil nutrients (N, S, Mn, Fe, Zn, etc.) Mn, Fe, Zn, etc.)

Balance between Balance between aerobic and aerobic and anaerobic conditionsanaerobic conditions

•Both conditions exist in Both conditions exist in soils at the same time. How soils at the same time. How is that?is that?

•Natural process that Natural process that alternates, often after rain alternates, often after rain events.events.

Soil Air Soil Air ContentsContents

Nitrogen: 79%Nitrogen: 79%

Oxygen: 5-15%Oxygen: 5-15%

Carbon Dioxide: 0.3-4%Carbon Dioxide: 0.3-4%

Trace gases: very small Trace gases: very small amountsamounts

Aeration and Soil Aeration and Soil BiologyBiology

Respiration vs. Respiration vs. FermentationFermentation

•Respiration: Oxygen Respiration: Oxygen Present; high metabolic Present; high metabolic efficiencyefficiency

•Fermentation: Oxygen Fermentation: Oxygen Absent; lower metabolic Absent; lower metabolic efficiencyefficiency

Fermentation Fermentation ≠≠ Anaerobic Anaerobic RespirationRespiration

Changes in Oxygen Changes in Oxygen StatusStatus

•Follow saturating rainsFollow saturating rains

•Follow heavy irrigationFollow heavy irrigation

•Follow tillage (no-till?)Follow tillage (no-till?)

•Compaction reduces (pore Compaction reduces (pore size declines)size declines)

3. Soil 3. Soil WaterWater

Solution containing Solution containing dissolved and dissolved and suspended materialssuspended materials

•Nutrients, some Nutrients, some nutrientsnutrients

•SaltsSalts

•Dissolved organic Dissolved organic materialmaterial

•Carries nutrients Carries nutrients toto plants plants

•Carries toxins/endproducts Carries toxins/endproducts awayaway

from plantsfrom plants

•Movement of some Movement of some microbesmicrobes

MovemeMovementnt

•Gravity; percolating waterGravity; percolating water

•Capillarity; adheres to Capillarity; adheres to surfaces surface, moves surfaces surface, moves under under tensiontension

Soil Air vs. Soil WaterSoil Air vs. Soil Water

Plant Available Plant Available WaterWater•Held in soil under Held in soil under tensiontension, but not too , but not too much.much.

•Plants must use energy Plants must use energy to pull water away from to pull water away from soilsoil

Drying soil has thinner Drying soil has thinner water films; plants work water films; plants work harderharder

•The thinner the water The thinner the water film on soil particles, film on soil particles, the more energy is the more energy is needed to pull it away.needed to pull it away.

•Plants must generate Plants must generate this energythis energy

4. Soil Organic 4. Soil Organic MatterMatter

A reservoir of carbon-A reservoir of carbon-containing compounds in containing compounds in various states of various states of decomposition.decomposition.• Remains of plant/animal/insect tissues Remains of plant/animal/insect tissues that have been chemically modified. that have been chemically modified. Some residues. ~50%Some residues. ~50%

•Newly synthesized carbon compounds Newly synthesized carbon compounds randomly attached to the residues. randomly attached to the residues. ~50%~50%

•Minerals bonded to the organic matrix.Minerals bonded to the organic matrix.

•Part polymeric, part random bondingPart polymeric, part random bonding

A reservoir of carbon-A reservoir of carbon-containing compounds in containing compounds in various states of various states of decomposition (cont.)decomposition (cont.)

•Range of molecules from Range of molecules from high high energyenergy** (fresh residues) to (fresh residues) to low low energyenergy (humus) (humus)•Microbial decomposition cycles Microbial decomposition cycles nutrients and taps the energy nutrients and taps the energy contained in organic matter. contained in organic matter. (energy does not “cycle”)(energy does not “cycle”)

** BiologicallyBiologically available available

Soil organic matter:Soil organic matter:•High CEC (also AEC, anion exchange High CEC (also AEC, anion exchange capacity)capacity)

•Energy and nutrient source for microbesEnergy and nutrient source for microbes

•Mostly insoluble forms, esp. laterMostly insoluble forms, esp. later

•Carbohydrates, proteins, fats, waxes, and Carbohydrates, proteins, fats, waxes, and many exotic molecules made by microbes.many exotic molecules made by microbes.

Organic Fraction of Organic Fraction of SoilSoil

Geography of Soil Organic Geography of Soil Organic MatterMatter

HumuHumuss

•Most decomposed material that Most decomposed material that remains “organic”remains “organic”

•Low energy material with long Low energy material with long residence time in soil. residence time in soil. Difficult for Difficult for microbes to decompose furthermicrobes to decompose further

•Foundation of good soil “tilth”, water Foundation of good soil “tilth”, water and nutrient holding capacities of a and nutrient holding capacities of a soilsoil

•Biological and Chemical BufferBiological and Chemical Buffer

HumuHumuss

•Contains few, if any Contains few, if any recognizable patterns of recognizable patterns of source materialssource materials

•Characterized by Characterized by Humic AcidHumic Acid, , Fulvic AcidFulvic Acid, and , and HuminHumin

•Operational definitions based on Operational definitions based on humus solubility in acids or bases.humus solubility in acids or bases.

Soil Soil StructureStructure

•Microbially-produced Microbially-produced slimes “glue” OM and clay slimes “glue” OM and clay particles into a particles into a 3-3-dimensional aggregatedimensional aggregate

•Increases average Increases average pore pore sizesize larger larger

•Improves aeration, Improves aeration, drainagedrainage

•Temporary; Temporary; needs needs constant renewalconstant renewal. How?. How?

FracturinFracturingg

Grains Grains stuck stuck togethertogether

Soil Soil ManagementManagement

•Control and exploit soil Control and exploit soil properties (Here is where we properties (Here is where we part ways with Naturepart ways with Nature))

•Natural soil fertility; based on Natural soil fertility; based on minerals (CEC, pH, OM, minerals (CEC, pH, OM, structure)structure)

Tillage controls weeds Tillage controls weeds and makes a nice and makes a nice seedbed…and leads to seedbed…and leads to cloddingclodding, more , more weedsweeds, , dryingdrying, organic matter , organic matter lossloss, , erosionerosion..

FertilizatioFertilizationn•Augments natural processes Augments natural processes

that supply plant nutrients; that supply plant nutrients; depleted soils have little ability depleted soils have little ability to feed plantsto feed plants

•Pathway to plants is through Pathway to plants is through water; fertilizer nutrients must water; fertilizer nutrients must get into solutionget into solution

•Chemical fertilizers are very Chemical fertilizers are very soluble; little soluble; little synchronysynchrony is is possiblepossible

•Excess lost to leaching, Excess lost to leaching, volatilization, immobilizationvolatilization, immobilization

Nutrient Use Nutrient Use EfficiencyEfficiency

Management GoalManagement Goal•MeteringMetering nutrients to plants, nutrients to plants, as they need it.as they need it.

•SynchronySynchrony is how it happens is how it happens in natural systemsin natural systems

•High NUEs can be attained by High NUEs can be attained by using organic nutrient sources, using organic nutrient sources, but it can be overloadedbut it can be overloaded