IDS Exam 2

Soil Colloids

• Small diameter (.1 to .011 um , high surface area, surface charte, adsorption of cations and anions + water

Inorganic Colloids

• Make up bulk, clay minerals, hydrous oxides, usually crystalline

Organic colloids

• highly decomposed organic matter called humus

• generally greated influence on soil properties per weight

Phyllosilicates

Clay Minerals constructed from tetrahedral and octahedral sheets. layered silicates.

• Tetrahedral sheets : Si^4+ , O²-

• Octahedral sheet: Al³+ , O²-

Phyllosilicates: The tetrahedral sheet

• Silicon Ion: shares its electrons evenly between the four oxygen ions, Each oxygen is left with an excess charge of negative one

• Orthosilicate anion: theoretically could be neurtalized by four hydrogen ions. The SiO₄^4- anion does have another option to satisfy the charges

Phyllosilicates: The octahedral sheet

• aluminum octahedral unit. Depending on conditions each 4 or 6 oxygen ions. Prefers 6 ions

Types of Clays

• 2:1 phyllosilicate type clays - two tetradehdral sheets to one octahedral. Ex) Illite , younger soils (Mollisols, Alfisols). Shrink swell soils (vertisols)

• 1:1- Ex) Kaolinite, Older/more weathered soils (Spodosols, Ultisols)

• Iron and aluminum oxides- just octahedral sheets. Highly weathered Soils (Oxisols)

Benefits of soil Organic Matter

• soil structure: bulk density aggregation

• Increased rainfall infiltration

• Aeration

• Water holding capacity

• Nutrient cycling

• Buffering capacity

• Insect/ Microbial activity

• Ion exchange & filtration

Up to 15% of organic matter is fresh residue

Decomposing organic matter

• As organisms continue to decompose organic matter, it becomes resistant to further changes by microorganisms

• Humus: highly stabilized organic matter (Resistant to further decomposition)

  • C₁₈₇H₁₈₆O₈₉N₉S₁ = Average chemical formula

Humus: Stable Organic Matter

Imparts the dark color to soils with high organic matter

• Increase in soil temp + oxygen = increased decomposition

The role of humus

• Very chemically reactive:

  • - High Cation exchange capacity (even higher than clay)

  • Binds nutrients to itself and associated soil particles

  • Enhances soil fertility

• Modifies and “stabilizes” toxic materials so that they are less reactive and/or dangerous

• Buffers soil pH - precents rapid changes in pH

Sources of charge

In the soil colloidal system, we can think of charge as originating in one of two places:

• Within the octahedral or tetrahedral lattice (isomorphic substitution)

• At the surface of the colloid (pH dependent)

Isomorphic Substitution

Process in which one element substitutes another of comparable size in the crystalline structure. Permanent change

pH dependent charge

broken ends of clays carry a charge. Charge can based on the pH. Lower pH = more H⁺ will attach to the broken ends.

Organic colloids only have…

pH dependent surface charge

What happens when an anion meets a cation?

Combine to become a molecule, the simplest compound: e.g. NaCl (Sodium chloride)

Cation exchange

interchange between a cation in solution and another cation on the surface of a negatively charged colloid

Cation exchange influenced by

• Strength of adsorption (Al>Ca>Mg>K = NH>Na>h)

Cation exchange capacity

Equivalent weight = molecular wt (g) / valence or charge

• EX) Calcium, Ca⁺⁺, atomic weight = 40g/mol, one Ca⁺⁺ replaces 2H⁺.

  • Thus Ca, required to replace 1mg of H is = 40/2 = 20 mg/meq

Origin of the cation exchange properties of soils

• Isomorphic substitution in clay minerals

  • Mg²⁺for Al³⁺ in the octahedral sheet

  • Al³⁺for Si⁴⁺ in the tetrahedral sheet

• Deprotonation reactions on metal oxide surfaces

  • Me-OH → Me-O^- (Me = metal oxide)

• Deprotonation of carboxylate and phenolate groups in soil organic matter

  • R-CO(OH) → R-CO(O^-) (R= soil organic matter)

  • Ar-OH → Ar-O^- (Ar= phenolate)

Base Saturation

The percent of the cation exchange capacity occupied by bases (2+’s : Ca, Mg, +’s: K, Na)

• low base saturation indicates an acidic soil with less plant nutrients

• high base saturation indicates a soil has a less acidic pH. and more nutrient retention

Ca+Mg+K+Na / CEC * 100

the equation for base saturation

Smectite

2:1 silicate clay. Charge: -80 to -150 cmol/kg (very high). 95% permanent charge (isomorphic substitution), only 5% pH-dependent

Kaolinite

1:1 silicate clay. Charge: -1 to -15 cmol/kg (low). Only 5% permanent, 95% pH dependent (comes from broken-edge OH groups, since there’s almost no isomorphic substitution in 1:1 clays)

Gibbsite (Al-oxide) & Geothite (Fe-oxide)

Charge: +10 to -5 and +20 to -5 (small, and can go net positive at low pH). 100% pH-dependent, 0% permanent- no isomorphic substitution at all

Humus card

Organic colloid. Charge: -100 to -500 cmol/kg - the highest of any soil colloid. 90% pH dependent (carboxylic/phenolic functional groups ionizing), 10% permanent

Effective cation exchange

sum of the permanent charge plus the pH dependent charge, ph up = cation exchange capacity up. Weathering up = decreased cec

Hydrometer method

Based on sedimentation from Stokes’ Law. Smaller particles settle slowly; larger particles settle fast

Aggregation

• dependent on soil colloids present, & the cations associated with the colloids

Flocculation

Individual particles stick together to form aggregates of particles or floccules. Such aggregates do not move in the soil solution and form the basis for soil structure.

Promoted by calcium, magnesium, and aluminum ions

Dispersion

Aggregates do not form. Each particle behaves as an individual.

• Water, air, and root movement in the soil are inhibited

• Dispersion is not a desirable characteristic of productive soils

Caused by sodium cations

Significance of soil pH

• Metals such as Fe, Mn, Cu, and Zn can be toxic to plants at low pH

• Plants can exhibit deficiencies of the same metals at high pH

  • Phosphorus:

    • Bound to Fe/Al at low pH

    • Bound to Ca at high pH

Sources of acidity: Al

Presence of this __³+ provides a mechanism for the soil system to buffer sudden changes in pH

Active acidity

H^+ or Al³+ in the water of the soil and a measure of how acidic the soil is

what the pH meter reads

Residual acidity

H^+ or Al³+ that are bound to clay or OM and NOT exchangeable

can be neutralized by lime

pH buffering

soils ability to resist change in pH

• H+ ions removed from solution are replaced by other H+ ions held on the exchange complex (and from reserve/residual acidity)

Soil texture & Buffering Capacities

Clay content or pH increases CEC, buffering capacity, and lime increases

Highest CEC = clay loams (25) = more lime less often

Lowest CEC = sands (5) = less lime more often

More weathered

the more acidic the soil becomes

Beneficial effects of lime

• Crop yield improvement

• Nutrient availability effects

• Improved microbial activity

• Improved legume fixation of N

• Ca & Mg addition

The Lime reaction

1) Ca²+ from the lime replaces Al³+ and H^+ on the cation exchange complex

2) The carbonate reacts with the H^+ ions, removing them from solution thereby raising the pH (lime works by turning H^+ into water!)

3) Al³+ is hydrolysed to form Al-hydroxides and H^+ ions

• Carbonate from the lime neutralizes the H^+ generated during the Al hydrolysis

Lime recommendation model

(target pH - current pH)/(6.6-current pH) * AC

AC= 4*(6.6-buffered pH)

Exchangeable acidity (AC)

Soil acidity is derived from pH and buffered pH. Acidity is exchangeable

Factors that change lime recommendations

• starting soil pH (lower pH = more lime = more acidic)

• Vegetation- each plant has its own pH requirement

• Soil texture- clayey soil generally needs more lime

• Buffering capacity and cation exchange capacity- higher buffering capacity and higher cation exchange capacity = more lime

• Type of lime used

• lime fitness (mesh size of lime): coarser lime = more lime

Particle Size, liming material

Surface area goes up, so does the reaction rate

Ag lime

• most common liming material used

• minerals mined from geological deposits

• Calcitic limestone: calcium carbonate

• Dolomitic limestone: calcium & magnesium carbonate

Quick lime

(CaO) obtained by heating calcium carbonate; burns off carbon dioxide

• extremely reactive

• extremely expensive, so only really used for greenhouse plants

Pelleted lime

created out of finely ground, good quality calcitic or dolomitic lime

Calcium carbonate equivalency (CCE)

neutralizing value of any liming material compared to pure calcium carbonate

Nitrogen exceptions

NH_{4⁺ (ammonium, cation) OR NO3^-} (anion) depending on soil conditions

Boron

pH dependent, can swing either way

Plant nutrition: structural elements

• Carbon: all organic compounds {air} most plant concentration

• Hydrogen: forms the main structural components {water} least

• Oxygen: main structural {water, air}

Plant nutrition: primary macronutrients

• Nitrogen: proteins, chlorophyll, nucleic acids. highest concentration

• Phosphorus: energy transfer, metabolism, nucleic acids, nucleoproteins. 0.05-1%

• Potassium: Protein synthesis; translocation of carbohydrates; enzyme activation. 0.03-6%

Plant nutrition: secondary macronutrients

• Calcium: structural component of cell walls; cell elongation; affects cell permeability. 0.1-3%

• Magnesium: component of chlorophyll; enzyme activator; cell division. 0.05-1%

• Sulfur: constituent of proteins; involved in respiration and nodule formation. 0.05-1.5%

Ion absorption by plants

• Passive uptake: Diffusion, Ion exchange

• Active Ion uptake: Ion Carriers, Selective/competitive

Nitrogen gases

• N2 = inert, 78% of the atmosphere

• Greenhouse gases! NO2→NO→N₂O

  • Nitrous Oxide (N₂O) is about 310x more efficient at trapping heat than is carbon dioxide

• Ammonia (NH₃)

• Haber-Bosch process: 2N₂+3H₂+heat = 2NH₃

Plant available soil N

• Nitrate (NO₃^-) is more likely to be lost from leaching

• Ammonium (NH₄⁺) is more likely to be lost from volatilization.

Symbiotic biological N fixation

When plants provide sugars to microbes and microbes provide nitrogen [Legumes, Bacteria, N₂]

Converting nitrogen into compounds, organic, and inorganic, that can be used by plants and other living organisms

Legumes

(soybean, alfalfa, peanut) are infected by bacteria (-zobium) to form root nodules

Non-legume

• Nodulated: alders and actinomycetes

• Non-nodulated: Cynobacteria

Mineralization

Biological conversation of unavailable organic N into plant available inorganic N

• Multistep process:

  • Ammonification- organic N to ammonium

  • Nitrification- Ammonium to nitrate (with an intermediary of nitrite)

Immobilization

Biological conversation of plants available N into unavailable organic N.

• Plant & Microbial uptake of: NH4+ and NO3-

Ammonification

• Organic N to ammonium (NH4+), and the first step in mineralization

• Warmer temperatures accelerate this biological process

Nitrification

• ammonium to nitrate, and the second step in mineralization

• NH4+ oxidized to NO2- (nitrite) by nitrosomonas

• NO2- oxiidized to NO3- by Nitrobacter

• Rapid process

Nitrification brought by

• microorganisms

• warm temperatures

• adequate supply of O2 (well drained soil)

The fate of ammonium (NH4+)

• Immobilized (taken up) by plants and microorganisms.

• Bound to the cation exchange sites within the soil

• Lost to the atmosphere as a volatile gas: NH4 + + OH- → H2O + NH3

• Oxidized to an anionic form of N (i.e., nitrate) and acidify the soil. NH4 + → NO3 + 4 H+

The fate of nitrate (NO3-)

• Immobilization - plant and microorganisms use it up

• Leaching - reduce health of crop and cause environmental problems (blue-baby syndrome)

• Denitrification. • Nitrification is an acidifying process (i.e., decreases soil pH)

• Pesticides slow down the rate of nitrification.

Nitrogen Gains

Fertilizers, organic amendments, net mineralization, rainfall, symbiotic biological N fixations

Nitrogen Losses

crop uptake and removal, denitrification, erosion and runoff, leaching, volatilization

Volatilization

• Loss of N in gas form

• Surface a[[lied N sources such as Urea [CO(NH₂)₂] and animal waste

• High pH soils

• Higher temperatures

Denitrification

• Transforms NO₃^- to N₂, N₂O or NO gases

• Bacterial process that occurs:

  • under low oxygen (anaerobic conditions)

  • under warm conditions

  • where there is a source of organic matter for microbial energy

Phosphorus

• needed 10 times lower than N,K

• Function = energy transfer + Growth regulation

  • Concentrated in seed and fruit

P deficiency symptons

• Small size (stunted)

• Abnormally dark green

• Reddish purple pigment (especially early season and cool weather)

Highly weathered Ultisols found in southeastern US

Solid form of Phosphorus

Adsorbed as an anion on positive sites (opposite of CEC)

• Bound with Al & Fe to form minerals

• Organic P

Phosphorus reactions in soil

lower pH = more Fe and Al fixation

higher pH = more Ca fixation

Eutrophication

(phosphorus additions to natural waters, stimulates weed and algae growth):

• Nutrient enrichment of a P limited ecosystem

• Algal blooms can occur

• Natural cycling of algae can delete dissolved oxygen

• Low dissolved oxygen results in fish kills

P loss from soil

• runoff of soluble P

• Erosion of soil bound P

• Leaching:

  • organic forms most mobile

  • also possible on deep sands, organic soils

Total runoff P = particulate + soluble

P in runoff

• dissolved (soluble P) (DP)

• Particulate P (PP)

• Total P (TP) = DP + PP

• Bioavailable P (BAP) = DP + part of PP

Potassium (K)

• Required by plants in approximately the same amounts as nitrogen

• Used in photosynthesis, water regulation, and protein synthesis

• Benefits of proper K nutrition include:

  • Drought tolerance

  • Disease resistance

K deficiency symptoms

Stunted growth.

• Poor root systems.

• Lodging of small grains and corn.

• Under severe deficiencies: Premature defoliation, Delayed maturity, Plant death

Comparing nutrient cycles

Nitrogen cycle is mediated by microbes.

• The phosphorus cycle is mediated by

equilibrium chemistry and is pH-dependent.

• The potassium cycle is either in solution or not

in solution

A bag of fertilizer has 15 lbs. of P_2O_5. How much elemental P is in the bag of fertilizer?

E𝑙𝑒𝑚𝑒𝑛𝑡𝑎𝑙 𝑃 = 𝑃2𝑂5 × 0.44 𝑙𝑏𝑠 𝑃 = 15 𝑙𝑏𝑠 𝑃2𝑂5 × 0.44 = 6.6 𝑙𝑏𝑠 P

Secondary nutrients

• Calcium (Ca) – most Ca in the soil comes from minerals or lime application

• Magnesium (Mg) – most Mg in the soil comes from minerals and dolomitic limestone application.

• Sulfur (S)- most of the S in soil comes from minerals, atmospheric deposition, and organic matter or fertilizers (ammonium sulfate).

• Critical to plant growth, but not often considered as important as N, P, & K in soil fertility management programs.

Calcium

Structural component of plant cell walls

• Deficiency:

  • Poorly developed root systems

  • Relatively little fruit of poor quality

  • Blossom end rot / bottom rot end

  • Black heart

  • Poor seed germination

  • Deformed cells, tissues

Magnesium

Major component of chlorophyll & especially important in oilseed crops

• Deficiency:

  • Interveinal chlorosis on lower leaves

  • Stunted growth

  • Reduced flowering / fruiting

Ca and Mg in the soil

• Sandy soils tend to leach Ca2+ and Mg2+

faster than clayey soils.

• Ca2+ and Mg2+ stick to cation exchange sites.

• Recall liming materials:

– Calcitic lime provides Ca

– Dolomitic lime provides both Ca & Mg

• Gypsum (CaSO4) can be used to supply Ca

when lime is not required.

Sulfur in plant

Component of amino acids required for building proteins

• Deficiency:

  • Yellowing, like N, but in newer growth

  • “grows” out as roots reach subsoil

Sulfur in Soil

• Associated with organic matter.

• Exists as an anion therefore can be lost

due to leaching.

• Sources:

– Atmospheric deposition.

– Irrigation water (5ppm S in irrigation water in a typical growing season could result in 15 lbs S being applied).

Micronutrient fertilization

• Increased interest in micronutrient fertilization has been brought about by:

  • Higher crop yields, resulting in greater nutrient removal.

  • Modern fertilizer use has tended to remove N, P, and K as limiting factors for growth.

  • New fertilization technology has resulted in higher-analysis materials with fewer impurities, including micronutrients

Micronutrients

What do they do?

• Involved in Photosynthesis

• Energy transport

• Enzymes and proteins

• Water regulation

• Fruit formation

• Disease resistance

What if there isn’t enough?

• Poor root growth

• stunted plants

• Poor or little fruit production

• Eventual plant death

What if there is too much?

• Reduced plant growth

• Poor root growth

• eventual plant death

Copper, zinc, and animal waste management

• Both are present in many animal feed.

• Both are passed through the animal and

in significant concentrations in manure.

• Both are immobile in soils maintained at

a typical agronomic pH (pH 6.0 to 6.8).

• Both accumulate in agricultural lands

where animal waste is being applied.

Copper

In the soil:

• Cu²+ is strongly fixed by soil organic matter and clay minerals (part of CEC)

• one of the least mobile trace elements in most soils

  • pH goes down, copper availability goes up (<pH 5)

  • org matter up = copper down

Plant toxicity:

• reduced plant growth and vigor

• leaf chlorosis

• reduced branching and thickening of roots

• symptoms often mimic Fe deficiency

Zinc

In the soil:

• Zn²+ is strongly fixed by soil organic matter and clay minerals (part of CEC)

• High phosphorus can decrease zinc availability.

• pH goes down, zinc availability goes up (<pH)

  • org matter up = copper down

Plant toxicity:

• reduced plant growth and vigor

• interferes with Fe adsorption leading to Fe deficiency

• pH dependent (example for peanuts);

  • NCDA index = 212 at pH 4.5

  • NCDA index = 425 at pH 6

Regulatory requirements

NCDA Agronomic Zn Index :

300: Peanuts are very sensitive

• Application should be limited.

• Seek alternative sites when possible.

• Zn toxicity risk is greater at lower soil pH and has been observed at Zn-I at 300.

500: Critical toxic level for peanuts. Cease application on peanuts.

2000: Caution - Seek alternative sites for all crops when possible.

3000: Critical toxic level for all crops. Manage pH to control availability.

Toxicity remediation

• Deep tillage can mix high Cu and Zn surface soil with lower-concentration subsoil

• Increase organic matter, clay, and pH = dec tox.

Index values

Used as a means of predicting soil fertility levels or potential heavy metal toxicities. No units associated with index values!

Composite sample

combine smaller samples into on sample for the field. average soil pH=4.8

grid sample

field sampled at regularly spaced grid points

• grid size depends on:

  • cost of labor for sampling

  • anticipated benefits of increased sampling

Nutrient management

Environmentally responsible application of fertilizers, manures, and composts requires a basic understanding of nutrient management principles.

• Questions: amount, timing, source, placement

Amount of fertilizer

Crops vary in their ability to use nutrients:

• Coastal bermudagrass has very high nutrient requirements

• a mature forest has much lower requirements

Since the amount of nitrogen varies directly with yield, there must be a way of estimating yields expected on different fields == Realistic yield expectations (RYE) = estimated crop yield for a given site

Nitrogen management: Realistic yields

• Nitrogen Application Rate = RYE * N

Factor

• RYE = Realistic Yield Expectations

(based on soil type).

• N Factor = amount of N needed per

unit of yield