Soils: Chemistry and Plant Nutrition

Soils: Chemistry and Plant Nutrition

Nutritional Requirements of Plants

• Elemental Composition:

  • Plants consist of >95% C, O, and H (organic).

  • The remaining composition includes N, P, K, and other inorganic elements.

• Essential Inorganic Nutrients:

  • Macronutrients: N, P, K, S, Ca, Mg.

  • Micronutrients: Fe, Cl, Mn, B, Zn, Cu, Ni, Mo, Na, Co, Si.

• Nutrient Deficiency vs. Toxicity:

  • Plants require a balanced range of the essential nutrients and can experience deficiency or toxicity with too little or too much of a particular nutrient.

Table 4.2: Elements Essential for Plant Growth and Survival

• Macronutrients:

  • Nitrogen (N):

    • Symbol: N

    • Form Absorbed: $NO<em>3^-$ or $NH</em>4^+$

    • Average Mass: 15.0 g/kg plant dry weight

    • Percentage of Total Mass: 1%-4%

    • Important Functions: Essential component of nucleotides, nucleic acids, amino acids, proteins (including structural proteins and enzymes), and chlorophylls.

  • Potassium (K):

    • Symbol: K

    • Form Absorbed: $K^+$

    • Average Mass: 10.0 g/kg plant dry weight

    • Percentage of Total Mass: 0.5%-6%

    • Important Functions: Involved in osmosis, ion balance, pH regulation, opening and closing of stomata; activator of many enzymes, protein synthesis

  • Calcium (Ca):

    • Symbol: Ca

    • Form Absorbed: $Ca^{2+}$

    • Average Mass: 5.0 g/kg plant dry weight

    • Percentage of Total Mass: 0.2%-3.5%

    • Important Functions: Strengthens cell walls and some plant tissues; involved in cell division and cell elongation, membrane permeability, cation-anion balance; structural component of many molecules; second messenger in signal conduction between environment and plant growth and developmental responses

  • Magnesium (Mg):

    • Symbol: Mg

    • Form Absorbed: $Mg^{2+}$

    • Average Mass: 2.0 g/kg plant dry weight

    • Percentage of Total Mass: 0.1%-0.8%

    • Important Functions: Essential component of the chlorophyll molecule; activator of many enzymes; involved in cation-anion balance and regulation of cytoplasm pH.

  • Phosphorus (P):

    • Symbol: P

    • Form Absorbed: $H<em>2PO</em>4^-$ or $HPO_4^{2-}$

    • Average Mass: 2.0 g/kg plant dry weight

    • Percentage of Total Mass: 0.1%-0.8%

    • Important Functions: Essential structural component of nucleic acids, proteins, ATP, and NADP+ critical for energy transfer and storage.

  • Sulfur (S):

    • Symbol: S

    • Form Absorbed: $SO_4^{2-}$

    • Average Mass: 1.0 g/kg plant dry weight

    • Percentage of Total Mass: 0.05%-1%

    • Important Functions: Component of some amino acids and proteins, coenzymes (including coenzyme A), and secondary metabolic products (including defensive compounds).

  • Carbon (C):

    • Symbol: C

    • Form Absorbed: $CO_2$

    • Average Mass: 450.0 g/kg plant dry weight

    • Percentage of Total Mass: ~44%

    • Important Functions: Found in all organic compounds

  • Oxygen (O):

    • Symbol: O

    • Form Absorbed: $H<em>2O$ or $O</em>2$

    • Average Mass: 450.0 g/kg plant dry weight

    • Percentage of Total Mass: ~44%

    • Important Functions: Major component of all organic compounds, including cellulose in cell walls, which makes up much of the plant's dry weight; backbone of other carbohydrates (sugars, starches) and lipids, which store and transport the energy captured in photosynthesis

  • Hydrogen (H):

    • Symbol: H

    • Form Absorbed: $H_2O$

    • Average Mass: 60.0 g/kg plant dry weight

    • Percentage of Total Mass: ~6%

    • Important Functions: Component of all major organic compounds; cellular respiration

• Micronutrients:

  • Chlorine (Cl):

    • Symbol: Cl

    • Form Absorbed: $Cl^-$

    • Average Mass: 0.1 g/kg plant dry weight

    • Concentration: 100-10,000 ppm

    • Important Functions: Used in stomatal regulation, proton pumps, and osmoregulation; critical for splitting water molecules in photosystem II.

  • Iron (Fe):

    • Symbol: Fe

    • Form Absorbed: $Fe^{3+}$, $Fe^{2+}$

    • Average Mass: 0.1 g/kg plant dry weight

    • Concentration: 25-300 ppm

    • Important Functions: Component of heme proteins and iron-sulfur proteins; necessary for chlorophyll synthesis and in the electron transport chain in light reactions.

  • Manganese (Mn):

    • Symbol: Mn

    • Form Absorbed: $Mn^{2+}$

    • Average Mass: 0.05 g/kg plant dry weight

    • Concentration: 5-75 ppm

    • Important Functions: Present in several enzymes; needed for activation of many enzymes.

  • Boron (B):

    • Symbol: B

    • Form Absorbed: $B(OH)<em>3$, $B(OH)</em>4$

    • Average Mass: 0.02 g/kg plant dry weight

    • Concentration: 15-800 ppm

    • Important Functions: Poorly understood; cell wall synthesis, nucleic acid synthesis, and plasma membrane integrity.

  • Zinc (Zn):

    • Symbol: Zn

    • Form Absorbed: $Zn^{2+}$

    • Average Mass: 0.02 g/kg plant dry weight

    • Concentration: 15-100 ppm

    • Important Functions: Present in several enzymes; needed for activation of many enzymes; protein synthesis and carbohydrate metabolism; structural component of ribosomes; important but not well understood role in plant hormone metabolism.

  • Copper (Cu):

    • Symbol: Cu

    • Form Absorbed: $Cu^{2+}$

    • Average Mass: 0.006 g/kg plant dry weight

    • Concentration: 4-30 ppm

    • Important Functions: Present in some proteins and in plastocyanin necessary for light reactions; enzyme activation; pollen formation and ovule fertilization; lignification of secondary cell walls in wood formation.

  • Nickel (Ni):

    • Symbol: Ni

    • Form Absorbed: $Ni^{2+}$

    • Average Mass: 0.0001 g/kg plant dry weight

    • Concentration: 0.1-5 ppm

    • Important Functions: Necessary for function of some enzymes; nitrogen metabolism.

  • Molybdenum (Mo):

    • Symbol: Mo

    • Form Absorbed: $MoO_2^-$

    • Average Mass: 0.0001 g/kg plant dry weight

    • Concentration: 0.1-5 ppm

    • Important Functions: Enzyme cofactor for N fixation and some other reactions; pollen formation and seed dormancy.

  • Sodium (Na):

    • Symbol: Na

    • Form Absorbed: $Na^-$

    • Average Mass: Trace

    • Important Functions: Osmotic and ionic balance, especially in some desert and salt marsh species.

  • Cobalt (Co):

    • Symbol: Co

    • Form Absorbed: $Co^{2+}$

    • Average Mass: Trace

    • Important Functions: Required by N-fixing microorganisms.

  • Silicon (Si):

    • Symbol: Si

    • Form Absorbed: $Si(OH)_4$

    • Average Mass: Variable

    • Important Functions: Not well understood; appears to have a role in disease resistance; important structural component of cells in grasses, Equisetum spp. (horsetails), and other plants; may reduce leaf water loss. Essential to some plants

Sources of Inorganic Nutrients for Plants

• Primary Source = Soil:

  • Weathering of rocks: Leads to mineral salts.

  • Decomposition of organic matter: Releases nutrients.

• Leaching/fragmentation: Initial stages of organic matter breakdown.

• Microbial respiration: Use and breakdown of organic matter by microbes.

• Mineralization: Conversion of organic matter to inorganic forms.

• Nutrient immobilization: Nutrients become unavailable to plants.

Uptake of Nutrients

• Nutrient Form: Nutrients are taken up as ions in solution.

  • Cations: Positively charged ions (e.g., $K^+$).

  • Anions: Negatively charged ions (e.g., $NO_3^-$).

• Nutrient Pools:

  • Dissolved pool: <0.2% of total nutrients.

  • Bound pool: ≈98% of total nutrients.

  • Exchangeable pool: ≈2% of total nutrients.

Cation Exchange

• Primary Ion Exchangers in Soil:

  • Clay particles: Negatively charged surfaces.

  • Humic substances: Negatively charged surfaces.

• Ion Attraction:

  • Cations are attracted more than anions.

  • Attraction is higher for highly charged cations (e.g., Ca^{2+} > K^+).

• Cation Exchange Capacity (CEC):

  • Displacement of cations with $H^+$ from roots.

  • CEC is linked to soil fertility.

Soil pH and Nutrient Availability

• Soil pH significantly influences the availability of nutrients.

• Different nutrients have varying levels of availability at different pH ranges.

• Maximum availability is graphically represented by the width of bars in relation to pH.

• Trends:

  • Nitrogen, phosphorus, and potassium availability are highest in a slightly acidic to neutral pH range.

  • Iron, manganese, copper, and zinc are more available in acidic soils.

  • Molybdenum availability increases in alkaline soils.

Adaptations to Nutrient Limitation

• Increased Nutrient Absorption by Roots:

• Alter Rooting Morphology:

  • Root:shoot ratio.

  • Rooting density (#roots/volume).

  • Rooting length.

• Modify the Rhizosphere:

  • Local acidification.

  • Root exudates stimulate microbial activity.

• Reduce Nutrient Requirements:

  • Increase nutrient retention (leaf life span).

  • Reduced growth rates.

Rooting Depth and Environmental Factors

• Vegetation Type and Climate:

  • Shrublands > Grasslands (in terms of rooting depth).

  • Deepest roots are found in water-limited environments.

  • Rooting depth is positively correlated with evapotranspiration.

• Soil Type and Texture:

  • High concentration of roots in surface organic layers.

  • Rooting depths decrease with increasing soil organic layer.

Adaptations to Low Nutrient Environments

• Evergreen Plants: Many evergreen plants are adapted to low nutrient environments (e.g., Black spruce, Rhododendron).

• Carnivorous Plants: Adaptations to low N availability through trapping and digesting insects (e.g., Sarracenia purpurea).

Nitrogen in Plants and Soil

• Plants have high nitrogen (N) requirements.

• N is often the most important limiting factor for plant growth (besides water).

• N availability in the soil is limited.

• Forms of N in the soil:

  • Nitrate ($NO_3^-$) -Generally preferred

  • Ammonium ($NH_4^+$)

The Nitrogen Cycle

• Overview: A complex biogeochemical cycle involving various transformations of nitrogen through different forms.

• Key Processes:

  • Nitrogen Fixation: Conversion of atmospheric nitrogen ($N<em>2$) to ammonia ($NH</em>3$).

  • Ammonification: Conversion of organic nitrogen to ammonia.

  • Nitrification: Conversion of ammonia to nitrite ($NO<em>2^-$) and then to nitrate ($NO</em>3^-$).

  • Denitrification: Conversion of nitrate to nitrogen gas ($N_2$), returning it to the atmosphere.

  • Assimilation: Uptake of inorganic nitrogen ($NH<em>4^+$, $NO</em>3^-$) by plants and microorganisms.

• Human Impacts:

  • Industrial fixation for fertilizer production.

  • Fossil fuel combustion and biomass burning, releasing nitrogen compounds.

  • Agricultural practices, including the use of nitrogen fertilizers.

• Fluxes:

  • Quantities provided related to different processes such as industrial fixation (80), biological fixation (140), denitrification (110), etc.

Biological Nitrogen Fixation

• Conversion of $N<em>2$ to $NH</em>3$ (ammonia).

• Exclusively carried out by prokaryotes.

• Free-living N2-fixers:

  • Photosynthetic cyanobacteria (e.g., Anabaena).

  • Heterotrophic bacteria in soil.

  • Cryptogamic soil crusts.

Symbiotic Nitrogen Fixation

• Symbiotic association between $N_2$-fixing bacteria and plants.

• Rhizobium and legumes: Root nodules provide an anaerobic environment for nitrogen fixation.

• Other associations: Frankia bacteria with plants like alders (Alnus spp.).

Importance of Nitrogen Fixation

• Use of stable isotopes of nitrogen to trace sources.

• $^{14}N$ = most abundant form (99.6%).

• $^{15}N$ = rare form (0.5%).

• $^{15}N/^{14}N$ ratios ($\delta^{15}N$):

  • Atmosphere = 0 o/oo.

  • Soil = enriched in $^{15}N$ = 9 o/oo.

  • $\delta^{15}N$ of plant tissue reflects how much is from atmosphere (fixed-N) vs soil.

Applications of Stable Isotopes in Nitrogen Studies

• $^{15}N$ Fixing Plant in Nitrogen-Free Soil:

  • Source of Nitrogen: Only fixed N from the air.

  • Expected $^{15}N/^{14}N$ Ratio: Low $\delta^{15}N$.

• $^{15}N$ Fixing Plant in Natural Soil:

  • Source of Nitrogen: Mix of both fixed N from the air and soil-derived N.

  • Expected $^{15}N/^{14}N$ Ratio: Intermediate $\delta^{15}N$.

• Non-Fixing Plant in Natural Soil:

  • Source of Nitrogen: Only soil-derived N.

  • Expected $^{15}N/^{14}N$ Ratio: High $\delta^{15}N$.

Anthropogenic Nitrogen Fixation

• Total “anthropogenic” nitrogen fixation = $150 \times 10^{12}$ g/yr.

• 1.5X “natural” terrestrial amount

• Teragram (Tg) = $10^{12}$ g.

Consequences of Altered N Cycle

• Global Nitrogen Fertilization: Enhanced Primary Productivity?

• Negative Effects on Biodiversity.

• Eutrophication of Estuaries & Coastal Waters.

• Groundwater Pollution.

• Air Pollution:

  • Tropospheric ozone.

  • Acid precipitation.

Mycorrhizae

• Mutualistic associations between fungi and plant roots.

• Two major types:

  • Endomycorrhizae: Hyphae penetrate root cortex cells.

    • 3 groups of endomycorrhizae

      • Vesicular-arbuscular

      • Ericoid/Arbutoid

      • Orchidaceous

  • Ectomycorrhizae: Hyphae do not penetrate root cells.

    • Form a “mantle” around root.

    • Common in woody plants (Pinaceae, Betulaceae, Fagaceae, others).

Table 11.1: Associations of the Five Major Types of Mycorrhizae

• Ectomycorrhizae (ECM):

  • Plant Taxa Involved: Dipterocarpaceae (98%), Pinaceae (95%), Fagaceae (94%), Myrtaceae (90%), Salicaceae (83%), Betulaceae (70%), Fabaceae (16%), and some others.

  • Fungal Taxa Involved: Basidiomycota (most), Ascomycota (less common), and Zygomycota (rare).

• Arbuscular mycorrhizae (AM):

  • Plant Taxa Involved: By far the most common mycorrhizae in angiosperms, except for nonmycorrhizal families (Brassicaceae, Portulaceae, Caryophyllaceae, Proteaceae, and some others); also common in many gymnosperms, except Pinaceae, and in some ferns and other taxa.

  • Fungal Taxa Involved: Phylum Glomeromycota (including families Gigasporaceae, Acaulosporaceae, and Glomeraceae and others).

• Arbutoid ectendomycorrhizae:

  • Plant Taxa Involved: Many species in the order Ericales

  • Fungal Taxa Involved: Basidiomycetes

• Ericoid mycorrhizae:

  • Plant Taxa Involved: Many species in the order Ericales

  • Fungal Taxa Involved: Basidiomycetes and some Ascomycetes

• Orchidaceous mycorrhizae:

  • Plant Taxa Involved: Orchidaceae

  • Fungal Taxa Involved: Basidiomycetes and some Ascomycetes

Types of Mycorrhizae

• Visual depiction of different types of mycorrhizae including Arbuscular, Ericoid, Ectomycorrhizae.

Significance of Mycorrhizae

• Improve uptake of phosphorous (immobile).

  • Increase absorbing surface area and soil volume exploited.

  • Overcome diffusional limitations of P.

• Enhance plant water relations.

• Facultative vs. obligative nature of mycorrhizae.

Mycorrhizal Networks and Plant Communication

• Inter-plant communication through mycorrhizal networks mediates complex adaptive behavior in plant communities

Salt Stress in Plants

• Ionic Stress (Salinity Stress):

  • $K^+$ deficiency/excess $Na^+$ influx.

  • $Na^+$ toxicity.

    • Leaf senescence.

    • Inhibitions of photosynthesis, protein synthesis, enzyme activity.

• Osmotic Stress:

  • Lowers water potential.

    • Dehydration.

    • Inhibitions of water uptake, cell elongation, leaf development.

• Adaptations:

  • Ion homeostasis, $Na^+$ extrusion/$Na^+$ compartmentation/Osmotic adjustment.

  • Accumulations of ions/solutes/organic compounds.

  • $Na^+$ reabsorption - store it in vacuoles to lower their own osmotic potential.

• Consequences: Cell death or recovery/adaptation.

Adaptations to Salinity

• Glycophytes = intolerant to salinity (0.1% salt).

• Halophytes = salt tolerant plants (>0.2 % salt).

• Exclusion of salts at roots or leaves (Atriplex).

• Sequestration of salts in vacuoles (succulents).

• Osmoregulation to allow for water uptake.

Common Halophytes

• Examples include Atriplex spp., Spartina spp., Rhizophora spp., Salicornia spp., Distichlis spicata

High Saline Environments

• Coastal salt marshes

• Inland salt marshes

Salinity and Coastal Marshes in Louisiana

• Map illustration salinity levels in coastal marshes of Louisiana from fresh to saline.

Salinization and Agriculture

• Salinization Processes: Water evaporates, salts remain behind.

• Water and salts move upward from a high water table; salts remain behind.