Soil Fertility: Exam 2

Nutrient Supply

Locked up in the structural framework of primary minerals, organic matter, clay, and hummus.

Adsorbed in a swarm of ions near the surfaces of clay and organic matter.

Released into the soil solution where plants can uptake them through roots.

Cation Exchange

Makes Ca²⁺, Mg²⁺, and K⁺ nutrients available.

Phosphorus in Soil

Exists in various forms of orthophoasphate (H₃PO₄, H₂PO₄^-, HPO₂^2-, PO₄^3-).

Has limited solubility.

Solubility Product Constant (K_sp)

Determines the concentration of ions in solution.

When ion concentration < K_sp → mineral dissolves.

When ion concentration > K_sp → mineral precipitates.

Eventually, equilibrium occurs where the solution is saturated.

Changing Mineral Solubility

Adding more water → more mineral dissolves.

Remove water (evaporation) → more mineral precipitates.

If another product containing the solubility product (e.g., SO₄^2-) is added, concentration will increase and cause more of the mineral to precipitate.

Solubility

Very important for P and many micronutrients.

Also important for buffering soil solution.

May be dependent upon pH.

Organic Matter

During degradation of plant and organic residues, organically complex ions can be mineralized including N, S, and P (organic → inorganic).

These minerals become available to plants.

If there is insufficient nutrients to meet microbial demand, inorganic nutrients can be immobilized into microbial tissue (available → unavailable).

Nutrient Uptake

Plant ion absorption requires contact between ion and root surface.

Ways Soil Ions Move to Roots

1. Root interception

2. Mass flow

3. Diffusion

Root Interception

Physical contact between the root and mineral surfaces.

More roots per volume → greater ion uptake.

If an ion held on a root hair and an ion held on soil exchange site are close enough, they can oscillate and exchange.

Enhanced by mycorrhiza growing inside the root structure, especially in low fertility soils.

Roots

Occupy less than 1% of soil volume.

Follow lines of weakness, similar to soil water; these are the areas with most soil nutrients.

Could contact ~3% of total available nutrients.

Mass Flow

Water is absorbed by roots, travels through the plant, and evaporates from leaf surface.

Ions in soil solution enter the plant through this water uptake process.

This is the most important for the uptake of most nutrients.

Can supply excess nutrients.

Factors Impacting Mass Flow

Less water → less mass flow (less nutrients).

Lower temperatures → less mass flow (less drive for plant activity).

Diffusion

Ions move from an area of high concentration to one of low concentration (passive).

Roots remove nutrients nearest to them, causing nutrients from the bulk soil solution to replace them; this creates a gradient driving nutrients to the roots.

Phosphorus and potassium are very dependent on diffusion.

Fick’s Law

Rate of diffusion is directly proportional to concentration gradient.

Increases in nutrient concentration between the roots and soil water causes increases in nutrient diffusion.

Effective diffusion coefficient (De) controls how far nutrients diffuse to roots.

Impacts of Water Content

As soil moisture decreases, nutrients become less available.

Water films become thinner, making movement of nutrients in films more difficult.

Highest nutrient transport occurs at field capacity.

Buffering Capacity

Higher BC increases the ability of soils to replace nutrients.

This reduces nutrient availability for plants.

De is inversely related to BC.

Factors Impacting Diffusion

Temperature: an increase from 10^°C to 20^°C will often double uptake.

Distance: 1.0 cm for N, 0.2 cm for K, 0.02 cm for P.

Back Diffusion

Occurs when nutrients entering the roots slowly create higher concentration around the root, which draws soil nutrients away from it.

Far less common than diffusion to the root.

Passive Uptake by Cells

Ions enter the root through diffusion and ion exchange (via concentration gradient).

Plant cell surfaces are usually negatively charged, attracting cations.

Plants release H⁺ to maintain neutrality.

Non-selective and requires no energy input.

Active Uptake By Cells

Nutrients reach plasma membrane.

Cannot enter passively because concentration is higher inside the cell than out.

Ion carrier escorts free ions across the membrane, then releases them; different transports for different nutrients.

Selective process that requires energy.

Reservoirs of Nitrogen

98% — Earth’s lithosphere (crust and outer mantle)

2% — N₂ in the atmosphere

0.01% — soil (even less in organisms).

Nitrogen Cycle

1. Nitrogen fixation — N combines with H₂ or O₂, then is added to the soil.

2. Mineralization — soil organisms mineralize organic N to inorganic NH₄⁺.

3. Nitrification — NH₄⁺ is converted to NO₃^- by bacteria.

4. Plant uptake — NH₄⁺ and NO₃^- in soil solution is taken up by roots.

5. Immobilization — some NH₄⁺ and NO₃^- is converted back to organic N.

6. Leaching — NO₃^- can be lost to groundwater or drainage.

7. NH₄⁺ fixation — occurs by 2:1 clay minerals.

8. Volatilization — NH₄⁺ may be volatilized to gaseous NH₃.

9. Denitrification — NO₃^- can be converted by denitrifying bacteria to N₂, N₂O, and NO that escapes back into the atmosphere.

N Inputs (Gains)

Fixation

Bacteria

Haber-Bosch

Lightning

Fossil fuels

Animal manure

Crop residue

N Outputs (Losses)

Plant uptake

Denitrification

Volatilization

NH₄⁺ fixation

Leaching

N Cycling (No Gain / Loss)

Mineralization

Immobilization

Nitrification

Nitrogen Fixation

Inactive N₂ in the atmosphere is converted to reactive N forms via symbiotic microorganisms on legumes, free-living microorganisms, lightning forming N oxides, and manufacture of synthetic fertilizers.

Lightning

Fixes N as oxides.

Splits N₂ and causes it to react with O₂ to form NO₃^-.

NO₃^- dissolves in rain and enters the soil during rain events (acid rain or atmospheric N deposition).

Sources of Atmospheric N Deposition

Combustion of fossil fuels.

Combustion of biomass.

Lightning.

NH₃ volatilization from fertilizer.

NH₃ volatilization from poultry litter and manure.

N Fixation (Legumes)

Symbiotic relationship with Rhizobium bacteria in the nodules.

Decreases as soil N increases.

Different crops require different species.

Active (working) centers are red; inactive centers are green or white.

Factors Impacting N Fixation

Soil acidity restricts the survival and growth of rhizobia.

Affects nodulation.

Al, Mn, and H toxicity; low Ca and P availability (liming can improve).

Mineral N status; excess NO₃ reduces fixation.

Micronutrients; requires Mo, Co, B, Fe, and Cu.

Photosynthesis and climate (high photosynthate production → high fixation).

Legume management (water stress, nutrient stress, weed or insect pressure, improper harvest).