Soil Fertility Lecture 13 Plant Nutrient Uptakes

What must be true of a mineral nutrient for a plant to absorb it?

It must be in a soluble form in the soil solution.

Where do plants obtain mineral nutrients?

From the soil solution, absorbing nutrients along with water.

Why must nutrients in the soil solution be continually replenished?

The soil solution contains a relatively small amount of nutrients that can be depleted within hours or days.

What are the three main sources that replenish nutrients in the soil solution?

Mineral particles through desorption and ion exchange; organic matter through mineralization; and external inputs such as fertilizers, manure, and liming materials.

Where is the bulk of soil nutrients stored?

In the structural framework of primary minerals, organic matter, clay, and humus.

Where are soil nutrients stored besides the structural framework of minerals and organic matter?

Adsorbed as ions near the surfaces of soil colloids, especially clay and organic matter.

Which soil nutrient pool contains the nutrients immediately available for root uptake?

The soil solution, where nutrients are dissolved.

What is desorption?

The release of ions from mineral or soil-particle surfaces into the soil solution.

What is ion exchange?

The exchange of ions between soil solution and charged surfaces of soil colloids.

What is mineralization?

The microbial conversion of organically bound nutrients into inorganic forms that plants can use.

What is immobilization?

The incorporation of inorganic nutrients into microbial tissue, making those nutrients temporarily less available to plants.

How do microorganisms affect nutrient availability?

They decompose organic residues, mineralizing nutrients; if microbial demand is high, they can also immobilize inorganic nutrients.

What is the relationship between nutrient pools and plant uptake?

Nutrients move from larger soil reserves into adsorbed pools and the soil solution, where roots can absorb them.

What is mineral solubility?

The ability of a mineral to dissolve and release ions into solution.

What does Ksp stand for?

Solubility product constant; it describes the equilibrium relationship between dissolved ions and a solid mineral.

What does Ksp help determine?

The equilibrium concentrations of ions that can remain in solution in contact with a particular mineral.

What happens when the ion concentration product is below Ksp?

The mineral tends to dissolve, releasing ions into the soil solution.

What happens when the ion concentration product equals Ksp?

The solution is saturated and at equilibrium with the mineral; there is no net dissolution or precipitation.

What happens when the ion concentration product exceeds Ksp?

The solution is supersaturated, so precipitation is favored.

What is precipitation in soil chemistry?

The formation of a solid mineral from dissolved ions in solution.

What happens when more water is added to a solution containing a dissolving mineral?

More mineral can dissolve as the solution becomes less concentrated, provided the mineral remains present.

What can happen when water evaporates from a mineral-containing solution?

Ion concentrations increase, potentially causing minerals to precipitate.

What is the common-ion effect illustrated by adding MgSO4 to CaSO4?

Adding a shared ion, sulfate, can favor precipitation of the original mineral, CaSO4.

How does mineral solubility help replenish soil nutrients?

When plant uptake lowers dissolved nutrient concentrations, minerals can dissolve and release additional ions into the soil solution.

How does mineral solubility help buffer the soil solution?

Minerals can dissolve when dissolved-ion concentrations fall and precipitate when concentrations rise, helping regulate solution concentrations.

How does pH affect FePO4 solubility according to the lecture?

Increasing pH reduces the concentrations of dissolved Fe and H2PO4− and favors FePO4 precipitation.

Why is pH important for phosphorus availability?

Changes in pH affect phosphorus solubility and the concentrations of dissolved phosphorus species.

What happens to FePO4 when plants remove H2PO4− from the soil solution?

The decrease in dissolved H2PO4− favors FePO4 dissolution, replenishing the soil solution with phosphorus.

What can happen when phosphate fertilizer increases H2PO4− concentration?

FePO4 precipitation can be favored when the solution becomes supersaturated relative to the mineral.

What are the two major forms of nutrient replenishment discussed in the lecture?

Release from mineral surfaces or minerals, and mineralization of organic matter.

What is root interception?

Nutrient acquisition through physical contact between growing roots and nutrient-bearing soil surfaces.

Why does having more roots per volume of soil increase root interception?

More roots explore more soil and contact more nutrient-bearing surfaces.

How much of the soil volume do roots occupy according to the lecture?

Usually 1% or less, although they can contact approximately 3% of total available nutrients.

Why can roots contact more nutrients than their small volume might suggest?

Roots follow lines of weakness in soil, which are also pathways for soil water and nutrient availability.

What are root hairs important for in nutrient uptake?

They increase root surface contact with soil and help exchange ions with soil and organic matter surfaces.

What are mycorrhizae?

Symbiotic associations between fungi and plant roots.

How do mycorrhizae improve nutrient acquisition?

Fungal growth increases the effective absorbing surface area of the root system and transfers nutrients to the plant.

By how much can mycorrhizae increase effective root surface area according to the lecture?

Up to 10 times.

What does the plant provide to mycorrhizal fungi?

Carbohydrates produced through photosynthesis.

Which soils may benefit most from mycorrhizal associations?

Lower-fertility soils, where obtaining nutrients is more difficult.

What is mass flow?

The movement of dissolved nutrients toward roots with water moving through the soil and into the plant.

What drives mass flow toward plant roots?

Water uptake by roots and transpiration from leaves.

How do nutrients enter roots through mass flow?

Dissolved ions travel with soil water as water is absorbed by roots.

Which nutrient movement process is important for the uptake of most nutrients?

Mass flow.

What three factors influence mass flow?

Water movement and uptake, nutrient concentration in soil water, and the rate of plant activity and transpiration.

What happens to mass flow when less water moves toward roots?

Mass flow decreases because less water carries dissolved nutrients toward roots.

How can temperature affect mass flow?

Lower temperatures can reduce plant activity and transpiration, reducing mass flow.

Why can mass flow sometimes supply more nutrients than a plant needs?

Water movement may deliver dissolved nutrients to roots in excess of plant demand.

What is diffusion?

The net movement of ions from an area of higher concentration to an area of lower concentration.

What drives nutrient diffusion toward roots?

A concentration gradient between the soil solution near the root and the surrounding bulk soil solution.

How do roots create a concentration gradient?

They remove nutrients from the soil solution near their surfaces, lowering the local concentration relative to the surrounding soil.

How does a concentration gradient affect diffusion rate?

A steeper concentration gradient generally increases the rate of diffusion.

Which two nutrients are especially dependent on diffusion according to the lecture?

Phosphorus (P) and potassium (K).

What is Fick's law used to describe?

The rate of diffusion in relation to the effective diffusion coefficient, root surface area, and concentration gradient over distance.

What is the effective diffusion coefficient (De)?

A measure of how readily a nutrient diffuses through soil toward roots.

What two soil properties influence the effective diffusion coefficient?

Volumetric water content and buffering capacity.

At what soil moisture condition is nutrient transport generally highest according to the lecture?

Field capacity.

Why does diffusion become more difficult in dry soil?

Thin and disconnected water films restrict the movement of dissolved ions through the soil.

How does buffering capacity relate to the effective diffusion coefficient?

The effective diffusion coefficient is inversely related to buffering capacity.

What does high buffering capacity mean for nutrient replenishment?

The soil can replenish dissolved nutrients more effectively from its reserves or adsorbed pools.

Why can high buffering capacity reduce nutrient availability through diffusion?

It lowers the effective diffusion coefficient, slowing nutrient movement toward roots even though it improves the soil's ability to replenish nutrients.

How does temperature affect plant nutrient uptake?

Higher temperatures can increase uptake; an increase from 10°C to 20°C will often double uptake, according to the lecture.

What are the approximate diffusion distances listed for N, K, and P?

Nitrogen: 1.0 cm; potassium: 0.2 cm; phosphorus: 0.02 cm.

If neighboring corn roots are 0.7 cm apart, how far must a nutrient diffuse to reach a root midway between them?

Approximately 0.35 cm.

Which fertilizer would benefit most from placement close to young roots, based on the lecture's diffusion distances?

Phosphorus fertilizer, because phosphorus has the shortest listed diffusion distance, approximately 0.02 cm.

What is back diffusion?

Diffusion away from a root when nutrients accumulate in the surrounding soil solution because they enter the root too slowly.

What are the three main processes that move nutrients from soil toward roots?

Root interception, mass flow, and diffusion.

What is passive ion uptake?

The movement of ions into root cells without direct energy input from cell metabolism.

How do ions enter roots through passive transport?

Through processes such as diffusion and ion exchange, following favorable concentration or electrochemical gradients.

What concentration relationship favors passive uptake?

The relevant gradient favors movement into the root, such as a higher ion concentration outside the cell than inside.

Why are plant cell surfaces important in passive ion uptake?

Plant cell surfaces are usually negatively charged, which attracts positively charged ions (cations).

Why do plants release H+ ions during ion uptake?

Releasing positively charged H+ ions helps maintain electrical neutrality as other ions are absorbed.

Is passive transport selective, and does it require energy?

According to the lecture, passive transport is non-selective and requires no energy input.

What is active ion uptake?

The energy-dependent movement of ions across the cell membrane when they cannot enter passively.

When is active transport needed?

When nutrients must move across a membrane against their concentration gradient, such as when their concentration is already higher inside the cell than outside.

Where does the energy for active transport come from?

Cell metabolism.

What role do ion carriers play in active transport?

They bind or escort specific ions across the membrane and release them on the other side.

Is active transport selective?

Yes. Different transport mechanisms can transport different nutrients.

What is the key difference between passive and active ion uptake?

Passive uptake requires no energy and follows a favorable gradient; active uptake requires metabolic energy and can move ions against their gradient.

How are mass flow and diffusion different?

Mass flow carries dissolved nutrients with moving water; diffusion moves ions down a concentration gradient.

How are root interception and diffusion different?

Root interception requires physical contact between roots and nutrient-bearing surfaces; diffusion transports ions through soil solution toward roots.

How are root interception and mass flow different?

Root interception acquires nutrients through root contact with soil surfaces; mass flow carries dissolved ions with water moving toward roots.

Why is nutrient replenishment essential for continued plant growth?

Roots deplete the small soil-solution pool, so minerals, exchange sites, organic matter, and external inputs must replenish nutrients.