Basic concepts of soil fertility

How plants absorb nutrients

Determining nutrient need

• movement to the root surface• absorption into plant

Essential plant nutrients• categories• effect of soil characteristics

Liming

Sources of nutrients to plantsSoil solution

- ionic form- low concentration- highly buffered

Contributors to the soil solution- exchange sites on clay and organic matter- organic matter and microorganisms- soil rocks and minerals- atmosphere and precipitation- fertilizer and other additions

Movement of ions from soils to roots

• Root interception

• Mass flow

• Diffusion

root tip

NO3-

Ca2+

Ca2+

NO3-

MASS FLOW – dissolved nutrients move to the rootin soil water that is flowing towards the roots

root tip

DIFFUSION – nutrients move from higher concentration in the bulk soil solution to lower concentration at the root;- In the time it takes NO3

- to diffuse 1 cm, K+ diffuses 0.2 cm, and H2PO4

- diffuses 0.02 cm

NO3-

NO3-

NO3-

NO3-

NO3-

NO3-

ROOT INTERCEPTION – roots obtain nutrients by physically contacting nutrients in soil solution or on soil surfaces; - roots contact ~1% of soil volume; - mycorrhizal infection of root increase root-soil contact

root tip

mycorrhizae

Zn2+

Zn2+

Mn2+

H2PO4-

H2PO4-

H2PO4-

Ion absorption by plants:

Passive uptake- diffusion- ion exchange

Active ion uptake- ion carriers- selective / competitive

Overview

Today I want to look at all aspects of plant nutrient uptake. This will cover the plants’ viewpoint, but also soil conditions and management.

Topics to cover:

The biology of nutrient uptake by rootsThe soil chemistry affecting this uptake.

Root uptakeIt has long been appreciated that roots in plants are like guts in animals – the site where nutrients are taken up. Because of this plant roots usually have an immense surface area caused by repeated divisions. Much of this area is due to root hairs, and it is these which are the main sites of entry into a plant for water and nutrients.

Root hairs adhere tightly to soil particles, which is where soil water tends to be bound.

Water enters through the epidermis of root hairs into the apoplast of the root (extra-cellular space). Here is is gradually taken up by cells and enters the symplast, from where it passes the casparian strips into the xylem vessels of the stele.

The flow of water into roots is controlled by a band of corky, water-impermeable cells lining the root cortex which force water to flow into the main vessels symplastically. This band of corky tissue (suberin + lignin) is the casparian strip, and is present in the endodermis of the root systems of most vascular plants.

The casparian strip ensures that all water entering the stele of the root (thence up to the main stem) has passed through a plasma membrane so has been regulated by transport proteins.

Cortex

stele

Casparian strip

Note that the tracheids and vessel elements of the xylem are dead and lack protoplasts, hence their lumen is apoplast, not symplast.

Minerals and water enter the xylem proper by being actively pumped from the walls of the endodermal (and stele parenchymal) cells. This way the xylem contents have been filtered through the plasma membranes of many cells, and are highly purified (of bacteria, mineral debris etc).

The Supply and Availability of Plant Nutrients in Mineral Soils

Factors Controlling the Growth of Higher Plants1. Light2. Mechanical Support3. Heat4. Air5. Water6. Nutrients

Principle of Limiting Factors• The factor which is least optimum will

determine the level of crop production

The Essential Elements– 16 - essential elements– Must be in farms usable by the plant– Optimum concentration for plant growth– Proper balance

Essential Nutrients Elements and Their Sources

Essential Elements Used in Relatively Large Amounts

Mostly fromAir and Water From Soil SolidsCarbon Nitrogen CalciumHydrogen Phosphorus MagnesiumOxygen Potassium Sulfur

Essential Elements Used in Relatively Small Amounts

From Soil SolidsIron CopperManganese ZincBoron ChlorineMolybdenum

Transfer of Plant Nutrients to Available Forms

Organic Nitrogen Ammonium Nitrite Nitrate(protein, amino acids) NH4

+ NO2- NO3

-

Ca3(PO4) + 4H2O + 4CO2 Ca(H2PO4)2 + 2Ca(HCO3)2

Insoluble Phosphate Water Soluble Soluble Calcium (Tri Ca Phosphate) Phosphate Bicarbonate

2KAlSi3O8 + H2CO3 + H2O H4Al2Sl2O9 + K2CO3 + 4SlO2

Microcline Carbonic Hydrated Soluble feldspar Acid silicate carbonate 1. Taken up by plants

2. Leached 3. Adsorbed

Transfer of Plant Nutrients to Available Forms

HCa + 2H2CO3 + Ca(HCO3)2

H

SulfurOrganic sulfur Sulfides Sulfites SulfatesProtein H2S SO3

= SO4=

ColloidalSurface

ColloidalSurface

Forms of Elements Used by PlantsTwo general sources of readily available

nutrients in the soil.1. Nutrients adsorbed on the colloids

-CaNH4 - - Mg

- K

2. Salt in the soil solutionKCl K+ + Cl-

Essential element must be in the ionic form

1. Cationic- Positively charged ions

2. Anionic- Negatively charged ions

The more important ions present in the soil solution or on the soil colloids may be tabulated as followsElements Symbol Form Used by

PlantsSulfur S SO3

=, SO4=

Carbon C++++ CO3=, HCO3

-, CO2

Hydrogen H+ H2OOxygen O= O2

Nitrogen N NH4+, NO2-, NO3-Phosphorus P+5 HPO4

=, H2PO4

Potassium K+ K+

Calcium Ca++ Ca++

Magnesium Mg++ Mg++

Cont.Elements Symbol Form Used by

PlantsIron Fe Fe++, Fe+++

Molybdenum Mo+6 MoO4=

Manganese Mn Mn++, Mn++++

Copper Cu Cu+, Cu++

Zinc Zn++ Zn++

Born B BO3=

Chlorine Cl- Cl-

Water H2O H+, OH-

O2 and CO2 come from the soil air or the atmosphere

Inorganic Salts -Forms in which you buy fertilizer

KCl K+ + Cl-

NaNO3 Na+ + NO3-

NH4NO3 NH4 + NO3-

Inorganic Salts -Forms in which you buy fertilizer

KCl K+ + Cl-

NaNO3 Na+ + NO3-

NH4NO3 NH4 + NO3-

Other Elements

K+, Ca++, Zn++, Mg++, Cl-, only one form presentFe, Mn and Cu - From depends on the oxidation

reduction condition of the soilFe+++ + e- Fe++

oxidation reductionic ous

Other Elements

Aerated soilsFe+++ (Ferric oxides)Mn++++ (Manganic oxide)

Poor DrainageFe++ (Ferrous oxides)Mn++ (Manganous oxide) Toxic

Micronutrients

• Micronutrient elements– Iron (Fe)– Manganese (Mn)– Boron (B)– Zinc (Zn)– Molybdenum (Mo) Zinc (Zn)– Copper (Cu)– Chloride (Cl)

• Usually supplied by irrigation water and soil• Deficiency and toxicity occur at pH extremes

Cations Anions

• Copper• Manganese• Nickel• Iron• Zinc

• Boron• Chloride• Molybdenum

Micronutrientsin solution

Inputs

Insoluble salts

Plant uptake

Soil OM

Losses

Exchangeable cations

Influence of pH on Nutrient Availability

Hydroponics

Recreational hydroponics

• Home hydroponics systems

Transport in plants

• Water and mineral nutrients must be absorbed by the roots and transported throughout the plant

• Sugars must be transported from site of production, throughout the plant, and stored

Transport and water potential

• Water potential (Ψ) of a cell:

Ψcell = Ψp + Ψπ + Ψm

p = pressure potentialπ = solute potentialm = matrix potential

Ψp - Pressure potential (turgor)

Low Ψp High Ψp

Ψπ - Solute potential

• Pure water Ψπ = 0

• All solutions, Ψπ < 0

• As solute concentration increases, Ψcell …

Water movement in plants

• Movement from high Ψcell to low Ψcell

• Occurs in the xylem

Early thoughts on water transport– Capillary action

Water transport utilizes a water potential gradient

Tension-cohesion theory

• Water is drawn up the plant by transpiration of water from stomata

low ψ

Transpiration creates tension

higher ψ

cohesion

higher ψ

lower ψhigher ψ

lower ψ higher ψ highest ψ

lower ψ

Importance of stomata• Regulate transpiration rate

– Controls rate of water uptake• Water required for photosynthesis• Water required to maintain turgor pressure

– Controls nutrient uptake• Regulate gas exchange

– CO2 required for photosynthesis

Ψ and transpiration rate

• In terms of ψ, can you explain how transpiration rate is influenced by:

– Atmospheric humidity?– Wind?– Air temperature?– Light intensity?

Transpiration and photosynthesis

• The dilemma of a hot, sunny day?

Good for photosynthesis, but…

Bad for water loss

Phloem transport• Pressure-flow

hypothesis

1M

10-50mM