Jan 18, 2016
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 potential
π = 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