Nutrients soil fertility
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SOIL FERTILITYSOIL FERTILITY
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Ability of soil to supply nutrients for plant growth
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Soil is storehouse of nutrients
Some available, some not
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Soil fertility: Soil fertility: quantity of nutrients soil
contains
how well protected from leaching
how available to plant
how easily roots can take them up
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Plant nutrients Plant nutrients
Essential elements needed for plant growth
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Plant nutrientsPlant nutrients
Plants absorb 90 elements
Only a few needed for growth Some not needed by plants but by
animals that eat plants (cobalt)
Others not needed, can be toxic (lead)
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How to determine which are essential?
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Essential plant elementsEssential plant elements
1. lack of element stops plant from growth or reproduction
2. element is directly involved in plant processes
3. shortage of element can only be corrected by supplying that element
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17 essential elements 17 essential elements
3 account for 95% plant needs
carbon, oxygen and hydrogen
come from water, air
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Non-minerals
Macronutrients Air and Water Carbon (C)
Hydrogen (H)
Oxygen (O)
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14 come from soil 14 come from soil
6 used in large amounts:macronutrients
Nitrogen CalciumPhosphorus MagnesiumPotassium Sulfur
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Primary Macronutrients Primary Macronutrients
Nitrogen PhosphorusPotassium
Not always available in large enough quantities
Add by fertilizing
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Secondary MacronutrientsSecondary Macronutrients
Calcium Magnesium Sulfur
Soil usually has plenty
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Micronutrients Micronutrients
8 nutrients left
Used in small amounts
Plant won't grow normally without them
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NUTRIENT IONSNUTRIENT IONS
Plants absorb some of the nutrients as ions instead of elements
Ion has positive or negative charge
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Ion has:
positive charge cationnegative charge anion
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Ions form in soil when compounds dissolve in water Example:
Potassium nitrate (fertilizer) dissolves in water,
molecule breaks down into potassium ion and nitrate ion
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Plant roots absorb ions - soak like a
sponge
Soil particles adsorb ions - hold on to them - stick to it
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SOURCES OF ELEMENTS IN SOURCES OF ELEMENTS IN SOILSOIL
Nutrient elements present in soil in four places (pools)
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SOURCES OF ELEMENTS IN SOURCES OF ELEMENTS IN SOILSOIL
1. soil minerals
major sourcereleased slowly by weatheringnot source of nitrogen
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SOURCES OF ELEMENTS IN SOURCES OF ELEMENTS IN SOILSOIL
2. organic matter
large amounts of nitrogennutrient anionsreleased by decay
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SOURCES OF ELEMENTS IN SOURCES OF ELEMENTS IN SOILSOIL
3. adsorbed nutrients
held by clay and humus particles
relatively available to plants
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SOURCES OF ELEMENTS IN SOURCES OF ELEMENTS IN SOILSOIL
4. dissolved ions
ions in soil solutionplants absorb directlymay be leached
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Soil colloidsSoil colloids
Tiny particles of clay and humus with slight electrical charge
This charge attracts plant nutrient ions
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CATION EXCHANGECATION EXCHANGE
Negative charge
on soil colloids:
attracts positively charge ions
repels negatively charged ions
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adsorptionadsorption
Negatively charged colloid attracts swarm of cations from soil solution
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Cation ExchangeCation ExchangeWhen one ion taken up by plant
(pulled off soil particle), replaced by another.
Replacement of one cation for another
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Ability of soil to hold nutrients –
directly related to the number of cations it can attract to soil colloids
Determined by the amount of clay and humus in soil mix
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Displacement of cations depends on:
Relative concentration
high concentration displaces low
The number of charges on a cation
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high CHARGE displaces low
Al>Ca>Mg>K>Na
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Plant roots have negatively charged surfaces
positively charged hydrogen ions attached
Cation exchange takes place when plant roots exchange positive hydrogen ions for cations on soil colloids or in solution
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Cation Exchange Cation Exchange Capacity Capacity (CEC) (CEC)
The ability of soil to hold exchangable ions
CEC expressed in milligram equivalents per 100 grams of soil (mEq/100g)
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Bonding strength Bonding strength
If two cations are present in soil in equal numbers
one that bonds most strongly will be adsorbed
others will be leached out
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Mass action Mass action more ions
in soil,
more exchange sites it will occupy
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Weakly
held cations are more available for plant uptake
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A clay particle is covered with negative charges
Opposites attract, ions with positive charge(s) stick all over surface of clay
root hairs secrete H+ into water around
clay particlesSmaller H cations replace larger cations
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Several nutrients available to plants as negatively charged ions – anion exchange
Negative charge means it is repelled from cation exchange site
Anion exchange greatest in acid soils
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Implications for Implications for GrowingGrowing
High CEC soils have more clay
Low CEC soils more sand
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HerbicideHerbicide
CEC determines how much herbicide should be used.
Colloids absorb pesticides also, tie them up.
High CEC, clay soils usually need more to get effect you want
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Fertilization Fertilization
High CEC soils have greater ability to hold nutrients - larger amounts, less frequently
Low CEC - smaller amounts more frequently - leach out
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Golf courses - all sand - low CEC - fertilize lightly and often
Greenhouses - soilless - low CEC - fertilize lightly and often
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Improve CEC by adding organic matter
Clay soils need less organic matter except to aerate soil
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NUTRIENT UPTAKENUTRIENT UPTAKE
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Nutrient absorption Nutrient absorption Nutrient ions
cross cell membranes of root cells and move into vascular system
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Some uptake is passive
Most uptake is active - takes energy to pull nutrients into high concentration already in plant
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Roots produce energy by respiration
Waterlogged soil limits respiration - limits nutrient uptake
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Root hairs
get ions from soil solution by their own form of cation and anion exchange
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As root tips
grow, move through solution, constantly finding more nutrients
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Capillary action moves nutrients through solution toward plant roots
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Diffusion
moves ions through soil solution
– higher concentration to lower concentration
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Factors affecting Factors affecting uptakeuptake
Anything interfering with photosynthesis - slows growth, slows uptake --low light--poor drainage--soil compaction--dry soils--soil temperature
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Luxury Consumption Luxury Consumption
Plants can sometimes store nutrients for when growth may be slowed
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Plants with deep roots, healthy roots need less fertilization