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SOIL FERTILITY SOIL FERTILITY

Nutrients soil fertility

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Nutrients - Soil Fertility Fruit and Vegetable ScienceK. Jerome

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Page 1: Nutrients   soil fertility

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