Nutrients soil fertility

SOIL FERTILITYSOIL FERTILITY

Ability of soil to supply nutrients for plant growth 

Soil is storehouse of nutrients 

Some available, some not

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

Plant nutrients Plant nutrients   

Essential elements needed for plant growth

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)

How to determine which are essential?

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 

17 essential elements  17 essential elements  

3 account for 95% plant needs

carbon, oxygen and hydrogen

come from water, air

Non-minerals

Macronutrients Air and Water Carbon (C)

Hydrogen (H)

Oxygen (O)

14 come from soil 14 come from soil   

6 used in large amounts:macronutrients

Nitrogen CalciumPhosphorus MagnesiumPotassium Sulfur

Primary Macronutrients  Primary Macronutrients  

Nitrogen PhosphorusPotassium

Not always available in large enough quantities

Add by fertilizing

Secondary MacronutrientsSecondary Macronutrients  

Calcium Magnesium Sulfur

Soil usually has plenty

    Micronutrients Micronutrients

8 nutrients left

Used in small amounts

Plant won't grow normally without them

  NUTRIENT IONSNUTRIENT IONS

Plants absorb some of the nutrients as ions instead of elements

Ion has positive or negative charge

Ion has:

positive charge cationnegative charge anion

  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

  Plant roots absorb ions - soak like a

sponge

Soil particles adsorb ions - hold on to them - stick to it

SOURCES OF ELEMENTS IN SOURCES OF ELEMENTS IN SOILSOIL

Nutrient elements present in soil in four places (pools)

SOURCES OF ELEMENTS IN SOURCES OF ELEMENTS IN SOILSOIL

1. soil minerals

major sourcereleased slowly by weatheringnot source of nitrogen

SOURCES OF ELEMENTS IN SOURCES OF ELEMENTS IN SOILSOIL

2. organic matter

large amounts of nitrogennutrient anionsreleased by decay 

SOURCES OF ELEMENTS IN SOURCES OF ELEMENTS IN SOILSOIL

3. adsorbed nutrients

held by clay and humus particles

relatively available to plants

SOURCES OF ELEMENTS IN SOURCES OF ELEMENTS IN SOILSOIL

4. dissolved ions

ions in soil solutionplants absorb directlymay be leached

Soil colloidsSoil colloids    

Tiny particles of clay and humus with slight electrical charge

This charge attracts plant nutrient ions

   CATION EXCHANGECATION EXCHANGE

  Negative charge

on soil colloids:

attracts positively charge ions

repels negatively charged ions

adsorptionadsorption    

Negatively charged colloid attracts swarm of cations from soil solution

Cation ExchangeCation ExchangeWhen one ion taken up by plant

(pulled off soil particle), replaced by another.

Replacement of one cation for another

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

Displacement of cations depends on:

Relative concentration

high concentration displaces low

The number of charges on a cation

high CHARGE displaces low

Al>Ca>Mg>K>Na

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

    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)

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

Mass action Mass action more ions

in soil,

more exchange sites it will occupy

  Weakly

held cations are more available for plant uptake

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

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

Implications for Implications for GrowingGrowing

  High CEC soils have more clay

Low CEC soils more sand

  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

Fertilization Fertilization   

High CEC soils have greater ability to hold nutrients - larger amounts, less frequently

Low CEC - smaller amounts more frequently - leach out

Golf courses - all sand - low CEC - fertilize lightly and often

Greenhouses - soilless - low CEC - fertilize lightly and often

Improve CEC by adding organic matter

Clay soils need less organic matter except to aerate soil

NUTRIENT UPTAKENUTRIENT UPTAKE  

Nutrient absorption  Nutrient absorption  Nutrient ions

cross cell membranes of root cells and move into vascular system

Some uptake is passive

Most uptake is active - takes energy to pull nutrients into high concentration already in plant 

Roots produce energy by respiration

Waterlogged soil limits respiration - limits nutrient uptake

  Root hairs

get ions from soil solution by their own form of cation and anion exchange

  As root tips

grow, move through solution, constantly finding more nutrients

Capillary action moves nutrients through solution toward plant roots

Diffusion

moves ions through soil solution

– higher concentration to lower concentration

Factors affecting Factors affecting uptakeuptake

  Anything interfering with photosynthesis - slows growth, slows uptake --low light--poor drainage--soil compaction--dry soils--soil temperature

Luxury Consumption Luxury Consumption

Plants can sometimes store nutrients for when growth may be slowed

Plants with deep roots, healthy roots need less fertilization