SOIL FERTILITY

Ability of soil to supply nutrients for plant growth 

Soil is storehouse of nutrients Some available, some not    

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   Essential elements needed for plant growth

Plant 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 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 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 6 used in large amounts: macronutrients Nitrogen Calcium Phosphorus Magnesium Potassium Sulfur

Primary Macronutrients Nitrogen Phosphorus Potassium Not always available in large enough quantities Add by fertilizing

Secondary Macronutrients Calcium Magnesium Sulfur Soil usually has plenty

Micronutrients 8 nutrients left Used in small amounts Plant won't grow normally without them

 NUTRIENT IONS Plants absorb some of the nutrients as ions instead of elements Ion has positive or negative charge

Ion has: positive charge cation negative charge anion

Ions form in soil when compounds dissolve in water Example:   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 SOIL Nutrient elements present in soil in four places (pools)

SOURCES OF ELEMENTS IN SOIL 1. soil minerals major source released slowly by weathering not source of nitrogen  

SOURCES OF ELEMENTS IN SOIL 2. organic matter large amounts of nitrogen nutrient anions released by decay   

SOURCES OF ELEMENTS IN SOIL 3. adsorbed nutrients held by clay and humus particles relatively available to plants

SOURCES OF ELEMENTS IN SOIL 4. dissolved ions ions in soil solution plants absorb directly may be leached

Soil colloids    Tiny particles of clay and humus with slight electrical charge This charge attracts plant nutrient ions

CATION EXCHANGE Negative charge on soil colloids: attracts positively charge ions repels negatively charged ions

adsorption   Negatively charged colloid attracts swarm of cations from soil solution

Cation Exchange When 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 –   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 Capacity (CEC) The ability of soil to hold exchangable ions CEC expressed in milligram equivalents per 100 grams of soil (mEq/100g)

 

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  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 particles Smaller 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 Growing High CEC soils have more clay Low CEC soils more sand

Herbicide 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   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   Golf courses - all sand - low CEC - fertilize lightly and often Greenhouses - soilless - low CEC - fertilize lightly and often

Improve CEC by adding organic matter   Improve CEC by adding organic matter Clay soils need less organic matter except to aerate soil

NUTRIENT UPTAKE  

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   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   Diffusion moves ions through soil solution – higher concentration to lower concentration

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

Luxury Consumption Plants can sometimes store nutrients for when growth may be slowed

Plants with deep roots, healthy roots need less fertilization