Fertilizers

Larry A. Stein

Professor and Extension Horticulturist

Texas A & M AgriLife Extension

Uvalde, Texas

Fertilizers

Not miracle products

Nutrition is just one of the components of a sound production program

What is a Fertilizer?

Fertilizer is any material of natural or synthetic origin that is applied to soils or to plant tissues (usually leaves) to supply one or more plant

nutrients essential to the growth of plants. (Kamas slide)

Well Drained Soil

50% soil

50% pore space

½ pore space has H2O

½ pore space has air

Oxygen is needed for:

Respiration – breakdown of sugar and release of energy

Absorption – Active transport across cell membrane

Plant hormone production

Exclude toxic ions

Fertilizers are Salts

Varies with the type

Too much – water moves out of the plant and burns the leaves

The Only Reason Fertilizers Work is Because of Water

Moves into the root zone

Root zone to root

Higher [ ] to lower [ ]

Nutrient deficiencies can be water deficiencies

Only New Growing Root Tips Absorb Water and

Nutrients

Requires oxygen

Top 3-6 inches of the soil

Excessive Fertilization Can Cause

Too much vigor

Delayed ripening – too much N

Leaf chlorosis – P ties up iron and zinc

Balance is the Key to Nutrition Management as Ions Compete

with Each Other

Too much K+

can affect uptake of CA

+2and Mg

+

and vice versa

Average N – P - K Content of Horticultural

Plants

% N % P % K

Ornamentals 2.0 – 6.0 0.2 – 0.7 1.5 – 3.5

Fruits 2.0 – 7.2 0.15 – 0.3 1.0 – 2.5

Vegetables 2.4 – 5.6 0.3 – 0.7 1.5 – 4.0

Generally a 3-1-2 ratio

In Order to Get the Most Out of Fertilization

Applications

Need to know which elements to apply based on soil test and crop

Timely applications

Accurate placement

Soil Type Will Dictate Frequency

of Applications

Foliar Fertilizers

Impossible to supply a significant portion of the macronutrient needs of most plants with foliar fertilizers

Micronutrient Vs. Macronutrients

Application Method Dictated by Amount Needed

Availability May Be Limited by Soil pH Will Dictate Form of

Fertilizer

Soil or Foliar Application

Kamas slide

Leaf Cuticle May Be a Barrier to Foliar Applications

Steps for Nutrient Entry Through the Leaf

Penetrate the cuticleMove through the apoplast

Absorption on membrane surface and transport into the leaf cells

Cuticle is a Major Limiting Factor to Entry

Stomata

Vein areas

Very young leaves

Base of trichomes

Key Role in Absorption is

Keeping the droplet wet for a long period of time

Not surfactants

Foliar Fertilizers Are Best Used To Supply

Micronutrients

Foliar Nitrogen Fertilization

• Remember, Nitrogen is a MACRO Nutrient

• May Make Sense in the Fall to Simply Maintain a Healthy Canopy

• Feed Grade Urea

– 5 lbs. per 100 gallons

Kamas slide

Soil pH

-Log [H+]

high pH = high Ca = tie-up of Fe and Zn

Nutrient Availability

Organic or Inorganic Sources?

It Does Not Matter to a Plant What Source the Nutrient Comes

From

There Can Be Down Sides to Long Term Use of Synthetic Fertilizers Fertilizers are Salts Failing to Add Organic Matter Back to Soils

Will Cause a Reduction in Soil Structure

There Can Be Down Sides to Use Of Manures Weed Seeds Salts

Composts? Content??? Cost of Product and Cost of Application(Kamas slide)

In Order to Understand Fertilizers and How to Use The

Effectively

Understand Elemental Chemistry and Mobility in Soils

Understand Nutrient Mobility in Plants

Kamas slide

Nutrient Mobility in Plants

Mobile

NitrogenPhosphorousPotassiumMagnesium

Nickel

Immobile

IronZinc

MolybdenumBoron

Calcium

Nutrient Mobility in Plants Is Primarily a Function of Their Solubility in Phloem Sap

Kamas slide

ModeratelyMobile

ManganeseCopper

What Elements Are Most Commonly Applied As

Fertilizers in Vineyards? Nitrogen

Phosphorous???

Potassium

Magnesium

Zinc

Iron

BoronKamas slide

Critical Characteristics of Nitrogen

• Very Mobile in Soils (neg. charge)

• Very Mobile in Plants

• Soils Typically Very Low in Nitrogen

• Native Nitrogen in Soils Consists of:

– Complex, Insoluble Unavailable Organic Compounds

– Simple, More Soluble, Available Compounds in Soil Solution

Kamas slide

Nitrogen

Usually deficient

Plant available forms;

Usually NO3- (nitrate)

Some NH4+ (ammonium)

Material %N Comments

Urea 46 Volatile Dry(NH2-CO-NH2) Material

Ammonium Nitrate 34 Dry Material(NH4NO3) Less Volatile

Ammonium Sulfate 21 Volatile on High(NH4)2SO4 pH soils

Nitrogen Solutions (UAN) 28-32 Volatile, UsuallyUrea +NH4NO3 + water Injected in Drip

Practical Nitrogen Sources

Soil Cations and Anions

Cations are Positively Charged (+)Anions are Negatively Charged (-)

Kamas slide

NH4+ not mobile because

it is a cation; + charges are attracted to soil

particles

NO3- can leach because it

is an anion - charges not attracted to soil particles

Denitrification

Conversion of NO3-

to N2 gas

Occurs under low oxygen conditions

O.M. + NO3- CO2 + H2O + N2

Volatilization of Ammonia

Soil ph > 7.5

Broadcast

Wet Soils

Percent Nitrogen in each fertilizer

Ammonium sulfate ( 21 %)

100 pounds ammonium sulfate = 21 pounds N

Percent Nitrogen in each fertilizer

Ammonium nitrate (33 %)

100 pounds ammonium nitrate = 33 pounds of nitrogen

Percent nitrogen in each fertilizer

Urea (45 %)

100 pounds of urea = 45 pounds of nitrogen

Percent nitrogen in each fertilizer

N – 32 (32 %)

One gallon of N – 32 weighs 11.08 pounds/gal.

Only 32 % N; 11.08 X 0.32 % = 3.55pounds N/gal

½ from urea; ½ from ammonium nitrate

¼ = ammonic; ¼ = nitrate; ½ = urea

Nitrogen Additions

Late Fall/Early Winter-Source? Wet Fall?

5”-10” Shoot Growth-Broadcast if Rain Likely

Mid Summer- Apply Through Drip

Foliar Applications-Late Summer/Early Fall if Needed (feed grade urea@ 5#/100 gallons of water)

Manures - Be Cautious• Little Control on

Nitrogen Availability to the Vines

• Weed Seed Contamination?

• Potential High Salt Content

Kamas slide

Situations to Avoid

• High Nitrogen Applications in Vineyards Still At Risk From Frost

• Excessive N Status at Bloom

– Shatter

– Poor Flower Bud Initiation

Kamas slide

Phosphorous

Literature Cites Grapevine Phosphorous Deficiency Only in Very Low pH Soils

Phosphorous Moves Extremely Slowly in Soils, So if Needed Should Be Added Pre-Plant

Added After Planting, Must Be Disked in Where the Roots Used To Be

Fertilizer %N Avail Sol Equiv Salt CommentsP acid K Acidity Index

Magnesium/Potassium

Competition for Uptake Between Ca, Mg and K

Petiole Values are Only Reliable Way to Balance Needs

Kamas slide

Sources of Mg

Dolomitic Lime on Acid Sites

(Calcium, Magnesium Carbonate)

Sul-Po-Mag

(Sulfate of Magnesium Potassium

0-0-22-22 [Mg]-18[S])

Kmag (Langbeinite ) Organic Form of Sulfate of Potash Magnesium

0-0-22- 11[Mg]-22[S]

Epsom Salts

(Magnesium Sulfate 0-0-0-10[Mg])

ChelatesKamas slide

Sources of Potassium

Sul-Po-Mag

(Sulfate of Magnesium Potassium

0-0-22-22 [Mg]-18[S])

Kmag (Langbeinite ) Organic Form of Sulfate of Potash Magnesium 0-0-22- 11[Mg]-22[S]

Muriate of Potash (Potassium Chloride 0-0-60)

Kamas slide

Band or Broadcast?

Nitrogen Typically Broadcast

Potassium/Magnesium Banded if Applied to Calcareous Soils

Kamas slide

Chelated Nutrients

Χηλή (chelè)

The bond between the organic chemical and theinorganic nutrient must be strong enough to protect

the nutrient, but must be weak enough to release thenutrient once it gets into the plant. In addition, the

chelating agent must not be harmful to plants. (Kamas slide)

Elements that Can Be Chelated

Iron

Zinc

Copper

Manganese

Calcium

Magnesium

CobaltKamas slide

There Are Many Products on the Market

Application Technology

Questions to Ask

Which Element is Deficient?

Length of Desired Effect?

Soil or Foliar? Element Mobility?

How Widespread is Problem?

What Chelating Agent is Called For?

Amount Needed? Economies of Scale Come

Into Play

When Are We Wasting Our Time

& Money? Trying to Apply Elemental

Forms of a Nutrient When Soil Conditions Impede Availability

Making Application After Damage is Done

Using Wrong Product for the Method/Point of Application

Using Products That Are Not Effective

Checks or Controls are a Major Key to Making Fertilizer Decisions

Just because your neighbor uses it does not mean it works

$1 or $2 per acre can add upHow can you recommend

when it MIGHT help

Do ..... Maintain a good soil/water

environment; avoid extremes Do a soil test every 3 to 5 years Avoid high amounts of

nutrients Use foliar fertilizer only to

correct micronutrients or in special situations