Soil Fertility

Fundamentals of Nutrient Management

Patricia Steinhilber

Department of Environmental Science and Technology

Ag Nutrient Management Program

University of Maryland College Park

Main Topics

•plant nutrition

•functional soil model

•soil chemical properties relating to soil fertility

Plant Nutrition Topics…

•growth factors

•plant nutrients

•mechanisms for nutrient delivery

•law of minimum

Growth Factors:What do plants need to grow?

1. water

2. oxygen

3. heat

4. carbon dioxide

5. mechanical support

6. nutrients

Definition of a Nutrient

•an element that has a direct effect on growth or metabolism

•required by plants to complete their vegetative and reproductive stages of life

•must be specifically required and cannot be replaceable by another element

Non-Mineral

Nutrients

Mineral Nutrients

carbon

oxygen

hydrogen

macronutrients micronutrients

primary secondary iron

nickel

zinc

chlorine

Manganese

boron

copper

cobalt

Molybdenum

nitrogen

phosphorus

potassium

calcium

magnesium

sulfur

What elements do plants require? (Table 4-2, p. 56, MANMH)

Forms in Which Nutrients Exist

•cation – positively charged ion

•anion – negatively charged ion

•neutral - uncharged

So which nutrients exist in what form? (Table 4-1, p. 55, MANMH)

• ammonium – NH4+

• potassium – K+

• calcium – Ca+2

• magnesium – Mg+2

• iron – Fe+2, Fe+3

• zinc - Zn+2

• manganese Mn+2, Mn+4

• copper – Cu+2

• cobalt – Co+2

• nickel - Ni+2

• nitrate – NO3-

• phosphate – H2PO4-HPO4

-2

• sulfate - SO4-2

• chlorine – Cl-

• borate - H3BO3, H2BO3-, B4O7

-2

• molybdate – MoO4-2

Plant Nutrient Terminology

•mineral nutrients

•non-mineral nutrients

•macronutrients

•primary nutrients

•secondary nutrients

•micronutrients

Relationship between Plant Growth and Nutrient Concentration

•What happens when a nutrient or nutrients are inadequate in supply?

•Can the concentration of a nutrient be too high?

•The next diagram addresses these questions.

How can you tell if a nutrient is deficient?

•visual symptoms−note location and type

OMAFRA

How can you tell if a nutrient is deficient?

•visual symptoms−note location and type−deficiency is severe if noticeable

•diagnostics via plant analysis−sample correct plant part at the correct time−see Soil Fertility Guide, PL-1, Plant Tissue Analysis

“hidden hunger”

What happens when more than one nutrient is inadequate?

Liebig-Sprengel Law of the Minimum

•Growth is limited by the growth factor present in the least adequate amount.

•Yield is proportional to most limiting factor.

•Barrel stave example is the most common representation.

Practice Question #1

Examples of macronutrients are the following:

A) nitrogen, iron and potassiumB) phosphorus, nitrogen and calciumC) sulfur, nitrogen and zincD) potassium, magnesium and cobalt

Functional Soil Model

•beyond the pie chart

•expand our vocabulary & concepts

•2-D slice of a soil

A

B

C

D

E

F

G

soil solution

soil air

primary

minerals

secondary

minerals

residues

and

by-products

humus

biomass

inorganic

solids

organic

solids

pore space

Surface Area and Reactivity

Pronk et al. SSSAJ, 2011

Soil Chemistry and Soil Fertility

•pH

•soil clays and reactive surfaces

•managing soil pH

What is pH?

•“p”

•H

What is pH?

•“p” – negative log−a mechanism to make communication about very

small numbers (decimal fractions) easier

•“H” – hydrogen ion concentration (activity) −[H+] or (H+)

pH = -log [H]

Some pH Facts

•pH and hydrogen ion concentration are inversely related.

•As pH increases, hydrogen ion concentration decreases.

•descriptors−acid (pH<7)−basic or alkaline (pH>7)−neutral (pH=7)

[H+] - pH relationship

[H+] (moles/L) [H+] (moles/L) pH

.1 10-1 1

.01 10-2 2

.001 10-3 3

.0001 10-4 4

.00001 10-5 5

.000001 10-6 6

.0000001 10-7 7

.00000001 10-8 8

.000000001 10-9 9

Practice Question #2

If the hydrogen ion concentration of a soil is 0.0000001, its pH is

a) 4b) 5c) 6d) 7

Cation Exchange Capacity (CEC)

•ability of a soil to hold cations on charged sites

•measure of the net negative charge of a soil

•expressed as meq/100g (old) or cmole/kg (new)

Importance and Consequences of CEC

•Exchange phase is the storehouse of cationic nutrients.

•Exchangeable cations are protected from leaching.

•Dynamic equilibria exist between nutrients in the soil solution and on exchange sites.

Mechanisms/Types of Charge Development

• isomorphic substitution (permanent charge)−main mechanism for clay minerals

•protonation and deprotonation of surfaces (variable or pH-dependent charge)

−gain or loss of a H+ at a surface−main mechanisms for hydrous oxides and organic matter−occurs broken edges of clay minerals

Building Blocks of Aluminosilicate Clays

Aluminosilicate Clay Minerals

Name Structure Layers CEC

kaolinite 1:1 3-15

montmorillonite 2:1 80-120

illite (hydrous

mica)

2:1 15- 40

vermiculite 2:1 120 -150

chlorite 2:1:1 15- 40

Variable (pH-dependent) Charged Sites

•hydrous oxides of iron, aluminum and manganese,

•humus, and

•edges of aluminosilicate clays.

How does pH-dependent charge arise?

On a Humus Molecule…

Cation Exchange Capacities of the Common Soil Colloids

Soil ColloidCation Exchange Capacity

(cmolc/kg of colloid)

humus

vermiculite

illite

montmorillonite

100-300

120-150

60-120

15-40

0-3* iron oxides

* at pH 7

Soil Texture and Cation Exchange Capacity

Soil TextureCation Exchange Capacity

cmolc/kg

sands

fine sandy loams

loams and silt loams

clay loams

clays

1-5

5-10

5-15

15-30

>30

Factors Affecting CEC of Soils

•amount of clay-sized particles (texture)

•kind of clay

•amount of humus

•pH

Just how much does a change in pH affect CEC?

Pratt & Bair,

1962

Base Saturation

•percentage of the exchange capacity occupied by basic cations

−calcium (Ca), magnesium (Mg), potassium (K), sodium (Na)

•hydrogen (H) and aluminum (Al) are acidic cations

•greater base saturation, the more fertile the soil

Base Saturation

• routine analysis by some soil testing labs

•Example: exchangeable cations were extracted and measured (cmoles per 100 grams soil)

H – 3Ca – 12Mg – 5Na - 1K - 4

What is the CEC of this soil?

What is the base saturation (%)?

Base Saturation

• routine analysis by some soil testing labs

•Example: exchangeable cations were extracted and measured (cmoles per 100 grams soil)

H – 3Ca – 12Mg – 5Na - 1K - 4

What is the CEC of this soil?

CEC= 3+12+5+1+4=25 cmole/100 grams

What is the base saturation (%)?

% BS=(sum of basic cations/CEC) * 100

(12+5+1+4/25) * 100

(22/25) * 100 = 88%

Types of Soil Acidity

•active acidity

•reserve acidity

Where/How does soil acidity originate?

•nitrification (oxidation) of ammonium−most fertilizers and all organic sources−2 H+ per 1 NH4

+1

•organic acids produced by plant roots and microbes

• rainfall −carbonic acid, nitric acid, sulfuric acid

•hydrolysis of aluminum −3 H+ per 1 Al+3

•oxidation of sulfur−2 H+ per 1 S

similar diagram on

p. 67 of the MANMH

Why Liming to Reduce Soil Acidity is Helpful…

Many crops prefer a specific pH range for optimal growth

Target pH in Maryland

•pH 7.0−alfalfa establishment

•pH 5.6−tobacco

•pH 5.2−potatoes, sweet potatoes

•pH 6.5−most other agronomic and horticultural crops

Adjusting Soil pH

•Lime materials are used to neutralize acidity and raise pH.

•Acid-forming materials are used to produce acidity and decrease pH.−elemental sulfur, iron sulfate, aluminum sulfate

Is lime required?

•Depends upon

−crop and its optimal pH range

−pH of the soil solution (active acidity)

How much lime is required?

•Depends upon−target pH of crop−pH of soil solution−reserve acidity

•“lime requirement” (LR) is a process or chemical test which estimates the amount of pure, fine limestone needed

UME Process for LR

•pH

•target pH of crop

•estimate of reserve acidity−soil texture (range of clay contents)−physiographic province (info about the kind of

clay)

Soil Tests for LR:Soil-Buffer Equilibrations

•mix soil and a carefully-designed buffer solution

•equilibrate (15 – 30 min.)

•measure pH of soil-buffer mixture

•the more the soil lowered the pH of the buffer mixture, the greater the lime requirement

Texture and Lime Requirement

How often is lime required?

Brady & Weil, 13th edition

When should lime be applied?

•2-6 months before most sensitive crop

•data from Alley at VPI indicate that application at planting improved yield

lime rate

(tons/A)

alfalfa yield

(pounds/A)

0 303

1 1,229

3 1,817

6 2,262

Review Question #3

Cation exchange is affected by a) amount of clayb) type of clayc) amount of humusd) pHe) all of the above

Questions?

Lemke, UWSP