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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−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
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
soil solution
soil air
primary
minerals
secondary
minerals
residues
and
by-products
humus
biomass
inorganic
solids
organic
solids
pore space
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
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.
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
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%
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
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
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
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