Title goes here Author and affiliations go here€¦ · When soils are tested for their nutrient content, we are largely testing the store of nutrients held in the ion exchange capacity

Soil Fertility

David Ruppert and Patricia SteinhilberDepartment 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

‘What factors limit Crop Production?’1. water2. oxygen3. heat4. carbon dioxide5. mechanical support6. nutrients*7. Lack of disease pressure8. Helpful symbioses

‘Soil Fertility’

Definition of a Nutrient

•Irreplaceable, exogenous element •required for life-cycle completion

If a nutrient is badly deficient in a plant you will be able to tell!

Plant Nutrient Terminology

•mineral nutrients•non-mineral nutrients•macronutrients•primary nutrients•secondary nutrients•micronutrients

Non-MineralNutrients Mineral Nutrients

carbon (C)oxygen (O)hydrogen (H)

Macronutrients MicronutrientsPrimary secondary iron (Fe)

nickel (Ni)zinc (Zn) chlorine (Cl)manganese (Mn) boron (B)copper (Cu)cobalt (Co)molybdenum (Mo)

nitrogen (N)phosphorus (P)potassium (K)

calcium (Ca)magnesium (Mg)sulfur (S)

Essential elements for all* plants(Table 4-2, p. 56, MANMH)

Examples of macronutrients are the following:

A. nitrogen, iron and potassiumB. phosphorus, nitrogen and

calciumC. sulfur, nitrogen and zincD. potassium, magnesium and

cobalt

A useful mnemonic for essential plant nutrients (Courtesy, R. Weil, UMCP)

C. B. Hopkins Café Co.Closed Monday Morning & Night.

See you Zoon, the Mgmt.

C. B. HOPKiNS CaFé Co.Closed Monday Morning & Night.

See you Zoon, the Mgmt.

‘Plant available forms’ (the way these essential elements typically occur when taken up by plants from the soil water*)

• 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

‘Cations’‘Anions’

(Table 4-1, p. 55, MANMH)

What happens when nutrients are lacking?Nitrogen – obligate in chlorophyll and every building block of proteinPhosphorus – obligate in energy delivery and DNA structure

Magnesium – The essential element in chlorophyll

Potassium – maintains salt balance within plant and between roots and soil

Other essential elements – like Mg – play indispensable roles in the machinery of cell function

How can you tell if a nutrient is deficient?Lab analysis of tissue samples

Visual Symptoms (if you can tell, its bad)

Sample correct part of plant at the right timeSoil Fertility Guide, PL-1, Plant Tissue Analysis

Note morphology(color, plant part, distortion, location on plant; landscape position)

***

Visual symptoms

‘Hidden hunger’*

How can you tell if a nutrient is deficient?

•visual symptoms−note location and type

OMAFRA

Deficiency on Older Leaves

Immobile nutrientsMobile nutrients

Deficiency on Younger Leaves

Sawyer, Iowa State Ext., 2004

Can the concentration of a nutrient be too high? Yes • High concentrations of one nutrient

can interfere with another.• In soil water nutrients are

ionic. If they are too abundant the soil water may be too salty.

• But practically speaking toxicity is only a problem with some nutrients.

Deficiencies and toxicities can occur even if soil test levels are ‘good’ or ‘optimal’

1. water2. oxygen3. heat4. carbon dioxide5. mechanical support6. nutrients7. Lack of disease pressure8. Helpful symbioses

Plants get their nutrients from what is dissolved in ‘soil water’

So if moisture levels are too low, low flow of nutrients in the soil water to plants may cause deficiencies

https://www.agric.wa.gov.au/mycrop/diagnosing-phosphorus-deficiency-field-peas

https://www.pioneer.com/home/site/us/agronomy/library/corn-drought-affects/

https://www.slideshare.net/viticulture/grapevine-micronutrient-use-deficiency-symptoms-in-the-sjv-of-california

Deficiencies and toxicities can occur even if soil test levels are ‘good’ or ‘optimal’

1. water2. oxygen3. heat4. carbon dioxide5. mechanical support6. nutrients7. Lack of disease pressure8. Helpful symbioses

Soil aeration can affect the ionic form and state of nutrients*

Soil saturation can prompt losses of nutrients or even asphyxiate root systems.

https://crops.extension.iastate.edu/what-about-n-losses-2000

Deficiencies and toxicities can occur even if soil test levels are ‘good’ or ‘optimal’

1. water2. oxygen3. heat4. carbon dioxide5. mechanical support6. nutrients7. Lack of disease pressure8. Helpful symbioses

At the start of the growing season low soil temps keep root systems from adequate development.

Small root systems may not be able to keep upwith nutrient demands of larger shoot systems.*

https://pss.uvm.edu/vtcrops/articles/EarlyCornProblems.html

https://petersonfarmsseed.com/blog/scout-now-for-early-season-sulfur-deficiency/

Early-season deficiencies often sort themselves out.

Deficiencies and toxicities can occur even if soil test levels are ‘good’ or ‘optimal’

1. water2. oxygen3. heat4. carbon dioxide5. mechanical support6. nutrients7. Lack of disease pressure8. Helpful symbioses

Clays and organic matter hold a lot of nutrients

Sandy, low organic matter soils are prone to deficiencies (see Cation Exchange Capacity, this talk).

Deficiencies and toxicities can occur even if soil test levels are ‘good’ or ‘optimal’

1. water2. oxygen3. heat4. carbon dioxide5. mechanical support6. nutrients7. Lack of disease pressure8. Helpful symbioses

Under our abundant rainfall, pH tends to drop over time

Outside of a ‘sweet spot’ for pH, nutrientsbecome deficient or toxic for most ag crops.

Where/How does soil acidity originate?

• nitrification (oxidation) of ammonium−most fertilizers and all organic sources

• 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

This diagram(plant-availability of nutrients vs. pH) was drawn up with agronomic crops in mind. Is there a sweet spot?

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

Deficiencies and toxicities can occur even if soil test levels are ‘good’ or ‘optimal’

1. water2. oxygen3. heat4. carbon dioxide5. mechanical support6. nutrients7. Lack of disease pressure8. Helpful symbioses

The ionic forms of plant nutrients are sometimes similar in size and charge; sometimes nutrients interact.

If a nutrient is overabundant it can be detrimentally substitute or react with another nutrient.

Nutrient in Excess Induced Deficiency

NH4+, Ca2+, Mg2+ K+

Potassium and/or Calcium Magnesium (Mg)

Chlorine (Cl) Nitrate and Sulfate

Nitrogen (N) Potassium

Calcium (Ca) Magnesium (Mg)

Magnesium (Mg) Calcium (Ca)

Calcium (Ca) Boron (B)

Phosphate (PO4) Iron (Fe), Manganese (Mn), Zinc (Zn),or Copper (Cu).

Iron (Fe) Manganese (Mn)

Manganese (Mn) Iron (Fe)

Molybdenum (Mo) Copper (Cu)

https://www.maximumyield.com/how-nutrient-antagonism-leads-to-nutrient-deficiency-in-plants/2/2092

Deficiencies and toxicities can occur even if soil test levels are ‘good’ or ‘optimal’

1. water2. oxygen3. heat4. carbon dioxide5. mechanical support6. nutrients7. Lack of disease pressure8. Helpful symbioses

If soil moisture, pH, temperature, aeration and nutrient levels are all adequate deficiencies can occur…

…in such case consider disease, or plant predators.

As I mentioned, the unfavorable conditions for crop growth this spring mean that observing some deficiency symptoms in young corn plants should not be cause for immediate alarm. However, if deficiencies continue after growing season conditions improve, it is important to confirm any deficiency before trying to correct the problem. Since the visual symptoms are sometimes not clear-cut, it could be beneficial to collect affected plants and conduct tissue nutrient analysis.--Fabián G. Fernández

http://bulletin.ipm.illinois.edu/print.php?id=1163

Liebig-Sprengel Law of the Minimum

•(to a point) Growth is limited by the nutrient present in the least adequate amount.

What happens when more than one nutrient is inadequate?

We have seen that

• Moisture• Aeration• Temperature• The nature of the soil itself• pH• Relative nutrient levels• Pests

All affect the ability of plants to obtain proper nutrition

*

All things being equal why is it that some soils are more fertile than others?

The beginning of the answer:

Some soils contain more nutrients anyway

Some soils can hold on to nutrients better

Q. Why are some soils inherently more fertile?A. Because they contain more nutrients; soils

developed from nutrient-rich geologies….mineral Formulaquartzite SiO2

olivene (Ca,Mg)2SiO4

Kaolinite Al2Si2O5(OH)4

Muscovite KAl2(AlSi3O10)(FOH)2

Montmorillonite (Na,Ca)0-3(Al,Mg)2Si4O10(OH)2•n(H2O)Limestone (Ca,Mg)CO3

….will contain more nutrients!

Q. Why do some soils hold more nutrients?

Organic matter contentA. Because soils differ in

Relative amount of sand, silt and clay (‘soil texture’)

Relative amount of absorptive clays

Soil component Relative ability to hold cations§

Organic matter 100-550Vermiculite clay 120-150Montmorillonite clay 80-120Illite clay 15-40Kaolinite clay 3-15iron oxides (at pH 7) clay 0-2silts 0sands 0

Surface Area and Reactivity

*

*

Factors Affecting CEC of Soils

•amount of clay-sized particles (texture)

•kind of clay

•amount of PrOM

•pH*

The relative ability of soils to be able to hold charged particles is called

‘ion exchange capacity’

‘Cation Exchange Capacity’(CEC)

Measured in the amount of charge that can be retained by 1 kg of dry material; typically centimoles of charge per kg.

‘Anion Exchange Capacity’(AEC)

†CEC>AEC for OM and most clays

Because all soils typically have a distribution of sand silt and clay and some organic matter, all soils have

some exchange capacitySoil Textural Class CEC§

cmoles/kg or meq†/100gsands 1-5fine sandy loams 5-10loams and silt loams 5-15clay loams 15-30clays >30†meq = ‘milliequivalents’

Exchange Capacity

• routine analysis by some soil testing labs• Example: exchangeable cations were extracted and

measured (cmoles per kg soil)H – 3Ca – 12Mg – 5Na – 1K – 4

What is the CEC of this soil?

Exchange Capacity

• routine analysis by some soil testing labs• Example: exchangeable cations were extracted and

measured (cmoles per kg soil)H – 3Ca – 12Mg – 5Na – 1K – 4

What is the CEC of this soil?CEC= 3+12+5+1+4=25 cmole/kg

The ability to retain ions means the ability to retain nutrients

Higher Lower exchange capacity

When soils are tested for their

nutrient content, we are largely

testing the store of nutrients held

in the ion exchange

capacity of the soil.

Plants get nutrients out of what is dissolved in the soil water

Soil fertility tests sample the nutrients adsorbed to the soil

Adsorbed ions > dissolved ions

While nutrient content can be tested we can also evaluate the share of

the ion exchange capacity occupied by certain elements.

There is particular interest in the share of adsorbed elements that are

not acidic.

Acid cations

Basiccations

H+ Ca2+

Al3+ Mg2+

Fe3+ Na+

Mn4+ K+

‘Base Saturation’= share of CEC occupied by ‘basic’ cations

=cmol charge / kg due to Mg, Ca, Na, K

CEC

Base Saturation

• routine analysis by some soil testing labs• Example: exchangeable cations were extracted and

measured (cmoles per kg 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 kg soil)H – 3Ca – 12Mg – 5Na – 1K – 4

What is the CEC of this soil?CEC= 3+12+5+1+4=25 cmole/kg

What is the base saturation (%)?% BS=(sum of basic cations/CEC) * 100

(12+5+1+4/25) * 100(22/25) * 100 = 88%

What is pH?•measure of acidity alkalinity•shorthand for the number of dissolved H+

ions in soil water

Acidic AlkalineMore abundant H+ OH-

Less abundant OH- H+

H+ and OH- play a zero sum game

[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

[OH-]

10-13

10-12

10-11

10-10

10-9

10-8

10-7

10-6

10-5

pH = -log [H]; ‘a logarithm is an exponent’

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

A. 4B. 5C. 6D. 7

Types of Soil Acidity

•active acidity•reserve acidity

How does pH affect CEC?

Soil MineralSurface Charge

Acidic AlkalineNeutral

Acid conditions diminish CEC

Just how much does a change in pH affect CEC?

Pratt & Bair,1962

Acid conditions diminish CEC