Chapter 10 - Soil Fertility

Essential Plant Nutrients - 14 are Mineral Based

Macros - N, P, K, Ca, Mg, S

Micros - B, Cu, Cl, Fe, Mn, Mo, Zn, Ni

Nutrients Exist in the Soil as Ions

Ions are charged particles and can be…

positively charged (cations): Ca+2, K+, NH4+

or

negatively charged (anions): Cl-, H2PO4-, SO4

-2

Ions are either adsorbed or absorbed

adsorbed - “like a magnet”

absorbed - “like a sponge”

Stored (held in reserve)• minerals (quartz, feldspar, etc.)• organic material• soil colloids (clay and humus

particles)

Readily Available• soil solution (ions + water)

Nutrient Sources

Soil Colloids

• very small particles (< 1 millionth of a meter)

• large surface area

• negative charges along surfaces

Mica

HumicAcid

Kaolinite

(kandite)

Montmorillonite

(smectite)

types of soil colloids: clays, oxides and humus

clay

clay

clay

humus

Soil Colloids

• Silicate Clays

- flat, plate-like crystals arranged in layers

- negative surface charge

• Oxide Clays (sesquioxides)

- iron and aluminum based

- red in color

- common in older soils with hot, humid climates

• Humus - decomposed organic matter

- dark brown to black in color

- mostly negative surface charge

- high water holding capacity

Tetrahedron:• Basic building block is 1 silicon atom, connected to 4 oxygen atoms

Octahedron:• Basic building block is aluminum or magnesium, connected to six hydroxyl groups or oxygen atoms

There are many layers in each micelle

Typical Clay Structure

silica

alumina

silica

Illite

-

--

-

Mg+2

Na+Mg+2

K+

Ca+2

Ca+2-

--

--

-

-

-

--

-

--

-

-Al+3

-

-

-

K+

Clay Particle

Clay and Organic Particles have Negative Charges

Cations are “attracted” and held to these particles with varying degrees of attraction…the order of relative strength of attraction is:

Al+3 > H+ > Ca+2 > Mg+2 > K+ = NH4+ > Na+

NH4+

CEC - the ability of a soil to hold and release nutrients (cations)

Roots Access Nutrients in Solution by…

• root interception

• mass flow

• diffusion