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