soil chemistry how nutrients, soil particles and chemistry fit together
TRANSCRIPT
Soil Chemistry
How nutrients, soil particles and chemistry fit together
Basic Chemical Charges
Ions are particles with either a positive or negative charge.
Cations are ions with positive charges– Ca2+, Al3+, H+, Mg2+, Fe3+, K+, NH4
+
Anions are ions with negative charges– OH-, Cl-, SO4
2-
Acids and Bases (pH)
pH is determined by the concentration of H+ (hydrogen)… and OH- (hydroxide)
H+ is very reactive
Acids = high H+ (low OH-) Bases = low H+ (high OH-)
Acids and Bases (pH)
1 7 14Acid
high H+
low OH-
Neutral
equal H+/OH-
Base
low H+
high OH-
Soil pH
Many soils have a pH at or just below neutral Some acidic soils may have a pH of 4 or lower pH > 8 is high for a soil
Most plants prefer a pH at or just below neutral
Where do soils get their pH?
Clay particles are negatively charged Organic matter is usually negatively charged
This means that positive charges (cations) are attracted to them
Clay Particle Organic Matter
Where do soils get their pH?
Cations (positive charges) could be…– H+ (making the soil more acidic)
Clay Particle Organic Matter
H+ H+H+
H+H+
H+
H+
H+
H+
H+
Cations could be…
Cations can be H+ Cations can also be nutrients
– Potassium (K+)– Calcium (Ca2+)– Magnesium (Mg2+)– Iron (Fe3+)– Nitrogen (NH4
+)Clay Particle
H+
H+H+
H+K+
NH4+Ca2+
Mg2+
Fe3+
Ca2+
K+
NH4+
Mg2+
Fe3+
Anions in Soil
Positive cations are almost always balanced with negative anions (electrically neutral soil)– Chloride– Bicarbonate– Phosphate– Sulfate– Nitrate
These attach to the positive cations
Clay Particle
H+
H+H+
H+K+
Ca2+ Fe3+
Ca2+
K+
NH4+
Mg2+
Cl-
Cation exchange
Cations can displace each other– Depends on concentration in soil (the more there
are in the soil solution)– Depends on how strongly they are attached to clay
or organic matter
Clay Particle
H+
H+H+
H+K+
NH4+Ca2+
Mg2+
Fe3+
Ca2+
K+
NH4+
Mg2+
Fe3+
H+
Frequent Watering Lowers pH over Long Period of Time
Rain tends to be slightly acidic Water is made of H+ and OH- H+ tends to be held very tightly to clay particles
Result: Areas that get a lot of rain tend to have more acidic soil!
Areas that get a lot of rain tend to have more acidic soil!
Clay Particle
H+
H+H+
H+K+
NH4+Ca2+
Mg2+
Fe3+
Ca2+ K+
NH4+
Mg2+
Fe3+H+
H+
H+
H+
OH-
OH-
OH-
Areas that get LITTLE rain tend to have more basic soils!
H+ doesn’t displace as many of the other cations present in the soil as in rainy areas.
Desert soils tend to be basic!– We don’t add lime or eggshells (bases) to our soil
like they do in the East where soils are acidic.– Instead we add coffee grounds (acid) to make our
soil more neutral.
The POINT?
Clay particles and organic matter are negatively charged and attract cations
Cations in soil can be H+ or nutrients
When there is a lot of rain, H+ cations displace other nutrients making the soil more acidic