1. How nutrients, soil particles and chemistry fit together
Soil Chemistry
How nutrients, soil particles and chemistry fit together

2. 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-, SO42-

3. 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-)

4. Acids and Bases (pH) 1 7 14 Acid high H+ low OH- Neutral equal H+/OH-
low H+
high OH-

5. 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

6. 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
Organic Matter
Clay Particle

7. Where do soils get their pH?
Cations (positive charges) could be…
H+ (making the soil more acidic) H+ H+ H+ H+ H+
Organic Matter
Clay Particle H+ H+ H+ H+ H+

8. Cations could be… Cations can be H+ Cations can also be nutrients
Potassium (K+)
Calcium (Ca2+)
Magnesium (Mg2+)
Iron (Fe3+)
Nitrogen (NH4+) K+ H+
Fe3+
NH4+
Ca2+ H+
Clay Particle
Mg2+ K+ H+
Fe3+
Ca2+ H+
NH4+
Mg2+

9. 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 K+
Cl- H+
NH4+
Ca2+ H+
Clay Particle
Mg2+ K+ H+
Fe3+
Ca2+ H+

10. 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 K+ H+
Fe3+
NH4+
Ca2+ H+ K+
Clay Particle
Mg2+ H+ H+
Fe3+
Ca2+ H+
NH4+
Mg2+

11. 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!

12. Areas that get a lot of rain tend to have more acidic soil!
NH4+
OH- H+
OH- H+ K+ H+
Fe3+ H+
Clay Particle H+
Ca2+ K+ H+
Ca2+ H+
NH4+
OH-
Fe3+ H+
Mg2+
Mg2+

13. 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.

14. 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