soil chemistry how nutrients, soil particles and chemistry fit together

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