Soil Chemical Properties
  1. Affect the ability to supply nutrients for
  2. Affect the fate of chemicals added to the soil.
    • fertilizers
    • organic wastes
    • lime
    • pesticides
    • herbacides .
Glossary of chemical terms
Element A single substance which can't be decomposed by ordinary chemical means. Ex. Ca, Na, S, N, P, K, Al, Mg, Mn. There are 118 known elements, singly or in combination. They constitute all matter.
Atom The smallest uncharged part of an element that maintains its chemical identity.
Compound A substance which is made up of 2 or more elements in fixed proportions. Can be decomposed by chemical means.
Molecule The smallest particle of an element or compound capable of retaining chemical identity. Ex. water (H2O) or table salt (NaCl)
Ion An atom or molecule that has electrical charge
Cations Positively charged ions --- Ex. Ca 2+, Mg2+, K+, Al3+,H+
Anions Negatively charged ions --- Ex. NO3-, (PO4)3-, SO4-2
Valence The charge of the ion is called its valence. Ex. Ca has a valence of 2+. Other examples are: Al+3 , Na+, Ca+2, Mg+2, NO3-, (S04)-2
Atomic Weight

 

The relative weight of the elements

H = 1g, Ca = 40g, O = 16g

Molecular Weight

The sum of the atomic weights of the elements found in a molecule or compound.

Ex. H20

H =1g x 2 = 2g

0 = 16 g = 16g

Molecular Weight = 2 + 16 = 18
Ex. CaCO3

Ca = 40g

C = 12g

O = 16x3 = 48g

Molecular weight = 100g

Equivalent weight

milliequivalent (meq)

 

Equivalent weight is the weight of 1 unit of charge expressed in milligrams
for equivalent weight = (atomic weight / valence )

For Ca+2 40 g / 2 = 20 g
For H+ lg/1 = 1 g

1 eq of any substance has the same amount of charge as 1 eq of another substance

Put these weights on a milliequivalent basis. Factor of 1000

Meq are always equal. The weights may differ but the charges are still the same. The weight represents the amount of material needed to have the same charge.

Al+3 = 27g = 9 g of Al+3 per eq. of charge

9 g of Al+3 has the same amount of charge as 1 g of H+ or 20 g of Ca+2.

1 Meq of Al+3 has the same amount of charge as 1Meq of Ca+2 or 1Meq of H+.

   

Soil Chemistry
Most of the chemical properties of soils are associated with soil colloids.

Soil colloid Opposite charges attract: colloids have the ability to attract (and hold) cations.

In order to understand the chemistry of a soil you must know what clay minerals are present and their contribution to the negative charge.