Writing the Soil Rx - The Ideal Soil 2014 - page 81

Solubility of Iron, Manganese, Copper and Zinc Amendments
Most Fe, Mn, Cu, and Zn soil amendments are applied as sulfates. The purified
metals have been reacted with sulfuric acid and water to form more or less soluble
compounds.As a rule, the more H 2 O molecules attached to the sulfate, the higher
the solubility. Zinc sulfate monohydrate, ZnSO 4 1H 2 O is practically insoluble in
water, Zinc sulfate heptahydrate, ZnSO 4 7H 2 0 is highly soluble. Which one we
choose will depend on whether we want slow release Zinc or highly soluble Zinc,
whether or not we will be able to till the amendment into the soil, and whether the
soil needs more Sulfur or already has too much. The oxide forms of the metals are
not soluble in water, but are a good choice for soils that are naturally high in
sulfates, e.g. gypsum soils.

Iron Fe
The worksheet calls for 39ppm Fe = 78# Fe

Standard feed grade ferrous sulfate monohydrate FeSO4 1H2O is 30% Fe and
18% Sulfur. It is only sparingly soluble in water. Ferrous sulfate heptahydrate ,
FeSO 4 7H 2 O (also known as “copperas” due to the green color of its crystals) is
highly water soluble but only contains 20%Fe and 11.5%S by weight.Athird
alternative for soils high in sulfates where one does not wish to add more S, is Iron
oxide Fe 3 O 4 , which is around 70% Iron .

We will choose to use the water-soluble heptahydrate, copperas, which is 20%Fe.

Iron Fe 50 Add 390# Ferrous sulfate 20%Fe

78# / 0.20 = 390# FeSO 4 7H 2 0

We need to add 390 kg/ha or lbs/acre Iron sulfate 20% Fe

In addition, this iron sulfate contains 11.5% Sulfur

390 x 0.115 = 44.85 rounded to 45 lbs or kg Sulfur.After calculating the
Potassium sulfate we needed another 63 units of S. 63 - 45 = 18. Now we only
need another 18 units, which will be taken care of with the addition of Manganese
sulfate and Copper sulfate. We will have more than enough Sulfur when all of

Boron B 0.21 Add 30# Borax 9% B

the amendments are added.

Note on Iron: Other common mineral amendment s such as greensand and many
rock dust powders contain significant percentages of Iron. If you have easy access