CEMENTING
[by John Taberna, Western Laboratories, Parma, Idaho]
Soils have two properties that are mostly affected by salts: physical and chemical. Both of these have distinct characteristics. The two physical problems affecting soil structure are crusting and cementing.
CEMENTING in salt-affected soils is due to CaCO3 (lime) soil + water. Unlike crusting, cementing is both a chemical and a physical problem. The chemical problem is lime reacting with soil and water to form a "brick yard." This cemented soil will not dissolve or disperse in the presence of water. It must be treated physically, such as sub-soiling, or chemically, such as elemental sulfur, or a combination of amending procedures.
Table 1. Guidelines for CEMENTING
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Three points to consider before selecting the proper amendment are: 1. is lime present, 2. is calcium greater or lesser than 4500, and 3. is sodium greater or lesser than 230. Cementing is a problem that affects the top foot of the soil.
Calcium at 4500 ppm becomes a salt problem. Calcium accumulates in soil in three ways: 1. mineralization of the soil, 2. irrigation water, and 3. calcium products applied to soil.
-- Calcium in a carbonate form is insoluble in water. Carbonic acid (H2CO3) is formed by the reaction of carbon dioxide (CO2) and water (H20). This develops due to respiration in plant roots and due to decomposition of organic matter. Calcium in the presence of carbonic acid forms lime.
Ca + H2CO3 = CaCO3 (lime).
-- Calcium in a sulfate form is soluble in water; however, calcium sulfate (gypsum) can precipitate at high rates. Gypsum (CaSO4) is used by many to treat a lime (CaCO3) problem in soil. In other words, calcium is added to treat a high-calcium problem. This is asking for trouble!! Lime or calciferous soils need to be treated with sulfur to form gypsum from the calcium already present in the soil. In other words, taking calcium out by solubilizing it in a sulfate form.
-- Elemental sulfur (S) is converted to sulfuric acid (H2SO4) by Thiobacillus bacteria in warm, wet soil. Sulfuric acid reacts with the free lime (CaCO3) which is insoluble in water. The reaction between sulfuric acid and lime to form gypsum takes minutes.
S + Thiobacillus + warm and wet soil = H2SO4 (sulfuric acid)
H2SO4 + CaCO3 (lime) = CaSO4 (gypsum).
Lime is insoluble in water.
Gypsum is soluble in water.
-- Besides sulfuric acid, phosphoric acid can be used on lime-containing soils. Phosphoric acid (H2PO4) plus lime (CaCO3) gives calcium phosphate (CaH2PO4).
-- Ironsul can be used in soils with or without lime, but it does work better in the presence of lime. It is an expensive product and should be included in a multiple-product approach.
FeSO4 (Ironsul) x H2SO4 (free sulfuric acid) + CaCO3 (lime) gives FeSO4 (iron sulfate) + CaSO4 (gypsum).
-- Acid-residue fertilizers assist in preventing cementation. In themselves, they do not resolve the cementing problem.