The main purposes for applying soil/water amendments are (1) to reduce sodium adsorption ration (SAR) or exchangeable sodium percentage (ESP) and (2) lower pH and release calcium in calculous soils.
Amendments for Sodium Control: Two types of amendments can be used for Na control: Ca-containing amendments and acids or acid-based amendments. Gypsum (CaSO4.2H2O) is by far the most common Ca-containing amendment. It is low-cost, abundant, and non-toxic, and can be added to either irrigation water or soil. Gypsum added to a sodic soil increases the soil water EC and reduces SAR (or ESP), thus improves soil permeability.
Gypsum application rate depends on the amount of initial exchangeable Na fraction in the rootzone (Naie), soil cation exchange capacity (CEC, mmol/Mg), soil bulk density (BD, Mg/m3) and the final exchangeable Na fraction (Nafe). The amount of exchangeable Na to be replaced per unit of land area for the depth of soil (Dr) to be reclaimed (ΔNa, molc/ha) can be calculated by:
ΔNa (molc/ha) = 104 (Dr) (BD) (CEC) Nafe – Naie
The gypsum requirement (GR, metric ton per ha) to reclaim a sodic soil can be calculated by calculated by (Keren and Miyamoto, 2012):
GR (ton/ha) = 86.1 X 10-6 ΔNa
Gypsum has no direct effect on soil pH, but it can slightly lower pH when added to soils with serious Na problems.
Acids or acid-based amendments can be used to prevent or correct Na problems in calcareous soils. Sulfuric acid is the most-commonly used acid amendment. Sulfuric acid and most acid-based amendments dissolve soil carbonates and yield gypsum: Sulfuric acid can be soil-applied or water-run.
CaCO3 + H2SO4 = Ca2+ + SO42- + H2O + CO2
Elemental sulfur (S) is an acid-based amendment that is 97% sulfur. Soil microorganisms transform sulfur to sulfuric acid. One potential disadvantage of S is its slow reaction time. In warm, moist soils, four to six weeks may be required for the complete transformation of S to sulfuric acid; in cold or dry soils this conversion will take even longer, as indicated by Dr. James L. Walworth, University of Arizona.
Nitrogen contained amendments can also be used as fertilizers. Nitro-sul® is an ammonium polysulfide material, which contains 20% NH -N and 40-45% sulfur (20 - 0 - 0 - 40S). It causes release of acidity after microbial oxidation. Thio-sul® is an ammonium thiosulfate material containing 12% NH -N and 26% S (12 - 0 - 0 - 26S). It also releases small amounts of acidity, but is used mostly as a fertilizer. N-Phuric® is a combination of urea and sulfuric acid that contains 10-28% N and 9- 18% S. This amendment releases acidity, is a safer way to use sulfuric acid, and can be safely used in drip or micro-sprinkler systems as an N fertilizer and to prevent clogging of irrigation lines (James L. Walworth, University of Arizona).
* Calcareous soils: soils containing free lime (often from 1 to 15% CaCO3).
Sulfur burners are generally cheaper than direct injection of sulfuric acid. This is how it works: a solid form of sulfur is added and then heated to form SO2, which then reacts with water to form sulfurous acid (H2SO3). i.e.,
oxidized sulfur creates sulfur oxide (SO2) gas. The SO2 gas is blended with water to create sulfurous acid (H2SO3).
Often times, when diluted with irrigation water, the Sulfurous acid seeks out a dissolved oxygen molecule and creates H2SO4.
As discussed above, sulfuric acid can lower pH. In sodium-affected soils that have adequate free lime present in the soils, it helps to release Ca2+ from CaCO3 to form gypsum, which improves infiltration.