Saline, Sodic, and Saline-Sodic Soils
Management of Sodic Soils
Sodic soils have limited crop production potential. The relatively high sodium and pH levels restrict the growth and yield potential of most annual crops and even a number of forage crops. Farmers with sodic soils are faced with decisions on how to best manage their land. Sodicity cannot be removed by leaching alone as in the case of saline soils. Some of the options for reclaiming sodic soils include reclaim the soil with chemical amendments, additions of organic matter, acidification of irrigation water, or leaving the land in its native state and utilize for livestock grazing.
Reclamation of Sodic Soils with Chemical Amendments
Reclaiming sodic soils requires a different approach than saline soils and can be considerably more costly. Prior to leaching, excess sodium needs to be replaced from the exchange site by another cation, namely calcium or magnesium, which will and reduce the ESP and SAR. Because calcium and magnesium have a stronger charge than sodium, they will replace sodium on exchange sites, causing sodium to be released to the soil solution as sodium sulfate (Na2SO4) which is susceptible to removal by leaching. Magnesium a divalent ion is about half as effective as calcium at displacing sodium. This is commonly accomplished by adding gypsum (calcium sulfate, CaSO4), since it is relatively soluble and inexpensive.
Types of Amendments
Chemical amendments for sodic soil reclamation can be broadly grouped into the following categories: (1) soluble calcium salts, for example, gypsum and calcium chloride, and (2) acids or acid-forming amendments, for example, lime sulfur, sulfur, iron sulfate, and aluminum sulfate. Application rates for these amendments will vary depending upon product properties (particle size, oxidation rate) and soil conditions (pH, buffering capacity, and minerals present).
Quantity of Amendment
The amount of amendment needed for reclamation depends on the soil’s initial ESP, the final level desired for exchangeable sodium, the soil’s capacity to adsorb sodium and calcium, the soil’s bulk density, and the total depth of soil to be reclaimed.
Additions of Organic Matter
The reclamation of sodium soil is slow because it can take a long time to rebuild the structure. One relatively inexpensive approach to improve the soil structure is to apply low sodium containing manure or to apply crop residues to these areas. The organic matter in these materials can help stabilize and improve soil structure. It must be pointed out that not all manures have low sodium concentrations.
Acidification of Irrigation Water
Some irrigation waters, typically those pumped from deep wells, contain high concentrations of bicarbonate and have a high SAR (high concentration of sodium relative to calcium + magnesium). Application of these waters can create sodic soils over time. Irrigation water that is high in bicarbonate or carbonate can react with calcium in the soil solution to form calcium carbonate. This process removes calcium from the soil solution.
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