What does gypsum do?

Gypsum is a soil amendment that may be recommended to treat poor structure and/or high sodium levels. But what is gypsum, how does it work, and when is it actually useful?

First lets straighten out two soil terms that are often confused: salinity and sodicity. Salinity refers to soil's overall salt content, and a saline soil is properly diagnosed by looking at the electrical conductivity of a soil extract. Sodicity, on the other hand, refers specifically to soil's sodium content. A sodic soil is defined by having a sodium saturation of 15% or higher in its cation exchange capacity. Sodic soils generally have a high pH greater than 8.5 (except for saline-sodic soils, which are both saline and sodic). Gypsum is used to remediate sodicity, but it can exacerbate salinity, so it should not be used for saline or saline-sodic soils with high calcium levels. Most saline and sodic soils in the United States are found in the western part of the country in arid/semi-arid regions and in the lower Mississipi River Valley.

What does gypsum do . . . and not do?

The chemical formula for gypsum is calcium sulfate (CaSO4). In soil, calcium is one of several positively-charged cations that bind to negative charges on soil particles, which make up the cation exchange capacity (CEC). Different cations bind more strongly or more weakly to the soil based on their atomic structure; they are essentially magnets that are harder or easier to pull out of the soil based on the strength of their positive charge relative to their size. Sodium (Na) has only one positive charge and binds more weakly to the soil than calcium (Ca), which has two positive charges. When we add gypsum to a sodic soil whose particle surfaces are loaded with sodium, the calcium from the gypsum replaces the sodium by binding more strongly to the soil particles. This makes it easier for the sodium to be leached out of the soil with percolating water. 

Sodium causes soil particles to repell each other, which is known as dispersion, so adding gypsum can improve soil structure in sodic soils by helping to clear out the sodium. However, if weak soil structure has another explaination, for example sandy texture, gypsum might not be the answer for soil aggregation.

Remediating saline and sodic soils: spotlight on Florida

The most effective method of reducing soil salts is to rely upon natural rainfall and/or low-saline irrigation. If clean water is not available, irrigate deeply with existing water to leach the salts remaining in the rootzone from prior irrigation cycles. Wetting agents may aid with moisture distribution but may also reduce infiltration rates. Despite claims, gypsum will not remove salts and does little to increase percolation in Florida’s sandy soils. Gypsum is a salt and, therefore, the application of gypsum increases salinity. However, gypsum can remediate sodium (Na)-related issues such as poor percolation resulting from deflocculated soils. However, Florida soils are predominately sands, which have a very poor capacity to retain Na and, therefore, Na-related soil problems are rare. However, gypsum has the additional benefit of reducing bicarbonates and carbonates (Figure 19), which can be toxic to many turf-grasses.

 

 

Figure 19. Bicarbonate (2NaHCO3) and carbonate (Na2HCO3) reductions following gypsum (CaSO4) application.

 

In locations where Na and/or bicarbonates are continually added to the turf/soil system, remediation may require regular gypsum applications over several years, in some cases. On established turf, gypsum application rates range from 200-500 lbs. per acre. If your normal fertilizer contains filler, your fertilizer distributor should be able to replace the filler with gypsum. In this manner, gypsum applications would then be a regular part of your nutrient applications with very little appreciable increase in cost. This method may be less expensive than sole gypsum applications, but it will require more time to achieve the same remediation effect.

 

o            Soil pH and Bicarbonates

Soil acidity and bicarbonate management are the primary purposes of Ca applications to Florida turf-grasses.  The application of gypsum does not increase pH as does limestone despite both materials containing Ca.  It is limestone’s counterion (CO3-) that reduces pH and because gypsum contains SO4-2 rather than CO3-, no increase in pH will occur.

The application of gypsum may lower pH if the soil pH is initially high due to the presence of Na.  The turf-grass response, if any, would likely be due to the increased biological activity that typically accompanies a reduction in pH i.e., increased microbial activity and increased micronutrient availability.  However, a turf-grass response to such a scenario has not been documented in Florida.

Direct toxicity to bicarbonates has been documented but tolerance limits for most turf-grasses remains unknown.  In most cases, bicarbonates are a concern due to their ability to bind with Ca and Mg and leave behind soluble Na.  Additionally, water low in Na and dissolved salts but high in bicarbonates may result in unacceptable pH levels (>8.0).  The application of gypsum will reduce the presence of bicarbonates.

o            Salt Remediation

Gypsum is a salt and, therefore, the application of gypsum does not remediate salt-affected turfgrass and, in fact, may exacerbate salt-related problems.