Gypsum effectiveness in loamy sand soils under saline conditions

¶ … gypsum on sandy loam and loamy sand-types of soil for erosion control purposes under saline condition. All soils are three-dimensional natural bodies comprised of both mineral and organic materials, with the specific composition of a soil at a given site and point in time being the result of the interaction of five interrelated factors-parent material, climate, plants and animals, relief, and time (Shahid, Aslam, Hashmi & Mufti, 2009). According to the Australian Soil Health Knowledge Bank, the integrity of the structure of soils is important because erosion represents a serious threat to agriculture and other human pursuits (Soil stability, 2011). Gypsum has been shown to be effective in improving the soil stability and reducing erosion in certain types of soils, such as those that contain large amounts of organic matter (Orme, 2001) as well as sandy and coarse-loamy soils (Shahid et al., 2009). These are important issues for soil management purposes because sandy and loamy soils are particularly susceptible to erosion which will require the addition of gypsum for erosion control and the promotion of soil integrity (Cochrane, Reichart, Eltz & Norton, 2005). There may be chemical as well as mechanical properties involved in the function of gypsum in controlling erosion. For instance, studies of the application of gypsum on soils have demonstrated effective stabilization of soil aggregates as well as the reduction of clay dispersion (Cochrane et al., 2005). This point is also made by James (2009) who reports that, "The binding of soil particles allows water and air to filter and collect in the soil. Soils with a higher percentage of clay (loamy) will hold more water and nutrients. Sandy soils do not have the same structure or holding capacity, so water and nutrients will leach quickly" (2009, para. 2). In these situations, water transports salt solutions to the deeper layers of soil where is can accumulate and produce sodic soil (James, 2009). In these cases, water is hampered from traveling downward from the sodic soil point onward and is forced to move horizontally through the other layers of soil (James, 2009). The process is described by this authority thusly: "The Calcium (Ca) in gypsum replaces the salt while the sulfate in gypsum reacts with the sodium forming sodium sulfate. Sodium sulfate is a highly water-soluble material that is easily leached below the root zone. This process also repairs soil structure so soil particles can bind with each other again. For severe cases the process may take several seasons" (James, 2009, para. 3).

In some cases, gypsum may not be as effective in improving soil stability in other types of soils without repeated applications. For instance, the authorities at the Australian Soil Health Knowledge Bank note that, "In sandy soils, soil particles are unable to form stable aggregates, but the soils are free draining. As the clay content of the soil increases, the particles are held together more strongly and structural strength increases" (Soil stability, 2011, para. 2). This means that over time, the organic content of the soil will increase while the sandy elements decrease, thereby contributing to the structural integrity of the soil. For example, according to Shahid and his associates, "An important feature of the soil is that it is not static, rather it is a dynamic natural body interacting in a complex manner with its environment. As a consequence, the soil changes through time and in space as a response to environmental changes" (2009, p. 20). This observation suggests that the addition of gypsum for erosion-control and soil-stabilization purposes may be more effective during some phases of the soils' evolution compared to others. In this regard, the Australian Soil Health Knowledge Bank adds that, "Soil slaking or dispersion is evident in soils with a high content of fine sand and/or silt (loamy soil) and low organic matter levels, with crusting and hardsetting most common in soils with 10 to 35% clay" (Soil stability, 2011, para. 3). In addition, the crusting and hardsetting processes can in turn affect the infiltration rate that determines a soil's composition, and infiltration rates are highly affected by the concentrations of saline that exist near the surface (Cochrane et al., 2005).

Although soils with high concentrations of saline may be responsive to gypsum applications, the gypsum will likely be required to be reapplied from time to time in order to remain effective (Soil stability, 2011). According to these authorities, "Gypsum (calcium sulfate) contains calcium which stops clay from dispersing when wet. Gypsum can be spread to manage surface soil problems, with more recent methods (injection into subsoils) used to counteract subsoil sodicity - often in combination with ripping" (Soil stability, 2011, para. 3). The effectiveness of gypsum in these applications, though, is only short-term and does not contribute significant improvements to the overall stability of soils that are not heavily salinated (Soil stability, 2011).

Some of the potential disadvantages of the application of gypsum for erosion-control purposes include the following:

1. Gypsum sold as an industrial by-product may contain heavy metals (cadmium) and other contaminants (fluoride).