Dairy Factory wastewater (DFW) can be a useful resource for pasture and crops but high sodium levels in the wastewater may accumulate in soil. Excess sodium can be detrimental to pasture and crop growth since it causes low soil water availability and poor soil structure. Soil structure effects may reduce the ability of the soil to receive DFW before surface ponding or runoff occurs. Gypsum is an ideal material to address any build up of sodium in soils receiving DFW, extending the number of years that affected land remains useful for receiving wastewater. Gypsum can also enhance soil structure, water infiltration and drainage in the face of high sodium levels. A gypsum programme should ideally start before critical sodium levels are reached.
Sodium
Sodium in wastewater largely comes from the use of caustic soda and sodium hypochlorite in cleaning and sterilising processes. Some soils receiving this wastewater will tend to accumulate sodium. The extent to which sodium accumulates in a soil depends on soil type, drainage properties and the level of other minerals in the soil and wastewater. Sodium is relatively easily leached and the role of gypsum is to accelerate that leaching to reduce the chance of sodium accumulation.
High sodium levels can impact on soil structure. Sodium is a monovalent cation (single positive charge) and competes for cation exchange sites in a soil with the more desirable calcium and magnesium cations (both divalent). While calcium and to some extent magnesium help flocculate a soil into a crumb or aggregate structure, high sodium levels can reduce flocculation particularly in what are known as dispersive soils (where clay content loses structure in contact with water). The subsequent loss of structure can impact on the ability of the soil to receive and drain large amounts of water as well as restricting roots and plant growth. High sodium can also increase soil crusting and reduce water infiltration, leading to increased water ponding and surface runoff problems.
How Does Gypsum Work?
Gypsum is hydrated calcium sulphate. Calcium from gypsum replaces sodium in the soil. The sulphate allows the sodium to be effectively leached out of the soil. The soil then has more ability to flocculate and form stable aggregates to improve drainage and soil quality.
The reaction of gypsum in the soil is
Na+ Na+ Ca++ leached
CaSO4 + SOIL CATION EXCHANGE => SOIL CATION EXCHANGE + Na2SO4
Why Gypsum?
Gypsum application is a standard practice worldwide for addressing the build up of sodium in soils including soils receiving wastewaters. The combination of calcium and sulphate effectively address sodium. Calcium release from the partially soluble gypsum is faster than from lime (calcium carbonate). Lime is also unsuitable in many cases as it acts to increase soil pH, pushing out acid hydrogen rather than sodium from the cation exchange. Although DFW can be acidic, the effect of adding it to the soil can increase soil pH over time, meaning that lime application would simply add to a future issue of overly high soil pH. Other calcium sources such as calcium nitrate and calcium chloride would provide a faster release of calcium than gypsum but, at the high levels required, they would have negative effects such as increasing nitrate leaching or increasing soil salinity to the detriment of plant growth.
Gypsum can maintain a higher electrical conductivity near the soil surface for a prolonged period of time resulting in improved infiltration of sodium rich DFW, thus further improving the ability of soil to receive the wastewater without ponding or surface runoff.
Application Method and Rates
Gypsum is typically and most easily applied as a broadcast application to the soil surface. This is also the recommended method if soil crusting is to be addressed. Incorporation of gypsum into the soil is not generally required as the gypsum can work through the profile of most soils.
Gypsum can be applied annually or every two or three years (at higher rates). Many soils will have quite high thresholds for sodium before soil structure is affected; this depends on the the balance of the monovalent cations (sodium and potassium) with calcium and magnesium, the electrical conductivity of the soil. It also appears that soil resilience to sodium is increased by the organic content (lactose etc) in DFW (Cameron et al., 2003). Application of gypsum will be most effective if a programme of application commences before soil structure is impacted by the sodium and potassium.
Regular soil testing will assist in calculating gypsum requirements. If sodium levels are high and increasing, gypsum application should be increased accordingly. The optimum amount of gypsum will depend on the current levels of sodium in the soil and the amount of sodium being applied each year, and complex factors affected by soil type and drainage properties of the site. A logical approach is to calculate gypsum requirement based on the annual increase in exchangeable sodium and potassium. Another approach is to identify the level of reduction desired in sodium and potassium to achieve reasonable or historic soil levels.
For each excess milli equivalent (me/100g) of sodium, at least 1.9 tonne/ha gypsum would be required to counteract that. (Based on calcium replacing of sodium on cation exchange sites, calculated for a soil depth of 150 mm and dry bulk density of 1 kg/L and assuming 33% of calcium is lost to greater depth or plant uptake).
Addressing Subsoil Issues
Sometimes subsoil structural issues can reduce drainage and impact on the ability of a site to receive wastewater without ponding or surface runoff. Gypsum may be helpful if the issue is related to sodium and dispersive clay content (clay that loses structure in contact with water), or if there is a problem with high aluminium availability in an acid subsoil. Applying gypsum to the soil surface for subsoil issues, requires generally large rates of 3 tonne/ha to 5 tonne/ha (even higher rates may be required with heavier soils or higher sodium levels). Allow sufficient time (at least six months) before checking any subsoil effect or conducting any deep ripping (the effect of which can be assisted by gypsum). Faster effects may be possible with incorporation of gypsum into the soil.
Gypsum Effect Requires Drainage
Drainage must be adequately addressed for gypsum to be effective at removing sodium. If a drainage issue is related to high sodium and clay dispersion, gypsum could assist, otherwise deep ripping, earthworks or artificial drainage may be required.
Gypsum may not be suitable for some soils with a high subsoil pH (>7.0) where the calcium could form calcium carbonate and reduce drainage.
Reducing Phosphorus Loss
Gypsum can reduce surface runoff of phosphorus and other nutrients by improving water infiltration (through soil structural and chemical changes), by binding organic matter and soil particles together better and by increasing the ability of soil to drain and thus cope with larger water inputs. Gypsum also increases the binding of phosphate to soil minerals including calcium and reduces the susceptibility of all forms of phosphorus to drainage losses. The multiple modes of action mean gypsum can be effective in a wide range of soil types. Rates of at least 1.25 tonne per hectare per annum (or 4 tonne/ha every three years) could address excess phosphorus availability and losses to the environment (see Jenkins 2014, for more detail).
References
Cameron, K.C., Di, H.J., Anwar, M.R., Russell, J.M. & Barnett, J.W. 2003. The “critical” ESP value: Does it change with land application of dairy factory effluent? New Zealand Journal of Agricultural Research 46:2, 147-154.
Jenkins, T. & Jenkins V. 2014. Use of gypsum to reduce effluent and fertiliser nutrient losses to waterways. In:Occasional Report No. 27. Fertilizer and Lime Research Centre, Massey Universityhttp://flrc.massey.ac.nz/publications.html.