Magnesium Adsorption by Bottom Soils in Ponds for Inland Culture of Marine Shrimp in Alabama Harvey J. Pine and Claude E. Boyd Dept. of Fisheries and Allied Aquacultures Auburn University Auburn, Alabama

Introduction -Low-salinity shrimp culture is cultivation in waters 10 ppt or less (below 1 ppt considered freshwater) (Boyd 2002). -Sources of low-salinity waters for inland shrimp production are typically diluted seawater or mineralized groundwater sources (Boyd and Thunjai 2002).

Introduction -The ability to produce shrimp under low-salinity conditions has allowed for its expansion to inland areas, which offers several advantages including: reduced land costs, removal from environmentally sensitive areas, and less exposure to potential pathogens -Inland low-salinity culture (ILSC) of marine shrimp occurs in China, Ecuador, Thailand, United States, and other countries.

Introduction -In the United States ILSC of shrimp is performed in several states including Alabama, Arizona, Florida, Texas, and others. -In Alabama ILSC of shrimp occurs in two counties (Greene and Lowndes, highlighted yellow) within the Black Belt region.

Introduction -Saline water obtained in these areas are mineralized groundwater sources with salinities ranging between 2 ppt - 9 ppt. -Groundwater sources in these areas are deficient in certain cations and require amending with potassium and magnesium fertilizers to achieve acceptable ionic compositions and ionic ratios (Saoud et al., 2003; McNevin et al., 2004; Davis et al., 2005)

Composition of water from eleven inland shrimp ponds before addition of mineral amendments. Variable Mean  SD Min.Max. TDS 3,888  150 2,0056,540 Bicarbonate 105  Sulfate 2.1  Chloride 2,274  105 1,8872,821 Calcium 86  Magnesium 21  Sodium 1,392  45 1,1251,665 Potassium 7.7  (Christopeher Boyd 2006)

Average deviation of concentrations of ions in inland pond waters (3.9 ppt salinity) from the seawater reference. Variable Seawater reference (mg/L) (mg/L) Average deviation (mg/L) Sulfate Chloride2, Calcium Magnesium Sodium1, Potassium (Christopeher Boyd 2006)

Introduction -Maintaining optimal potassium and magnesium concentrations are complicated by the adsorption and non- exchangeable fixation of cations (notably potassium) by the soils (Boyd et al., 2007). -The ILSC of shrimp farms in the Black Belt region of Alabama are situated on soils that are dominated by smectitic mineralogy, which are 2:1 expansible clays that typically have high cation exchange capacity (CEC) and the ability to fix cations between adjacent layers of tetrahedral sheets.

Introduction -Continuous adsorption by these soils requires continuous monitoring of ionic concentrations and subsequent additions of fertilizers such a muriate of potash (KCl) and K-mag® (K-MgSO 4 ).

Objective The objective of the current study has been to monitor the loss of magnesium in the water column to bottom soils from the Black Belt Region of Alabama. The objective of the current study has been to monitor the loss of magnesium in the water column to bottom soils from the Black Belt Region of Alabama. Determine whether or not the loss of magnesium is via exchangeable or non- exchangeable processes Determine whether or not the loss of magnesium is via exchangeable or non- exchangeable processes Determine if the capacity of soils to take up magnesium can be overcome Determine if the capacity of soils to take up magnesium can be overcome

Materials and Methods - Three different soils from an inland shrimp farm located in Forkland, Alabama USA, were collected. -Soils were air dried and sifted using No. 8 US standard sieve(2.36mm)

Materials and Methods Saline well water from the farm was collected and 56 L placed in each tank over 5 cm of soil Saline well water from the farm was collected and 56 L placed in each tank over 5 cm of soil 16 tanks (control - MgSO 4 not added) 16 tanks (control - MgSO 4 not added) -Soil A, 3 replicates, one control -Soil B, 3 replicates, one control -Soil C, 3 replicates, one control -No Soil, 3 replicates, one control Dosed to ~40 mg/L Magnesium Dosed to ~40 mg/L Magnesium Sampled regularly and analyzed for Mg by atomic adsorption Sampled regularly and analyzed for Mg by atomic adsorption

Soil sub-samples were collected from each tank before and after the trial.  Soils were dried at 55°C, pulverized, and sieved to < 2.0 mm for determination of: Soil pH Extractable Cations Cation Exchange Capacity Base Saturation Materials and Methods

Results Water Quality Water Quality  Well Water pH pH Salinity -4.1 ppt Salinity -4.1 ppt Initial [Mg 2+ ] mg/L Initial [Mg 2+ ] mg/L (Seawater of average composition at 4.1ppt would have ~156 mg Mg 2+ /L)

Results

Results Before Exposure After Exposure Soil A Soil B Soil C Mean soil pH of exposed soils

Results Magnesium loss from water and adsorption by soils over 11 months in laboratory soil-water systems with 56 L of water Variable Soil ASoil BSoil CMean Mg 2+ Loss from water (mg/tank) ± 155 Ex Mg 2+ Ads by soil (mg/tank) ± 274 Non-Ex Mg 2+ Ads by soil (mg/tank) ± 166

Discussion and Conclusions The Exchangeable Mg 2+ capacity of the soil is saturated rather quickly and accounts for nearly 92% of the adsorbed Mg 2+ The Exchangeable Mg 2+ capacity of the soil is saturated rather quickly and accounts for nearly 92% of the adsorbed Mg 2+ The loss of Mg 2+ to non-Exchangeable processes is slower and will take longer to saturate these sites within the clay minerals The loss of Mg 2+ to non-Exchangeable processes is slower and will take longer to saturate these sites within the clay minerals  Saturation of exchange sites in these soils will be determined using serial exposures to high Mg 2+ solutions.

Discussion and Conclusions Compared to K + adsorption on these soils, which is dominated by non- Exchangeable processes (~72%) and sustained uptake (Christopher Boyd 2006), Mg 2+ adsorption should diminish rather rapidly. Compared to K + adsorption on these soils, which is dominated by non- Exchangeable processes (~72%) and sustained uptake (Christopher Boyd 2006), Mg 2+ adsorption should diminish rather rapidly.

Discussion

Discussion

Discussion Kmag® (K∙MgSO 4 ) Kmag® (K∙MgSO 4 )  ~10.5% Mg 2+  $145/tonne Kmag®, or $1380/tonne Mg 2+

Discussion and Conclusions The CEC and adsorption capacity will vary spatially over the region and ILSC farms will experience different levels of losses of dissolved ions to the pond bottom soils. The CEC and adsorption capacity will vary spatially over the region and ILSC farms will experience different levels of losses of dissolved ions to the pond bottom soils. Mg 2+ concentrations need to be determined prior to stocking and amended to desired levels ?? Mg 2+ concentrations need to be determined prior to stocking and amended to desired levels ??

Discussion and Conclusions When applying Mg 2+ fertilizers to ponds there is a need to monitor Mg 2+ concentrations regularly When applying Mg 2+ fertilizers to ponds there is a need to monitor Mg 2+ concentrations regularly Reuse water fertilized with Mg 2+ to conserve the cation Reuse water fertilized with Mg 2+ to conserve the cation

Thank You