Magnesium Adsorption by Bottom Soils in Ponds for Inland Culture of Marine Shrimp in Alabama

1. 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

2. -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

3. 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. http://www.usmsfp.org/news/Newsletter/4-2004/images/oilrig.jpg

4. 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.

5. 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)

6. Composition of water from eleven inland shrimp ponds before addition of mineral amendments. (Christopeher Boyd 2006)

7. Average deviation of concentrations of ions in inland pond waters (3.9 ppt salinity) from the seawater reference. (Christopeher Boyd 2006)

8. 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.

9. 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-MgSO4).

10. 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. • 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

11. 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)

12. Materials and Methods • Saline well water from the farm was collected and 56 L placed in each tank over 5 cm of soil • 16 tanks (control - MgSO4 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 • Sampled regularly and analyzed for Mg by atomic adsorption

13. Materials and Methods • 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

14. Results • Water Quality • Well Water • pH -8.07 • Salinity -4.1 ppt • Initial [Mg2+] -15.35 mg/L (Seawater of average composition at 4.1ppt would have ~156 mg Mg2+/L)

15. Results

16. Results Mean soil pH of exposed soils

17. Results

18. Discussion and Conclusions • The Exchangeable Mg2+ capacity of the soil is saturated rather quickly and accounts for nearly 92% of the adsorbed Mg2+ • The loss of Mg2+ 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 Mg2+ solutions.

19. Discussion and Conclusions • Compared to K+ adsorption on these soils, which is dominated by non-Exchangeable processes (~72%) and sustained uptake (Christopher Boyd 2006), Mg2+ adsorption should diminish rather rapidly.

20. Discussion

21. Discussion

22. Discussion • Kmag® (K∙MgSO4) • ~10.5% Mg2+ • $145/tonne Kmag®, or $1380/tonne Mg2+

23. 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. • Mg2+ concentrations need to be determined prior to stocking and amended to desired levels ??

24. Discussion and Conclusions • When applying Mg2+ fertilizers to ponds there is a need to monitor Mg2+ concentrations regularly • Reuse water fertilized with Mg2+ to conserve the cation

25. Thank You