Many farmers are confused about gypsum sources. Are they all the same? Our testing has shown that like limestone, not all gypsum is the same. One of the newest “gypsum" sources to come on the market recently is Flue Gas Desulphurization (FGD) from coal combustion.
- Coal contains small amounts of toxic metals. Some of these metals are captured in FGD gypsum.
- Wet FGD technologies can remove highly soluble oxidized mercury (from burning coal); about 85%-90% of the mercury can be captured in wet FGD gypsum.
- Therefore, mercury content in FGD gypsum is of major concern in FGD by-products.
- Mercury is more volatile than other elements and can readily evaporate into the atmosphere.
- It has been reported that mercury in the soil can emit into the atmosphere and be absorbed by plants.
- Mercury in the soil also can be up taken by plants.
- When it comes to mercury distribution in plants, the mercury concentration in the stem is much lower than in leaves and roots. (Mercury is up to 6 times higher in the leaves and roots than stems.)
- When a plant defoliates, the leaves fall down, and mercury enters the soil and is absorbed by the plant’s roots.
- Mercury in the plant’s roots stays in the plant’s roots.
- More FGD added leads to more mercury in the soil. (30% to 300% depending on depth sampled.)
- Over dosage of FGD gypsum resulted in a negative effect on growth of tall fescue.
- Mercury could also poison the roots, leading to low yield.
- Plants may absorb mercury because the mercury size and valence state is similar to plant nutrients.
- It could be inferred that mercury penetrated into the subsoil and finally leached out of the soil.
- The mercury concentration in FGD gypsum is 300 ppb. The initial mercury concentration in the soil is 28 ppb. Mercury in seed is 65 ppb. (This means in a standard application of 1 ton of FGD you are applying 3# of Mercury.)
- One possible reason for the increase is that mercury species in FGD gypsum was in ionic form, and therefore is able to dissolve in the water
- Using water evaporation as a mode of transportation, mercury was released from the soil.
- With the increased addition of FGD gypsum, more mercury was dissolved in the water, and thereby more mercury evaporated into the atmosphere. (8-15% of applied mercury vaporized into the atmosphere. The higher the application rate the more that evaporated into the atmosphere.)
- FGD gypsum only affected the H+ concentration in the soil at massive doses, which is most probably due to unreacted CaCO3 in the FGD gypsum. (In this study (page 51) large amounts of gypsum raised pH significantly. This suggests the conversion of the limestone and sulfides to gypsum is not a stable conversion and could cause soils to be “over-limed” in already high pH soils.)
- In this field study, the FGD gypsum used had the same mercury concentration as that of mined gypsum.
- The mercury concentration of the whole stalk was higher in FGD gypsum plots than the mined gypsum plots.
- The FGD gypsum mercury may be more easily absorbed by the plant than mined gypsum.
I leave you with this... SuperCal SO4 is naturally mined gypsum, has no heavy metals and had been proven effective.
The Blogronomist is maintained by Craig Dick, head blogronomist and VP of Sales and Marketing. Here you will find a wide array of blog articles from Craig and expert guests on topics related to soil and crop health, farming, and so much more. If it’s not here, ask us!