Pond Boss Forums INTRO Questions & Observations Alum/Hydrated Lime and Ag Lime questions

I am going to try the aluminum sulfate treatment in my pond. The Texas A&M article mentions adding one half part hydrated lime for every one part alum to buffer it. Will ag lime provide the buffering I need?

Buffering your alum treatment is a safeguard against drastic pH change. I learned the hard way. If you add the "right" amount of aluminum sulfate, pH won't be affected. But, add just a little too much, and pH will plummet. Aglime won't be as responsive to a mistake as hydrated lime. If you choose to use alum, keep an accurate pH measuring device, and proceed with caution. Have hydrated lime on hand, in case you boo-boo. Read the pH before you start, and as you go.
When applying alum, watch the water. Clumps of dirt will form as you reach the "right" amount. It's okay to stop applying, as you reach saturation. Watch the water. It will tell you

Hydrated lime is used to offset or buffer the pH shift caused by aluminum sulfate. Agricultural lime's slow action is much less capable, if not completely incapable of balancing aluminum sulfate pH swings. So, don't use it in place of hydrated lime.

The primary reason for avoiding a pH shift is fish-safety. Aluminum sulfate is notorious for prompting rapid and, often times, severe pH shifts. I've been told that a pH shift of 2+ points is lethal to fish. I'm not sure what the exact tolerance threshold for fish might be (more or less) or if it varies by specie. But, I know of several instances where fish were floated by non-buffered aluminum sulfate treatments.

Hey Bob. We must have been typing at the same time, but you pulled the trigger first.

Yep...but, I like your answer better.

Thanks for the advice. I have now found a supplier for hydrated lime. Should I just go ahead and use it at the 1/2 part per 1 part alum just to be safe or should I just use it if the pH drops? My pH is already on the low end.

Also, how does gypsum compare cost-wise and as a binding agent? I may just try that instead.

First off: have you confirmed that your presumed turbity problem is due to a water-chemistry imbalance? If not, the cause of the problem could be physical (rooting fish, wave-action or run-off erosion) - and will not likely to be remedied with either alum or gypsum.

Gypsum is a safer option for correcting chemistry-related turbidity problems - though it takes more gypsum (and a longer period of time) than with alum.

For application rates of alum, you can have your local pharmacist measure some alum into one gram increments. Then collect a one liter sample of your pond water--ideally getting a little from several different areas of the pond. It is probably wise to obtain your pond water early in the morning when pH levels are already low from plant respiration. Obtain an accurate pH measuring device and incrementally mix the alum into the water while measuring pH change. When reaching desired safe pH levels you now have a g/L or ppm value for application rate. Now calculate number of liters of water in your pond to estimate safe number of grams for total application. You probably don't want your pH to go under 6.5. If, for example, your estimated application value is 100 kg you could theoretically apply 80 kg first and check pH values after a couple of days before applying the last 20 kg to build in a safety net. If your starting pH is above 9 to start, you may get the added benefit of reducing your amount of unionized ammonia, which is toxic to fish. The alum will also bind with available phosphorous to yield aluminum phosphate which will settle to the pond bottom. Clarity will improve because fewer nutrients will be available for aquatic plants to use. A company known as General Chemical in New Jersey can help you get large quantities of alum possibly for a lesser price.

Yes, on the water chem inbalance.

Bruce, I see you are from Lincoln. Apparently you have used this method before. How well does it work in our area?

My alum application worked extremely well. Unionized ammonia was reduced to virtually nil and my water clarity, i.e. secchi readings improved from .5 m. to 1.1 m. I also did not have a summer kill this year as I did the previous two years. My ponds are fed by well water which comes in at pH 8.1 which means I have the ability to buffer the acidity produced by the alum reaction. I also feed my fish which eventually starts to increase NH3 or ammonia to high levels. My problems appear to be temporarily solved. I don't have any experience on how alum works on inorganics such as suspended silt particles. I only have to worry about organic buildup from feeding.

Kelly, I'm interested in your comment on application methodology and how it affects the ultimate pH and ammonia values.

How slowly (days) can I apply alum so that I can get the desired effect yet keep a close eye on the pH?

Here's some practical hints on application methodology.
Know your pH, first. If above 7.0, proceed with application. If not, rethink.
Alum comes in powder/granular form, or liquid. I like liquid. We apply with a pump, mixing alum 25-30 parts water per part alum. Spraying across the water, we observe several ways. First, we measure pH every 15-20 minutes, depending how fast we apply the material. Then, we watch the water. As inorganic matter (suspended solids/soils) begin to react, they flocculate, like dirty little magnets attracting to each other. Watch closely, you will see dirt begin to clot, then separate, then clot, then separate. As bigger "chunks" of solids form, you can see them grow larger. Stop applying, and watch. Check pH. As these small dirt clumps form, pH will show a temporary change.
Here's what you will see. pH the same, pH the same, same, same...then it begins to plummet, in areas where flocculation begins. Stop applying. Measure pH again. If it is lower than "normal", add hydrated lime cautiously, to bring pH back up. Did I say to monitor pH? Monitor pH.
Let the lake sit still, overnight. If still turbid, add a little more alum.
Using powdered alum? Mix it with water, then mix with lake water 25-30 to 1 and spray over the lake.
Don't have a pump? Get one. The mistakes you might make without it easily covers the cost.
Gypsum has similar effects as alum, but much slower, with considerably less risk.
Regarding ammonia, or ammonium, your water de-nitrifies it quickly, into nitrites and nitrates. However, alum can bind. My experience with alum tends to lean toward suspended soils mostly affected by the application. I haven't measured alum's affect on ammonia.
Alum works great, but risks are high. I would recommend using it, with proper caution. By the way, the stuff is caustic, be sure to clean equipment, boat motors, boat, etc., after use.