Pond Boss Magazine: Pond Liming Rates

The world recently changed forever for better and for worse, and I believe more for the worse. I am referring to the development of artificial intelligence (AI) and the ease with which anyone can avail themselves of this application. This amazing, yet dubious, technology can be applied to most anything, even to pond management.

I recently responded to a question about pond liming that had been answered by AI. It was concluded by “young” AI that alkalinity is measured in calcium carbonate (CaCO₃) equivalents and that agricultural limestone is typically about 100 percent CaCO₃ equivalent. This then suggested to AI that one weight unit of agricultural limestone would increase the alkalinity in pond water by the same amount.

As many pond owners should know, 1 part per million (ppm) of a substance is achieved by dissolving 2.71 pounds of that substance in 1 acre-foot of water. AI knows this also, and its response was, one needs only add 2.71 pounds of limestone in each acre-foot of water to raise its alkalinity by 1 ppm.

Ponds usually are around 4-6 feet in average depth, the liming rate usually should increase the alkalinity by at least 25 ppm. This would be 271-406 pounds per acre. Now, I think that this may cause pond management professionals some consternation in convincing an AI-wise pond owner that he or she should contract with them to apply 2-4 US tons/acre (4,000-8,000 pounds per acre) of agricultural limestone to their ponds when only a fraction of that much is indicated by AI.

I want to explain a bit of water and soil chemistry as related to liming that can help with this dilemma that may arise because AI, at least at this stage in its development, is dumb about pond liming.

In a pond with low alkalinity, below about 50-60 ppm, agricultural limestone dissolves very slowly and the limestone particles scarcely dissolve while settling to the bottom. In fact, the increase in alkalinity in a pond for a few days after liming will be only a few parts per million. If alkalinity of the pond water is higher than 50-60 ppm, the granular particles simply will not dissolve because the water is saturated with calcium carbonate.

The particles that settle to the bottom will continue to dissolve, but the problem is that there will be competition for the dissolved limestone in the pond bottom soil. The calcium carbonate dissolves in water that contains carbon dioxide to yield calcium ions and bicarbonate. Those ions are free to enter the water column above as the pond water mixes and circulates. That mixing occurs by wind action, and sometimes aided by mechanically-forced circulation in aerated ponds.

But, there is much acidity in the pond soil of ponds with low alkalinity water.

The upshot of this is that the bottom soil acidity is neutralized by reacting with, and consuming, the bicarbonate ions from the dissolving limestone laying on the bottom. This deprives the water column of the beneficial effect of increasing its alkalinity. The liming rate (application rate for agricultural limestone) must take into account the amount of limestone necessary to overcome the bottom soil acidity in the upper 2 to 3-inch layer of sediment or original soil so that limestone dissolving in the pond bottom may enter the water column to increase the alkalinity.

Methods for estimating the lime requirement of pond bottom soils have been developed, and most soil testing laboratories can make this analysis. The usual pond lime requirement is based on the lime requirement of the soil plus half again as much to assure that sufficient limestone particles remain to provide alkalinity to the water column. The usual rate is about 2-4 tons per acre, and this application usually must be repeated at 2 to 4-year intervals.

There are some drastic deviations from the normal situation of which I will give some examples. The first is that some pond bottom soils may require more agricultural limestone simply because they may be somewhat more acidic than most others, but this would not increase the liming rate for ponds above 5 or 6 tons per acre.

The other two examples involve more unusual situations. The most common one relates to ponds that have much larger watersheds than normal or more stream or spring inflow than normal. If ponds have a water retention time less than about two or three pond volumes per year, a higher lime application rate than suggested by the lime requirement test may be needed. This is because pond water of suitable alkalinity is flushed out and replaced by lower alkalinity water. In fact, it usually is not feasible to lime ponds when the water retention time is less than two months.

The second situation is in ponds where the bottom soils contain iron pyrite depositions. Iron pyrite oxidizes to form sulfuric acid that lowers alkalinity. Such ponds may have lime requirements of 10 tons per acre or even higher. It rarely is economically possible to lime such ponds. Such ponds typically have no measurable alkalinity if they have not been limed previously. The best advice usually is to forget about developing a fishpond if soil contains iron pyrite. Such soils are rare, usually in coastal areas or in ponds developed in surface coal-mining pits. The best way of identifying such soils is to make an analysis of the total sulfur concentration. If it exceeds about 0.75% of the soil weight, it will usually contain iron pyrite.

It seems to me that those who recommend liming and will contract with pond owners to apply the limestone should be diligent to provide good evidence for the reasons for any agricultural liming rates above 5 tons per acre or more frequently than at 3-5/yr intervals. Water and bottom soil analyses are critical to the evidence. In my experience, most ponds will require less than 5 tons/acre of agricultural limestone.

Someone out there needs to tell AI a little bit more about calcium carbonate and how it actually works.

Dr. Claude Boyd is a renowned aquaculture water chemistry expert retired from the School of Fisheries, Aquaculture and Aquatic Sciences, Auburn University, Auburn, Alabama 36849. Dr. Boyd’s latest book, Handbook for Aquaculture Water Quality, is available in the Pond Boss online store, or you can order one by calling Pond Boss, (800)687-6075 or (903)564-6144.