Mineral content and pH readings are created by interaction of soil beneath a pond and water that fills the pond. Ponds must have clay basins to retain water. Clay soils often are acidic. When filled with poorly mineralized water, the result is usually low alkalinity and hardness.

William A. Wurts and Michael P. Masser from the Southern Regional Aquaculture Center share important findings on this subject in an article “Liming Ponds for Aquaculture”. According to research, when total alkalinity and hardness fall below 20, pH and productivity are reduced. Alkalinity concentrations below 20 mg/l often lead to large swings in daily pH values, which stress aquatic animals. Acidity in pond soils can be neutralized and productivity improved by liming with compounds of calcium or calcium and magnesium.

Liming has three important benefits:

  • Enhancing effects of fertilization.
  • Helping prevent wide swings in pH.
  • Adding calcium and magnesium is important in animal physiology.

The most important reason to lime ponds is improving fertilizer response. Fertilizers containing nitrogen, phosphorus, and potassium (especially phosphorus) stimulate growth of microscopic plants (phytoplankton) and animals (zooplankton) which serve as forage for other animals in the aquatic food chain. In recreational ponds, abundant plankton supports larger populations of largemouth bass and bluegill. Healthy phytoplankton blooms also absorb toxic nitrogen wastes and raise daytime dissolved oxygen important to water quality. Without normal alkalinity, phosphorous cannot support phytoplankton growth and greatly increase pond productivity. Without proper hardness, fish may experience poor bone and scale formation.

To determine if a pond should be limed, check total alkalinity. Collect a water sample from the first several inches below the surface. Make sure the sample contains no bottom sediment. Be sure the container contains no chemical residue. Samples can be processed with a swimming pool test kit or university lab. Contact your county Extension agent or our office for forms to the Soil Lab at Texas A&M in College Station. The A&M fee is only $20. Consult a professional for interpretation of test results. If findings indicate your pond should be limed, you will need assistance determining the type and amount of material for treatment.

To be effective, liming materials should be applied evenly over the pond bottom. The best, and easiest, time to lime is before it is filled with water. A truck or tractor pulled liming wagon can be driven around the dry bed to spread material evenly over the entire bottom. It’s not necessary to disc lime into the soil, but it will accelerate neutralizing activity. If the pond contains water, lime should be applied evenly over the entire surface. Material is loaded onto a boat or barge and shoveled or washed uniformly into the water. Agriculture lime does not dissolve quickly, will sink to the bottom, and may have immediate affect on water quality. Increasing pH may cause water to clear suspended particles (mud) and help increase light availability to plants. It’s best to apply lime in fall or winter. The pond will equilibrate within several weeks. Ponds requiring lime usually need repeat treatments every three to five years. If a pond needs lime, it will not respond well to fertilizer. If alkalinity concentration is below 50 mg/L, agricultural limestone can be used to increase alkalinity and hardness. If alkalinity is above 50 mg/L, adding ag limestone will not be effective. Similarly, if pH is stable at 8.3 or greater, limestone will not dissolve. In such cases, review treatment options with an experienced biologist.

Water should be tested periodically so hardness and alkalinity can be managed properly. Apply limestone materials as needed and most importantly—keep good records to improve water quality and pond productivity.