Robert Romaire | 11/28/2007 9:22:18 PM
Crawfish have a requirement for calcium. They obtain some of the calcium they require through the food they eat, but much is extracted from the water. The amount of calcium present in the water in a crawfish pond is related to the amount of calcium present in the soil with which water is in contact. In the case of well water, calcium content is dependent on the geological makeup of the aquifer. Calcium is the major mineral compound contributing to water hardness. Hard water has high levels of calcium while soft water is low in calcium. The calcium-rich alluvial soils of south-central and southwest Louisiana have significant amounts of calcium carbonate (limestone) minerals, and water in contact with these soils also usually has high calcium levels. Groundwater in south-central and southwest Louisiana are also usually high in calcium.
What are the calcium, water hardness and liming requirements for crawfish? One of the first studies conducted on crawfish by the LSU AgCenter in the late 1960s was to determine calcium and water hardness requirements for crawfish production and the amount of agricultural limestone that would be required should water hardness levels be too low for optimal crawfish production. The study found a significant reduction in crawfish growth and production occurred at water hardness levels below 50 ppm (which is equivalent to about 20 ppm of calcium). Highest crawfish production was observed when total water hardness was 100 to 150 ppm (equivalent to 40 to 60 ppm of calcium), and no significant increase or decrease in crawfish production was observed at hardness levels that exceeded 200 ppm.
We still use these criteria as guidelines for water hardness and liming recommendations for crawfish ponds in Louisiana. Virtually all waters and soils on which crawfish are grown in southwest and south-central Louisiana meet the minimum criteria of 50 ppm water hardness, and most are within the optimum range of 100 to 150 ppm, so in most farms, liming is not required to increase water hardness and calcium content of the water. If a water test shows that the hardness of water in which crawfish will be grown is less than 50 ppm, then a soil analysis should be conducted to determine if lime is required and how much. More on that later.
Will harvest of crawfish deplete the water and soil of needed calcium over time? Some believe that unless soils are limed on a regular basis, the continual removal of crawfish will eventually deplete the soil and water of calcium sufficiently to reduce crawfish production. Let’s see if this is likely. Crawfish have 4 percent to 5 percent calcium in their shell on a wet-weight basis. The amount of calcium in other body tissues is negligible. If we harvested 1,000 pounds per acre of crawfish from our pond or field we would be removing 50 pounds per acre of calcium from the field on an annual basis.
1,000 lbs/acre of crawfish x 0.05 = 50 lbs/acre of calcium removed in crawfish harvest
If hardness of the water in which crawfish being grown is 120 ppm – which is a good average concentration for crawfish ponds in Louisiana – that water will have a calcium level of about 50 ppm. With an average pond or field water depth of 12 inches, the concentration of calcium in the water would be 136 pounds per acre, or nearly three times the amount that would be removed from harvest of the annual crawfish crop. It makes no difference if the water comes from a surface water source – bayou, stream, ditch or river – or if it is water pumped from an underground aquifer (well water). Crawfish will extract calcium from the water in which they live as needed.
The agricultural soils in south-central and southwest Louisiana will usually have calcium levels 20 to 50 times higher than calcium levels in the water. As calcium levels in the water are reduced, calcium will move from the soils into the water to replenish that which has been depleted. The movement of calcium from the soil to the water is a slow process, but even under those conditions it is not likely water calcium can be depleted by removal of the crawfish crop. In fact, the only factor that we have observed that significantly reduces the water calcium and water hardness in a crawfish pond is heavy and continuous rainfall, which flushes the ponds of its resident concentration of calcium. Rain has very low levels of calcium and hardness.
Since crawfish ponds or rice fields used to cultivate crawfish are drained and replenished annually with “new” water, there is no evidence at this time to indicate that crawfish harvesting has any noticeable or measurable effect of reducing calcium to a level that would reduce crawfish production in the following season or in subsequent years provided that the water has a total hardness level that exceeds 100 ppm during the crawfish production season.
What is lime and why do we use it?
The primary purpose of liming soils for agricultural crops or even recreational fish ponds is to raise acidic soils to a pH level (pH = measure of the acidity or alkalinity of a soil, water or solution) that provides maximum nutrient availability or release, usually of phosphorus, for the crop being grown. Optimum level of soil pH is usually about 6 to 7 for most agricultural crops. If a soil is highly acidic, then critical nutrients needed by the plant are tied up with iron and aluminum, making them less available for the crop being grown, and crop production will suffer. However, equally important, if lime is used in excess of what is needed and soil pH is increased above the level of optimum nutrient release, then critical nutrients required by the plant are tied up with calcium and crop production suffers. Adding lime in excess can be as equally detrimental to crop production as no liming at all.
In a flooded pond or field with low water hardness, liming pond soils at the appropriate application rate will also increase the level of alkalinity (alkalinity is a measure of bicarbontates and carbonates) of the water, stabilizing water pH at levels necessary for good crawfish production (water pH 6.5-8.5) in addition to maximizing nutrient release and increasing water calcium and hardness concentrations. So as it pertains to crawfish production, it must be determined if lime is needed for both the forage crop being grown as well as if sufficient hardness and calcium are available in the water.
Although several different chemical compounds are referred to as “lime,” the liming agent used in agricultural applications is agricultural limestone. Limestone contains calcium carbonate minerals and magnesium carbonate minerals. Calcitic and dolomitic limestone are the recommended liming agents. If liming is called for, do not use quick lime or hydrated lime. This is extremely caustic and could kill crawfish by raising the water pH to excessively high levels.
If I need to lime, how much should I use? We do not recommend that ponds and fields be limed unless a water and soil analysis test specifically indicates that it is called for. As stated earlier, most waters and soils used for crawfish production in Louisiana are sufficiently high in hardness and alkalinity and do not require additions of agricultural limestone other than what may be recommended by the LSU AgCenter for the forage crop being cultivated for the crawfish. If the pH, hardness and alkalinity of the water in which crawfish are being grown are low (and that is easily determined from a routine water analysis), then soil samples should be taken and a soil test conducted to determine the lime requirement. The lime requirement cannot be accurately determined solely from a water analysis. If the soil test analysis recommends a specific amount of agricultural limestone for the forage crop, the recommended amount of lime should be disked into the pond or field soils during the next dry cycle prior to planting the forage crop. Do not apply lime to the crawfish pond while flooded. The amount of agricultural limestone required must be determined from a soil test analysis for the kind of forage crop to be grown during the summer.
Are there other important considerations to consider in liming crawfish ponds? YES! The decision to lime should consider additional factors. For most crawfish farmers, rice is the forage of choice, whether it is used in rice-crawfish double cropping or a rotational cropping system, or whether it is planted in late summer strictly as forage for the crawfish (crawfish monocropping). Rice is an “acid loving” plant that grows best at pH below 6.0. Rice grain production can be significantly reduced when soil pH approaches 7.0. Adding lime in excess of what is considered optimal for the rice crop will increase soil pH and potentially reduce rice forage and grain production, thereby potentially reducing grain yield as well as the crawfish forage and food supply. Other agricultural crops recommended for crawfish such as sorghum-sudangrass perform best at a higher soil pH than rice, so the liming requirement would likely be different. Another consideration is that lime persists for several years in the soil and consistent annual additions of agricultural lime to ponds or fields will raise pH to levels that may not be conducive to good crawfish forage crop production. Before lime is applied to a crawfish pond, the crawfish farmer should know the type of soil, the soil pH, the hardness and alkalinity of the water and type of plants that will be cultivated in the pond in current and future years. County agents with the LSU AgCenter can be of great assistance in helping you obtain this information.
So should I lime my crawfish pond or not? The decision to lime should be made only after soil and water are analyzed with consideration to the forage crop that will be planted. Results of the tests will tell you if lime is recommended or not. If the results of the test indicate that lime should be applied, the estimated amount of lime required for your crawfish forage crop will be provided with the results of the soil analysis.
Where can I get my water and soil tested to determine calcium, total hardness, total alkalinity, pH and lime requirement? Water and soil samples can be analyzed by the LSU AgCenter’s Soil Testing and Plant Analysis Laboratory, located on the Louisiana State University campus in Baton Rouge, for a nominal fee (currently $7 per sample), and recommendations will be provided as to the suitability of the water and soil for crawfish production with the results of the analysis. The routine water test package will provide you with results for hardness, alkalinity, pH, calcium, conductivity, magnesium, manganese, sodium, potassium, iron, sulfur, chlorides and nitrates. The soil routine test package will provide results for pH, extractable phosphorus, exchangeable calcium, magnesium, potassium and sodium, in addition to soil texture and lime requirement. Contact your local LSU AgCenter extension office for instructions on collecting and submitting water and soil samples for analysis. County agents or crawfish specialists in the LSU AgCenter can assist you in interpreting the findings and making a decision to lime or not. The phone number for the LSU AgCenter’s Soil Testing and Plant Analysis Laboratory is 225-578-1261 or visit the web site.