Guest Blog: The Difference Between Calcium and Gypsum
After last week’s guest blog by agronomist Craig Dick, a SafeLawns member asked us to elaborate the differences between gypsum and calcium. Here is Craig’s answer:
Calcium carbonate is the active ingredient in agricultural lime . Calcium carbonate, CaCO3, is a common substance found in all parts of the world, and is the main component of shells of marine organisms, snails, pearls, marble and eggshells. Calcium carbonate can change soil pH due to the CO3, carbonate molecule. The carbonate molecule reacts with hydrogen on the soil particle resulting in water and carbon dioxide, liberating the hydrogen from the soil, leaving the calcium behind, thus raising soil pH. CaCO3 is not highly soluble, it needs to be ground very fine for it to work in the soil and be used by plants.
Calcium sulfate is a soluble salt (this is different from sodium). It can be found in three forms: anhydrite (CaSO4, the nearly waterless form), which is used as a desiccant and as a coagulant in products like tofu. The hemihydrate (CaSO4•~0.5H2O) is better known as plaster of Paris and used to make drywall. The dihydrate (CaSO4•2H2O) occurs naturally as gypsum. Gypsum does not typically change pH since it is a neutral mineral without a carbonate molecule. Since its sulfur is already oxidized, it doesn’t lower pH. Di-hydrate Gypsum is 160 times more soluble than limestone and can be an effective calcium and sulfur fertilizer.
Are these two ingredients interchangeable?
If you have an upset stomach you would take a Rolaids. Its active ingredient is calcium carbonate and it will reduce the acid in your stomach making you feel better. Gypsum will not do this. So the simple answer is no. If you have acid soils and you need to correct the pH you must use limestone.
If you need calcium for your soil either gypsum or limestone or both may be appropriate, depending on your circumstances. Anytime soil pH is under 7, small applications (2-3 pounds/1,000 sq ft.) of limestone can provide necessary calcium and keep pH in an acceptable range. If your pH if low (under 6 pH), then larger applications (5-20 pounds/1,000 sq ft) of limestone can be made to correct the pH.
Soils with varying pH can generally be improved with gypsum. Gypsum’s most widely accepted excepted use is on high sodium soils. The sulfate in gypsum reacts with the sodium to leach it from the soil improving plant growth and soil properties. Magnesium acts like sodium in soils and the sulfate from gypsum can help leach magnesium into the subsoil soil, resulting in soil that has better infiltration and drainage.
Gypsum is an effective sulfur fertilizer. It provides sulfur in the sulfate (SO4) form, the only form plants can take sulfur up in. It is also pH neutral and generally won’t cause acidity like elemental sulfur (90 percent sulfur), ammonium sulfate or aluminum sulfate will. Elemental sulfur is widely sold but is a poor choice for a fertilizer since it needs to steal calcium and oxygen from the soil (oxidize) to form gypsum to be plant available. This process may or may not happen depending on soil pH, biological activity, temperature and moisture levels.
Gypsum is also an effective calcium fertilizer. The calcium in gypsum works to keep clay soil particles flocculated or clumped together making soil more porous. Calcium in soils can leached out by rain (nearly pure water) which will naturally reduce soil soluble salt levels over time. The term soluble salt refers to the inorganic soil constituents (ions of nitrogen, potassium, calcium, sulfur, etc.) that are dissolved in the soil water. The reduction of salts in the soil can causes soils to form a crust layer of de-flocculated soil particles.
Excessive rates of fertilizer application can create high soluble salt problems. Pet urine and feces can also increase soluble salts in localized areas. The use of deicing salts (primarily in the form of sodium chloride) on streets and sidewalks frequently results in high soluble salt levels in area adjacent streets and sidewalks. High soluble salts can reduce water uptake by plants, restrict root growth, cause burning of the foliage, inhibit flowering, and limit fruit and vegetable yields.
Use of gypsum (calcium sulfate) can be an effective means of correcting low or high soluble salt problems. Fall is a great time to apply gypsum as a winterizer and can help prevent snow mold and quicken recovery from de-icers.
NOTE: NatraTurf, has the country’s, purest, finest ground and most uniformly pelleted organically listed gypsum on the market. HydroSave is 95 percent pure mined gypsum, finely ground and pelletized so there is no dust. It can be found on our website at www.natraturf.com and it can be found in many home improvement and lawn and garden centers throughout the Midwest.
Craig Dick, is the Blogronomist and Sales and Marketing Manager at NatraTurf. NatraTurf is a manufacturer of high quality organic products for the lawn and garden. Find other articles by Craig and guest writers at http://natraturf.typepad.com/.