Gypsum

There are many claims about gypsum in the garden, but how many of them are true?

Gypsum from Poland (Elade53) Public Domain

Used to make sidewalk chalk, plaster of Paris, and sheetrock, gypsum (CaSO4) is a soft mineral (along with silica and other minerals) that coats everything and everyone who attends Burning Man. Before we decide whether or not it is beneficial in the garden, let’s learn a little more about gypsum.

Gypsum crystals are plastic enough to bend under pressure of the hand (Rama) CC BY-SA 3.0 fr

Gypsum description

Gypsum, also known as calcium sulfate, is a salt that forms crystals. These crystals are particularly sharp. In Old English, it was known as “spear stone”.

Gypsum crystals in the Cave of the Crystals in Mexico. Note person for scale (Alexander Van Driessche) CC BY 3.0

Unlike other salts, gypsum becomes less water soluble as temperatures rise. Gypsum can take a variety of forms and, depending on the presence of other minerals, it can take on many different colors.

Gypsum mixed with copper (Rob Lavinsky) CC BY-SA 3.0

Desert rose gypsum (Didier Descouens) CC BY-SA 4.0

Selenite gypsum from Canada (Rob Lavinsky) CC BY-SA 3.0

Gypsum in the garden

Gypsum is made of calcium and sulfur, two nutrients important to plant health. In the early 1800’s, gypsum was considered such a fertilizer miracle that smugglers battled local authorities in what became known as the Plaster War. At that time, gypsum was also known as sulfate of lime or lime sulfate.

Plants use calcium to maintain cell walls.  Calcium in the soil helps build healthy soil structure by binding tiny clay particles into larger clumps called aggregates. Sulfur is an important component of proteins used by plants. As such, gypsum can be beneficial in the garden, but not always.

False claims about gypsum

Like most other quick fixes, many of the claims about gypsum are based in fact, but taken too far.

• Acidification - Normally, elemental sulfur is used to acidify soil as it reacts with water to form sulfuric acid. The sulfate ion in gypsum, however, does not turn into sulfuric acid, so it does not acidify the soil.
• Blossom end rot - This condition is not, as popularly thought, always the result of insufficient calcium in the soil. Very often, it is due to water stress. Adding gypsum will do nothing for your irrigation schedule.
• Drainage - Unless you have clay soil with high salt levels (see below), gypsum will not improve drainage.
• Soil fertility - Beside adding calcium and sulfur, which may or may not be needed, gypsum does nothing for overall soil fertility. Soil fertility is largely a function of its cation exchange capacity (CEC), pH, and organic matter content, not the presence of gypsum.

Applying gypsum unnecessarily can cause leaching of aluminum, iron, lead, manganese, potassium into local lakes, rivers, and underground water stores. It also interferes with the beneficial soil microorganisms responsible for helping plants absorb nutrients. Applying gypsum to sandy soils can slow the transport of copper, phosphorus, and zinc.

Benefits of gypsum

All that being said, there is one situation when gypsum can be helpful in the garden. This only occurs when clay soils contain high levels of salt or, more accurately, sodium. These sodic soils can benefit from gypsum applications, in moderation. High salt levels in clay compound poor drainage, often causing heavy crusts to form. Adding gypsum in this situation allows the calcium to bind to the clay, replacing the salt, which is then leached out of the soil over time through cation exchange. Ultimately, this improves soil structure and drainage and reduces salt levels. Adding gypsum to clay without high sodium levels is a bad idea, as it can make alkaline soils even more alkaline. In most cases, plants need a soil pH of 5.5 to 7.0 to thrive.

Rather than simply adding gypsum because you heard it was a good idea, get your soil tested, determine your soil structure, and mulch everything with coarse wood chips to improve your soil and help your plants grow. If you happen to find a large, fine-grained seam of gypsum, you are in luck. Because that particular form of gypsum is more commonly known as alabaster. Now you know.