People struggle with clay soil and try all kinds of quick fixes including gypsum which is regularly touted as as a clay buster, but does it really work? Will gypsum make clay soil easier to dig? Will it improve drainage? Should you add it to your soil?
Maybe!
There are good reasons for using gypsum but you have to know when and when not to use it. Most gardeners should not use it. Don’t listen to marketing hype about this product – much of it is wrong.
What is Gypsum?
It is a simple chemical called calcium sulfate (CaSO4) and it’s used to make drywall gypsum board. It contains calcium ions and sulfate ions, both of which are plant nutrients.
The Claims for Gypsum
Gypsum is a favorite amendment for soil, especially clay soil where it is claimed to do all kinds of wonderful things.
- Improves the structure of clay
- Improves drainage
- Increases pH
- Adds nutrients
- Improves compaction
- Reduce aluminum toxicity
- Neutralizes dog urine
Understanding Sodic Soil
From my book, Soil Science for Gardeners:
” Sodic soil has high levels of sodium, and low levels of calcium and magnesium. The excess sodium combines with the negative charges on clay which disrupts the clay structure, making it fall apart into extremely small particles,\ which pack close together and fill pore spaces. These soils become difficult to till, have poor seed germination and make it difficult for plants to grow.”
Most soil is NOT sodic.
Soil in costal regions can be sodic, as can agricultural land that has been watered with sea water. Arid regions with very low rain fall can also be sodic because the salt migrates from lower levels to the surface where evaporating water leaves the salt (sodium) behind. A SAR lab test determines if soil is sodic.
Understanding Dispersive Soil
Dispersive soil contains clay that will disperse when aggregates are exposed to pure water. The clay particles are held together so weakly, that water molecules break them apart. “The terms sodic and dispersive are frequently used interchangeably, however, not all dispersive soils are sodic and not all sodic soils are dispersive”.
There is a simple test for dispersive soil.
Gypsum Improves the Structure of Clay
In sodic soil, the sodium ions disrupt the clay structure. The soil drains poorly, is sticky when wet, hard when dry, and contains very little air for plant roots. In such a situation, the calcium in gypsum knocks the sodium ions off the clay and replaces them with calcium ions. The sodium gets washed deeper into the soil profile by rain and irrigation. The result is soil with much better structure.
Dispersive soil is also improved with the addition of gypsum. The calcium ions help hold the clay particles together, even in pure water.
Gypsum does NOT improve the structure of other types of clay, which accounts for most of the clay soil in North America.
Gypsum Improves Drainage
For the reason just discussed, gypsum will improve drainage on sodic soil, but it does nothing to help drainage on non-sodic soil.
Gypsum Increases pH
This is a very common myth that is simply not true. It probably stems from the use of lime to raise pH. Lime is calcium carbonate which sounds a lot like calcium sulfate (gypsum), but they are very different.
The calcium in both of these does increase pH, but the sulfate in gypsum reduces pH. The net effect is that gypsum does not change pH.
Gypsum Adds Calcium
This is true for all types of soil, but gypsum may not be your best option for adding calcium.
Many garden soils are not deficient of calcium, in which case adding any form of calcium is a waste. How do you know your calcium level? Get a soil test done.
If your soil has low levels of calcium you can add more using either lime or gypsum; each offers different advantages.
Lime is ground limestone and the calcium in it becomes plant available slowly over several years. It is a cheaper source of calcium and is effective for a longer period of time. It is also a good choice for acidic soil because lime increases soil pH.
On the other hand, gypsum is much faster acting because it more easily dissolves in water, but for this same reason it is only effective for a few months and needs to be reapplied. It is a better choice for soil with a pH above 6.5 since it does not raise soil pH.
Adds Sulfur
Gypsum does add sulfur to soil, and plants can use the sulfate ions.
The easiest and cheapest way to add sulfur to soil is to use agricultural sulfur. It is a bit slower acting since it needs to be converted to sulfate ions by microbes before plants can use it, but it is also longer lasting.
Sulfur will lower pH, which is a bonus in alkaline soil, but is problematic in very acidic soil. In the latter case gypsum might be a better options for adding sulfur.
Gypsum Improves Compaction
This claim is not true. Gypsum will not improve compacted soil.
It might make it easier to improve compaction in sodic soil, but compaction is a physical problem that will not be fixed with any chemicals. In lawns, core aeration works well for compaction, and in ornamental beds mulch is your best option.”
Gypsum Reduces Aluminum Toxicity
This claim is actually true.
Aluminum can become toxic to plants in acidic soil (below pH of 5). Calcium sulfate helps knock aluminum ions off clay, allowing water to wash them away, thereby reducing the aluminum level around roots.
Gypsum Neutralizes Dog Urine
” Both winter salt and pet urine cause ugly, patchy spots in the lawn. But don’t despair. All your lawn needs to recover is a little help from its friend, Garden Gypsum“.
You will have no trouble finding claims on pet related sites, that gypsum fixes urine damage on lawns, or that applying gypsum will reduce future damage, but it is all wishful thinking.
Lawn damage from dog urine is the result to too much nitrogen being applied to a small area of soil. Adding calcium sulfate will have no effect on this.
Should You Add Gypsum to Your Soil?
Using this soil amendment will have no effect for the majority of gardeners. Read and understand the above uses, and use gypsum only if you have a valid reason for doing so. Adding it with no clear problem to fix, can damage your soil.
Don’t believe what you read on product advertisements.
After our building site was graded, the area surrounding our home was poor quality subsoil. We spread gypsum over it, and added several inches (6-9″?) of bulk planting mix on top. (We were told that this is what local housing subdivision developers did before planting lawns.) A year or two later, digging in the garden revealed that the rich, darker soil had seeped down into the poor subsoil. So I plan on using gypsum again when developing new garden areas. Southern California.
I have taken over my parents urban property where my father was a strong believer in artificial fertilizer, and weed mat with bark on top.
The result is fine smelly grey sludge that won’t drain and goes rock hard in summer. I used lime and compost to break it down and the old fashioned digging it in. After 2 years I now can grow vegetables, but not to the size I’m used to. More compost, and mulch I suppose. New Zealand
Lime does not break up clay.
Will Gypsum Improve Clay Soil?
It’s hard to know what to believe, the marketing hype is so misleading. Thanks for the more detailed, nuanced version.
What about dolomite to amend clay soil?
I have a lot of red clay in my back yard and wish to do a vegetable garden. This doesn’t do well in clay so is it better just to have he clay removed and dirt brought in or is there something I can do get the soil able to plant ?
Add 6″ of compost and dig it in. Then plant. Mulch well.
Thank you for clearing up the gypsum myth. I live in NE Ohio where the soil is mostly heavy clay. The only remedy I’ve found so far is to till in a lot of organic material and give it time.
Thank you, Robert. This was one of your best presentations! It was made even better with your video link: How to improve Clay Soils, plus the article on female dog urine. With those, you’ve effectively “busted” several myths.
In Minneapolis, I have compressed clay in my lawns and gardens largely as a result of my home being built in 2001 when heavy machinery was used and the basement was dug. That clay subsoil was then spread on the soil. Each year I’ve added “tons” of leaves and compost to my vegetable garden. The result is: very happy earth worms and night crawlers. But my soil remains “heavy”, just getting microscopically lighter and less compacted each year. Perhaps I need to add sand too.
I stopped rototilling a few years ago. But in the spring, if the leaf layer is too thick, I shovel it over to plant rows. But after heavy rain in the summer, I still see puddling between plants where the water isn’t penetrating the soil. However, I’ve piled compost mulch and leaves between all rows.
Progress is being made, but it’s microscopic given the huge volume of biomass I place in the garden each year.
It is a slow process. YouTube videos that see big changes in a year are nonsense.
Using a fork, like a potato fork or broadfork really makes a difference. It’s labor intensive, but it doesn’t all need to be done in a day.
I don’t see how a fork can improve clay soil. It might reduce compaction temporarily, but it does not change soil texture.
The local stores stock a brand called “Soil Magic”. What could be better than a big bag of pixie dust to apply to the soil?
I have even heard / seen gypsum recommended on a TV garden programme for sandy gutless soils – go figure that one out ?
Gypsum is a good soil amendment for the renovation of baseball infield skins where the the practice of frequent shallow irrigation often with reclaimed water creates extremely high SAR’s above 20. I have data from infield skins with extreme salinity and SAR’s and or both which negativity affect the PI index of clay portion of the soil mix. . Acids are also valuable on infield skins to get the pH down below 6 and prevent carbonates. Often the soil is not reclaimable and it has to be removed and thrown away. But yes agree that gypsum is shrouded in myth.
The Myth
Upon continued prodding from one of my university extension colleagues, I recently watched several
episodes of a well-known gardening program on television. My kids joined me, alerted by my animated
responses to the host’s non-stop torrent of advice. Among many amazing discoveries I learned that by
adding gypsum to my yard or garden I would improve my problem soils by changing the particle size and
loosening compaction. Further searching on the web revealed that gypsum would also improve drainage,
decrease acidity, and eliminate soil salts. Previously, I had heard of gypsum for use in soil reclamation
projects, but not for a typical urban landscape. Since gypsum is simply calcium sulfate, could this
chemical truly transform soil structure and serve as a fertilizer for yards and gardens?
The Reality
This myth falls into the category of agricultural practices misapplied to ornamental landscapes.
Gypsum effectively changes the structure and fertility of heavy clay soils, especially those that are heavily
weathered or subject to intensive crop production. Gypsum also improves sodic (saline) soils by
removing sodium from the soil and replacing it with calcium. Therefore, one can see improvement in
clay soil structure and fertility, and desalinization of sodium-rich soils, by using gypsum