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?
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.
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.