How Much Gypsum to Add to Clay Soil?

When I was a kid, we tried to start a little vegetable garden in the back yard…it didn’t really go all that well. One thing that I remember most about that project was the near-constant need for fertilizer.

On one of those bags, I remember seeing gypsum listed in the ingredients section on the back of the bag and found myself wondering about that. If this stuff was supposed to help the plants grow, why was nothing growing.

We’d picked things that required little sunlight and they were well cared for, but nothing really grew in that patch as far as I remember. Were we adding too much or too little fertilizer? Was it possible to add too much?

Exactly how much gypsum should be added to clay soil? 12.2 pounds (1 kilogram) per 10 square feet (1 square meter) of land, mixing it into the first 5 inches 10 – 15 cm of topsoil. This breaks up the clay soil and makes it easier to work with. It also leeches excess aluminum and sodium from the ground, and helps prevent rotting in the blossoms of your plants.

What is Gypsum?

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Another name for gypsum is calcium sulfate, and it’s used primarily as a fertilizer to leech excess sodium from the ground – replacing it with calcium.

In its native or raw form, it’s a sulfate crystal which is typically mined in Brazil, the USA, the UK, Spain, Thailand and other places.

What is Clay Soil?

Put simply, clay soil is a type of soil which is heavy in mineral content, but lacking in organic content (meaning there isn’t much organic matter in the soil). It’s sticky and largely impermeable as the mineral structures are so tightly packed together.

As such, it doesn’t drain well, and water tends to form puddles. This means it’s not exactly ideal soil for gardening.

Ideally you want loamy soil which has a mixture of mineral and organic content. The reason for this is that if you’re trying to grow flowers or vegetables, they won’t grow well in the densely compacted clay; their roots aren’t strong enough to get through it.

Advantages and Disadvantages

Clay soil is very dense and retains moisture well. It also tends to be richer in nutrients than other types of soils.

With that said, however, clay soil is slow to drain and is easily compacted – which may negatively impact your plants. It also tends to be very alkaline.

Why Add Gypsum to Clay Soil?

In agriculture its used to change the chemical structure of the soil by, as I mentioned, replacing excess sodium in the ground with calcium and breaking down the mineral components of clay soil to make it easier to work with.

Adding a layer of organic matter (i.e. grass cuttings, leaves, etc.) helps to make it more permeable. You can also de-tox the soil your garden if needed by leeching out excess aluminum.

While these are certainly advantageous, there are some problems related to working with gypsum.

Problems with Gypsum

There is a myth around the use of gypsum that it changes the acidity of the soil and improves the fertility of acidic and / or sandy soils; it’s also supposed to improve the water retaining properties of sandy soil.

Reality is quite different…

  • ❌ Leeching the excess aluminum from the soil can detox your soil but it can also contaminate nearby watersheds.
  • ❌ It can lead to deficiencies in iron and manganese.
  • ❌ It can cause magnesium deficiencies in your plants when applied to acidic soils.
  • ❌ Gypsum does not, in fact, improve soil fertility in acidic or sandy soils.
  • ❌ It doesn’t improve the water retaining properties of sandy soils.
  • ❌ The effects are short-lived.

What are some Safety Hazards of working with Gypsum?

As far as safety hazards, calcium sulfate, in its solid form is non-toxic and is an approved food additive. Powdered gypsum is the opposite: it can cause irritation to the skin and mucous membranes of those exposed to it.

Final Thoughts

Okay, so to recap:

  • Using gypsum in clay soil changes the soil’s chemical structure by breaking up the mineral particles, and making the soil more permeable.
  • You can use it to detox your soil, but it may contaminate nearby watersheds.
  • It can cause mineral deficiencies.
  • It doesn’t improve the fertility of either acidic or sandy soils.
  • It doesn’t improve the ability for sandy soils to retain moisture.
  • The effects that gypsum has are short-lived and you’ll have to apply it more than once.

Happy gardening!

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