Should I add gypsum to my soil?
Updated: Oct 19, 2018
If you head to the nursery and tell them you have clay soil, 9 times out of 10 they will walk you over to the soil amendments and recommend that you add some type of gypsum product to “loosen” up your heavy clay. While this recommendation may not be detrimental to your garden or landscape, it is a little misguided. Although gypsum is a common amendment to agricultural soils, it is rarely beneficial or necessary in residential settings.
One of the benefits of having clay is that clay soils are better at retaining water and nutrients. Clays contain charged particles that help bind and hold nutrients that are important for plant growth. Unfortunately these same charged particles can also bind elements which are not good for plants. Agricultural soils with high amounts of clay often develop high sodium (Na+) levels from excessive irrigation. Sodium ions bind to the clay particles and over time high sodium concentrations can severely damage crops. Enter gypsum.
Gypsum, or calcium sulfate dihydrate, when incorporated into soil releases positively calcium ions which can displace sodium that has built up in the soil. Additions of gypsum allow the sodium to be leached out of the soil and thus make it much more hospitable for most agricultural crops. In clay soils where sodium is not an issue, additions of gypsum may have little or no benefit. Additionally, in soils that already have high levels of calcium, gypsum will not be very effective at leaching out sodium. In San Diego, our irrigation water tends to have high levels of calcium already, so in most cases additions of gypsum are not necessary. The only way to know for sure if your soil has high sodium and or low calcium is to have a soil test done.
Before you amend with gypsum, here are a few step you can take first to make sure it is the right choice for your soil:
1. Determine if your soil is predominantly clay. This can be done with a basic soil texture test as described HERE or by setting up a Mason Jar Soil Test
2. Observe any plants that are already growing in the areas. Do they look healthy? High sodium is toxic to nearly all plants so if your soil has high levels, most existing vegetation will be exhibiting signs of stress.
3. Consider having a soil test. A basic soil test will give you information about Na+ levels in your soil and can be done for $12-15 through A&L Laboratories or other soil testing labs.
4. If a soil test indicates high Na+, then additions of gypsum may be beneficial. However, if you have clay soil with low/normal levels of Na+ the best way to improve your soil is by adding compost and maintaining a 3’’-4’’ layer of organic mulch on top of the soil at all times. Because clay soils drain slowly, be sure to avoid over watering. Using a low flow drip system and spacing out watering times is the best way to effectively irrigate in a clay soil. It is also worth remembering that having some clay in your soil is actually good because it helps retain water and nutrients.
If you need help identifying your soil type or interpreting results from a soil test, call or email Revolution Landscape to schedule a consultation. For additional information about how to manage clay soils, check out this article from UC Davis about “Managing Clay Soils in the Home Garden.”