Myth #3: Gypsum for lawns is a great liming material and will effectively raise soil pH.
While gypsum is useful in many ways, this gypsum myth applies only to gypsum as a soil acid neutralizer. While there are reports of gypsum raising pH, this is mainly restricted to agriculture and not so much for lawns. I talk to people all the time who use gypsum in an attempt to neutralize acidic soil, not realizing it will not raise the soil pH at all.
Gypsum is a great product and a good soil conditioner, but there are no miracle cures. It is great for adding calcium and for solving some problems lime alone can't do. These are explained below.
At times the benefits of applying gypsum for lawns can be countered by a few disadvantages. Some of the possible disadvantages is nutrient leaching leading possibly to nutrient deficiencies. Before adding anything to the soil, it is always advisable to make sure it is needed first.
What is Gypsum for Lawns?
Gypsum that is used for lawns (CaSO4) is the same material used in gypsum board, also called sheet rock. Like limestone used for lawns, gypsum, too, is a Ca+2 (calcium) material. However, gypsum also contains sulfur (SO4) that acidifies the soil.
Dr. Nick Christians, University of Iowa, Professor of Horticulture, states: "Whereas the Ca+2 [calcium] in gypsum has the potential to neutralize H+ [hydrogen] in the same way as lime, the sulfate forms sulfuric acid (H2SO4), which acidifies the soil and balances the effect of the Ca+2." (Fundamentals of Turfgrass Management p.102) In the end, the gypsum neither raises nor lowers soil pH.
In other words, when used as a liming material, the sulfur cancels out the benefits of the calcium and you end up with little or no pH gain.
What is Gypsum for Lawns Used For?
Gypsum has three primary uses on lawns.
The first purpose for gypsum is when it is used on soils with excessive amounts of salt (Na+). Too much sodium in soil will damage good soil structure by displacing other elements needed by plants. Salt is probably most damaging on clay soils.
Applying gypsum for lawns will dislodge the sodium which slowly restores the proper soil structure. Once dislodged, the sodium leaches below the root zone and away from plants. It is an important part of improving soils affected by salt.
Sodium can enter soil from many sources. In parts of Texas, septic tank overflow water must be sprayed via a sprinkler system (Aerobic Treatment Unit) instead of the conventional leaching system. Septic water contains sodium and heavy metals that may need to be monitored.
Other sources are runoff from winter road salt trucks and along coastal areas. Many western states also have problems with excessive salt.
The second purpose for using gypsum for lawns is when calcium is needed, but you do not require a change in the soil's pH. Calcium (lime) is needed by plants in large amounts and, therefore, calcium makes up about 65% of the soil's cations. Only Magnesium can equal calcium in total amounts needed by plants.
However, you may have sufficient calcium (lime), but the calcium may not be available to plants. Solid lime from granulated limestone applications must be dissolved first before it can be used. That can take up to two years. Gypsum doesn't take as long as limestone to break down and is more quickly dissolved, so it can be used by plants. You can also try liquid calcium, such as CaCl (calcium chloride,) which is immediately available and can be taken up by plants from the moment it is applied. (Super-Cal® from AgriGro is a good source for liquid calcium. Super-Cal® also contains organic acids that dissolve solid lime, so it, too, can be used.) However, CaCl will slightly affect soil pH, unlike gypsum.
The third purpose for using gypsum is to reverse soil aluminum toxicity in soil. Aluminum is not an essential element for plant or animal life even though aluminum is found in soil in large quantities. Aluminum only becomes toxic in acidic soils. Only acidic soils below 5.0 pH and especially below 4.5 pH are greatly affected. Gypsum for lawns applied to the soil can quickly penetrate to the sub-soil area in the root zone where surface applied lime usually can't reach.
Soluble aluminum toxicity damages root growth and decreases root penetration, thus affecting plant health. Only the few acid loving plant species (blueberries, etc) have high aluminum tolerances.
Soluble aluminum testing is not always part of the standard soil testing regimen. If not, and you would like it done, you will need to request it.
Another misconception About Gypsum
If you search the internet you will find many sites on gypsum use. University tests conducted on gypsum's ability to soften landscape soils almost always differs from the many claims in home landscape forums. For that matter, even some researchers sometimes disagree on the benefits of gypsum.
Keep in mind that gypsum works differently in agricultural areas due to extensive and aggressive soil maintenance. The soil is often turned over to prepare for planting new crops, compared to lawns which are never turned once the lawn is established. In agricultural settings gypsum has proven itself and can improve heavy clay structure, improve plant root depth, drainage, etc.
However, gypsum does little to soften hard soils in home landscapes, as is often believed. Home soils are not easily affected by gypsum due to soil layering, heavy compaction, high organic content, etc. Coarse soils can actually be damaged by applying gypsum. Magnesium deficiency from gypsum for lawn application can result in some situations.
Gypsum will increase water filtration in saline soils, however. Salt prevents water absorption and blocks absorption into the roots. When the salt is removed by gypsum, the soil can again do its job in absorbing water and making it available to plant roots.
Here is a True-Life Example
I was recently speaking to a person living along the Gulf Coast of Texas who applied gypsum for lawns and spoke about how it helped break up his hard clay soil. He claimed he now had better water filtration. He didn't realize that it was the salt from the Gulf, especially salt water from the last hurricane that affected his soil and the gypsum corrected the problem.
To learn more about soil salinity and how gypsum can help, click on the link: Soil Salinity Problems and Cures
Special thanks to Anthony Meldal-Johnsen, Soil Scientist from the Western Cape, South Africa. He contacted me and pointed out some of the uses and benefits of gypsum, especially on agricultural sites.
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Did you know?
Gypsum is a great product when used properly. However, when using gypsum for lawns, the number one reason it is applied will almost never achieve its objective. Read this page to see what gypsum can do and what it can't do.