Gypsum has something of a reputation as a miracle cure for a myriad number of soil ills. However, gypsum is best applied only if certain conditions in soil exist. If it is applied outside these certain conditions, it may do more harm than good. In many cases, other soil amendments are better solutions to problems than gypsum.
Gypsum, also known by its chemical name, calcium sulfate, is used to improve soils with an excess of sodium. Gypsum attracts sodium, removes it, and leaves calcium in its place. In heavy clay-type soils, application of gypsum also improves soil structure.
Gypsum application can help leach aluminum out of soils. If they have a high enough aluminum content to be toxic to plant life, this can be a positive feature. However, that excess aluminum must go somewhere. When it rains, that excess aluminum ends up contaminating local water sources. In some instances, gypsum leaches or otherwise negatively impacts nutrients ordinarily necessary to healthy plant growth. Manganese, magnesium and iron deficiencies may occur, as may an inability for plants to uptake copper, zinc and phosphorus.
Gypsum applications in areas where trees were later planted have yielded instances in which usual and expected mycorrhizal inoculation (establishment of beneficial mycorrhizae fungi in a symbiotic relationship with young tree roots) is either halted or severely hindered. This leads to poor establishment of trees during the critical period when they are newly planted.
Gypsum application should only be done after a gardener has tested the soil to see whether or not it needs it.
Gypsum has been touted as helping to favorably adjust soil acidity, both up and down, but it does neither. It also does not affect soil fertility in any noticeable way. It also does not in any way increase any soil's capacity to hold water.
Dr. Linda Chalker-Scott, an extension horticulturist and associate professor at Washington State University, advises that gypsum's reputation as something of a wonder amendment for soil is simply not true. Furthermore, she advises that unless a soil is extremely sodic (salty) or is heavy clay, gypsum has no place as a soil amendment. Instances where sulfur content needs boosting can be better addressed by addition of ammonium sulfate. Improving soil texture and friability can similarly be more easily accomplished by judicious use of compost and mulch. Agricultural lands are different, and may find adequate need for gypsum. However, most urban and suburban gardeners will find that gypsum is more trouble than it is worth, and may do more harm than good to the soil's health.