Gypsum has been touted as a solution for various problems ranging from fertility levels to lack of soil structure. It has been in use as an agriculture product for over 250 years. In fact, two famous farmers, Thomas Jefferson, and Benjamin Franklin recognized the positive impacts of gypsum and used it in their own operations. This blog post will cover some of the uses and considerations for gypsum applications.
Soil Aggregation
Gypsum is well known for its ability to improve soil aggregation and soil structure. Improved soil structure can help improve water movement, nutrient availability, aeration, germination, and root penetration. Part of this fix happens through flocculation. This is the process by which attractive forces draw individual clay particles together to form small aggregates. Gypsum promotes the flocculation process. Gypsum also reduces the dispersion of clay particles as well which is usually caused by ions such as Na+ or Mg2+. Gypsum is also helpful in reducing crusting on the soil surface which is generally caused by clay particles dispersing, largely from water impact. Because the processes of flocculation and dispersion are more prevalent with clay particles, soils high in clay may respond better to gypsum application.
As a Calcium Source
Gypsum is made up of both sulfur and lime and has been shown to be a good source for soils that are low in calcium. Gypsum typically contains between 22-24% calcium. Gypsum however, is not able to change pH and should not be used as a substitute for lime. Gypsum is also much more soluble than lime and can penetrate farther into the soil profile that liming materials. This can be beneficial in reducing Al toxicity. A study from central Kansas showed that gypsum applications can increase the soil calcium levels by 883 ppm at a 4 ton/ac application rate.
As a Sulfur Source
In some instances, gypsum is primarily used as a sulfur source. Gypsum is about 16-18% sulfur which equals about 320 pounds of sulfur for each ton of gypsum applied. Over the last several decades, the amount of sulfur supplied atmospherically has decreased dramatically with new restrictions on industrial processing facilities. Additionally, with increasing crop yields, more sulfur is required to produce an increasing level of grain. With the high availability of gypsum, it is often considered a good sulfur source.
As a Way to Reduce Al Toxicity
High Al levels in soils can cause toxic effects on plants. Gypsum however will react with the Al and essentially push it down through the soil profile and away from the rooting zone. While some of this happens with lime applications, lime is not effective at displacing Al from lower than the soil surface. Gypsum is 200 times more soluble than lime and can penetrate much farther into the soil. This pushes the Al ions farther than what would be possible after a lime application and allows for better rooting in the subsurface.
To Improve Soil Properties
Gypsum has also been touted as a way to improve drainage and infiltration in poorly drained soils by reducing surface sealing. Additionally, it has been known to reduce soil erosion by the increased amount of flocculation. Better aggregation means less erosion. This also helps to reduce amount of phosphorus loss through surface erosion. The calcium in gypsum also binds with the phosphate to form a precipitate and improve water quality usually subject to eutrophication.
Rates
A quick way to test if your soils might respond to gypsum is to mix two tablespoons of soil in a straight walled quart jar that is 2/3 full of water. Shake the jar up and let it sit for at least two hours. After two hours, if the water near the top is still cloudy, your soil would likely respond to gypsum.
So, what rates should you apply? This helpful chart from the Ohio State University outlines application rates for different objectives for gypsum application. Are you applying for soil property improvement? Or for sulfur application? Check the chart for the right amount and timing of application.
Resources