By Carl Wilson, Horticulturist with Denver Cooperative Extension
Gypsum is a mineral found in large quantities in most Colorado soils. The belief persists that adding gypsum can "break up" the compact clay soils so typical of the Front Range area. This is not true and here is why.
Calcium sulfate or gypsum when added to our clay soils only increases the already high calcium content. Since plants growing in our soils already have all the calcium they need, the added gypsum does nothing to improve plant growth. Spending money and time to add gypsum to a soil that is already high in calcium is a waste of money and effort.
As far as relieving soil compaction, gypsum has no effect. Loosening soils is a physical process, not a chemical one. The way to break up tight, clay soils is through adding and mixing in organic amendments. The amendment holds the clay particles apart creating more space for the air so critical to plant root growth.
Improve the aeration and drainage of your clay soils with organic amendments such as compost, peat and manure. Add a two inch layer of the organic amendment on the surface of the soil and till or spade in to a four to six inch depth. When purchasing organic soil amendments, buy four cubic yards for every thousand square feet of area.
It's easy for gardeners frustrated by hard, clay soils to grasp at anything that sounds like it might work. With gypsum, the expression "If it sounds too good to be true, it is too good to be true" applies. Decades of research has proven gypsum is not the answer for us. The secret to developing a good soil for gardening in our area is one organic gardeners have known for years - organic amendments.
Photograph courtesy of Judy Sedbrook.
© CSU/Denver County Extension Master Gardener 2010
888 E. Iliff Avenue, Denver, CO 80210
Date last revised: 01/05/2010