How Can I Improve My (Expletive Deleted) Clay Soil?

From the garden e-mail bag, Gordon of Lodi writes: "I've about had it with clay/hard pan soil. The soil map for Lodi/Stockton says we are supposed to have sandy loam.  Give me a break!  I bought a 30 lb. bag of gypsum pellets and I'm willing to dig it in everywhere. But, how much do I use? I found an article online. It says to add 20-30 lbs. per 1000 sq. ft. of garden area. I'm not going to work up 1000 sq. ft.!  How about a cup full in 10-20 sq. ft. every three years?"

Gordon, how about getting a soil test done first? California's Central Valley is made up of many different soil types, so yours may not be representative of what you researched. 

Gypsum is effective on soils high in salts; it does nothing to improve the permeability of our clay soils, most of which do not have a salt issue. Still, only a soil test can determine that. By the way, gypsum is very slow to work. It needs to be added (for that salinity problem) on a regular annual basis, not once every three years.

Usually, the reasons for the expletives used against clay soil include symptoms such as slow draining soil, difficult to dig soil, or hardpan layers.

This time of year, especially after a rainy winter, compaction of clay soil is not uncommon (quit walking on wet clay soil!).
  

Don't want to be a scientist? Then, build a raised bed and plant in that, using a high quality commercial soil mix. To improve water penetration, be sure to mix in some of the new soil into the existing soil base at the bottom of the raised bed.

Raised Beds, Fair Oaks Horticulture Center

If you want to improve the soil drainage and permeability of your existing clay soil, add quality compost, top the soil with several inches of mulch, and grow a cover crop. 

The Yolo County Master Gardeners put together an easy-to-understand chart of soil symptoms and possible solutions. More info is available in their online publication, "Using Soil Amendments in Yolo County Gardens."

A word of warning about one of those suggestions: adding sand, perlite or vermiculite to your soil may be counterproductive. According to retired college horticulture professor Debbie Flower, "I would not add those to my field soil: Too little sand added to clay = cement. Perlite is ugly (and it tends to rise to the surface). Vermiculite just compresses when soil is worked or walked on."

Elsewhere in that publication, they explode some of the myths about certain soil amendments: 

Myth #1. Gypsum softens clay or loosens compacted soil. 
Gypsum (calcium sulfate dihydrate) is effective in counteracting the effects of sodium, which in excess (in soil or in irrigation water) causes soil aggregates to disperse, sealing the soil surface and reducing water infiltration. The calcium in the gypsum replaces the sodium, making for more stable soil aggregates, which do not disperse and form a seal so readily. This process is the basis for the misleading claim that gypsum “loosens” the soil. Gypsum does not reduce or prevent soil compaction, dissolve hardpan, soften clay soils, or convert clay to loam. 

Myth #2. Gypsum lowers the pH of alkaline soils. 
In very high pH soils (pH > 8.5, characteristic of high- sodium soils), gypsum will lower the pH, but only slightly. At lower, though still alkaline, soil pH values, gypsum has little or no effect on pH. Chemically, calcium sulfate is a “neutral salt”, i.e., when it dissolves in water, it does not change the pH of the solution. If the objective is to lower soil pH, elemental sulfur, iron sulfate, or aluminum sulfate should be used.

Myth #3. Inoculating soil with microbes or microbial preparations, will improve “dead” or infertile soil. 
A wide variety of “microbial” products that contain (or claim to contain) microorganisms are commercially available for farm and garden use. A review of the claims made for such products is beyond the scope of this bulletin. Keep in mind that most soils, including soils that have been fertilized only with synthetic fertilizers, or have suffered years of abuse (compaction, erosion, loss of organic matter) still contain a great diversity of microbial species. To establish or re-establish a healthy soil food web, all that is needed is to apply a variety of organic amendments and ensure that pH and nutrient levels are adequate for plant growth.

More information about: 
The benefits of adding compost to the soil.
The benefits of topping garden soil with mulch.
The benefits of cover cropping.