I reside in the North Carolina Triassic Basin having veins of red, white, blue, gray and nearly black clay. The Triassic Basin contains mudstones and claystones that are the mainstay of the North Carolina brick industry. The North Carolina State University Horticulture Notes clearly state that for our area there are three soil amendments that should always be avoided; peat moss (water retention), gypsum (good for alkaline clay soils, not acid clay soil) and sand (any mixture less than 70% sand in 30% clay actually packs more densely that straight clay, you have brick). For the lawn I would consistently use compost and organics. For planting holes I have tried and failed with amending deep holes with anything in my clay. I end up with a deep clay pot in the ground with nowhere for the water to go. The plant roots then rot from sitting in water. I have now had success digging wide shallow saucer shape holes and then mounding that up using 50% bark fines along with the usual topsoil/compost, etc.

I didn't see your 10:21 post. Sand, silt and clay can be divided according to size from larger (sand) to smaller (clay) (albeit ignoring other important characteristics like element/chemical content and physical structure-clay being platelets) . It appears to me that your jars are showing a distinct horizon between particle sizes and the "2" mark. I may be wrong. The differences are important as to the different characteristics. For instance, among other traits, sand imparts no structure, has poor water retention, high leaching, low TEC (nutrient holding capacity); while clay has high (but slow to absorb) water retention ability, large capacity for holding nutrients, and its structure holds together so well that it is susceptable to compaction.. Your soils TEC is likely to fall in the 7-10 range. Guessing is fun, but not very useful. I hate to keep advising people to spend money, and I have no connection to any lab, but it is always advantageous to get a soil test (loganlabs.com) so you have as much reliable information about your soil as possible to help in making decissions rather than guesses. There is a guy who visits now and again who is very knowledgeable on all things soil. If you do get a report from logan and post it, he might give you some pointers--or not. Worth a try. This post was edited by grass1950 on Mon, Aug 12, 13 at 16:50

In case the link is ever broken, cut and paste the PDF file here: Linda Chalker-Scott, Ph.D., Extension Horticulturist and Associate Professor, Puyallup Research and Extension Center, Washington State University The Myth of Gypsum Magic âAdding gypsum to your yard or garden will improve soil tilth and plant healthâ The Myth Upon continued prodding from one of my university extension colleagues, I recently watched several episodes of a well-known gardening program on television. My kids joined me, alerted by my animated responses to the hostâs non-stop torrent of advice. Among many amazing discoveries I learned that by adding gypsum to my yard or garden I would improve my problem soils by changing the particle size and loosening compaction. Further searching on the web revealed that gypsum would also improve drainage, decrease acidity, and eliminate soil salts. Previously, I had heard of gypsum for use in soil reclamation projects, but not for a typical urban landscape. Since gypsum is simply calcium sulfate, could this chemical truly transform soil structure and serve as a fertilizer for yards and gardens? The Reality This myth falls into the category of agricultural practices misapplied to ornamental landscapes. Gypsum effectively changes the structure and fertility of heavy clay soils, especially those that are heavily weathered or subject to intensive crop production. Gypsum also improves sodic (saline) soils by removing sodium from the soil and replacing it with calcium. Therefore, one can see improvement in clay soil structure and fertility, and desalinization of sodium-rich soils, by using gypsum. What other effects will gypsum have on soil and plant health? There are a number of scientific studies on gypsum usage both in the literature and on websites. Briefly, researchers have found: ⢠Gypsum does not usually change soil acidity, though occasional reports of both increasing and decreasing pH exist; ⢠Gypsum can increase leaching of aluminum, which can detoxify soils but also contaminates nearby watersheds; ⢠Gypsum can increase leaching of iron and manganese, leading to deficiencies of these nutrients; ⢠Gypsum applied to acid soils can induce magnesium deficiency in plants on site; ⢠Gypsum applied to sandy soils can depress phosphorus, copper and zinc transport; ⢠Gypsum can have negative effects on mycorrhizal inoculation of roots, which may account for several reports of negative effects of gypsum on tree seedling establishment and survival; ⢠Gypsum is variable in its effects on mature trees; ⢠Gypsum will not improve fertility of acid or sandy soils; ⢠Gypsum will not improve water holding capacity of sandy soils; and ⢠Gypsumâs effects are short-lived (often a matter of months) With the exception of arid and coastal regions (where soil salts are high) and the southeastern United States (where heavy clay soils are common), gypsum amendment is just not necessary in non-agricultural areas. Urban soils are generally amalgamations of subsoils, native and non-native topsoils, and - in home landscapes - high levels of organic and non-organic chemical additives. They are also heavily compacted and layered (and gypsum does not work well on layered soils). In such landscapes, it is pointless to add yet more chemicals in the form of gypsum unless you need to increase soil calcium levels. This nutrient deficiency can be quickly identified by any soil testing laboratory for less than a bag of gypsum costs. (If you need to improve sulfur nutrition, itâs wiser to use ammonium sulfate). To reduce compaction and improve aeration in nearly any landscape, application of an organic mulch is more economically and environmentally sustainable. The Bottom Line ⢠Gypsum can improve heavy clay soil structure and remove sodium from saline soils ⢠Gypsum has no effect on soil fertility, structure, or pH of any other soil type ⢠Most urban soils are not improved by additional gypsum ⢠Before adding gypsum or any chemical to a landscape, have soil analysis performed to identify mineral deficiencies, toxicities, and soil character ⢠Adding gypsum to sandy or non-sodic soils is a waste of money, natural resources, and can have negative impacts on plant, soil, and ecosystem health For more information, please visit Dr. Chalker-Scottâs web page at http://www.theinformedgardener.com.

A big part of your problem is the raised bed format. Any soil that is raised up out of the surrounding median grade-level will warm/dry quicker. Even the top of natural hills display this phenomenon. Raised garden beds are notorious for this. With sandy soil, you've got a double-whammy. There really are very few reasons to not use/improve your native soil at your feet. Adding organic matter is key here. The roots of plants are the best way to inject carbon INTO the soil. Use cover crops whenever possible (I'm a huge fan of white clover, a nitrogen-fixer), and don't rip out the roots if you don't have to. I've had good results with hugelkultur as well. Otherwise continually add mulch to the surface, and let the bugs and worms pull it down. Also, where drought is a significant pressure, space your plants out 1.5-2 times normal; that extra space goes a long ways toward drought-tolerance for many plants (especially vegetables). S'all for now, good luck.