The effect quoted here is called buffering, which is the tendency to modify excesses of high or low pH towards a middle value. The value depends on the type of buffer, for gypsum I think it is around neutral. To have a significant effect will probably take a great deal of gypsum and the closer the pH gets to neutral the less change adding more gypsum will produce. I would expect as buffering substances go gypsum would be quite a poor one.
So while you are technically correct that gypsum _can_ alter soil pH but practically speaking if you wanted to modify soil pH you wouldn't choose gypsum to do it as there are much more effective ways. If you wanted to modify pH *away* from the buffer level, from neutral to either acid or alkali, you are completely wasting your time with gypsum.
I would like to see this vendor produce figures showing how much gypsum would be required to raise the pH of a variety of common soils from say 5 to
6, which would be a typical sort of thing you would want to do.
This is a complex issue. From the point of view of the practical gardener, if I had to choose between two simplified views of the world (1) "gypsum is useful to modify soil pH" and (2) "gypsum has no significant use in modifying soil pH" I would take the second every time.
You should keep in mind that these people are trying to sell gypsum!
The reference to it, originally, was that it would raise the pH, which was not correct. It can neutralize it, but not raise it in already high pH soils. I suppose I should have been more specific. I have gardened in Dallas soils, so know the problems.
Correct. I mean, because it says so on the Internet doesn't always mean it's so. Gyp does not truly alter the pH in clay, rather, it helps the particulate with cation exchange capacity, rendering the soil easier for nutrients to become available. As you or someone said, buffer to neutral. Nothing will raise or lower pH, permanently. I've used pellet gyp in clay and it is very effective, along with compost, in making soils friable when they are basically scraped off sub-soil, aka, second horizon. In Dallas, they scrape all top soil off, bag it and sell it back.
Go visit your local cooperative extension agents and talk with them about your particular yard. Perhaps take up a section of soil by cutting around it first with a shovel or whatever will penetrate it, disturbing the natural lay of the soil -- otherwise don't turn it over, and put it in a box just as an example of what you're dealing with. Through the example, talking about what you want to do with your ground, the extension agent should be able to give you some constructive suggestions.
While you are there talk to the agent about the proper methods for taking a representative soil sample for testing, this is different than taking in the chunk of soil I suggested to begin with. When taking a sample for testing, you take it from various places in the yard, and the instructions for taking the sample will tell you at what depth.
Getting the results of the soil test will tell you what's there, what's missing, and will give you suggestions of how to amend your soil.
I know gypsum is recommended for our clay soils here in Idaho to help in breaking it up, making it more workable. The soils here tend to be more alkaline than acid overall, but those pockets of clay soil aren't quite as alkaline as others. I personally have not used gypsum, and I think you should talk to someone in your area who does not have a monetary stake in whether you buy it or not, such as your extension agency. If there are garden associations/clubs in your area which aren't ones that are all snooty, they may be a resource. Another place to ask, if your soil is pretty representative of the city's as a whole is the parks and recreation folks if there are public plantings around town in parks and on the grounds around public buildings. I suspect they will tell you add as much organic matter that you can lay hands on though.
Clay soils overall are more desirable in the long run than light sandy soils where added organic matter just burns out over a season. I suggest making the collection of organic matter your new hobby for awhile, particularly if you have a pickup or larger vehicle and some strong family or friends and pick up lawn trimmings (from places which have not used weed & feed recently) if you 're close enough to some rural areas where you may come across spoiled hay or straw, bedding from the race track stables can sometime be had, and in the fall, get every leaf you can lay hands on and mow them up or if you have a shredder, shred them.
Before I'm shouted down, I should warn you that hay and to a lesser extent, straw (you might get some wheat or oats if it's oat straw), but to me the need for organic matter outweighs the weed issue, I had weeds to begin with, I'll always have weeds, seeds are everywhere and I have flood irrigation so I get exciting new weeds every few years that way ;-) If you balk at the weeds, then do a real compost layer green grass and dryer materials, toss in some of your clay soil here and there, and let it heat and turn it. I'm too lazy though.
My dad read Ruth Stouts book years ago something like the no dig or lazy person's garden (look up ruth stout and gardening in google you'll find it) which is basically what they're calling lasagna gardening now. It's sheet composting. Lay down your organic matter, whatever it may be in sheets, toss a bunch of red worms night crawlers or whatever the best worm varieties you have there in your area, and they will go to work on it all. Night crawlers here come up and get organic matter and try to pull it down into the ground and succeeds most of the time. I've seen them trying to pull dandelion leaves, which are still attached to the plant into the ground. LOL
Worms and organic matter will improve the soil, but if you want to garden soon, the sheet composting will allow you to just pull the compost apart and place transplants into the soil, and over time push it back. The fertility zone is in that top few inches of soil, the added layers of compost as it decays keeps the top of the soil in that fertility zone, but as you keep gardening keep adding more compost, the worms will gradually pull the compost down. And you'll dig some of it in from time to time.
My mother said they raised a big garden on old blue adobe clay in southern Utah, They added manures from the farm for organic matter and fertilizer and plowed it up, planted it, and the kids hoe hoe hoed a lot, which is why she was never much of a gardener later in life, she got sick of all that hoe work ;-)
I own them both. She says to lay a thick layer of hay on the soil, make way for the seeds, plant seeds in soil, let plants grow up through the hay mulch. It's not the same as sheet composting. It is sort of the idea, not the same. The hay shades the soil keeping weeds from germinating, it cools the soil, and provides a very slow nutrient base for plants. I recommend the use of alfalfa hay. I use it all the time and I've yet to see an alfalfa plant.
Ruth's books are long gone, but she was brilliant. I'm glad you brought her up.
There are some books out there on the web for sale. Just have to plug her name into the search engines, although some are kind of high priced!
I used to go looking for spoiled hay stacks, alfalfa is the most prevalent, and it's supposed to have about the same fertilizing value as horse manure. I covered my entire yard with hay. Don't know if my neighbors liked it, at least at first, but there were squash vines covering it in the front yard by the end of the summer. They were pretty lush looking, nearly tropical.
But all that hay was gone, disappeared into the soil in a few years time, worms and chickens worked it into the ground over time. Tons of hay!
I bought mine at antique shops for a few dollars. I collect antique and old garden books and was amazed at the price of these. Both still have the jackets (in rough shape).
Today it's raining, which is a great thing where I live...
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