The only thing clay soils need to make them better is organic matter, compost, vegetative waste, and that rice hull based amendment could be part of that. Start with a good reliable soil test, although since your University of California does not do them you will need to look around some. The University of Connecticut, UCONN, does do out of state soil testing.
In addition these simple soil tests may be of some help,
1) Soil test for organic matter. From that soil sample put enough of the rest to make a 4 inch level in a clear 1 quart jar, with a tight fitting lid. Fill that jar with water and replace the lid, tightly. Shake the jar vigorously and then let it stand for 24 hours. Your soil will settle out according to soil particle size and weight. For example, a good loam will have about 1-3/4 inch (about 45%) of sand on the bottom. about 1 inch (about 25%) of silt next, about 1 inch (25%) of clay above that, and about 1/4 inch (about 5%) of organic matter on the top.

2) Drainage. Dig a hole 1 foot square and 1 foot deep and fill that with water. After that water drains away refill the hole with more water and time how long it takes that to drain away. Anything less than 2 hours and your soil drainsâ too quickly and needs more organic matter to slow that drainage down. Anything over 6 hours and the soil drains too slowly and needs lots of organic matter to speed it up.

3) Tilth. Take a handful of your slightly damp soil and squeeze it tightly. When the pressure is released the soil should hold together in that clump, but when poked with a finger that clump should fall apart.

4) Smell. What does your soil smell like? A pleasant, rich earthy odor? Putrid, offensive, repugnant odor? The more organic matter in your soil the more active the soil bacteria will be and the nicer your soil will smell, to9 a point. Too much organic matter can be bad as well.

5) Life. How many earthworms per shovel full were there? 5 or more indicates a pretty healthy soil. Fewer than 5, according to the Natural Resources Conservation Service, indicates a soil that is not healthy.

Here is a link that might be useful: UCONN soil testing

Other than the treatment of clay being a different issue than the treatment of alkalinity, I have three things to add.

One, read up on gypsum and what particular variety of clay soils it is effective in before using it. I HATE to suggest soil tests, but one might be a very good idea if you think gypsum could be a remedy.

Two, manure (composted if possible) can work very well with clay. However it tends to be alkaline too in its fresher stages, so watch your pH levels if you use a lot of it. If you have the room and are willing to take a long term approach to it, pile the stuff up somewhere with any other organics you can find, let it rot away, and revisit it next year. It's also a good passive way to insure against possible manure carried pathogens, if any.

Third, before buying bagged remedies for bad soil, think about how much soil you need to affect. If you have a thousand square foot garden and need to treat the soil to a depth of eighteen inches, that's fifteen hundred cubic feet of soil. How many two cubic foot bag$ of rice hulls are you going to need to have any effect on fifteen hundred cubic feet of clay? The Earth is big, and the efforts of us humans is puny indeed. Horses can help!

The answer is OM, yes. Perhaps in semi-arid areas is not as easy as it is here to get a few free tons of tree products (chips or leaves). Proper amending of clay, IMO, requires of order 1-2 tons per 200 square feet, with tilling. If that is the case, buying and seeding tillage radish is the next best option. The soil will be ready in March after you whack the tops. Territorial Seeds has it, product WW4429 and it should be seeded ASAP. I am also very fond of hugelkultur, it has done wonders in the very poor clay soil of my orchard, but again, I have a large supply of rotten logs.

Gypsum is useful in sodic (salty) soils. Those would generally be found were rainfall is minimal and California is such a place. Otherwise Gypsum has little value in clay soils in spite of all the hype the sellers of that product use.

If you are using any lime in the barn to control odors the pH of manure may be high, and if it's hydrated lime it can have a VERY high pH. So watch out for that. Also, where do you get your hay, and are you confident it isn't treated with persistent pasture weedkillers? Such as picloram. These can survive a livestock gut and appear in the manure. They are not usually found in lawn weed killers but more often in pastures.

sabine, I have horses as well and use the resulting compost on my native clay soil here in the southeast with excellent results. I compost manure with bedding waste, usually a fine pine shavings or pelleted straw.

I buy hay locally and always check with my supplier concerning any herbicides, and it has never been a problem.

When I create a new bed I will dig in compost and if I've planned ahead let it sit for a few months. After that I use my compost on top of the soil as mulch. In some areas I may put an inch of more traditional mulch on top of the compost if I want a more polished or finished look.

I don't purchase any other soil amendments except alfalfa pellets which I use for the addition of nitrogen--but only for heavy feeding roses.

Although I have experience with clay soil in a dry climate, I think the constant warm temperatures of SoCal makes sabine's situation different than that in Southern Idaho.

I like the idea of using tons of wood chips to help condition the soil. Freshly chipped ramial mulch is the best. Do a search on this site for more information about ramial mulch.

Manure needs to be composted before it can be used for plants. Also, be aware that a lot of hay is grown in soil that has been treated with picloram, which is a very persistent broadleaf herbicide and will ruin your soil for a long time.

" Also, be aware that a lot of hay is grown in soil that has been treated with picloram, which is a very persistent broadleaf herbicide and will ruin your soil for a long time."

Aren't alfalfa and clover broadleaf legumes that would be killed by those herbicides?

'A long time' may be overstated. One season is typical, I think I read somewhere that in very bad cases it can last for two.

I will link the Oregon Department of Forestry's link for picloram below.

Here are quotes from that link. Picloram is toxic to some plants at very low concentrations.
Picloram is moderately to highly persistent in soils.
A typical half-life in soils is 90 days. PicloramâÂÂs half-life may be substantially longer in dry regions.

My intent is to inform. Composted manure can be an optimal soil amendment, but there are pitfalls. Good luck.

Here is a link that might be useful: picloram information

What I was getting at was that unless the hays were grass, timothy, and such, they would have alfalfa, clovers, and such legumes and how could those legumes have grown in the contaminated soil to start with and be in the manure?

Are pure timothy and grass hays common?

Oh, I get it. Alfalfa feed would not be at risk of picloram contamination. But, yes, there is hay that is strictly grasses. There are many warnings about 'ditch grass'. County road crews spray picloram on ditches and then allow local farmers to cut the grass that grows in the ditches for hay. That stuff then gets fed to cows and horses, where it passes through the animal gut without any decomposition. The result is manure with picloram.

Do a google search on the term 'ditch grass picloram' and read about problems with residual herbicide contamination from manures.

We have alkaline clay here in north central Texas, too;
even our water is alkaline, & our summers are hot & dry.

I've had great results with sheet composting.

I just spread out OPBL (other people's bagged leaves) & put vegetable peels etc under the layer of leaves.

When neighbors bring me their lawn clippings, I spread them out & add leaves or wood chips on top.

When I get the opportunity, I add pine needles, sawdust from a woodworking shop, cedar shavings, etc.
Over time, the materials seep in & improve the clay, & they've even raised the level of the ground, which improved drainage.

When you start with slow-draining alkaline clay, you just can't go wrong;
anything is an improvement!

I also do not plant anything that *has* to have a more acid environment.

no pine trees or azaleas or trilliums!

To sum up, pretty much everyone is saying add organic matter, vegetative waste and animal manures while being careful of the source so no toxic materials are included.
My soil, when I started here, tested at a pH of 5.7 and after a few years of adding organic matter to that soil the pH tested at 7.2, where it has stayed for many years now. People with soils that tested at a higher pH found that adding organic matter would lower that soils pH as well.

Two, or even ONE, ton of organic matter per 200 sq feet?

At that rate, you're not so much amending the existing soil as replacing it altogether!

When you have horses at home (which I'm assuming is what "unlimited supply of horse manure at my house" means) I have found that doing the follow up with your hay supplier about herbicides is not that big of a deal--you just need to actually do it. And yes, if horses are eating a hay with a legume (alfalfa, clover being the main two) you don't need to worry about those hays.

Also for the record, I use manure at varying levels of composted. Horse manure doesn't get very hot. I've thrown it fresh from the stall on to roses before with no issue. All I do is check that it is not touching the stems. With food crops you need to give it a few months. (Look up organic standards for manure and compost if you want the details.)

"Horse manure doesn't get very hot" Not the experience of many others. The Nitrogen in horse manure is quite readily available and spread, fresh, on a garden can cause Nitrogen burn of plants. But then again no animal manures should ever be spread, fresh, uncomposted, on the garden.

Well, at least not if you're planning to plant within a couple months. I would think you could spread fresh manure in fall and by spring it would have mellowed enough to be OK. But other than that, I totally agree, in view of the high N and potentially harmful bacteria.