I am typically a fan of the Kellogg soil products - Gardner&Bloome and Master Nursery are the brand names most common here but I can't imagine they are significantly different from their other brands.

Kellogg's Amend is intended for just that purpose - amending clay soils. It sounds like you are simply intending to fill in a large ditch with the product? That is not its intended purpose and you may very well not get optimum results following this method.

What I would suggest is that you work this product into the existing soil all along/through the area you intend for planting. Blend it thoroughly with the clay soil and not just in the 12'x2 area -- you don't want to create a ditch of enriched soil bounded with clay that will work as a bathtub when it rains, collecting water and preventing proper drainage.

Kellogg's Amend is a blend of composted forest products (bark), rice hulls, chicken manure and gypsum -- materials intended to loosen and aerate heavy clay soils and provide some fertility. Bagged soil amendments and other soil products are always going to be a lot more expensive than purchasing something similar in bulk. If you need more amendments, you might want to look into just locating some bulk compost. Incorporating this into your soil will produce the same results as using the more expensive bagged product.

I also wasn't aware gypsum had a particular odor associated with it......OTOH, chicken or poultry manure does, even after composting :-)

OP said concerning her soil, "but in the summer it is rock hard."

That's because it contains clay. Not necessaarily a lot of clay, but some clay. In other words, that's normal.

To dig and/or plant in a clay-based soil, one must make certain it is properly moist -- a handful won't drip when squeezed and will begin to rupture when released then poked with a finger.