The other downside of burying organic material in soil which doesn't drain is that it sours. Like a container whose drain holes clog, the organic material begins supporting the growth of anaerobic bacteria, those which grow in the absence of oxygen. If you've ever smelled "soured soil" you know precisely how to tell if that's happened. There must be somewhere for water to drain out before organic material in it begins to sour or anything planted in it is doomed. Loosening the clay, perhaps adding gypsum as you wrote you did, and digging that clay as deeply as possible to provide air space for the water to penetrate, will help. Mulching well with organic material which will digest and release Humic Acid which will flush into the soil with irrigation and rain will help to loosen the clay further. Those organic particles flush through and separate the "flakes" of aluminum silicate (clay), providing air space and drainage. They must be continually replenished or they eventually wash through leaving clay behind. So, keep mulching. If you can obtain horse manure, use it. Keep it away from the crowns of the plants and out under their drip zones, where rain falls off the canopy of the plants, and keep it watered so the bacteria, fungi and insects digest it, releasing its nutrients and Humic Acid. It's what Nature has done since "day one". It works.
