If the soil is as hard as you say it is, I'm afraid even a broadfork won't be stout enough.
It is in some places, but not everywhere. I'm guessing that the compaction is due to 50 years of horse hooves pounding on it, and it's worst right in the middle of the field/paddock.
At the edges it's somewhat more reasonable. I dug out a furrow near one edge with my pickaxe this morning, and I sorta got the feeling that a broadfork might work in this area.
I found [url=http://meadowcreature.com/broadfork.php]this scary-looking broadfork[/url] for $250, and the blade-like construction looks like it might be more resistant to bending than a tine with a round cross-section. Here's [url=http://www.groworganic.com/deep-spader.html]another bladed design[/url] for $229 (+S/H). Pricewise, I'm moving in the wrong direction!
But I'd be OK with spending $250 if it works and is durable.
The ground you're dealing with is, unfortunately, very common in California: adobe clay. The stuff the missions and Spanish colonial buildings were made of. The buildings that have weathered two and a half centuries of storms and earthquakes. Pretty durable stuff. People (like me) who have moved here from Georgia's famous "red clay" have found the adobe to be an entirely different--and much more difficult--clay. It may be like the New Mexico caliche; I haven't fought with caliche, so can't give a personal comparison.
Now that you know what you're up against with the adobe, consider the design
of all the broadforks you've looked at.
1) Personally, I would remove from consideration *any* broadfork I could not stand on, like the "deep spader" you found. The broadfork/U-bar is not
meant to be an upper-body workout device. Women can and do regularly break ground with this tool, and it's not because of our superior upper-body strength relative to men; it's because we can exert *all* of our body strength/weight on this tool by standing on the crossbar safely and in an ergonomically effective way.
The strength and width of the crossbar
, into which the tines are set, are critical for this purpose. Consider both how robust the crossbar is and how wide the tool is. 14", for example, wouldn't work for me, as my personal "cross-section" in the hips is (shall we say?) more than 14". I wouldn't be able to stand on such a narrow crossbar in a safe *or* effective manner. Only a very slender person could stand on a 14" crossbar and be able to move his/her arms as well. A broadfork/U-bar with a crossbar at least 18" or 19" wide and preferably wider would be great; I could stand on these quite sturdily and exert my weight very effectively while keeping a good arm position.
2) The length of the poles + tines
is important, especially for tall (> 5'6") individuals. Some of the broadforks are 48" from the ground to the end of the handles (poles). That sounds terrific for me, but I'm 5'4". For my DH, who's 6'1", it would be a particularly difficult form of torture; he would have to stand on the broadfork in a very cramped, bent-over position. Not safe for him, and certainly not healthy for his back/shoulders or effective in breaking ground via use of his body weight.
Some broadforks come with longer poles, and the length of the tines varies from maker to maker, so measure yourself and estimate what total length of broadfork would be comfortable for you (and your gardening partner, if you have one) to use. Also read the makers' descriptions carefully; some refer to the length of the poles alone; others refer to the length of the poles + tines.
3) The last factor I recommend looking at for ergonomic/ground-breaking reasons is the number and strength of the tines
. Some of the broadforks have widely spaced tines, approx. 5" apart, while others are much closer together, approx. 3". How finely do you need your ground broken up? Maybe you'd like to approach this project in two phases: this year, for the "cooperative" soil, and then next year--after the deep-rooted plants have done their work--for the very compacted center soil.
I'm pleased to see that the market has expanded in the past two or three years so that there are many choices available. Unfortunately, it also makes decision-making more difficult!
Just read what all the websites have to say, and make liberal use of toll-free phone numbers and other interactive methods the makers have available to inform you about their products. At least one of the makers (Johnnys) has videos and instruction sheets on the website to assist in understanding how a broadfork/U-bar works; this may help in decision-making as well.