To carry on from my previous post:
In and after the 1920's people could choose between locally grown produce and that grown further away. Over time people chose the salad vegetables grown in California (mostly) over their local produce. Why? Presumably because the CA produce was available through more of the year and perhaps because it was cheaper and more consistent in quality.
In the 1920's "organic" wasn't a category, wasn't a characteristic that would have played into anyone's buying decision. I don't know when people actually ended up with salad vegetables that weren't organic. I suspect chemical fertilizers came more slowly to local farmers than to the larger concentrated farmers in California.