Hey Eric,
Good question.
There are a few good liquid calcium products on the market from companies like cropping solutions and sustainable farming solutions, but they are expensive. Nicole Masters recommends making whitewash with hydrated lime, other farmers are trying micronised products or using strong acids to solubilise the calcium in micronised calcium carbonate etc…
I’ve yet to see much proper research or testing on the effectiveness of these measures but a few of us have started doing some so will keep you posted.
The other thing is that you can only get so much calcium into plants via foliar sprays and it’s not mobile in the plant so can’t be redistributed. We really need to look to soil uptake for the bulk of our calcium but soil uptake can be sluggish unless there’s good biological activity, hydration etc…
Like phosphorous, it’s best applied at the beginning of the season and I think upfront Calcium is as, if not more, important than the other macro elements as it’s so important for new growth. Its pretty hard to play catch up once calcium deficiencies start to show up.
The application of slowly soluble sources like single supa, and/or gypsum at, or prior to, planting is a strategy that’s somewhat effective for meeting growing season requirements along with the usual liming required to get soil calcium levels up.