It could be a "But if your green leaves are suddenly turning purple, it could be a sign of phosphorus deficiency, as a result of low soil temperatures early in the season. This condition typically affects younger plants."...

https://readytodiy.com/top-problems-growing-kale-and-how-to-fix-it-0052/

Phosphorus deficiency is one possibility. But lots of things will turn brassica leaves purple. Anthocyanin is a purple/red-tinted pigment that can build up in response to cold, disease, and lack of water. Anthocyanin is actually what makes many tree leaves red in the fall. In many brassicas magnesium deficiency will do the same thing.

Fascinating. Thanks for the quick replies! It has been unseasonable cold here the past week, so the temperature could be the culprit.

1. What tips would you suggest for reversing phosphorus and/or magnesium deficiency?

2. After last season's kale stunted, I was told I probably overwatered. How often would you recommend watering kale in these conditions/maturity?

Well, it is a little odd that you're only seeing this problem in some plants. That is, you'd think any deficiency would be common for everyone in the bed. There are many strategies for adding these nutrients Certainly just applying some broad spectrum fertilizer like MiracleGro can't hurt. But if you're serious about gardening, I'd recommend that you get a soil test done. Then you can work with facts. Now, I don't believe CA extension services do these tests, so you'll have to find another provider. Maybe someone from CA could weigh in here.

FWIW, pure seaweed fertilizer has ZERO phosphorus. I'd look at the NPK rating on it, and see what it's got. Because of that, phosphorus is sometimes added to make it more complete.

As to water, you don't want soggy soil, but Brassicas do like moist soil. Now, overwatering in a grow bag is kind of hard to do. They drain readily.

What sort of kale is it? Many have a natural purplish tinge which may account for some of what you're seeing.

I agree - it may just be a natural variation - or accidentally mixed seeds. Did you start them yourself? I've grown purple Russian kale and it looked a lot like that.

It is strange I'm not seeing the same coloration/deficiency in the other two plants I've got. But they're in a different grow bag, so maybe somehow that soil has more magnesium/phosphorous in it? I'll see what my options are for soil tests and try to get my hands on the right fertilizer to add.

Also, I planted these as seedlings from a nursery up the road. Forgot to write down what type of kale I'm working with...