Purple leaves: Cold damage, Insect stress, too much light, magnesium or phosphorous

[Ray]

Originally Posted by K-Sci

I suspect you will agree that most orchids grow bigger, faster, and flower more profusely with nutrient levels far higher than in the rain forests.

Absolutely. I think that's the difference between a subsistence diet and a healthy one. But...again, it says nothing about the ratios.

These two levels, sufficient and toxic, appear to allow for a very wide range of the nutrients to be beneficial in greenhouse and home culture. People are successful using ratios such as 10-10-10 and even with bloom boosters that are as P-rich as 10-52-10.

Unfortunately, we know little about the true uptake dynamics, which is bound to have an impact. I just recently read an article from some college extension service that talked about how calcium, magnesium and potassium are taken up in an uncontrolled fashion if they are in solution, how they can compete for "capture sites" (my term) within the plant, and how overdoing any of them can cause issues due to the "blocking" interference they can cause. That is, too much of any of them can result in a deficiency in the others.

Phosphorus is a whole different case however, as the plant will take up as much as it can get, whether it needs it or not.

That may be, but isn't it also fair to say that the much higher levels of P and K widely used by commercial greenhouses for many more decades have not resulted in issues that can be attributed to those fertilizer formulas. The only one I can think of is the expense of using unnecessary nutrients.

I think we have to consider that most commercial nurseries don't actually keep plants around for very long, precluding the potential issues, and the breeding stock plants they do grow long-term get repotted regularly, thereby dumping the accumulated minerals in the medium.

Cool discussion - Happy Thanksgiving!