Hi peeps! I've got an interesting example to add in the hopes that perhaps someone down the line might be helped if they come across this strange looking situation. It looks scary, and I can imagine if I saw this a couple years ago as a newbie I would be freaking out haha.
This is what can happen when you mix up a fresh batch of soil (I'm organic but this could happen otherwise too) and if you don't give the soil time to "cook" and stabilize. (couple weeks to couple months, depending)
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It can look like this:
Very strange indeed huh? Notice a couple things, the plant is otherwise healthy, and different leaves are only half or partially affected.
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Cause? What is happening here is this: Plants uptake nutrients and water from the roots through the fibrous material which acts like highways. In the root system each little root is like a road which individually feeds up to specific places, and thus collectively the root system all together feeds the plant. (You can test this by that old trick of taking a stemmed white rose or carnation, splitting the stem into fourths, and placing each into a different colored water, in a few days different parts of the flower will be different colors.)
Here, a few of those root roads encountered a very strong "hot spot" in the soil where far too much of one or more nutrients are too concentrated. Thus this hot spot will lock out other nutes by clogging up the traffic at those roots. This is what caused the "chlorosis" or yellowing of just parts of some leaves.
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Fix? By the time you notice it, probably a few weeks into the grow, it's too late to do much except ride it out. If you're lucky like I was, just a few leaves in one small area will be affected. If you're unlucky, well it could render the plant incapable of surviving.
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Prevention? Be sure to do a couple things if you're mixing your own soils (actually probably good advice for store-bought soils these days with all the probs members are having). First things first, test pH and PPM of some good distilled water runoff. It'll probably be crazy high on both, but just so you have it recorded. Time!! Let it sit and "cook" for a time (time determined by many factors like temp, strength of amendments, ect ect) but at least a couple weeks. During this time you want the myco and good bacteria to be doing their hard work breaking things down, making nutrition more bio-available, and starting to normalize the pH. Stir it up (Marley song!) and stir it up some more, like every few days or twice a week or so, just stir up the soil with a shovel or pitchfork. It needs to breathe some too, and be moist during this time. Some warmth helps, so I keep mine in big huge rubbermaid bins outside in the sun with some air holes drilled. This will help cook out the hot spots and get the beneficial soil life working for you.
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Too late! Oh no! If you've got this happening now, you've got options. 1.) let it ride 2.) attempt a transplant 3.) cull the plant and start again with diff soil. I'm always for letting it ride. The plant will try it's best. Transplanting autoflowers mid grow is just not really good practice, it shocks the plant a little bit (or a lot) and can throw off its' timing for everything. Culling is your choice, but unless it's really truly completely doomed, it's best to let it play out. Good luck gardeners!