I’m on week 6 of flower on my first grow in Fox Farm Ocean Forest soil. I am using Fox Farm nutrients as well at half strength. I suspected my plant might be nutrient locked because it’s showing signs of calcium deficiency but also has slightly burnt tips. Other than slightly burnt tips and mild calcium deficiency, it looks healthy and the buds are developing well.
To confirm my suspicions, I gave it plain PH balanced water measured at ~200 PPM and the runoff measured at ~900 PPM so I’m pretty sure it’s safe to say that there is a build up of nutrients in the soil.
My question is how do I fix this issue so late into flower? Should I continue feeding plain water until the PPM of runoff more closely matches the feeding water? Or should I flush the next time I water until the PPM is lower, then continue feeding on subsequent waterings?
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Sorry, I was stuck in trim jail last night. I actually started to reply last night but was too tiered and opted to hold off rather than give a half-assed response.
First thing to address- any true soil mix that isn't a large quantity coco is going to have the ability to hold significant quantities of nutrients. This is called cation exchange capacity. Essentially, your feed water passes off particles to your medium when you water your plant. Because of this you would have a very hard time actually flushing out nutrients from "real" soil. You can remove some of the mineral salts but it's never going to remove what you want. You could water until the ppms are close to feed water but you'd be drowning your plants to the point that your medium is so saturated the water just passes through (think of cleaning a sponge without ringing it out). What you are actually measuring is your feed water at this point because your medium has already reached its point of maximum saturation. Once it dries out the minerals are still there. If you are flushing with ro or distilled water then this isn't the case since these are like an empty train. As the train goes through your medium it picks up passengers or strips your medium of material to create more stable compounds because it's unnatural for water to exist in this state, which is why it only exists in a few places in nature. Unless you see white salt buildup on the surface of your soil I don't ever reccomend flushing a true soil. Coco is different because it doesn't hold onto nutrients with such a strong bond. This is the main reason coco can be used in a hydroponic setup. People often get these confused and flush soil like they would coco and end up hurting more than helping.
Next thing- Cap Stone is correct. Your ppms are actually a little low for this stage normally, but since you have tip burn it means you are dialed in for your environment. Soil itself is constantly breaking down into smaller and smaller particles and as it does so compounds that we're trapped get released. This will make ppms creep upward. These could be literally anything from arsenic to potassium etc.
Lastly, returning to the point above- yes, your plant has what most people consider to be a very acceptable amount to nute burn. That's a sign that you are pushing them the perfect amount based on your environmental parameters. I believe you are also correct in assuming a calcium deficiency. In soil you want to shoot for a ph of 6.5 to 7.5 and I believe that calcium is absorbed in the higher band more effectively, though I need to look at the absorption chart.
Since you are already feeding calmag I would suggest trying to raise your ph to around 7 at a minimum since this is where the optimal absorption of calcium begins.
Your plant looks great for a first soil grow so kudos on that. You are so close to the finish line. Stuff like this happens as the plant responds to the reconciliation of its life coming to an end. It's all part of the process. Sometimes it's just not your fault. Either way I think you'll be pleased based on what I see.