This is not true in hydroponics and coco is hydroponics. Seedlings need to be fed from day 1. 25% strength balanced nutrients along with kelp and Humic Acid, 50% strength week 2 along with Bio-stimulants, Silica, B-Vitamins Kelp and Humic Acid. Then bump the nutrients up 10% per week until you get a bit of tip burn. Then back off 10% and you will be dialed in on what this plant likes.
Never feed a single element. You should always feed a balanced fertilizer. If you need to add Cal-Mag because you see the need add it to your regular feeding. Where did you get your Feeding schedule? New growers always want to reinvent the wheel?
When you feed a single element it is 100% of what is presented to the roots. Elements do not blend in the media (except water) So by presenting only Cal-mag you will be locking out Potassium, Magnesium and Boron from the plant.
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IMHO it is best to use a vendors entire line ONLY. They spend millions of dollars developing nutrients to work together to achieve the best possible results. It is all about Liebig’s law of the minimum. When you understand this you will understand why you should not mix-n-match your own. The only exception to this rule is when you have deficiencies (it won't happen often if using nutrients correctly). There are a couple of additives I do like and use: Azos and Mykos are microzae and beneficial fungus, Botanicare HydroGuard protects roots in Soil/DWC. I use the Bio-stimulants Kelp, Humic Acid, and B-vitamins. Other than this I use a vendors entire line only on their schedule for what I am doing. As to the strength, the vendors charts are a beginning point and for many strains or phenos it is too strong but not all strains or vendors. So I have a system where I start seeds at 1/4 strength for the first week with a pinch of kelp and humic acid then 50% for the next and I continue to increase by 10% each week until tip burn is observed then back off 10%. This gets you to where the plant likes it not just blindly following the chart.
What you need to learn about watering will come with practice. Here are the basic rules: Never let the soil dry out. Soil and or coco can become hydrophobic if allowed to dry. This means it repels water. This in turn will create dry pockets in the soil and roots there will die. If your soil - coco have accidentally dried out use a surfactant to help re-wet it. I like yucca powder. Don't let soil remain soggy by watering too much too often. Root rot, damping off, molds, fungus gnats and other problems start in soggy soil. When you do water water the entire pot. How to learn when to water starts before you plant the seed. Fill your container with fresh soil/coco and weigh it (heft it) this is the lightest weight and consider it a dry pot. Now slowly water until the soil/coco will no longer absorb the water and run-off begins; weigh the pot (heft it) this is the maximum water, the wettest the pot can get. The difference between wettest and driest is the maximum water weight, for ease of explanation lets just say the water weighs 20 pounds. When the pot loses 10 pounds (half of the water weight) it is time to water again. There is an art to watering.