The thing you need to get a full understanding of is the chemistry that is involved with growing, using synthetic nutrients(man-made). In nature, microbes do all the work of breaking down raw materials and feeding them to the plants. They also do all the work of controlling the pH in the soil. When you grow using "synthetic" nutrients (typically known as fertilizer) YOU have to assume the role of balancing the nutrients and pH in the soil so that the plants are able to absorb the nutrients.
Plants have to have certain pH present in the soil in order to be able to absorb all of the different elements that we call nutrients. In nature or in "organic" growing, the microbes take care of the pH so that the plant can access all of the elements it needs.
For the "synthetic" grower this pH range has to be controlled by us. We also have to control the amount of elements that we put into the soil at any point because each element can and will affect the pH. But the elements can also affect each other.
Too much of any one element within the soil can bind chemically with other elements and prevent the plant from having access to either element.
When I say the soil is "locked up tight", that means the pH is so far out of the acceptable range that none or few elements within the soil are able to be absorbed by the plant. Also, if an element such as magnesium is said to be "locked up", that means either the pH is out of the range for the roots to be able to absorb magnesium, or there is something that is chemically binding with the magnesium which prevents the roots from absorbing it. This often happens with coco coir. There is an element within the coco that chemically bonds with magnesium and calcium(some). If large amounts of coco are used in soilless application then magnesium deficiency can occur if the coco isn't pretreated some.
Now another term here is "the soil/medium is too hot" That means the soil/medium has too much nutrients and their presence is literally able to chemically burn the roots and leaves of the plants.
If you are using soil that has lots of nutrients already available in it and then you toss in a strong dose of synthetic nutrients, along with heavy water(high mineral content), it can cause multiple issues from nutrient burn to toxicity. Also, soil that has nutrients already may not have the balance that your plants like. They then can leave these elements in the soil which can react to the elements that you are adding with your nutes and water.
If you have RO I would use it for these. I would also suggest that you go back to neutral soil/medium by flushing them with straight water. Then start over with fresh nutrients set at the proper pH. This will eliminate any chemical issues by removing all of the chemicals with the flush. But be sure to aerate the new nutrient batch before giving to the plant.
Now some people will say the soil "looks too tight", that means the soil is compacted. Plants need soil or medium that is easily broken and moved to allow plenty of air to get down into it as the water drains out or is used. Often people use pearlite to break up the soil so that it isn't able to compact.
I hope this helps.
