The testing of EC and PPM and all the other numbers are for people who are not growing in soil. Hydroponic styles of growing do not rely on the plant being able to get the nutrients they need from the soil since they rarely use soil. Instead the grower has to add fertilizers on a regular basis, sometimes as often as once a day or every couple of days. They test before adding the nutrient water and they test the water that comes out to get an idea of how much the plant is using. A lot of record keeping involved. Have to keep track of which nutrients are used, how often and how much of each one. Then keep track of the numbers coming out the bottom of the pot.
In a natural soil with dirt and compost like the Fox Farm Ocean Forest and many, many other soil mixes it is the micro-organisms that are doing all the work. They eat and reproduce and die at fantastic rates. They release the nutrients and the plant often will absorb these nutrients within minutes to hours.
Using the bacteria in the soil as an example, they can double their population within 15 to 30 minutes under the right conditions. Then there some organisms that also live in the soil which will eat the bacteria and that helps release the nutrients that the bacteria made.
All these micro-organisms help to create a change or adjustment in the soil which will change the pH of the water to match which nutrients are becoming available. There are several different charts available which will show which nutrients become available at specific pH levels. When growing in soil many new gardeners will become frustrated when they get different pH numbers each time they test the water that came out the bottom of the pot. It can change as often as every 15 minutes give or take to as long as every couple of hours. That is why there is the recommendation for soil growers to not bother with testing the pH of the saucer water. It will change quickly so the only number that is important is the pH of the water when it is poured on the soil.
