Ah this old chest nut always makes me chuckle
Largely it is a misconception between hydroponic water based systems or with the use of inert growing mediums vs soil/compost grows, it is really just decades of bad advice or lack of understanding where the lines have become blurred.
Well hydroponics water based / inert mediums used such as rock wool, perlite, clay pebbles to name a few all work on the sample principle it is the water PH & solution which holds the nutrients to delivery of such aprox around PH 5.8 for most water based systems for growing MJ. Most of the micro nutrients are in a chelated form which means they can work on a wider PH range so a swing in water/nutrient mix PH may work.
With soil/compost mix's it is basically the PH value of medium used providing it is between PH 6 & 7 you should be fine being that you really don't have to PH correct your nutrient solution/water as the soil/compost will natural buffer back to to its ambient PH value with in 24 hours which can be observed & pretty easy to do.
Now measuring the run off aka the leached remains of water/nutrients put through the soil/compost it is hardly ever going to test right !
1. Leaching aka run off - it is only going to pick up waste nutrients/salts with in the medium in the run off... a plant is only going to use what ever it needs the rest just sits in the soil/compost over time this may build up which may cause nutrient burn, lock outs & a increase of PH more likely acidic.
Most nutrients are acidic in nature even the organic ones !
So what what ya looking at is a sensible feeding watering regime to prevent this
Just would like people to think about this, MJ is plant it has been growing dirt for thousands of years in all parts of the world the soil PH may change between regions it is just another plant with basic needs & at what point did they need to change the PH value of water/nutrients used to make em grow ?
Now by real life trade i work in horticulture for the last 20 years i must say... & i must admit that i have never PH corrected any plant food/nutrients in my entire life time for any plant when it is down to the basic PH level of the growing medium used !
Never ever have i read a gardening book or taken a professional course in, has ever mentioned PH correcting of water/nutrients for soil/compost growing when it is the ideal range of the soil/compost PH value most plants thrive in !
So really how do think agriculture works aka farmers field... do they buy a 50 gallon drum of PH up/down to correct PH value of water to make nutrients work ?
I don't think so !
How do granule fertilizers work, i'm sure people don't use PH corrected water for this...
I mean if i grow veg or what ever plant in my back garden do we really PH correct the water/nutrients to make em grow... it is big point...
So why is MJ grown in soil/compost & the use of PH corrected water/nutrients so important ? well it is not... Just my 2 cents worth...