Unsolved problem -SOS-

[fhydro]

Hello,
i've read the "awesome plant problem guide" and it seems that one plant in my garden is suffering Mag def. But the soil has got mag because good soil and good nutrients. I think that mag is locked by something like PH too low or too much N because i added some "brown gold" (that is a mix of sea algaes and manure) on the top of the container and I watered the plant giving water on top of the brown gold. At first time watering over the brown gold, the water that goes out of the bucket is black.
So how is it possible to fix that mag def ?
How can i know my soil's PH using a digital ph meter?
Thanks for aswering.
Fhydro

[Mr.pH]

How can i know my soil's PH using a digital ph meter?

General idea is to mix soil with water and then to measure pH of the water. Trick is to always repeat the same procedure as precisely as possible - the same amount of soil, water, same time of mixing, electrode immersed to the same deep. Not to mention electrode calibration, but that's part of the procedure that have to be followed every time, regardless of pH of what you want to mesure. See pH of soil (http://www.ph-meter.info/pH-measurements-other-procedures) measurement description for more details.

Note: there are soil pH meters with metallic electrode that you can stick into soil and read result on a small amperometer placed on top. Their readings are very sensitive to the soil hydration - but as these are not digital, I suppose you don't want to use them ;)

[stinkyattic]

I think that mag is locked by something like PH too low or too much N because i added some "brown gold"


That's a logical conclusion...
You could foliar-feed a solution of 1/4-1/2 teaspoon/gallon of Epsom salts to try and correct the def, bypassing the roots.
But yeah you still need to address the cause as mr pH describes.