Possible magnesium deficiency?

Its a shame my first post couldn't be something more useful however i've started hitting a problem on one of my plants.The strain is critical kush, just into week 2 of flowering in some batmix soil. I started with oldtimer bloom nutes(3ml/litre) probably a bit prematurely at the end of week 1 flowering. For water i use regular tap water, perhaps a bit on the hard side if anything.

In the last couple of days i noticed this slight yellowing / bronze patches on the bottom sets of leaves. After researching a bit i came to the conclusion that it could be magnesium deficiency, however with it being my first grow and all i thought it would be best to get someone with a bit more experience to confirm it.

Would someone please be able to take a look and see what they think? I really don't want to treat it for the wrong thing and make it worse. Any advice would be greatly appreciated, hope i attached the picture correctly!

What is the ph of your water? I'm not a.pro by any stretch but doesn't look like mag Def to me. Usually that is tiny brown spots, don't take my word for i though.


Did you start feeding 1 week into flower or one week after you switched the light cycle?

Cal

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DurbanP said:

What is the ph of your water? I'm not a.pro by any stretch but doesn't look like mag Def to me. Usually that is tiny brown spots, don't take my word for i though.


Did you start feeding 1 week into flower or one week after you switched the light cycle?

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So it turns out my PH meter wasnt right, i got a second one and it now reads a PH of 7.5 from the runoff. Water PH is 8.5, after nutes are added 6.8. I started feeding 1 week after i switched the lights.

Since the difference between runoff and input is so large im suspecting i have some kind of buildup in the soil. Is it correct to flush them at this point?

s121 said:

So it turns out my PH meter wasnt right, i got a second one and it now reads a PH of 7.5 from the runoff. Water PH is 8.5, after nutes are added 6.8. I started feeding 1 week after i switched the lights.

Since the difference between runoff and input is so large im suspecting i have some kind of buildup in the soil. Is it correct to flush them at this point?

Click to expand...

It looks like mg def. Add 1/2 to 1tsp epsom salt to your next feed. If you use calmag you'll be adding calcium when it doesn't appear to be deficient.

Do you calibrate your ph pens? You said you got a new one and it sounds like you're comparing the old to new, as if the new one is more accurate. You need calibration liquid. Using two pens can be useful to take the average between the two (if they're off .2 or so). But, you need to know they're both calibrated.

If the runoff ph is higher than what you pour in, I doubt it's salt buildup. That goes the other way (acidic soil). Runoff isn't very accurate. It's useful to to track as a trend when you start growing (or change nutes, soil). The trend can be informative. But, as a one-time measurement there are two many variables which influence the ph of the runoff. How long it sits in the soil and equalizes before being displaced into runoff. How much displacement which can "dilute" the equalized liquid with unequalized.

Google for "NCSU Pour-Thru Method." That's the most precise way to measure soil ph. But, most of us don't have time to do that too often. (I bought a $60 Control Wizard Accurate Eight soil probe. It's useful to confirm runoff, how I interpret the results according to how long I let the water sit in the container, etc. But, again, it's more about the trend, not an individual measurement. And, once you have a system dialed in there's no reason to measure this stuff.).

If I were you, I'd do the pour-thru method once because higher runoff ph doesn't sound right. (I'd be sure the pens are calibrated.). But, regardless, I'd do the epsom salt in the next feeding. Salt buildup can produce mg def, but it usually causes general lockout and the multiple deficiencies. If you have salt buildup it could end up looking like that. But, for now, your runoff ph doesn't confirm it. (If the runoff were 5.5, then I'd worry.).