HELP! Week 4 flowering yellow leaves. Suspect Ca deficiency. Thoughts?

[rangerjake]

One of my White Widow autos - the best performing of the three- recently demonstrated a rapid yellowing of bottom leaves with some peppered brown spots on those leaves (can be seen in the two pictures). Seems like it closely aligns with symptoms of Ca deficiency. Here's my situation:

Coco grow, 40% perlite. Unbuffered (by me) so I feel like I have been battling my medium a bit the whole time.
Been doing alternating nutes and plain phd water feedings over the past couple weeks after a brief go at fertigation which I found to be causing more nutrient buildup.
Most recent feeding was done at 6.2 540ppm with runoff at 6.4 508 (time to up the nutrients). The 2nd biggest of the WW (back left) most recent runoff (fed at same as #1) was 5.6 366ppm - would expect more of a Ca deficiency to show there.
Been feeding 2.5ml/gal of calmag per feeding.
Generally feedings have been at 6.0-6.2 ph so that should be close to the good range for Ca, runoffs consistently between 6.4 and 6.8.
There was a recent plain water feeding that was at about 5.5pH. Could that drop in pH be the culprit?

[Obi-Wan]

Given the symptoms of the sick plant and pH of both plants I would suspect it’s a manganese deficiency rather than a calcium deficiency. Try feeding the sick plant at pH 5.8 with twice the normal volume of nutrient solution. This will bring the root zone pH down under 6 and flush away any unwanted nutrient buildup.

[rangerjake]

Is your belief that it is Manganese instead of Calcium solely based on the runoff pH? It seems that every symptom is shared by Ca and Mn, the only difference being that Ca is accessible to the plant at 6.2-6.5 and Mn is accessible 5.5-6. If I just alternate my feedings between 5.7 and 6.2 I should cover my bases for these two nutrients, no?

By and large my runoff pHs have been in the mid to high 6s. I was surprised to see that 2nd plant runoff at 5.6. That 2nd plant also seems to be a solid week plus behind its compatriots in flowering progress.

[rangerjake]

When you say

"Try feeding the sick plant at pH 5.8 with twice the normal volume of nutrient solution."

Do you mean use the same mixing ratios of my nutrient suite and just feed 2x the amount of fertilized water to create excessive runoff?

Not that I should double my nutrients per gallon (thus doubling my PPM). Or, conversely, halving my nutrients per gallon (thus halving my PPM). Just making sure I understand your suggestion.

[Obi-Wan]

Yes, it is the shared symptoms coupled with the deficiency showing in the plant with a higher pH runoff that drives my conclusion.

Yes double the volume of feed not the ppm.