Just came up with this also.
NITROGEN (N)
Pale plants, red stems, smaller growth. Rapid yellowing of lower leaves progressing up the plant. Add any chemical fertilizer containing N. Treated plants recover in about a week.
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POTASSIUM (K)
Affected plants are usually tallest and appear to be most vigorous. Necrotic spots form on lower leaves. Red stems. Leaves appear pale or yellow. Add chemical fertilizer containing K.
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I dont think your plant is showing these symtoms so this may not be of any use to you.