Nutrient Lockout

Nutrient lockout is the chemistry trap that catches more cannabis growers than any other issue — the plant shows clear deficiency symptoms despite perfect feed schedules and the right nutrient products, because the nutrients in the root zone are biologically unavailable. Lockout has three primary mechanisms: (1) pH lockout — when root zone pH drifts outside 5.5-6.8, individual nutrients become chemically unavailable to roots even when present in solution. Calcium and magnesium lock out below pH 5.5; iron and manganese lock out above pH 6.5; phosphorus locks out at extreme pH on either end. (2) Salt buildup — accumulated nutrient salts in the growing medium raise the EC to a level where osmotic pressure prevents root uptake even of nutrients the plant needs; soil and coco show this most often when growers feed without sufficient runoff. (3) Antagonistic ratios — excessive concentration of one nutrient blocks uptake of another. Excess potassium blocks calcium and magnesium uptake. Excess phosphorus blocks zinc and copper. Excess calcium blocks magnesium and potassium. Excess nitrogen blocks calcium. Visible symptoms vary by which nutrient is locked: nitrogen lockout = yellowing of older leaves bottom-up; calcium lockout = brown necrotic spots on new growth; magnesium lockout = interveinal yellowing on lower leaves; iron lockout = yellowing of newest growth tips. Diagnosis: test runoff pH and EC, compare to feed-in values — runoff EC significantly higher than feed-in confirms salt buildup, runoff pH outside 5.8-6.5 confirms pH lockout. Treatment: flush the growing medium with 2-3x container volume of pH-balanced water, then resume feeding at 50% strength with corrected nutrient ratios.