Phosphorus Deficiency

Phosphorus deficiency is the plant running short of the element it uses to move energy around and to build flowers. True phosphorus shortage is uncommon in bagged soil. What is common is phosphorus that is present but unavailable.

What you are seeing#

Lower and middle leaves first, because phosphorus is mobile and gets pulled upward to the new growth.

The color goes wrong before anything else. Affected leaves turn a dull, dark blue-green with no shine, and they look slightly smaller and thicker than they should. Then bronze, brown or dark purple blotches appear across the leaf blade, not in a neat pattern and not confined between the veins. Those patches dry, go brittle and the leaf curls and drops.

Stems and leaf petioles often turn red or purple. On its own that means very little, because plenty of healthy strains have red stems genetically. Paired with dark dull leaves and bronze patches it is meaningful.

Growth slows across the board. In flower you get small, airy buds that stop filling out.

Why it happens#

Phosphorus uptake is very sensitive to two things: temperature and pH. Below about 60 F at the roots, the plant can barely absorb phosphorus even when the pot is full of it. Below pH 5.5 or above pH 7.0 in soil, phosphorus binds to other elements and becomes unavailable.

Beginner causes:

• Cold nights, especially pots sitting on a concrete basement floor.
• pH drift, which is the single biggest cause of what people call phosphorus deficiency.
• Heavy overwatering, which chokes the roots and stalls uptake.
• Running a grow-only nutrient deep into flower, when phosphorus demand roughly doubles.

How to fix it#

1. Put a thermometer at the base of the pot tonight. If root zone temperature drops below 62 F, raise it. Get the pots off the cold floor onto a 1 inch foam board or a shelf, and raise the lights-off temperature to at least 64 F.
2. Test runoff pH. Correct to 6.2 to 6.8 in soil, 5.8 to 6.2 in coco and hydro. Phosphorus availability peaks in that band.
3. If pH and temperature are fine, switch to a bloom nutrient with a higher middle number and feed at half strength, roughly EC 1.0 to 1.2.
4. Over the next week, keep watering less often rather than less volume. Let soil dry until the pot feels noticeably light before you water again.
5. Do not add extra bloom booster on top of a complete bloom feed. Excess phosphorus locks out zinc and iron and you will trade one problem for another.

How to tell it apart#

Versus cold shock. These overlap heavily and often coexist. Cold shock purples the leaf blades themselves, especially the undersides, and affects the whole plant including new growth at the top. Phosphorus deficiency concentrates in older lower leaves and produces dull bronze patches rather than an even purple wash. If it started within a day or two of a cold night, treat it as cold first.

Versus pH lockout. In soil, most apparent phosphorus deficiency is lockout. The discriminator is a runoff pH reading. Out of range means fix pH and wait, do not add phosphorus.

Versus nitrogen deficiency. Opposite color. Nitrogen deficiency makes leaves pale and yellow. Phosphorus deficiency makes them darker and duller than normal, with distinct blotches. If the plant looks washed out, it is nitrogen.

How long until it looks better#

The blotched leaves are finished. The bronze tissue is dead and will not repair, and those leaves will eventually yellow and fall whatever you do.

Correcting root temperature shows up fast, within three to four days, because the phosphorus was already there. Correcting pH takes about five to seven days. New growth should come in a normal medium green with no blotching, and in flower the buds should visibly resume swelling inside ten days. If new growth is still blotching after two weeks, recheck pH rather than adding more phosphorus.