Phosphorus supplies the energy that powers root development and budding. When phosphorus is deficient root and bud development will be slow and stunted. Plants grow slowly, with small leaves.
A plant with a phosphorus deficiency will show it in their older leaves first. Leaves become dark green and weak. Then they develop brown and yellow patches and limp downward. Younger leaves will pull phosphorus from the older leaves, showing symptoms last.
Extra phosphorus is beneficial during the first week of growth to aid in root development. Phosphorus can also be increased from the last week of vegetative growth to the 2nd last week of bloom to support flower development.
Cold weather below 10°C, 50°F slows the absorption of phosphorus. Increase phosphorus when the grow room is cold.
Dr. Seeds Weeks 5-10, 1&4 and 11&12 are all high in phosphorus and can help correct deficiencies. Corrective measures will not fix existing damage, but will restore plant health. Symptoms will clear up on the new growth.
If phosphorus is toxic, copper and zinc get locked out. Symptoms of a phosphorus toxicity look like a deficiency of these other minerals. This can make a toxicity diagnosis difficult.