What does it do?
Phosphorus is an integral player in photosynthesis, and plant/flower growth and structure.
What do you see?
Slow growth, small leaves with purple/black necrotic leave parts. Leaves eventually become malformed, twisted and shrivel. Deficiencies are most common in the flowering growth stage.
What can you do?
Add a phosphorous specific fertilizer to your nutrient solution, such as Herculean harvest or Seabird Guano.