Phosphorus helps produce healthier seeds and strengthens the plants root structure. It is necessary for photosynthesis and respiration, cell division and energy transport. Plants use phosphorous in multiple ways and could not exist without it. The highest levels of phosphorus are used in the plants early stages - seed germination, seedling growth and flowering.
A deficiency in Phosphorous is usually detectable by the following symptoms.
1. Unusually Dark green foliage caused by accumulation of chlorophyll in leaves.
2. Stunted growth
3. Red and purplish hues on leaves
4. Substandard small roots
5. Substandard small flowers
6. Leaf tips curling downwards
7. Necrotic spots occur on leaf margins - advanced stages
8. Leaf tip burn - advanced stages
Phosphorus deficiencies will first appear in older growth. Leaf damage initially appears as dark green to bluish green patches. At times when the deficiency is severe foliage turns a red - purple hue.
New growth leaves when lacking in phosphorous will display dark green veins. As some symptoms of nitrogen deficiency are mimicked by phosphorous deficiency the two can at times be differentiated by purple-red veins on yellowish green new growth leaves when nitrogen is lacking.
A Phosphorous deficiency is not always due to a lack of phosphorous in the nutrient solution. In fact adding more phosphorous when your plants are displaying signs of phosphorous deficiency can lead to problems with other nutrients. Excessive phosphorus causes a decrease in the uptake of iron, zinc and copper which could easily lead to more deficiencies while not curing the initial problem.
On the flipside - if your plant has excess iron or zinc , it could prevent the plant from absorbing adequate phosphorus.
At times environmental factors hamper the plants ability to uptake / distribute phosphorus. Phosphorous is used by plants in their early growth cycle and reproductive stages, in nature this a warm period - spring and summer. Plants have evolved to take up more phosphorous during these times to facilitate your seasonal development. As the weather turns cold they stop taking up phosphorous. In an artificial environment such as a grow rooms or greenhouse , if the nutrient solution is too cold a decreased uptake is common. Temperatures below 55 degrees F are conducive to this situation.
Ph is another factor that hampers plants ability to uptake phosphorous. When ph is below 5.5 or over 7 phosphorous uptake problems are amplified.
If you are certain your phosphorus deficiency was caused by a pH problem, flush the system with fresh water [I prefer distilled Water] that is at the correct pH with nutrients blended in. The flushing will also help to eliminate nutrient salt buildup that could be hampering phosphorus uptake. It will bring the pH level back to normal too
If you believe the temperature was the problem that caused your phosphorous problem that is perhaps the easiest to rectify - adjust the temperature - ya think.
If neither of the above is the issue and you are absolutely certain that they are not the causes, check your TDS and EC. If they are in the proper configuration goody goody for you - how much of the total Parts per Milllion in your solution are Phosphorous - you don't know. There are no meters that will indicate what elements are in your solution, or in what quantity. They can't even tell you if the elements within the parts per Million are useful or harmful elements to your plant. For a nutrient solution to do it's intended job , all the mineral elements within it need to be in balance.
Obviously if you have all the symptoms of a phosphorous deficiency and you were unable to ascertain that it was due to a phosphorous intake / uptake issue - See step 1. ^^^^ Flush the system ...
Another suggestion in helping to maintain a healthy uptake of Phosphorous is Mycorrhizae. Mycorrhizae, like trichodema are a beneficial fungi useful in horticulture. They emerge from a dormancy when phosphorus levels are low. A primary function of Mycorrhizae in nature is facilitating the uptake of phosphorous and they are just as relevant in hydroponic scenarios. See Mychorizae
Hydroponic Nutrients Hydroponic Silicon
pH in Hydroponics Salt Build Up Algae Buildup