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Phosphorus
PHOSPHOROUS:
Phosphorous levels are represented as ??% P? on fertilizer labels. Note that fertilizer labeling laws are very old, and not based on current knowledge of plant elements. The ??P? on the fertilizer label is often represented as ??P2O5?, which is phosphoric anhydride; a form plants don??t even use. So if a fertilizer label reads:
4% P (as P2O5)
What you actually get is:
1.75% P
To further confuse matters, phosphorous is most commonly absorbed by plants in the phosphate form, which is PO4. So if there was 1.75% actual P in the fertilizer, there would be 5.35% potential PO4 (phosphate).
You need not overly concern yourself with this for practical purposes, just recognize that the ??P? listed on the label is generally not telling you exactly how much ??useable? phosphorous is in the solution.
Phosphorous levels are required in relatively lower amounts for vegetative growth, and in higher concentrations for flowering/fruiting. The ratio of phosphorous to other elements may also increase slightly during the rooting phase of cuttings.
During early growth, levels of phosphorous at 15 to 30 ppm are adequate for most crops. Over applications of phosphorous will trigger imbalances of iron (Fe) and calcium (Ca), and possibly zinc (Zn). Iron and zinc are very closely associated with keeping the plant ??green?. When increasing phosphorous levels, calcium, iron, and zinc levels should be increased proportionately. Calcium should be maintained in a 1.5 : 1 ratio with phosphorous. Commercial hydroponic liquid calcium supplement products often contain levels of iron and other trace elements, making them ideal to use when increasing phosphorous levels for blooms.
Phosphorous levels may be increased to levels upwards of 250 ppm during the peak phase of bloom for heavy feeding crops, provided that it is done while maintaining important ratios such as calcium and micro-nutrients. Note that a strong ??spike? of phosphorous levels in hydroponic crops during the first week of flowering may induce a slight stress/trigger; in some strains this helps control the height of the crop and flowering structure. This is an aggressive practice and is not recommended for the novice grower. You have to really know your plants before you consider subjecting them to short periods of stress for the long-term benefit of the crop. For example, increasing phosphorous levels to 200+ ppm during the first week of flowering and then gradually increasing phosphorous levels over the course of the crop after dropping to 100 ppm in the second week.
Taken from Maximum yeild By Erik Biksa
