Phosphorus in Aquaponics
Phosphorus is one of the primary plant nutrients- the “P” in NPK ratings.
It is often overlooked in aquaponic systems simply because it is seldom truly deficient. However, many aquaponic practitioners are not getting the best possible yields out of their flowering and fruiting crops because phosphorus (while not deficient) is not present in high enough amounts during critical stages in production.
Phosphorus is very soluble, but also very sought after in the world of plants and algae.
Phosphorus and Algae Blooms
The cause of many algae blooms (see: eutrophication), is phosphorus from industrial agriculture that has washed from cropland into rivers and streams, and eventually the ocean. Along the way, algae, often inhibited by lack of phosphorus, bloom, consume oxygen, and turn once vibrant, ecologically diverse waters into green, dead sludge. As a result, a lot of scrutiny has been directed at phosphorus. Whether it was soaps, sewage or runoff, anything with excessive amounts of phosphorus in it was closely regulated.
“Peak Oil?” How About “Peak Phosphate?”
While there is a lot of phosphorus in circulation in nature, natural sources for agriculture are limited. In fact, while the world considers “peak oil” at length, most people are unaware that phosphorus in the form of rock phosphate (the primary source of phosphorus for industrial agriculture) is rapidly being consumed. We are at “peak phosphate,” a truly concerning point, as there are possible replacements for oil, but no great replacements for rock phosphate. But I digress…
Phosphorus and Plant Growth
Because phosphorus is very soluble, and rapidly consumed by algae, it is often present in systems as soluble phosphate PO4-3 ions as well as in organic forms. While these background levels are adequate for typical growth and development, when plants are flowering and trying to set fruit, more phosphorus is needed.
A lack of phosphorus is one reason why many aquaponic tomatoes and peppers bear only small to moderate amounts, even when supplied with plenty of nitrogen rich water.
In essence, Nitrogen is the most important element (quantity-wise) for vegetative growth (leaves, stems and roots) while Phosphorus and Potassium become much more important for setting flowers, seeds and fruits.
Phosphorus for Aquaponic Systems
So, for the aquaponic practitioner wishing to grow rich tomatoes, cucumbers and peppers (all big fans of phosphate), how do you supplement phosphorus? There are many possibilities- most of which are too risky, or labor or money intensive.
Rock phosphate comes in a granular or dust form and can be sprinkled along the top of your growbeds containing your fruiting crops once they start to flower. It will remain for some time, slowly releasing phosphate into solution.
When done correctly however, much of the phosphate will be immediately consumed by the plants in the bed, reducing the amount that bleeds out into your system. Even if background levels of phosphate rise, this isn’t typically a problem in systems where all of the crops are fruiting crops.
If however, you have vegetative crops (think basil, lettuce, kale, etc.) then excess phosphate (in part) can stimulate some of these crops to flower, which isn’t very desirable.
So, I recommend that fruiting and vegetative crops are kept separate and growing in different systems. This is usually better anyway as fruiting crops are well adapted and appropriate for bed production, whereas vegetative crops are best grow in towers.
Phosphate Additions and Fish Tanks
One thing to keep in mind, if you do choose to supplement phosphorus, is that your fish and sump tanks must be shaded, as additional phosphorus can cause an algae bloom in unshaded systems- especially at higher levels.
If you’re able to successfully manage phosphorus in an otherwise healthy aquaponic system you will be able to bump up your fruit and flower production significantly. If you have questions about what crops need phosphorus at different times in their development, check out one of the many great books on hydroponic nutrient solutions, or feel free to ask!
Phosphate Levels in Aquaponic Systems
For most aquaponic systems shoot for phosphate concentrations between 10 and 20 ppm for light feeders (vegetative crops) and between 20 and 40 ppm for heavy feeders (tomatoes, cucumbers, etc.).
Note: If you plan on running higher levels of phosphate in your system, prepare to supplement other nutrients at higher levels to balance your solution out. But more on that later. . .