Testing for Phosphate in Aquaponics

This is part of a series on Aquaponic Water Testing.

Phosphates are one of the three main nutrients for plants. Phosphorus deficiency symptoms are not very distinct. The most common symptom is dwarfed/stunted plants. Tomato, lettuce, and brassicas can develop a distinct purpling of the stems and underside of leaves. Leaves may develop a blue/gray luster under severe deficiency conditions.  A brown netted veining can occur in older leaves under severe deficiency.

Test for Phosphate in Aquaponics

All tests measure only “Total Reactive Phosphates”, which is roughly equal to Orthophosphates.

Plants can only take up Orthophosphates
Total Phosphates can only be measured with a high temperature chemical digestion.
Total Phosphate – Orthophosphate = “Organic Phosphate Bank”

Tests can provide results in Phosphorus (P) or Phosphate (PO4).

P = PO4 / 3.07

Most of these test methods will suffer from Fe-EDDHA interference. The API kit, and Hach Method 8048TNT have proven free of Fe-EDDHA interference in my testing. Look forward to an upcoming post regarding Fe-EDDHA testing interference.

  1. API Phosphate Test. (0, 0.25, 0.5, 1, 2, 5, 10)
    $7/150 = $.05ea (@Amazon)
  2. Hanna Checker HI736 Phosphorus (0.0 – 0.1ppm P)
  3. Hanna Checker HI713 Phosphate 0.0 – 0.814ppm P
    $46.38 (@Amazon)
Hanna Checker HI717 Phosphate 0.0 – 9.7ppm P
  5. Hanna Checker HI706 Phosphorus 0.0 – 15ppm P
  6. LaMotte Smart3+ 3655-SC (0.0 – 70.0ppm; MDL 5)
    $22.10/50 = $0.44ea
    Lamotte Smart 3 + Lamotte_3655sc
  7. LaMotte Smart3+ 3653-SC (0.0 – 3.00ppm; MDL 0.05)
    $38.45/50 = $0.77ea
    Lamotte Smart 3 + Lamotte_3653sc
  8. Hach DR900 + 8048 (0.02-2.5; MDL .02)
    $31.29@Hach / 100 = $0.31ea
    HachDR900 + Hach8506
  9. Hach DR900 + 8048TNT (0.06 – 5.00)
    $44.25@Hach / 50 = $0.89ea
    HachDR900 + Hach8048TNT
  10. YSI Phosphate HR (0-100; MDL1; ±3)
35.67 / 50 = $0.71ea OR 79.17 / 250 = $0.32ea

Know another way of testing for phosphate in Aquaponics? Let me know in the comments and I’ll update this post.

4 thoughts on “Testing for Phosphate in Aquaponics”

  1. Hi, Scott. This series on testing has been invaluable! I can’t access some topics like “Why we test” and dissolved oxygen.
    I had to search for information about what levels we are aiming for, in order to know which range tester to purchase.

    As far as I know, we should be aiming for 10-20ppm Phosphorus for leafy vegetables, and 20-40ppm for fruiting and seeding plants.
    If that is correct, then the Hanna Checker HI717 Phosphate **0.0 – 30ppm P, according to their site, doesn’t quite cover that range up to 40ppm.
    I ordered a HI706 Phosphorus 0.0 – 15ppm P, and then do a PO4 / 3.07 calculation. I would appreciate your thoughts on the target levels.

    1. Hi Barnabas. If you’re using the HI706, then the results will already be in Phosphorus – no need to do any calculations. The results from the HI717 need to be divided by 3.07 to read in P. Neither meter will read up to 40ppm P directly. That’s where having a bottle of distilled water comes in handy. Diluting the sample by half will double the range the meter is capable of.

      1. Correction: to determine Phosphate level from my Phosphorous checker, multiply the result by 3.07

        So if my HI706 Phosphorus checker has a range up to 15ppm, then effectively it tests up to 46.05ppm Phospate (P04), right?

        Missing from your article is the target levels for Aquaponics, which affects the decision of which tester to purchase.

  2. Scott,

    I love the work you have done with this blog. I see so many AP enthusiasts swear by the API master test kit and proclaim all additional testing and supplementation as heresy. I prefer to test everything I can and correct issues before they show symptoms. The first symptom of Phosphate deficiency is low fruit/flower production. If someone waits until their leaves show symptoms then they have already missed out on the higher yields that come with optimal levels. This has given AP a bad reputation in some circles in my area. They say, AP has low yields on flowering plants. I detected a Phosphate deficiency without seeing discernable symptoms and began supplementing steamed bone meal and I have already noticed an increase in fruit/flower production after just a few weeks. I ordered a few other tests after reading some of your other posts and have added a photometer (Hach or YSI) to my “someday” wish list (can’t quite pull together $1300 at the moment). Thanks so much for the information – I am looking forward to reading more.


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