Rapitest for Hydroponic NPK Analysis

Discussion in 'Hydroponic Growing' started by trojangrower, Mar 11, 2021.

  1. This report is an evaluation of the Rapitest Digital NPK testing gear, and whether or not it works for determining NPK values in hydroponics. I've recently been wondering about the sudden drops in my daily EC/TDS values, where I've seen up to 200 ppm drops on some days, normally after a res change, and it makes me wonder, what the plant absorbed.
    I am currently using Holland's Secret three part formula at a 3::2::1 ration (Grow:Micro:Bloom) as my fertilizer base along with Calnesium (2-0-0). I will consider this line to be equivalent to GH's Flora three part solution. I just changed the res and held off adding any beneficial organisms for the purpose of this test.

    I used 30L RO water + 7.5mL silica (boost) + 11.5 mL calnesium solution + 90 grow + 60micro + 30bloom which resulted in an EC of 2.1 mS. This is according to the manufacturer's instruction (kinda strong imo).

    Freshly calibrated Apera pH and EC meters were used throughout this work.

    I decided on a quick fix Rapitest Kit with a digital monitor that reports levels in a semi-quantitative manner, but I will point out that many more test kits exist, and maybe more accurate and more feasible (where you get more bang for your buck).
    upload_2021-3-10_21-36-9.png
    This particular kit only comes with enough reagents to conduct 5 tests for each N, P, and K. There is also ten pH test reagents, but I don't really care about those, although I did test the pH with the kit, to compare with the Apera meter.
    upload_2021-3-10_21-40-5.png
    I followed Rapitest kit instructions and mixed up the pH, N, P, and K tests (as shown respectively). The kit did not elaborate whether or not I should use the shaken or settled solutions. The above picture shows the samples following the initial reaction (shaken).

    upload_2021-3-10_21-37-41.png
    The pH test is a disappointment. The Rapitest kit reported a pH of 4.5, whereas the Apera stabilized at 5.32. I would have accepted a 5.0 or a 5.5 from the Rapitest, but at 4.5, with a difference of 0.8 units, its just a fail in my books, ie. the Rapitest pH test is unreliable.

    Next I tested for N(itrogen), and after the allotted 10 minutes elapsed, I placed the sample into the device, which reported surplus. Go figure. I removed 0.5 mL of the sample and diluted in 4.5 mL of RO water (a 10x serial dilution). I checked the results which indicated a sufficient score.

    I repeated the process for P(hosphorous), which also indicated a surplus from undiluted sample, but scored sufficient after a 10x serial dilution.

    Lastly, I attempted to analyze the K(potassium) values using the Rapitest, but it wasn't clear if I needed to shake it after ten minutes or just test it as it was. An undiluted and settle K test reported a score of deficient. So I shook the sample and tested again, which then reported surplus. I conducted a 10x serial dilution and tested the shaken sample which then reported sufficient.

    This is an ongoing test, and the results will be reported in the following days. Will the EC climb tomorrow, will it drop? And more importantly, did I just waste $50?

    upload_2021-3-10_21-50-51.png
    10x serial diluted samples for Nitrogen (red), Phosphorous (blue), and Potassium (yellow) used during this analysis.
     
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  2. Oh yea, these the plants that are being tested, or more accurately, the plants that are fed the hydroponic solution that is being tested.
    upload_2021-3-10_21-53-43.jpeg
    Fem'd Blueberry and White Lightsaber (obtained from ACSB.CO)
     
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  3. Nice, I'll be following along.
     
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  4. This happened to me last night. Res change set at 800ppm ( I lowered it from 860 because I was starting to concentrate 950ppm at the end of 10 days of add backs) less than 12 hours later it was at 640ppm plant was definitely eating as it put on some noticeable growth in the same time period. It will be great if this test can give a direction to go in. As my reaction to this currently is to add a stronger solution of the same ratios as my original. Im sure that's not ideal
     
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  5. #5 trojangrower, Apr 12, 2021
    Last edited: Apr 13, 2021
    I'm now into the second week of flowering. I have gone through the 3:2:1 veg and 2:2:2 transition ratios and have moved onto the 1:2:3 ratio. This is actually 3:6:9 (mL) G:M:B per gallon (~4L). I have fed these nutes for almost a week, topping the solution up with regular feeding. The EC was lower using 1:2:3 than the other ratios (by ~500 uS) but has started to climb in the last couple days, enough so that I just added RO (with calmag) to dilute back to ~1600.
    I prepped a 4L solution (2mL calmag + 1mL K-silica + 6mL Micro + 3mL Grow + 9mL Bloom) and tested this using the Rapitest kit. The solution was measured with the Apera pH pen at 5.60 and EC 1528 uS. The results are as follows:
    • Rapitest pH: 4.5
    • Nitrogen (undiluted): surplus (highest score)
    • Nitrogen 1/10 dilution: depleted (lowest score)
    • Phosphorous (undiluted): lost sample, but would have been surplus.
    • Phosphorous 1/10 dilution: adequate
    • Potassium (undiluted and settled): deficient
    • Potassium (undiluted and shaken): surplus
    • Potassium 1/10 dilution (shaken): depleted
    I then tested the (used) reservoir solution. The apera instruments indicated a pH of 5.39 and an EC of 1763 uS
    • Rapitest pH: 6.0
    • Nitrogen (undiluted): adequate
    • Nitrogen 1/10 dilution: not tested (as it would have shown depleted)
    • Phosphorous (undiluted): surplus
    • Phosphorous 1/10 dilution: deficient
    • Potassium (undiluted and settled): depleted
    • Potassium (undiluted and shaken): surplus
    • Potassium 1/10 dilution (shaken): depleted

    What do these results tell us.
    • Rapitest method of determining pH is flaky at best. I don't think I would use this kit as a reliable source for pH. I think a $10 ph pen would probably be more accurate.
    • Nitrogen testing seemed ok, at first, regular 1x solution show surplus, but surprisingly, the reservoir solution show adequate, suggesting that nitrogen is actively being taken up by the plants, and would not be contributing to a rising EC. If anything, more nitrogen is required during these two weeks of blooming.
    • Phosphorous testing was also straight forward, but the amounts taken up by the plant don't seem to match the manufacturer's recommendations. Compared to nitrogen uptake, phosphorous has hardly been taken up. These are surprising results considering I have been following the manufacturers recommended feeding, and maybe even less so considering I am only following the basic regimen instead of the professional regimen, which would have called for more KP solution.
    • Potassium (Potash) depending how you read this, the instructions say just let it settle, so shaken seems unwise, but potassium levels in settled samples are already deficient in 1x (brand new) solution, so no surprise that it is depleted in the reservoir's solution. An increase of potassium might benefit the plant.
    If this assay is correct, results suggest that phosphorous is accumulating in the solution because the plants don't want to absorb it. As well, the res has been inoculated with a plethora of beneficial bacteria, so P assimilation should increase. I suppose the plant just doesn't need it. And it makes sense that the plant isn't absorbing all this phosphorous because the reservoir's pH is dropping (5.39 today). Thus backing off on phosphorous might be a better tactic during these feeds.

    Given this information. the 4L solution was added to the reservoir and 6mL Micro was also added. The EC was then backed to 1600 uS using RO water. The reservoir will be changed soon, so I'm not too worried.

    I believe a 6:6:6 solution would probably be best for the next two weeks. I will test this again later on, maybe a couple weeks or so.
     
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  6. Trojan on the front lines!!!! I hope this kit pans out cause it could become the hydro growers secret weapon to hit the npk’s at the right exact time each phase
     
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  7. yo bro, yea, it already has said that P levels too high. If I re-did it though I would invest in a better kit.
     
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  8. Well it’s a start and idk about it telling me it’s high, I’d rather have watch one tell me where it’s at, and then check a day or so latter and see what fell and how hard is the best case imo
     
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  9. I did that, not in triplicate or anything, but yea. I think I should color code the results in a table for clarity.

    The test I conducted showed a very sharp drop in nitrogen between the freshly prepared and used solutions, but not so much in phosphate levels, but since the pH portion was horribly wrong lacking precision and accuracy, how much can I rely on this product? Ideally, one should conduct this trial in a series of identical experiments, but I'm not spending my little money being the consumer report.

    I was eyeballing the LaMotte soil NPK test kit, (50 tests for ~$90) for strictly NPK, but it costs more upfront. I checked their site, and there's a 4-way test specifically for hydroponics, it just costs too much (~$400). It looks more reliable though.
     
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  10. This entire experiment was started after reading "Impact of Phosphorus on Cannabis sativa Reproduction, Cannabinoids, and Terpenes (2020)
    upload_2021-4-13_10-37-59.png
    I pulled that from this article.
     
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  11. The res has been jumping higher over the last couple days, although the pH hasn't strayed much, just inching upwards. So I've been adding a bit of new solution each day, about 4L of {1:1:1} ratio, mostly depending on the EC at 6:6:6 (mL) per 4L of RO water. Also add 2mL of calnesium to that. But the last few days the EC has been climbing, so I've adjusted the solution to 5:5:5 for the last couple days. *note, the ratio I am using is no longer in accordance to the manufacturer's directions.
    Its also been nine days since the last res change, so I decided to change the reservoir, but first, I conducted another Rapitest and will again share the results.
    DSC00546.JPG
    DSC00548.JPG
    These are the plants that are fed from the solution. I think they are in 3rd or 4th week now, can't remember and too lazy to go check atm. They are very healthy, although I am tempted to add some side lighting.

    I did not conduct a 10-fold dilution this time, nor did I perform a positive control (ie. newly made solution at time=0). I only tested the (end) solution at the end of its use (following daily top-ups).

    The Apera pH meter was calibrated (three point calibration) prior to use. It recorded the (end) solution pH as 5.55.

    Each sample was vigorously shaken (and flicked to suspend the powders) and allowed to settle prior to analysis.

    Rapitest results:

    • pH test (green) = 5.5
    • Nitrogen (purple) = sufficient (2)
    • Potassium (orange) = adequate (3)
    • Phosphorus (blue) = surplus (1)
    The results are numbered from highest rating (1) to lowest (5).
    Capture.JPG
    I've reposted the figure for reference to the scores I listed.

    Seems to me the {1:1:1} diet is still working fine. I say this because all the values are in range, but this is at 1x dilution, not 10x dilution, (like I had to conduct for the solution at time=0). Nevertheless, even though the solution is now low (compared to T=0), the values are still somewhat balanced... except for phosphorous, which is still high (maybe this one should have been diluted and read). And if there is an excess of phosphorous, it suggest no more is needed. *If I follow the manufacturer's recommended dosages, there would be more phosphorous added, and less N and K (since the recipe calls for 1G:2M:3B instead of 1:1:1).

    Also, prior to today's experiment, I conducted the Rapitest pH test alongside the other tests, but I did not follow the proper instructions and measured the reaction after 10 minutes, as you should for N, P, K, but this is not the way it should have been done. The (re-read) instructions state a two minute wait time for the pH test, which I followed this time around and got a result that reflected the Apera pH meter results. This difference in time (10 min vs 2 min) might be why the earlier results were off.

    None of these results definitely explains why there is a rise in my EC in the last few days, unless the phosphorous is getting too high, but the solution was not becoming acidic, although the pH rise did slow down near the end, so maybe we can blame the phosphorous (again) being too high.

    I have enough to conduct one more test and I will try to do this for next rez change, which should coincide with the mid-point of the flowering cycle.
     
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  12. Its been a week since I changed the res out, and kinda wanted to this time since the EC has been starting to climb again. Maybe I should just back it off... but before I do that, I decided to try the last Rapitest in the kit.
    The pH meter was recalibrated and the solution pH was 5.62, which was 0.02 different from the uncalibrated pH pen. Anyways, here are the results:

    • Rapitest pH 6.0
    • Nitrogen Surplus (1)
    • Potassium Sufficient (2)
    • Phosphorous Surplus (1)
    Based on these results, I think I would be ok just backing off the solution with just water, but did a res change and changed my ratios to {8:4:6} (G:M:B) per gallon. I did this so to increase potassium levels, while dropping nitrogen slightly and maintaining the (high amounts of) phosphorous. I don't mind losing some nitrogen since the plants are now in later flower.

    Review of the test kit.
    Well, its difficult opening the capsules and placing the powder into the tubes. The tubes are a drag to clean. And I don't really know if the results are accurate enough. The test solutions were flocculent and some tubes, well I just shake them again half way thru the wait times. The pH part also worries me, since the pH was 5.60, and the Rapitest pH was 6.0, so... I would have been more convince if it had read 5.50 instead. So its 'ok' for a test, which I'm using like I'm not suppose to... I guess. Finally, I feel that investing in a better kit would be the better way of going about this kind of testing.

    Final thoughts.
    The phosphorous, I think I just been using too much. My plants look good enough without needing to pour in more phosphorous when the testing indicates its in excess. Note* that the amounts of nutrients I have been using (ie: {6:6:6} per gallon) is much lower than recommended amounts (which wants like almost 20 instead of 6 for phosphorous, or something stupid high like that). So I will moderate the phosphorous from now on, or maybe conduct a side by side test later in the future.

    Plants at time of testing.

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    A scrogged Blueberry is one of the plants drinking from the reservoir.
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    Closeup of Blueberry highlighting trichome development.
    DSC00564.JPG White lightsaber is the other plant sharing the reservoir.
     
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  13. Nice work, appreciate the contribution.
     
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  14. Those plants are beautiful, top part of the top shelf.
     
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  15. thank you bro
     

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