Phosphate from tap...usable?

[Dmaaaaax]

I tested my tap and found I have at least 2ppm phosphate according to my test kit. Is this phosphate usable by my plants? I thought I read somewhere that KH2PO4 was easier for the plants to use than the form typically found in your tap, but I can't seem to find that article anywhere nor find any confirmation on that subject.

I am hoping it is one more chemical I can stop dosing but, I wanted to be sure first.

 

[VaughnH]

If you were to add KH2PO4 to your tank to get 2 ppm of phosphate, would you then stop dosing KH2PO4, figuring you had enough now? When you fill the tank with tap water containing 2 ppm of phosphates that is no better than having done one 2 ppm dosing of KH2PO4. We want phosphates in the water so plants can consume it in order to grow. Once they consume it, it is no longer there, so we have to dose it again. At best, tap water lets us skip a dose or two, but the plants will soon use up what is in the tap water, and we have to start dosing again.

If you are using a low enough light intensity that you will only dose fertilizers once a week, you might be able to use the phosphates in the tap water, which are replenished with each 50% water change, and not dose phosphates. But, why take the chance that the tap water condition will be changing from week to week?

 

[abcemorse]

Keep in mind also that if you do say a 50% WC, and the PO4 from the last WC has been consumed, you're only dosing the equivalent of 1ppm.

 

[Dmaaaaax]

I know a lot of people who keep an eye on their nitrate and phosphates the first couple of week then set their weekly dose based on that. For instance I have found that for nitrate, I can start with 1/2tsp on day 1 then do 1/4tsp on day 3 and day 5. But for phosphate, it seems to hold at 2ppm or more the whole time. When I do a 50% water change, I am just adding more phosphate. Others I have seen start with 1/8tsp then just and another 1/8 at day 5 because plants don't seem too eat through it like they do nitrate.

If on top of this I dose phospate blindly I would be adding another ~1-2ppm every other day.

 

[Tom Barr]

As long as you do large water changes, you should be okay without dosing it, adding more will not hurt though either, and sometimes the tap drops the PO4 if the KH is good.

So you might run out when they do that and you do not test to make sure each water change adds enough.

So add a bit and account some for the tap.

Regards,
Tom Barr