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Thread: Discussion: Magnesium phosphate precipitate from dry fert mixing.

  1. #1
    Join Date
    Apr 2017
    Location
    Victoria Park, Western Australia
    Posts
    29

    Default Discussion: Magnesium phosphate precipitate from dry fert mixing.

    Hi guys,

    I've been using Dennis Wong's (https://www.advancedplantedtank.com/...rtilizers.html) DIY recipe for a couple of months now and it's working pretty well for me. The macro recipe calls for, among other things, Potassium Nitrate KNO3 and MonoPotassium Phosphate (aka MKP or KH2PO4) to be mixed with Magnesium Sulphate (MgSO4) aka Epsom Salts.

    According to this agricultural fertigation guide by incitec (https://www.incitecpivotfertilisers....0Agritopic.pdf), page 12, MgSO4 should NEVER be mixed with KH2PO4, and it also precipitates in the presence of KNO3 in high concentrations (above 10%).

    For the lazy, I will quote excerpts from the document:

    "Examples of products that should not be sprayed together, as insoluble precipitate may form and settle to the bottom of the tank, are:-
    Magnesium sulfate - Do not mix with MAP, MKP, calcium salts or boron fertilisers.

    A precipitate may form when potassium nitrate (solubility in water of 32 kg/100 L) is mixed with either ammonium sulfate (75 kg/100 L) or Epsom Salts (71 kg/100 L). At potassium nitrate concentrations much above 10% (10 kg/100 L), potassium sulfate will be precipitated. This is about one-third of the concentration of a saturated solution of potassium nitrate."


    I've been using this recipe for around 3 months and it's been doing wonders. I did notice a little precipitate/undissolved crystals when I first mixed the solution but it disappeared 3 days after mixing.

    Any ideas on why the supposedly insoluble precipitates disappeared?

    Cheers
    Khee

  2. #2
    Join Date
    Jun 2013
    Location
    ACT
    Posts
    13,778

    Default

    Quote Originally Posted by Khee View Post

    Any ideas on why the supposedly insoluble precipitates disappeared?

    Cheers
    Khee
    Probably because the concentrations of the compounds have remained below threshold for precipitation. Precipitation can be affected by impurities, formation of complexes and chelation.

    It's pretty difficult to predict definite rules AFAIK but 'precipitation' of K2SO4 would be fine because in the tank, it will become soluble again!

    Beyond that, dunno!

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