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Which Fall Fertilizer Should I Use?

Discussion in 'Fertilizer Application' started by joed, Sep 7, 2012.

  1. Skipster

    Skipster LawnSite Bronze Member
    Messages: 1,085

    I'm not sure where this thread has gone, but a few points may need to be considered.

    1) P has always been regarded as relatively immobile in the soil. Leaching has never been a worry for water contamination with P.

    2) The environmental worry about P applications are off-target apps that put P fertilizers directly in surface water (like getting fert on driveways, sidewalks, roads, and misapplications). Because P is tightly bound to the soil, the only way it can get into water after a proper application is if the soil ends up in the water.

    3) K isn't volatile. Urea is volatile. Ester formulations of herbicides are volatile. Gasoline is volatile. K is not. "Volatile" simply means that a compound's vapor pressure is low and it evaporates quickly.
  2. phasthound

    phasthound LawnSite Fanatic
    Messages: 5,138

    I agree with Skipster on this. And I will add that thick healthy turf helps to prevent erosion which is a major source of P affecting our water quality.
    Here is a link on K that may be of some help.

  3. Smallaxe

    Smallaxe LawnSite Fanatic
    Messages: 10,082

    The question was whether K leaches from the soil... volatility is not the question...
  4. phasthound

    phasthound LawnSite Fanatic
    Messages: 5,138

    From the link I just posted:
    Losses of potassium from soils are caused primarily by crop removal, fixation by clay minerals and leaching. ...leaching may be a minor factor in very sandy soils.
    Yes, it's an agricultural article.
  5. Skipster

    Skipster LawnSite Bronze Member
    Messages: 1,085

    This was your quote the other day. As far as I know, P has never been thought to leach at all, so I don't think that anyone is highlighting conditions under which P may leach.

    You are right to recognize that K is not as tightly held on cation exchange sites as P. But, soil K is a different animal.

    K usually hangs out in four places in soils.:

    1) Unexchangeable K is held in K-bearing minerals, like micas and
    feldspars (90-98% of soil K). This is NOT leachable and NOT plant

    2) Unexchangeable K can be held inisde layers of 2:1 clays (1-10% of soil
    K). This is NOT leachable and NOT plant available.

    3) Exchangeable K can be adsorbed to soil particles on CE sites and in soil
    solution (1-2% of soil K). This is somewhat leachable and is totally plant

    4) Exchangeable K can be bound in OM (<1% of soil K).

    A study by Jerry Sartain on sandy soils at University of FL in 1998 showed K source to be an important factor in leaching loss from turf. K2SO4 required 50 inches of water to move any K below the rootzone, while K3PO4 required 100 inches of water to move any K below the rootzone. Maximum leaching depth was 10 inches.

    So, K can be leached, but it doesn't move far.
  6. turfmd101

    turfmd101 LawnSite Bronze Member
    Messages: 1,375

    Sorry. I used volatile meaning easily depleted. The last 13 years I read about 120 customer soil samples per year. They were from all over central Fl. These were 90% serviced lawns & 10% customers who were just starting applications. Round about average yearly. I promise less than 5 ever showed sufficient K. These customers admitted a recently applied application. Everyone else's showed almost no K available... even after 16 days testing after app of K. Just my experience. Alot of these customers used 9-2-24 religioy.

    All samples were done at CLC LABS in Ohio.
    Posted via Mobile Device
    Last edited: Sep 13, 2012
  7. turfmd101

    turfmd101 LawnSite Bronze Member
    Messages: 1,375

    To add. I'm not sure If any of these customers took samples deeper than 4" or what the root systems depths were at time of sample.
    Posted via Mobile Device
  8. Kiril

    Kiril LawnSite Fanatic
    Messages: 18,334

    Solution P is an anion. It is incorrect to speak of P being bound by the CEC.

    Further, P can and does leach. A review of available published literature on P mobility in soils will reveal this.
  9. Skipster

    Skipster LawnSite Bronze Member
    Messages: 1,085

    Kiril, you are kind of correct, but you're forgetting your soil knowledge here.

    1) In acidic soils, mineral surfaces have a net positive charge (both + and - exist, but the +s outnumber the -s), so some phosphate ions are adsorbed electrochemically.

    2) Phosphate ions don't exist as free H2PO4- and HPO4-- very long. They react very quickly and readily with Fe, Al, Mn, and Ca ions on CE sites and in solution. In acidic soils, Fe and Al are the main bonds. B/c the Al-O-P and Fe-O-P bonds are very strong, desorption is very difficult.

    3) In alkaline soils, Ca and Mn bond more readily than Fe or Al and sequester phosphates on cations attached to CE sites.

    So, while phosphates don't directly adsorb to CE sites, they bond with cations that are directly adsorb to CE sites and they adsorb to AE sites.
  10. Kiril

    Kiril LawnSite Fanatic
    Messages: 18,334

    I forgot nothing skip and there is no "kind of" about it You are the one who was talking about P being bound by the CEC ... or have you already forgotten.

    CEC = cation exchange complex ... not anion exchange complex.

    Errr Skip, when P become fixed to select cations it precipitates out of solution as a compound with variable solubility ... some of which may become part of the active pool, some part of the fixed pool. This has nothing to do with the CEC.

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