Which Fall Fertilizer Should I Use?

[Skipster]

Originally Posted by turfmd101

I consider K the most volatile. Unless it's sulfur coated.
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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.

[phasthound]

Originally Posted by Skipster

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.

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.

http://www.ianrpubs.unl.edu/epublic/...ld/ec155-3.pdf

[Smallaxe]

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

[phasthound]

Originally Posted by Smallaxe

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

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.

[Skipster]

Originally Posted by Smallaxe

That would be interesting to know... They've recently shifted the the idea that P doesn't leach,,, yet now they are highlighting the conditions in which there may be some leaching...

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
available.

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
available.

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.

[turfmd101]

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.
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[turfmd101]

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.
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[Kiril]

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.

[Skipster]

Originally Posted by Kiril

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.

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.

[Kiril]

Originally Posted by Skipster

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

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.

Originally Posted by Skipster

You are right to recognize that K is not as tightly held on cation exchange sites as P.

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

Originally Posted by Skipster

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.

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.