Adding phosphorus

mauirose(11)March 9, 2008

What is the best amendment to use to supply phosphorus in new/existing planting area? Seems i recall that this nutrient moves slowly through the soil so i'm not sure if topdressing with bonemeal (under a protective layer of mulch) would be effective. Soil is very low in calcium as well.


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Woof! 5.4 pH.

I would get some superPhosphate as well as the bonemeal.

And lime for the pH.

Going forward, I would suggest using only bonemeal because it's easier on the microbes, but you need Phosphorus now to get this year's crop, so use the superPhosphate. And lots of compost.

    Bookmark   March 10, 2008 at 12:47AM
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Start with a good, reliable soil test because in most places phosphorus is in the soil but not very readily available and the very soluble forms of phosphorus, such as superphosphate, are a major cause of water pollution today. That soil pH may be a limiting factor in your soils phosphorus avaiability and even superphosphate is not the "stuff" to correct a low pH.
Because of this pollution problem from phosphorus Minnesota has very strict requirements on the sale of any fertilizer containing P and many others are working on similar restrictions. Several Michigan counties have placed similar severe restrictions because of that pollution problem because they cannot wait for the state legislature to function.

    Bookmark   March 10, 2008 at 6:56AM
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woof? is that like-holy cow? funny

i love compost, almost as much as mulch. Usually i plant adapted species, mulch, and don't worry too much about it but this space will be a little different so i did start with a soil test. Don't have the numbers with me but P scored 'low'. I think Ca was 'very low'. pH was merely 'low'. Depressing really since the only thing that was 'sufficient' was K but it did explain some things.

The report puzzled me a bit because county recommended gypsum AND lime (why both?) along with 10-30-10. Guess i was hoping for a more organic suggestion.

So it sounds like superphosphate moves through the soil pretty easily, especially if i work in some lime/dolomite? And i could add some bonemeal which might start to become available next year?

    Bookmark   March 11, 2008 at 12:33AM
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Where you are might help explain the use of gypsum because that does have value in sodic soils in aiding the removal of the "salts", the chemicals that accumulate in some soils due to lack of rainfall to wash them out of soils.

    Bookmark   March 11, 2008 at 8:09AM
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The use of gypsum and lime may be due to the low Ca result. Gypsum will give you Ca without raising the pH while the lime gives you Ca and raises the pH. Gypsum also has some sulfur for what that's worth.

Where are you located anyway? All I see is zone 11.

Hum, how quickly do you need this phosphorus to be available? As in, when are you planting or are the plants already in and suffering? I personally would be careful with the super or triple phosphates. I know lots of the normal soil amendments and manures are often pretty good sources of phosphate as well as nitrogen. My issues are the soil has plenty of phosphate and I need potassium so now I'm racking my brains for the stuff that I was avoiding. Bone meal is of course a good choice. Granted it is slowly available but that doesn't necessarily mean that putting it down won't help at all now and then suddenly start working in the fall. It is just not an instant shot in the arm for plants right now.

Here is a link that might be useful: My garden

    Bookmark   March 11, 2008 at 8:33AM
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The reason why both gypsum and lime were recommended was likely because calcium was interpreted as being 'low' for plant nutritional purposes and your pH is low.
A pH of 5.4 is not incredibly low and it won't take huge amounts of lime to get it up into the preferred slightly acidic range. Yes, some calcium is provided by lime but since they are also recommending gypsum, I would guess that they ran the numbers and found that the calcium provided by the amount of lime required to adjust the pH would not be sufficient to provide enough calcium for nutritional purposes. Hence the lime topped off with some gypsum.
"Why not just use all gypsum then?", you may ask. Because gypsum is added to provide calcium. Calcium will not change the pH.
On the topic of salts, since it was brought up.... Having a saline soil does not necessarily mean you have a sodic soil. On the flip side, having a sodic soil does not necessarily mean you have a saline soil.
Calcium does help with the reclaimation of sodic soils. It specifically replaces sodium, not all salts. Gypsum is a salt. If your soil salinity is elevated due to high calcium, magnesium, potassium, sulfate and chloride (or any combination of those) adding gypsum won't remove the salts. Gypsum applied to a saline, non-sodic soil with do precisely nothing to help the situation.

    Bookmark   March 11, 2008 at 8:36AM
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Wow-thanks for the clear answers. i knew gypsum wouldn't change pH but didn't recall that it was so high in calcium. Makes sense now.

The area i'm preparing will be planted in a few weeks and this is the area i'm interested in amending. As far as existing plantings i'll probably stick with mulching and maybe a little compost for the laggards.

I'm writing from windward HI where if you run low on Na+ you can just scrape some from the window :) Doesn't stick around in the soil much though.

I passed by the dump yesterday and noticed they were grinding sheetrock up in the mulcher. Asked the guy about it and he said they were grinding it up for-you guessed it-GYPSUM! So-can i use sheetrock scraps for gypsum?-and what is the best way to apply it?

    Bookmark   March 12, 2008 at 3:54AM
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Sheetrock, Drywall, plaster board does use gypsum as a base, just like any other plaster. However, just like any other plaster, the plaster board has additives that you do not want in your soil, binders, adhesives, maybe fire retardents. It is not a good idea to use this material in the garden, and may be prohibited by building codes in your area, too.

    Bookmark   March 13, 2008 at 9:35AM
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colloidal Soft Phospate is a great way to add phosporous safely to the soil. It is safe and mild enought to actually placed in direct contact with tender roots with no risk of burning. I use it in planting holes when setting out transplants.

    Bookmark   March 13, 2008 at 1:17PM
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Superphosphate 0/20/0 and triple superphosphate 0/46/0 are safe and ready use forms of phosphurus which can be used to advance rooting and budding for most any plant material.

Have a slow flowering tree, give it superphosphate for each of the early three months when the soil has warmed up.
Repeat the application in the fall for the last 3 months that the soil accepts water given.
Planting roses, be sure use superphosphate in the planting hole and mixed well with the soil there.
Planting bulbs, superphosphate will ensure good rooting which ensures livability through winter's worst.

Phosphurus used in fertilizers by the average gardener poses no danger whatsoever to ground water and scare tactics by certain poster who says it does is way out of his depth since he is forever against anybody using any fertilizer. His concept of feeding a plant is to do nothing or maybe use some compost which can amount to very little of elemental value and is a question mark even then.

    Bookmark   March 13, 2008 at 1:34PM
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Some people seem to never allow science to get in the way of long held beliefs, but pollution of water from Phosphorus as well as Nitrogen is a well documented problem. The latest test results from the EPA Groundwater people not only shows nitrates and phosphates in increasing levels in our drinking water but prescription drugs and antibacterials from soap.

Here is a link that might be useful: Nutrient pollution of water

    Bookmark   March 14, 2008 at 8:32AM
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fertilizersalesman(z6 PA)

Ok, I am going to have to make a few comments here; regardless of how soluble the form of phosphorus it is going to rapidly react as soon as it hits to soil and turn into a much less soluble form. Changing pH does not significantly affect phosphorus availability; at different pH levels it simply reacts to form different minerals. Phosphorus is often banded to delay this reaction with soil, but it is never going to readily leach into the soil. If you want phosphorus in the root zone you are going to have to put it there. Water pollution from phosphorus is almost exclusively associated with erosion. When soil is washed into surface waters the bound phosphorus goes with it.

Calcium is a base and most certainly raises pH. The reason gypsum (calcium sulfate) does not raise pH is because the associated sulfur reacts with water to form sulfuric acid which essentially balances the base (calcium) resulting in an insignificant change in soil pH.

    Bookmark   March 14, 2008 at 12:38PM
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annebert(6b/7a MD)

Changing pH _does_ affect availability of P. At pH below 5.5 it forms insoluble compounds with Al and Fe and is poorly available. I learned this at my father's knee (really!).

What I can't remember and can't find out, even at useful site such as the link below, is what changes in chemistry occur as you raise the pH near neutral that make it more available.

Depending on how your soil test was performed, it may or may not be an accurate reflection of the P that would be available if you raised the pH.

Here is a link that might be useful: the nature of phosphorus in soils

    Bookmark   March 14, 2008 at 1:16PM
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fertilizersalesman(z6 PA)

It is true that there is maximum H3PO4 in solution at a pH of 0, maximum H2PO4 at pH 5, maximum HPO4 at pH 10, and maximum PO4 at pH 14, however when you look at the total P from all species in solution it is roughly the same at all pH levels.

I would agree that P availability is not terribly accurate when soil testing, but it is still better than a wild guess.

    Bookmark   March 14, 2008 at 1:51PM
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Always good to hear from you. I recognize that you are very knowledge and I respect your input a lot. However, I do have to disagree with the statement that calcium will increase a soil's pH and that sulfate will decrease the pH to balance out.
This could end up being way too long of a post so I will present a couple of things here real quick and we can discuss foggy points in more detail if you like.
First of all, although calcium is a "base" cation. It is not really a base. In fact, calcium is actually a Lewis acid. That is, an electron acceptor from the metal section of the periodic table. They call it a "base" cation because higher amounts of Ca on the exchange correlates with a decreased chance that the soil is acidic. Low calcium means that the exchange is most likely taken up more by Al and H. That of course would be an acidic soil.
To say it quick and dirty: Less calcium means it's probably an acidic soil. More calcium means it's probably a basic soil. Ca indicates base but does not form a base.
Of course, you see Ca product raising the pH as in CaNO3 in nurseries, but it is the NO3 sequestering H+ in that case.
In the case of CaCO3, it is CO3 sequestering H+. And to be fair I cut that post short but there are a lot of other reactions raising pH. For example, when HCO3 is formed, the other biproducts of the reaction are Ca+ (which doesn't increase pH) and OH- (which does increase pH). There are also reactions between CaCO3 and H+ (or hydronium to be more correct) to form water (neutral) and CO2 (which gasses off)which takes 2H out of the picture. There's more, but I won't go into it.
The plain and simple fact is that other factors are at work and Ca+ does not increase pH because it is not truly basic.

    Bookmark   March 14, 2008 at 3:28PM
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I love chemistry-(really love bio-chemistry)-just wish i could remember more of it :) Thanks for the refresher link Annebert.

Seems like the short answer for me is bump up the pH to improve availability and then put the P in the root zone.

Kimmsr-seems like i need to do more reseach on the drywall-sure would be nice to keep it out of our island land fill.

Thanks everyone for your answers.

    Bookmark   March 15, 2008 at 8:07PM
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what is the safest and none chemical way to raise p h just green sand and rock phosphate???

    Bookmark   June 14, 2008 at 3:05AM
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Hilorain, you should probably start a new thread for your question.

In the meantime, neither greensand nor rock phosphate is going to raise your pH.

What is your soil's current pH?

    Bookmark   June 14, 2008 at 3:27AM
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shebear(z8 NCentralTex)

So back to the phosphate thing. How does super phosphate effect the soil bacteria and microbes?

Do root crops stay in the soil so short a time that no relationship occurs between plant and soil fauna?

    Bookmark   June 14, 2008 at 1:32PM
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