of the 3 main nutrient type NPK, the P is the least needed macro nutrient for any type of vegetable even for tube vege such as Potato , and it is the most common nutrient to build up in the soil to a degree that often become serious headache for most gardener. Please try to avoid high P fertilizer at all cost. No vege needed it enough to really call for fertilizer 18-46-0.

Among the things that affect soil structure is organic matter and a synthetic fertilizer does not add any organic matter which will over time cause soil compaction due to loss of soil organic matter.

Why does a high level of phosphorous become a headache for gardeners?
is it toxic if your soil levels indicate: "very high"?

If you know you need phosphorus, kokos, why not try it and see what the plants have to say about that? Plants love phosphorus: the run-off issue with phosphorus is not that it kills anything, but rather that it grows water plants very strongly in bays and such and changes the ecosystem.

A lot of people fear change, despite the fact that change is the only constant.

I use inorganic AND organic fertilizers plentifully and I have never had a "headache," but rather fertile soil that grows big veggies and flowers, which I'm quite sure is the whole point.

There's theory and numbers from the Internet and radical environmentalism ---

And then there's gardening or farming. These are quite different points of view, and in my opinion gardening or farming works best by trying things out for one's specific situation and maximizing crops.

Excess levels of any nutrient in soils will inhibit a plants ability to uptake other, necessary, nutrients and often that can mimic nutrient deficiencies. Excess Phosphorus in the soil can inhibit the uptake of Zinc, Iron, and Cobalt and while these micronutrients may only be required in very small amounts they are essential to plant health and lack of them can increase a plants attractiveness to insect pests and plant diseases.

"the run-off issue with phosphorus is not that it kills anything..."

In 2002, a Dane County teen died from ingesting these algae-produced toxins while swimming in an area lake.

But it's just theory and numbers from the internet.

Lloyd

If it is a run-off issue. I'm no where near lakes or ponds. There is 0 danger off phosphorous harming anything where I am. It is at higher elevation anyway.
So to sum it up 18-46-0 or 11-52-0 not much difference?

I've seen phosphorous run off at a chicken farm where the manure was piled and the run-off was green in color.

"There is 0 danger off phosphorous harming anything where I am. It is at higher elevation anyway. "

There is always a potential for pollution if you are adding nutrients. It is not just "run-off", you have to consider that water moves within the soil (and flows downhill).

I don't care what you do, it's your choice, but at least try and make an informed choice. Even basic searching reveals lots of info and a forum such as this is a poor place to gain knowledge. Don't trust anyone, including me!

As you said you were in Toronto...

Eutrophication and toxic contamination have been major pollution problems in Lake Ontario

Lloyd

"Among the things that affect soil structure is organic matter and a synthetic fertilizer does not add any organic matter which will over time cause soil compaction due to loss of soil organic matter."

Fertilizer causes you to lose organic matter? lol. Ok buddy. You add organic matter when you till your crop over every year.

kokos: I'm not familiar with either of those products, and it's hard to imagine how a sprinkling of one phosphate or another is going to severely affect soil texture. I would ask whoever told you that why they think so. I will try to do a little research on it.

All: This is really an interesting thread. A farmer says he needs to add P to his soil and is met with everything from 'vegetables don't need P' to 'it will run off and produce toxic algae.' Seriously?

It's a well known fact that P is often a limiting nutrient in farming. It's fairly expensive so farmers don't add it unless they've done tests and know they need it. As kokos has apparently done.

After a bit of googling, DAP (diammonium phosphate) has a higher pH in water solution - above 7. MAP (monoammonium phosphate) has a lower pH, in the 4-5 range. This is probably temporary and the amount you're adding is quite small compared to the mass of soil, but all else being constant, I would think if your pH is high you would want MAP rather than DAP.

There may be other factors I don't know about.

The reason that I advice against adding chemical P is that first of all phosphorous is a very different nutrient. If you google phosphorous cycle you will see that phosphorous virtually never evaporate in any weather and almost never leached by water too. They just stay there. Runoff is minor too if tilled into soil. Kokos soil has 7.7 ph which indicates that, P in his soil might be tied up and unavailable chemically so that the first thing one should do to make P available is not just add more P but to lower ph with compost. P built up is serious problem in some places I read, you have no easy way of using them all up in the soil.

I would think in farming situation (large areas) that sulfur would be a good option for lowering pH. Nothing wrong with the organic matter from compost, but it won't lower pH as much as sulfur, and it will likely add even more P, which you are suggesting is a bad idea.

OTOH, the soil may actually be low in P and not just have a pH problem.

ceth k couldn't be more confused. First, minerals don't evaporate. Second, runoff is a big problem - phosphorus runoff causes over-fertilizing of fresh water plants, which thrive and then rot causing eutrophication.

Phosphorus doesn't leach down to the water table, but it certainly does run off at the surface. And as a macro nutrient, it gets used up and commonly needs to be replenished when heavy feeders are grown or land is cropped intensively.

As always, first get a soil test. If they soil test says you have enough P, don't add more. If the test says you have enough P but your pH is out of range, adjust your pH. Adding compost or manure in particular can add to the pH problem - have you had your compost tested? You should before you start trying to treat your soil with it.

>>ceth k couldn't be more confused. First, minerals don't evaporate. Second, runoff is a big problem - phosphorus runoff causes over-fertilizing of fresh water plants, which thrive and then rot causing eutrophication.

Perhaps she was referring to nitrogen, which can be lost to the air in various forms. Otherwise, I agree.

And yes, P will run off although it tends to bind tightly to soil and does not leach downward very fast.

As far as soil tests, it appears the OP *did* have a test, has high pH and low P, and is deciding which kind of P to use on his farm.

Other than flinging manure, do farmers use compost on their fields? It seems it would be an extremely expensive proposition, but I'm not that familiar with farming, either conventional or organic.

Yes, it would be an extremely expensive situation to cover a farm with compost.

With larges acreages you about have to go to cover crops and green manuring which is fine.

I don't build compost piles except for horse manure. I return crop residue immediately back to the soil right where it grew. I also spread leaf mulches and boughten leaf composts on the soil. I also raise some cover crop.

For the original poster...I don't know the best form of super phosphate to add. toxcradr says that the MAP would be better for you and that sounds like good advice to me.

Many people still adhere to the idea that the only way Phosphorus enters a water system is via runoff, even though research over the last 20 years, or so, has found high levels of P in the groundwater.

That may be true, but even calcium leaches down [probably much more than phosphorus]. Yet we don't want to ban calcium and a host of nutrients...just manage them well.

A farmer says he needs to add P to his soil and is met with everything from 'vegetables don't need P' to 'it will run off and produce toxic algae.' Seriously?

This is an incredible misinterpretation of what I posted and who I said it to. I'm surprised Zog, you don't normally mis-read comments like that.

Lloyd

Another factor to consider is soil texture and structure. If you pour lots of fertilizer on sandy soils, it can leach through it much faster than in soils with more clay and silt and organic matter. These soils are able to absorb and retain much larger quantities of both water and nutrients. If a heavier soil is poorly drained, nutrients can diffuse in that saturated water. If the soil is drained well, water percolates through without much of this diffusion. Sandy soil on the other hand couldn't hold the nutrients to start with if heavy rains fall.

This post was edited by wayne_5 on Sat, Feb 2, 13 at 17:43

Wayne just hit on why maintaining adequate levels of organic matter in your soil is important and it matters not whether you have a clay soil or a sandy soil.
Clay soils do tend to hold onto nutrients even in the absence of organic matter, to the point that those nutrients are not available to plants. Sandy soils have particles too large to hold much of anything and the organic matter fills in those pore spaces and then holds both nutrients and moisture.
There is a whole body of research that shows that soils with adequate levels of organic matter do a better job of filtering water then do soils with little organic matter.

If you can't readily change the soil mineral texture, then it behooves us to keep plenty of organic matter as kimmsr says.
Sand has larger particle sizes and the total external area is so much smaller than for the fine particles like clay and silt. Each particle has a film of water and nutrients around it.

One strange thing though...sand will release water down to about 1% [water moisture] before plants wilt while they will wilt in clay at about 10%. Still, the clay released a lot more volume. I believe the reason clay won't release that last amount of water is because every particle of soil holds onto the last thin film of moisture [I believe it is called hrdostatic] and cannot release it to plants. Since clay has so many more particles holding onto that hydrostatic film, it adds up to quite a bit of moisture.

This post was edited by wayne_5 on Mon, Feb 4, 13 at 12:38

reading this thread reveals one thing clearly: the study of soils is incredibly complex, and the lack of comprehension surrounding the subject is impossible to understate.

as the wise man said, knowledge defies the laws of mathematics: the more it divides, the more it multiplies.

Plants wilt in clay soils quicker then in sandy soils because the clay locks in soil moisture so it is not avaolable to the plants. Clay also locks up soil nutrients so they are not available to the plants attempting to grow in that clay. Organic matter changes clay so both the moisture and nutrients are more readily available.

Lloyd: Apologies if I misrepresented your post. I was simply trying to point out that the OP asked a simple question, and I thought a lot of the responses went pretty far afield without even attempting to answer it. If I hyperbolized in the process of making that point...Zog very sorry.

I probably should have stayed out of this one, not being a farmer or a soil scientist, but I only wanted to help the poor guy.

BUT I still don't believe anyone else has attempted to address the idea that ammonium phosphate will destroy soil structure, which was the original question.

welcome to the wonderful world of agronomics. there is nothing that can be suggested that cannot be refuted by someone else (specially if they authored the research grant proposal), no product that exists that cannot be replaced by one far superior (particularly if there is a commission to be made), and no methodology that cannot be exceeded using the techniques used generations ago (righteously so when you wrote a book detailing your experiences doing just that), all verified by independent research paid for by interested parties.

and what's funny, based on a certain set of circumstances, they are all right.

this particular question is a good example of this.

and I'm not sure the question can be answered without a number of caveats and conditions specific to the conditions of the location in question.

did find reference to a study indicating that clay content increases with duration of continual P application.

did find reference to some interesting data re: purity of MAP formulation.

did find an interesting recommendation re: P use and replenishment from a consulting co that basically said MAP is what they recommend, in rather modest quantities. I think they took about two thousand words to explain it, and were quite thorough in walking through the process.

but I found nothing to answer the question directly. some of the stuff referenced above leads you to be able to infer that soil type, duration of use, and perhaps rate of application over time, may be influencing factors, but no way would I try to answer that question without running it past someone with way more expertise than I.

I did not address the original question because I don't know the answer. I addressed some misconceptions (italicized quotes) about the potential problems that runoff of applied fertilizers may cause. Here in Manitoba we have almost killed the sixth-largest freshwater lake in Canada with nutrient pollution. It may be a good idea to consider where these nutrients we are adding may end up.

Lloyd

I know people who like high P, but they are terrible gardeners. They only think "I want more flowers now", but I don't care about the over all plant health. It is like trying to raise a kid on junk food. If you want a healthy you need to spend the money and do the work to make the soil good by adding organic matter/compost hopefully that you made yourself. Just buying a bunch of bagged stuff is another quick fix. I add bagged stuff to my home made compost. Once the soil is good and you do other things weeding etc.. 46 is so high you will get out of balance. If you want to grow vegetables they won't like high P. I would never use that stuff in my garden.

Strob: Excellent and very penetrating post about agronomics. As a relative outsider - but a scientist and researcher - I totally get that whole thing.

Lloyd: Noted! We certainly have P runoff problems, no question about that. That whole thing goes way back to the dawn of the US Clean Water Act in the early 70s. I hoard my kitchen box of sodium phosphate for certain cleaning projects, but when used it doesn't go down the drain, but into the compost.

TT: Agreed on the dangers of overshooting on P for the garden. Our soil was deficient when we moved in, and I used 10-10-10 or 13-13-13 for quite awhile on the lawn and garden. Gotta stop on the garden, since last spring's test showed it as very high. Trouble is most people don't do the test. Glad I did.

Kimmsr did bring up a good point about excess soil P tying up micros in some soils, I remember learning this in a soils class long ago but can't remember any of the specifics.

Another point I remember from soils class is that some soils (calcareous included) can actually tie up P as P reacts with lime rendering it unavailable to crops. Organic matter can inhibit this P fixation to some extent. In these soils, it is common to side band apply P fertilizers near to the root zone to allow the fertilizer to sort of supersaturate a small band, thereby allowing for the presence of available P in the band for the roots to grow to and into. If the same amount of fert. was broadcast and incorporated in the crop row it would be exposed to a lot more soil and become tied up.

Darned memory!!!