It is sure not anything I would recommend!!

First, most soils have sufficient levels of P already. Increasing these levels unnecessarily results poor plant performance, possibly death if too high. High levels of P will also kill off the microherd.

TSP is highly water soluble and so will move through the soil profile and leach into ground water. It is highly toxic to aquatics, accounts for massive eutrophication and is one of the largest polluters of streams and waterways. Many lawn fertilizers are now being marketed without P for this reason - the application is generally unnecessary and the P just contributes greatly to water pollution.

It is also quite alkaline and will affect soil pH, making it difficult for acid loving plants to thrive.

If you don't need it for its normal cleaning purposes, dispose of it in an accepted hazardous waste collection station. Or offer it on craigslist to someone(s) who might have an appropriate use for it. Which is NOT on garden soil!!

TSP is a powerful degreaser and hydrocarbon remover. I don't understand why you'd even consider throwing on your garden. Why not laundry detergent? If you consider killing your plants to be preferable to throwing it out then, well, go for it.

I guess it is possible that in suitably tiny quantities it might be helpful, but unless you know for sure what those suitably tiny amounts are, you're just playing with fire.

Bottom line. If you don't know that your soil lacks phosphate, putting on a phosphate supplement of any kind makes no sense. It's not like "the more the better".

Sell it on Craigslist in the "building materials" section.

It's an excellent degreaser and wall cleaner.

Josko... I think you've asked a thoughtful question in light of your situation... TSP is used for cleaning but also as a food additive among other uses. TriSodium Phosphate... I'd be concerned about the addition of so much sodium. Though I'm certainly no authority about this, I believe adding sodium would adversely affect soil structure (tightening it up) plus sodium really isn't even a plant nutrient. How much could be added to your soil without ruining the soil (turning it sodic) would depend upon how much is already there but really sodium is not something that gardeners try to amend their soil for. The other part of TSP is Phosphate and that, I believe, is where your focus lays... Though it is a plant nutrient, too much and it becomes detrimental to soil and plants alike... tying up other minerals and affecting pH. Whether your garden would benefit by the addition of the phosphate would depend again upon what's already present in your soil. However even if the addition of phosphate would be beneficial for your garden soil, I'd hesitate on using TSP to supply it because of the aforementioned addition of so much sodium as a consequence.

As to your situation which prompted your question, I really like gardengal's suggestion of craig's list.... sure sounds like a win/win solution to me... allows you to dispose of it and make a little money and someone else gets to use it in an appropriate manner.

And I would not even add that unless a professional soil test indicated P levels were inadequate. Which they seldom are.

Which they seldom are.

That would depend on the region.

Unless a previously heavily farmed tract of land or very sandy or volcanic soils. P is usually available in adequate amounts. However, there are other factor that affect its availability to plants but not necessarily its concentration.

The phosphorus cycle.

There is a pervasive garden myth that if something is considered a nutrient, having a lot of it is a good thing. That's sort of like consuming a bottle of vitamin tablets every day because, hey, vitamins are a good thing! I am always suspicious of adding amendments to soil when it isn't clear the soil really needs it.

And I would not even add that unless a professional soil test indicated P levels were inadequate. Which they seldom are.

Inaccurate generalization.

Unless a previously heavily farmed tract of land or very sandy or
volcanic soils. P is usually available in adequate amounts.

Interesting that you don't mention OM content. Still inaccurate generalization.

However,
there are other factor that affect its availability to plants but not ...

Hmmm, Do you mean like...wait for it... those factors that define regional soil differences? Why do you think the Bray and Olsen P extraction methods among others, were developed, employed and are recommended for different regions of the US?

Here is another Fact sheet from the U of MN Extension:

Understanding Phosphorous...

"Soils throughout most of western Minnesota usually have natively low
levels of available P because of the materials the soils where formed
in. Therefore, appropriate management of phosphate fertilizers is a
major concern for these soils. On the other hand, soils in southeastern,
central, and east-central Minnesota usually have a natively high level
of available phosphorus. For these regions, phosphate is usually not
needed in large quantities in any fertilizer program."

...necessarily its concentration.

You referring to a soil's phosphorous "sink"? Now you want to switch gears and qualify what you think you meant to say? Fine, if the total amount of P in the soil (the total P sink) plays this significant roll in plant health, what is the test used to determine the total P sink? Can you site a study that correlates the total P sink with plant health? An agronomist that uses total P sink in making P recommendations?

I'm sorry if I come off testy, but if a person is going to "qualify/correct" a remark of mine, I ask that they be accurate.

I'm always happy to learn new things.

Western MInnesota is a pretty small tract of land compared to the rest of the US..........so no, I don't care to qualify my comments any more than I already have. If you read soil analyses from across the country, most will state " there is a large amount of phosphorus in most soils." (Penn State) And that happens to be confirmed by most studies across the country. And while most soils DO have phosphorus in adequate to more than adequate levels, much of it is immobile and unavailable to plants until converted into solution. And that depends on a variety of factors, including soil pH, rainfall levels, the percentage of OM in the soil, other nutrient levels, temperatures and the type of plants that are being grown.

All of these factors, even under optimum management, have a consistent impact. The efficiency of plant uptake of phosphorus is very low—usually less than 20 percent. But at the same time as the soil solution phosphorus is depleted by crop uptake, unavailable phosphorus can slowly be released to more available forms to replenish the soil solution. This slow release can sustain plant growth in many natural (garden) systems, but is usually not rapid enough to maintain adequate phosphorus availability in intensively managed cropping (farm) systems without some supplemental application.

What it boils down to is that unless your soil has been tested to determine if P levels are adequate (probably) or deficient (less likely) there is no compelling reason to add P and more than a few good reasons not to. Nitrogen and phosphorus, primarily from fertilizer applications and as leachates from feedlot manure piles and compost, are the largest contributors to water pollution in the US. Currently there are 12 states that restrict or ban sales of phosphorus fertilizers unless a soil test indicates a deficiency or for agricultural production, and more are taking the issue under consideration.

Lol. You can quit your squirming, I'll let you down off the stove. :)

If it was a pound, I'd say go for it, but a few dozen pounds is a LOT in terms of both P and pH. Much more useful as a cleaner and degreaser. I keep a box under the kitchen sink to soak burned-on pots and pans. It only takes a spoonful. After use I throw that water into the compost. That box has lasted me for years.