Triple Superphosphate

The only nutrient my soil is deficient in to any notable degree is P (and some of my amended soil is fine there, too) – I have a bag of Triple Super Phosphate.

If I’m fertilizing specifically for P and use it – I have a question about its use…

It’s 90% water-soluble P from what I’ve read. If that is the case, does the dissolved phosphate work its way down into the soil profile at all, or does it stay in place? To me, ‘water-soluble’ means, at least for a time, it will move down the profile as rainfall and irrigation dissolves it and works its way down, although I realize P binds itself to soil and persists as well.

I guess what I’m trying to figure out is – I realize incorporating it into soil is always best and that is the intent, but, will it work its way further down either if surface applied or only worked in an inch or so, or is it more like lime which tends to stay pretty put?

P is strongly absorbed by clay and organic matter. It won’t move very far into anything that has good levels of those. But if it’s even just placed under mulch the plant will have many opportunities to take up the P.

Furthermore most fruits don’t need much P. Too much is bad as it causes interference and such with uptake of other elements. I don’t think I’ve ever applied P to my fruit trees, at least not much. And I’ve started about 8 home orchards as I’ve moved around TX and once to CA.


The question is not about what the soil tests tell you, it is what the leaves tell you. I have never seen trees suffer from P deficiency in 50 years of working with trees and soil- many, many types of soil (mostly fruit trees). It can happen- especially in soil that lack the proper mychorizal populations to lend trees a helping hand (or strands), like in subsoils extracted deep down and dumped on the surface or mining slag- even in Midwestern prairie soils I could imagine a problem- the fungus that works best with trees may not be there. In these rare cases inoculation is probably more practical than amending with P- a few quarts of forest soil should do the trick- or purchase the commercial stuff.

It is in vegetable gardening where P can become an important issue, As I understand it it takes a while for the mychroizal relationships to click in and available P amendments can accelerate establishment of annuals.


Even if it’s THIS low? It’s virtually nonexistent.

Sure put some on. The amount should be recommended on the soil test report. Just remember more than recommended isn’t better. And don’t apply it every yr. Get another soil test in about 5 yrs before applying more.

Some of my soil has 15 times the recommended rate of Mn. It’s nearly toxic. I can see it on the leaves. I don’t think I applied it. The prior homeowner did.

1 Like


Wonder if the very low PH of 4.8 causes the P level to report lower than expected?

Are you growing Blueberries?

The pH has since been raised to 6.5, and where I’ve added fertilizers and compost the numbers are better, but there is an area that has only had lime and nothing else.

It won’t hurt. All I’m saying is P deficiencies for trees are quite rare- Whitcomb suggests it is virtually nonexistent except in very unusual circumstances and even your low number is probably not going to stop trees from getting what they need through mychorizal symbiosis.

I’m just a stickler for accuracy and Cornell and other ag U’s still take P numbers very seriously in terms of orchard establishment recs, but I think it is just one of those things where the science just doesn’t penetrate the general literature.

I’m just a dumb farmer though and am open to any real world explanations for why we need to worry very much about P. Homeowners end up overdosing their lawns and erosion takes the stuff to our waterways creating certain environmental problems such as algae bloom and lethal oxygen deficits for aquatic species.


All valid points, Alan.

FWIW, my veggie and fruit gardens are adjacent anyway and my fruit trees are planted on previously amended soil with adequate to high nutrients including P.

Excessive amounts of phosphorus will also kill beneficial fungal Mycorrhizae in addition to everything else mentioned above.

Triple Super Phosphate is inorganic phosphorus. Plants can only readily absorb inorganic phosphorus. Spraying it on your plants this year will feed them and the rest will likely wash away with the rain. This is the problem is phosphorus runoff. Phosphorus does not stay in place in soil unless its been metabolized into organic forms. There is a whole cycle by which phosphorus moves back and forth from organic to inorganic states. Spraying on your plants and removing the fruit and plants when done is not going to enrich your soil, but you can supply what they need this season.

1 Like

If I have a tree that’s a shy bloomer, I’ll give it some Triple SP the following winter. It will usually increase bloom production. If it doesn’t, I get rid of the tree. It’s also good for roses.

1 Like

My homemade container soil tested (after a couple of years’ use) shy of P. A “2” where “5”-“7” is optimal. So here I am. It’s possible. I had hoped the continued breakdown of the compost-based container soil would free up some P and K, but evidently not so much. Or at all.

The ground tested OK.

Having read all this, I don’t think I would add P without testing first. I don’t think I would add P beyond the top end of optimal in an orchard situation.

I thought P binded tightly with soil fairly quickly, becoming mostly unavailable and that P pollution involved a certain amount of erosion from this bound P on the very surface to which it is applied. Because it is so immobile and remains locked, surface apps can easily erode, especially because it fixes to the finer particles of soil that are most soluble. This is why it has been removed from most lawn fertilizers.

I also believe that the association with P to stimulating flowering of plants has been debunked. The early experiments were not correctly evaluated and it turned out surplus N. reduces flowering in favor of more vegetative growth and they were failing to take in consideration the decrease in N in the studied plants. Of course, I might be unaware of more recent research- but it would seem P deficiency would be a very common problem in commercial fruit production if low levels of the nutrient reduced fruit set.

1 Like

Potassium K is what’s important for fruit and flowers. Nitrogen of course is important. I do not recall ever seeing any mentioning of why P is very important, yet oddly a lot of fertilizers have P stronger than K.

1 Like

It does seem upon research that P is more important for veggies and some herbaceous plants vs woody plants.

I do grow those too so that’s worth knowing.

Yes you are right (although it does depend somewhat on the soil and the type of P). Triple P is a very reactive form of P (perhaps the most). The P tends to bind to Ca in the soil and then is immobilized. It can become plant available again via the actions of fungi and bacteria (mostly the former), but is much less available (and its use tends to kill off the microbes you need to mobilize P).

I generally steer away from Triple P, because it can run off in high rains and only is really available for a short time. But if you have it, use it. Ideally sometime when you won’t be getting any downpours for a couple of weeks. And/or apply fractional amounts every few weeks.

Signs of P deficiency are typically reddish-purple color on the stems and leaves.

Is this a deficiency you’ve ever observed in trees?

Not in fruit trees, at least the ones I grow (apples, pears, cherries). One can often see it on tomato starts that are on display in the big box stores (or even your own home grown ones if you’ve been slow with the fertilizer) especially if its been cold out. A reddish-purple color to the underside of the leaves