Balanced Fertilizer and Peak Phosphorus

When I first started reading extension recommendations for plants around 15 years ago now, I read a lot about “balanced fertilizers”.  Today I still see balanced fertilizers recommended, but not nearly as frequently, which is a good thing.

At this point you may be asking yourself “what is a balanced fertilizer?”  A balanced fertilizer is one that has three numbers on its label which are the same — such as a 10-10-10 or a 14-14-14 (Nitrogen, Phosphorus and Potassium are the three nutrients indicated by
these numbers).  Although it might seem like equal numbers would mean equal amounts of these nutrients, these numbers actually indicate percent Nitrogen, percent P expressed as P2O5 (in other words, if you took all of the phosphorus present in the fertilizer, made it into P2O5 and then added it back to the fertilizer and figured out what percent of the fertilizer that made up — that would be the second number) and percent potassium expressed as K2O (basically the same as the phosphorus example you saw previously).

But here’s the problem.  The amount of phosphorus, and often potassium, that is added when you use a balanced fertilizer is typically out of line with the amount that the plant needs.  This is because fertilizers are usually applied based on the amount of nitrogen that a plant needs.  The reason that a balanced fertilizer was usually recommended was that phosphorus and potassium levels in these fertilizers is high enough that they provide all of those elements that a plant needed without being toxic to the plant.  The problem is that, while the levels of these nutrients added might not be toxic, they are in excess of what is usually needed.

OK, so we’re adding excess phosphorus and potassium, what’s the problem?  Well, for the potassium the problem isn’t usually that big a deal.  In fact, a fertilizer bag with the first and third numbers equal may be what’s called for in many cases — fruits and nuts in particular often like a higher level of potassium.  Additionally, the world has a pretty big store of potassium so we’re not likely to run out any time soon.  Phosphorus, on the other hand, is a little bit different.

Phosphorus is a bit more hard to find in large quantity than either of the other elements in a bag of fertilizer.  Because of this it is often the element that limits the growth of plants, for example algae.  When phosphorus runs off into a lake or other body of water it can allow algae to go crazy and use up all of the oxygen in the water killing fish and other creatures (actually it’s the dead algae that do this — bacteria use oxygen while breaking the algae down).  Most of you are familiar with this and know that it’s the reason why Minnesota and now Wisconsin have banned the use of phosphorus fertilizers on lawns without a soil test.

But here’s what you may not know.  Most of the phosphorus which we use for fertilizers comes from rock phosphate.  Rock phosphate is mined in only a few places around the world, Florida being one.  Just as it is a foregone conclusion that we will run out of oil someday, it is also a foregone, but lesser known, conclusion that we will run out of rock phosphate.  Estimates are that we will reach “Peak Phosphorus” (in other words maximum phosphorus production.) in 20 years or so and that we will run out in 50 – 100 years.  Here’s an interesting article on the situation

So conserve our natural resources and skip the balanced fertilizer.  When asked for a general use fertilizer recommendation I usually recommend something with a ratio of roughly 5-1-2, with a higher potassium content if you’re growing fruits or nuts.

9 thoughts on “Balanced Fertilizer and Peak Phosphorus

  1. It’s also worth mentioning that these NPK fertilizers are based on intensive agricultural production – not the needs for home landscapes and gardens. NPK tend to become deficient most quickly in agriculture fields and have to be replaced routinely. Home landscapes aren’t “harvested”, so P in particular is rarely deficient.
    Moreover, phosphate toxicity is increasingly common in nonagricultural landscapes where it’s been overapplied for years. It’s just one more really good reason why you should have a soil test before you indiscriminately apply this chemical. (I’m sure readers of this blog understand it makes no difference whether it’s an inorganic or organic source: too much is too much regardless of source.)

  2. I’ve recommended to gardeners before that they should first get a soil test to determine if they NEED phosphorous (in our part of the state, it’s often more than adequate). I also point out that lawn fertilizer (typically high in N, and by law in MN, often with zero phosphorous) works just fine for gardens, though ‘weed and feed’ is probably not a good idea. Just because the label says it’s for your lawn doesn’t mean that’s where it needs to be put.

  3. I recently was given this advice: In the case of a vegetable garden, it’s not necessary to put all of the recommended fertilizer that’s in the soil test results on at the same time – spread it out over the season: At planting, at first flower, and periodically thereafter, until first ripening fruits (I have tomatoes in mind). I’ve seen some detailed formulas for varying the amount of K during the season to aid in the ripening process, but too involved for most home vegetable gardening.

  4. Granted this is anecdotal evidence. My irises only bloom if provided with P. I apply it in late winter in front of the advancing rhizome.

  5. Most Australian Proteaceae have adapted to our naturally P-deficient soils. They excrete citric acid from their roots to make what little P exists in our indigenous soils more available to them, which is pretty clever.
    The only place I apply any kind of fertiliser is my vegetable patch. To be honest, if I had to apply P to my ornamentals to have them grow successfully, I wouldn’t have planted them to begin with. I’m more of a ‘right plant, right spot’ kind of guy.

  6. Phosphate ore must be chemically processed with sulfuric acid. When sulfuric acid reacts with the phosphate it produces a slightly radioactive byproduct known as phosphogypsum. According to the Florida Department of Environmental Protection, there are a billion tons of phosphogypsum stacked across the state and 30 million tons are generated each year.

  7. It is surprising to read such an intelligent discussion of P without a single mention of mychorizal relationships which allow most species to thrive in conditions where a soil test would indicate a need for P.

    My favorite arboriculture book, “Arboriculture” Harris, Prentiss Hall mentions research in the chapter on tree nutrition that soils where trees benefit from the application of P are extremely rare in this country, whatever the soil test may say.

    This is because of hyphae (hope that’s the right word) networks deliver P to trees from a much greater area than occupied by the plants root system in exchange for carbohydrate to beneficial fungus from the plants roots.

    I’m sure Linda could explain it better.

  8. Alan, you’re exactly right about the importance of mycorrhizae in nutrient acquisition of all woody plants (and many nonwoody ones as well). I wrote an article on mycorrhizae for the now-defunct MasterGardener Magazine a few years ago. Adding unnecessary phosphorus to soils interferes with this fungal-plant relationship. (We’re in the process of trying to post the magazine archives on the web, at which time you’d be able to read it.)

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