The Myths of Phosphorus and its Overuse in Indoor Horticulture

I can't dispute that phosphorus is an essential element in plant growth: it is present in every cell, and is directly involved in many processes in the plant, including, energy generation, respiration, nitrogen fixation, and most importantly photosynthesis. A plant can't do much without it. There are a lot of old ideas, though, that are repeated in the horticultural industry, despite the fact that they have been obviously disproven. In the interest of attempting to dispel the dogma and disseminate some valuable insight about how plants actually make use of the phosphorus that we provide them (or, more importantly, how they often don't), what follows are a few points that hopefully will help professionals and amateurs alike choose the best fertilizer for the needs of their plants.

Note that I put the word 'interior' in the title, hoping to imply that the plants I'm discussing are growing in a soilless growing mix indoors, and not in the ground: that's a whole other can of worms, and the concepts here will be greatly simplified without having to worry about it. We're working with basically a blank slate here, nutrient-wise. It's also easier for us here in Toronto because the water is fairly low in minerals save the bloody bicarbonates, which I'll likely talk about at a later time. 

An old-timey picture of a rock phosphate mine, because this post, like many others, has gotten pretty long and I need to break it up a little. That and actual rock phosphate isn't really much to look at. Public domain image; retrieved from altervista.

An old-timey picture of a rock phosphate mine, because this post, like many others, has gotten pretty long and I need to break it up a little. That and actual rock phosphate isn't really much to look at. Public domain image; retrieved from altervista.

The phosphorus used in fertilizers for the most part is derived from rock phosphate, which is becoming more scarce with time (here's an article that discusses a few aspects of that whole thing as it pertains to global agriculture: Phosphate: A Critical Resource Misused and Now Running Low). There are other sources, notably from organic sources such as bonemeal and bat or bird guano, which can be combined with a growing media and which makes itself available to plants gradually and thus acts as a kind of organic slow-release fertilizer for plants.

As mentioned above, phosphorus is one of the macronutrients that are required for healthy plant growth (as opposed to the micronutrients, which, while also essential, are required by the plant in much smaller quantities). In most fertilizers, the three primary macronutrients are listed on the packaging as a ratio in the order of nitrogen, phosphorus and potassium, such 20-10-20 and 15-30-15. These may or many not have the additional macronutrients calcium, magnesium, and sulphur, as well as the fairly long list of micronutrients. In most fertilizers that you can buy 'over the counter', so to speak, the amount of phosporous will typically be equal to or greater than nitrogen or potassium. There are several reasons why this is ridiculous.

Plants do not use most of the phosphorus that is contained in these typical fertilizers. These high-phosphorus recipes have their origins in field crop production, where phosphorus behaves quite differently than in our soilless media, and where yields can be significantly affected by the availability of this nutrient. Producers of fertilizers for domestic use typically market high-phosphorus fertilizers as producers of better root growth and better blooming, when in fact the extra fertilizer is of no use whatsoever. They make quite a killing at it, too, I'm sure. It's pretty wasteful, though, as it happens, and it seems that, at least in some cases (see below), lower phosphorus can produce better flowering and better quality crops in general.

Additionally, the fact that the extra phosphorus doesn't leach away and remains present in the soil can have some negative effects: too much can inhibit the uptake of other negatively-charged elements such as iron and manganese. It also readily precipitates with other elements, forming insoluble compounds which are unavailable to plants, such as calcium and magnesium phosphates, particularly at higher pH levels.

But so how much phosphorus do plants actually use? Not very much! This paper provides a good look at the use of phosphorus by azalea plants, and indicates that the addition of phosphorus above a certain (low) threshold made no significant difference to the growth of the plants in the study: Nitrogen and Phosphorus Uptake Efficiency and Partitioning of Container-grown Azalea During Spring Growth. And here's another paper that shows that lower phosphorus levels can actually produce better quality crops, with more flowers that held for longer time, as well as increased drought tolerance: Improving Bedding Plant Quality and Stress Resistance with Low Phosphorus.

Figures 1 and 2 from the bedding plant study linked above, showing increased flower production and less wilting in plants grown with less available phosphorus.

Figures 1 and 2 from the bedding plant study linked above, showing increased flower production and less wilting in plants grown with less available phosphorus.

So from these and other studies, we can determine that phosphorus seems not to promote better blooming, yet somehow these fertilizers can seem to be effective, hence their continued use. What is it about them that makes them work? The answer is in the number to the left: nitrogen. Many of the 'bloom booster'-type fertilizers have either reduced nitrogen or increased phosphorus/potassium levels, and a reduced nitrogen to potassium ratio is one of the ways to shift plants more into reproductive mode, wherein they obviously produce more flowers at the expense of foliar growth. Balancing plants between vegetative and reproductive growth is the art of the commercial grower, and it is a fine art indeed, and worth examining to the amateur grower. 

Recommendations for commercial crop production of most tropical plant species (which, to be fair, are mostly grown for their foliage) is for the use of a fertilizer with a 3-1-2 ratio, though even this seems a bit high: compare it, for example, to the MSU orchid fertilizer, which is 13-3-15, and which is a fantastic recipe, in our opinion. This article from the American Orchid Society discusses some of the reasoning behind the ratio they use, and how it works to create beautifully balanced plants that grow and bloom at an optimum: Without High Phosphorus A New Fertilizer Proves Itself with Orchids. It's important to note that this fertilizer was not developed specifically for orchids, but is rather a well-suited mix for plants of any type, being built on sound principle and good science. For those in the business, or those who really go through the stuff, it would be fairly easy to reverse-engineer it from the guaranteed analysis on the label if you have access to the raw materials. (A hint if you don't want 200 lbs of elemental fertilizer sitting around- hydroponics stores often sell smaller bags of potassium nitrate, etc., though for an inflated cost.) If anyone is interested in doing this and doesn't know how, say so in the comments and we'll see if I can't help you sort through it. 

Remember, the job of the fertilizer company is, first and foremost, to sell fertilizer. Properly managing plant nutrition is the responsibility of the grower.

Remember, the job of the fertilizer company is, first and foremost, to sell fertilizer. Properly managing plant nutrition is the responsibility of the grower.

I find it a little funny that even though the recommendations for production are to use a 3-1-2 ratio, I see professionals further down the line such as interior landscapers or garden centres using something like 20-20-20 (or even 10-52-10 for new transplants). Phosphorus is about four times more expensive than it was ten years ago, and I don't presume the cost will be going down any time soon. Given the plants aren't using it, why throw your money into the dirt, so to speak? Even local orchid societies are applying high-phosphorus fertilizers to their collections, despite vendors at their meetings carrying the MSU feed! Old habits die hard, I suppose.

I'm not recommending that anyone rush out, buy a reverse osmosis filter and start mixing up batches of MSU feed to start doing their houseplants (well, I sort of am, though it is expensive and maybe almost as wasteful as the extra phosphorus I'm bitching about here, due to the waste water that RO filters produce- your plants would love you for it, though, particularly those that struggle with the ever-present nasties like flouride [like, say, every Dracaena], and with high total dissolved solids in general [like some orchids and most carnivorous plants]). But it is worth considering when choosing or mixing your own fertilizers. There are a lot of ones that you can find on the market that are in and around the 3-1-2 ratio (particularly if you're not looking at the major brands, which have got a good thing going with their 'root-' and 'bloom-boosters'), and you can give yourself a pat on the back for having used less of an increasingly scarce fertilizer. You may also save yourself some nutritional problems down the road. And if your plants aren't blooming as well as you'd like, there are other avenues to experiment with besides lowering your nitrogen (though it will probably help), like lowering your night temperature a little (tricky out of season, unfortunately), giving the plant more light, or reducing somewhat the amount of water you give it, all of which are known to promote reproductive growth. Some plants won't even set buds until some triggers are hit, like shortened day length or extended drought. Another reason to know what you're growing, I guess.