Why is diluting fertilizer 1/2 to 1/4 strength so common?

You would think that houseplant fertilizer manufacturers mixing instructions would be technically accurate since they are supposed to be the 'fertilizer gurus'. Why then do so many websites, for so many types of plants, recommend only 1/2 or even 1/4 strength? Are these sites generally erring on the side of safety? How would you know if you missing out on the full potential of your plant if your under fertilizing?

Sincerely,
Beginner gardener who may be thinking too much :-)

This message was edited May 14, 2015 1:32 PM

They want to sell you more ferts, so they label it so you use the maximum possible dose. The plant grows faster, and you run out of ferts sooner. So you go buy more.
The dosing is appropriate to commercial greenhouse use, where they grow the plants under bright light and make them grow as fast as possible to sell them quickly.

In fact, most plants do better if they grow somewhat slower. Extremely fast growth is often soft, succulent growth. Very susceptible to pests and diseases. Slower growth is more resistant to pests and diseases. In the case of house plants a slower growing plant will not outgrow the container or the location so fast. House plants are often in lower light, so they cannot make use of luxury amounts of fertilizers.

To get the optimum growth of the plant, make sure it is in the maximum light that is appropriate to the species. If you are supplementing the sunlight, or growing in a dark room and just using grow lights, then make sure the bulbs have the highest PAR rating- this is a special rating for plants, it means Photosynthetically Active Radiation.
Plants use a lot of the light waves in the red part of the spectrum, and a lot in the blue. They use less in the yellow to green areas.
Our eyes see the yellow to green parts of the spectrum best, so lights designed for human use may not be so rich in the colors plants use. Conversely, a grow light (high PAR) may not look very bright to us. May look more pink-purple.
If you arrange lighting like this, then use the fertilizer at the maximum dose.

Diana has it nailed. In my mind the recommended dose might be considered the most you would ever find useful, which is to say that most of the time it's not what you need or want. Not being an expert grower, I have never gone above about 1/4 strength (50 ppm N), and that has always been plenty for the plants I have (almost all succulents, all growing in the sun).

One distinction to bear in mind is that a stronger dose might be appropriate for one-time delivery, to be followed up by plain old water the next several times around. It sort of works its way through that way. If you like to fertilize regularly, a much lower dose makes more sense. It might work out to some sort of time average equivalent. I like to give a little bit regularly and the plants I grow tend not to be needy with respect to nutrients, so it turns out well.

Depending on your location and the quality of your water, it may actually be more important to pay attention to the pH when you're feeding your plants. Our tap water comes out really alkaline, in the range where nutrients become unavailable to plant roots, and a quick shot of acid can correct that quite easily. It's not much more complicated than what you might do for a planted aquarium (and aquarium products work perfectly for this purpose).

This message was edited May 14, 2015 10:30 PM

OK Diana / Baja.....I'm a believer. Thanks.

It never hurts to do a few experiments here and there to know where you stand. When in doubt you can always try growing two plants side by side with different treatment, to know what works best for you. Over time that knowledge sort of snowballs.

I like to monkey around with various things to see what sorts of corner cutting I can get away with (being fundamentally cheap and lazy) vs. what parts need to be fussed over (wanting the plants to do well). Doing this tells me what actually matters from the plant's point of view, and illuminates areas where I might be (to use your words) thinking too much.