Fertilizing orchids properly makes them grow faster and bloom more. Feeding orchids is important to keep them healthy and growing: a common phenomenon if orchids are underfertilized is that they don't get any bigger year after year, they stay the same size or slowly diminish. For example, Phalaeonpsis will drop old leaves as fast as they grow new ones, and have 5 leaves or so at any given time. (A really healthy, mature plant will have a dozen leaves or more.) Providing fertilizer consistently (at least weekly) will make sure that plants stay healthy and grow larger each year.
Orchids, especially epiphytic orchids, often have limited access to nutrients in nature, so they are vulnerable to "fertilizer burn" if overfertilized, causing browning of the leaf tips and/or root damage. To avoid such issues, the standard advice is to fertilize orchids "weakly, weekly." Typically, fertilizer is dissolved in the plant's water, then it's watered normally. It's a good idea to start using an orchid fertilizer at half the recommended concentration, and experiment from there to see what gives the best results. Usually that will be enough fertilizer to use. If you see salt buildup on the pot or potting medium, this salt buildup is excess fertilizer. Many orchid growers alternate waterings with fertilizer with waterings that don't have it, to prevent this salt buildup. If you see significant salt buildup, it's probably time to repot the plant.
There are a few things you should know about fertilizers before you start feeding orchids. First, you'll want a urea-free fertilizer. Urea is a form of nitrogen that's bad for orchids, but commonly used in fertilizers. Many fertilizers provide nitrogen in the form of nitrite. For plants growing in soil, this is fine as the soil particles provide lots of surface area for bacteria that convert the nitrites into nitrates that the plant can use. But most orchids are epiphytes that grow on the bark of trees; terrestrial orchids also need a loose potting mix. So there is much less surface area for the bacteria that produce nitrates, and the best orchid fertilizers provide nitrogen in the form of nitrates rather than nitrites. Shop around to find a brand of orchid fertilizer that does this: it will work much better.
When reading a fertilizer's label, you'll see some numbers listed, in a format like "20-20-20". The first number is the amount of nitrogen, the second is phosphorus, and the third is potassium, all listed as percentages by weight. (Liquid fertilizers will have lower numbers than solid fertilizers, due to the weight of the water, so don't be surprised by this.) These are all important elements in plant nutrition, but there are lots of other nutrients needed in small amounts, too, so you want a fertilizer with plenty of micronutrients. It should probably be formulated especially for fertilizing orchids. Let's talk more about the macronutrients, and what they're good for.
Nitrogen, the first number, encourages overall growth of all plant parts, because bacteria around the plant's roots use it to make amino acids. Contrary to popular belief, orchid bark does not rob nitrogen from plants, so there's no need to raise the first number relative to the others and use something like a 30-10-10 fertilizer.
Phosphorus, the second number, encourages flowering. Many orchid growers use a fertilizer with a high middle number when a plant has a flowering spike. Unfortunately, this can backfire: orchids often have too much phosphorus, which can inhibit blooming! Some commercially-available orchid fertilizers have actually started using low middle numbers relative to the other two.
Potassium, the third number, is good for root growth. You probably want about the same amount of potassium as nitrogen in most cases.
If you grow a lot of orchids, you can save money on fertilizer by first watering your plants thoroughly to make sure all the potting mix is wet, then watering the plant again with the fertilizer-water. By running less fertilizer out the bottom of the pot this way, you'll only need to use about half as much to get the same results.
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