Advertisement Plants are pretty magical. They seem to eat nothing--all they need, at least for a while, is air, water and sunlight. However, after a few months of normal growth, many houseplants seem to grow without growing. They grow new stems and leaves, but they drop old ones; they stubbornly refuse to flower; they seem to sulk, despite no changed conditions. The reason for this is that plants can't actually just eat air, water and sunlight. Just like you, plants produce proteins, and proteins take atoms other than oxygen, carbon, and hydrogen (the atoms bonded into glucose in the principle reactions of photosynthesis). Plants usually get these nutrients from the medium they are grown in (with a few exceptions I will get to shortly). Plants need these nutrients just as much as they need sunlight, water, and air; otherwise, you are essentially feeding them nothing but sugar, and it's honestly astonishing how long they can go from just that. For long term survival and flourishing of plants, you will need to fertilize. The traditional division of plant nutrients is into macronutrients--those used in large quantities--and micronutrients--those used in small quantities. However, when using fertilizer, growers tend to worry principally about the three following macronutrients: nitrogen, phosphorous, and potassium. When you see a fertilizer rating, you will notice three numbers, arranged as follows: x-y-z (for example, 10-10-10). What this means is actually fairly simple: the first number is the percentage of nitrogen by volume, the second is the percentage of phosphorous by volume, and the third is the percentage of potassium by volume. For this reason fertilizer ratings are often known as NPK, after the acronyms for these elements on the periodic table. Most fertilizers contain the other macronutrients plants need (calcium, magnesium, and sulfur). However, most do not contain the necessary micronutrients--chlorine, iron, boron, manganese, zinc, copper, molybdenum, and nickel (knowing what these are is usually not necessary, but can be useful for seeing if a fertilizer is really complete). Micronutrients are just as important as macronutrients; they are simply not needed in as high quantities. Usually they are present in sufficient quantity in tapwater, and for this reason, I recommend watering almost all plants with tapwater where possible. If the tapwater in your area is unsuitable (usually because it has too many dissolved solids) and you need to use bottled water, I recommend using distilled water and a fertilizer that includes micronutrients, since it's essentially impossible to know the quantities of various dissolved solids in other kinds of bottled water. Now, the obvious question is: how much and how often should you fertilize? This, of course, depends on what you are trying to grow. Many sources will talk about dilution to "half strength" and then tell you how often to fertilize; I, however, prefer to work with parts per million (ppm), simply because it is universal. Here is an example of where this matters: I have two fertilizers. Fertilizer A is rated 10-10-10, and fertilizer B is rated 20-20-20. Suppose I dilute to half strength (usually the strength recommended on the package) and fertilize once a week. The plant receiving fertilizer B is receiving a doubly high load of fertilizer! If I use ppm instead, however, I will dilute fertilizer B in twice as much water, thus watering the same amount. Ppm is measured in terms of volume. As a simple example: suppose I have a 10-10-10 fertilizer, and I dilute one mililitre of it in one litre of water. In that case, I would be dissolving one tenth (10%) of one one thousandth of one litre of nitrogen into a litre, so the concentration of nitrogen would be one ten thousandth, or one hundred ppm. The same math can be done for the concentrations of all of the nutrients. It is important not to over-fertilize, because fertilizer usually comes in the form of salts, which can burn plant roots if they are present in overly high quantities. What I have used with pretty good success is 30-10-10 fertilizer diluted to 50ppm, with which I water my entire collection aside from carnivorous plants about once a day. However, though I grow a wide diversity of plant species, the ones I fertilize all have one thing in common: they are epiphytic or lithophytic, so in the wild they grow on exposed tree bark or rocks. Thus, they have low nutrient requirements and they grow slowly. My guess would be that most house plants would like a higher concentration of fertilizer, but I don't really know what it is. What I would do is start with 100ppm, see how the plant reacts, and go from there. You can look up signs of over-fertilizing online, and I've already described the signs of under-fertilizing. I've significantly damaged at least one plant through over-fertilizing, so do be careful (although it's worth noting that that one was carnivorous, and many people don't fertilize carnivorous plants at all). I know of two groups of plants that don't absorb nutrients primarily through the roots. Carnivorous plants, of course, tend to use their traps, whatever those are; I usually fertilize those traps where possible, although often carnivorous plants do just fine on whatever insects are around (with the notable exception of king sundews). Tillandsias are able to absorb nutrients through their leaf surface (despite the name air plant, they, like all living things, need more than just air to survive), so you can just spray the leaf surface like you do when watering. If there's any information I've missed, or anything I should clarify, please tell me. I hope this is helpful and not too confusing.