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Fertilizers
Ideally, a plants required nutrients are supplied in a soil mix. Yes, I said ideally, but often it isn't the case and manufactured fertilizers must be used.
20-20-20 
30-10-10
12-14-12
All of these are numbers you see on boxes of fertilizer. What does it mean? And how to choose? Fertilizer can be a confusing subject, but I will outline some of the basics here.
First, the numbers. 20-20-20, for example. The numbers represent the percentages of usable nutrients in any given formulation. The corresponding initials are N-P-K, and stand for the chemical abbreviations for Nitrogen-Phosphorus-Potassium
They are always listed in that order. Each performs in different ways when taken up by the plant, and although they will sometimes perform similar functions, they each have major functions we rely on. They are also referred to as macronutrients.
N is for Nitrogen
Nitrogen has several major functions. It promotes vegetative growth, it is needed for the formation of chlorophyll (which provides a nice healthy green color), it is required by soil micro-organisms for growth and they in turn then aid the decomposition of organic matter. It is available in either organic or inorganic forms, and either fast release or slow release.
Common nitrogen fertilizers are:
Ammonium nitrate - less expensive to manufacture, readily available to the plant even in cold temperatures. It can cause "physiological drought" in amounts too high. Physiological drought is when the roots are unable to use available nutrients and wither and die.
Urea formaldehyde (UF) - is a largely
water-insoluble slow release form of nitrogen. This is generally a little more
expensive to make. It has less potential to "burn" roots, and is less
likely to be leached from the soil. It relies on enzyme activity of the
soil micro-organisms. Release will be faster at soil temperatures of 86 degrees,
slower at 50 degrees.
Sulfur-coated urea (SCU) is fast release urea that is covered with sulfur to
make it more impenetrable and more time-released. Higher soil temperatures will
affect the breakdown of the sulfur (making it faster), and because the coated
particles are not uniformly covered, some particles break down sooner than
others.
There are others, such as ammonium sulfate and "straight" urea, but I am doing more research on those.
P is for Phosphorus
Phosphorus is vital to the development of roots in young seedlings and it is required for the flowers and fruit to form. It may become "bound" in soil and is only available for the plant to use in a slow way. This means it does not leach out of a soil quickly and could build up to potentially toxic levels. If using a fertilizer with a high Phosphorus number on container plants, care must be taken to periodically leach the soil by running water through a pot for several minutes. Superphosphate is the most common fertilizer phosphorus.
K is for Potassium
Many metabolic actions rely on potassium, and it also acts against disease. By strengthening plant cells, it can increase a plants cold-, heat-, or drought-tolerance. Without it, starches would not be formed, sugars would not make it from the photosynthesizing leaves to the other cells, and chlorophyll development would be hampered. Muriate of potash is the most common form of potassium fertilizer.
Okay, so you still ask, what does this mean? If you want a basic fertilizer for a variety of houseplants or a garden bed, look for something "balanced" with the three numbers equal or close to equal (20-20-20). Balanced means that the formula will address each of the three nutrient requirements mentioned above. If you want to promote extra bloom, look for one that has a higher middle number, such as 12-14-12, or 15-36-15. Once you know what you want in your plant, and you know what each macronutrient can do, you can choose the formulation that suits your conditions best.
Plants that are epiphytic (absorb nutrients through roots held in the air) or those that grow in non-soil materials, such as orchids, need higher nitrogen levels in the fertilizer as there is none in their growing media. A typical orchid fertilizer may be 30-10-10.
Currently for my orchids I use a 20-20-20 that is diluted to 1/4 strength and applied at each watering. For my geraniums and other house or garden plants, I use 20-20-20 mixed at the recommended (full) strength. For my cacti, when I do fertilize them, which is once in the spring after growth has started, I use the 1/4 strength diluted 20-20-20. I also don't currently buy specialized fertilizers, except for Optimara brand African violet fertilizer. This is formulated at 14-12-14. I dilute this to half-strength and use it every time I water.
Keep in mind that temperature not only may affect the rate of fertilizer available, it often affects the rate of plant growth, and thus the nutrient needs of the plant. Other factors such as soil or water Ph also affect fertilizers.
There has been discussion as to whether urea nitrogen is good for plants or bad for plants. I think it depends on the plant and the growth condition, and the type of urea used. I personally do not worry about urea build-up in my container plants as I TRY (really I do) to repot them on a regular basis, which alleviates any build up of anything. For gardens and lawns urea is a valued fertilizer for its ability to be long-acting.
Micronutrients
The little guys. Essential to plant growth but not always mentioned. These are mainly small amounts of minerals that plants require for proper function. Most commercial fertilizers will have some micronutrients, but read the label to be sure. They are also referred to as "trace elements" and some of them are:
Boron (B)
Copper (Cu) (can be toxic in high amounts, some plants very sensitive)
Iron (Fe)
Manganese (Mn)
Molybdenum (Mo)
Zinc
(Zn)
Whenever
purchasing any fertilizer, read the label for
the breakdown of ingredients. By law, it has to be listed. Of
course, I did not address other more natural forms of fertilizer such as
compost, dehydrated cow manure, bone meal, bat guano, fish emulsion, etc. but
that will be another page, as will, at some point, a page on foliar feeding :)