Have you ever noticed that every package of fertilizer (“plant food”) somewhere on the label has listed three numbers separated by two hyphens? After years of answering people’s plant questions it still amazes me that most people have no idea what these numbers represent or what it means for their plants. Therefore, here’s a brief tutorial about plants’ nutrients requirements and what you should know when buying yourself fertilizers.
All plants rely on certain macro- and micro-nutrients to live. There are a total of six macronutrients. Three of these are readily available in nature (carbon, oxygen, and hydrogen), and three we usually have to provide as a supplement (nitrogen, phosphorous, and potassium). The three numbers on any given bottle of fertilizer are referred to by horticulturists as the N-P-K. These numbers refer to the percentage of the three different macronutrients present in a single dose of the given fertilizer. The first number is going to be the percentage of nitrogen in a single dose of the fertilizer. Nitrogen is responsible for helping a plant to maintain its lush green appearance. For foliage plants especially you want to make sure that they are getting an adequate dose of nitrogen during their active growing season so that they produce chlorophyll and photosynthesize as optimally as possible. The second number is the percentage of phosphorous. Phosphorous is the macronutrient most responsible for flower and fruit production. If you look at a “bloom booster” fertilizer you will typically see that they have a higher middle number. You might also find that orchid fertilizers have a higher percentage of phosphorous as well. In order to flower, orchids and other plants must use a lot of energy to produce their flowers and attract their pollinators. After the flowers pass, these reserves of nutrients need to be restored. By fertilizing your houseplants or containerized annuals during these active growing months from spring to early fall you can be sure that they are getting the right nutrients to keep putting out a prolific bloom. The last number to discuss is the percentage of potash, potassium in a form that can be readily taken up by plants roots. Potassium helps a plant to maintain overall cellular strength and rigidity, from its roots all the way up to its outermost stems. When trying to establish plants in containers or the landscape, you want to make sure that they have an adequate amount of potash so they are strong and able to deal with all the natural elements they will be exposed to. You will also find that cacti usually like a fertilizer with a decent percentage of potash which makes sense because they are usually pretty firm and rigid to be able to stand up to the pounding of the suns rays.
I hope this helps you to be more of an educated consumer. As I said, this is a very basic overview. If you are curious about soluble vs. insoluble nitrogen or other specific topics feel free to email me your questions. To best care for your plants do a little research and find out the best time to fertilize. Most plants benefit from being fertilized during the active growing season, from spring through to early fall, but should not be fertilized through the colder, darker months. A “Balanced” fertilizer has equal portions of nitrogen, phosphorous, and potassium in a single dose and usually can be used for a number of different houseplants. Some plants, like succulents and cacti, may prefer a lighter application, so again, do a little research. For orchids, many experts suggest applying your orchid fertilizer “weakly weekly” to provide a small boost of nutrients at all times. Follow the instructions and never apply a fertilizer more concentrated then suggested on the label. Excess fertilizing can lead to buildup of soluble salts in your containers or your turf and this can hurt a plant just as fast as you’re trying to help it.