Feeding Plants

This is an article on correctly feeding houseplants and plants in general. Correct feeding reduces waste of fertilizer and build up of damaging salts in the potting soil. Some plants are actually harmed by incorrect feeding.

Fertilizers come in a confusing array of analysis, type, organic, inorganic, specific to species, application rates and frequencies, etc. The fertilizer you choose can follow your philosophy of organic or not. My preference is to fortify the soil to provide feeding over a long period of time and I will address recipes and amendments in future articles. I also provide supplemental feedings of liquid fertilizer during strong growing periods or just as a pick me up. There are also slow release fertilizers that can be added to the soil which provide feeding over the course of a growing season.

A quick course on fertilizer analysis. On all fertilizer sold, somewhere on the label you will find 3 numbers in a form like this: 20-20-20. The numbers can be any combination, but the formula means something specific. It is the ratio of nitrogen, phosphorous, and potassium, always in that order. So, a fertilizer with 20-20-20 is 20% nitrogen, 20% phosphorous, 20% potassium. These are the nutrients that plants need a lot of. They also need several others and the best fertilizer to choose is one that provides as many of the others as possible. Organic fertilizers like fish emulsion and seaweed provide micro-nutrients as well and they are not usually listed on the package. You will notice that the organic sources usually have lower numbers in the formula. This is because that is the natural amount of nutrients in the product. Chemical fertilizers can be concentrated and combined in many ratios.

So, what fertilizer to choose? First, decide organic or inorganic. This is a people preference because it doesn’t matter to the plant what the source is as long as they get it. Fish emulsion can be very smelly. Keep that in mind when you consider it. When feeding houseplants with it, you are living in the same place as the fish emulsion and you and anyone living with you will smell it for at least a few days. I’ve not heard of divorces occurring over the use of fish emulsion, but keep in mind divorces cost a lot of money, and other organic sources such as fortifying the soil and seaweed can be used. Inorganic fertilizers are not smelly and quite concentrated. They also come in specific formulations for certain species of plants. African violet food usually has lower nitrogen, high phosphorous, and an average amount of potassium. A typical analysis looks like this 10-15-10. They don’t like to be overfed and do poorly if they are. They do need regular feeding. The middle number, phosphorous, is needed for flower production. Can any other fertilizer be used? Yes, as long as it has a bit higher proportion of phosphorous in it. Foliage plants need more nitrogen, so something like 10-10-10 or 20-20-20 is appropriate to them. Triple 20 is especially good for fast growing plants that also need to bloom and produce fruit like tomatoes. For blooming annuals, a fertilizer analysis of triple 20 or something especially formulated for blooming plants (other than African violets), with an analysis of 10-54-10. Again, the high middle number is a large amount of phosphorous for blooming.

More philosophy. You can always follow the label instructions and mix the concentration they suggest and the frequency of application as suggested. I, however, do not like to blast the plants with fertilizer. They can only use so much at a time and the rest will eventually be washed out of the soil with watering or will build up as salts in the pot and root zone which will definitely harm the plant in the long run. I mix the fertilizer at half strength if I am going to fertilize weekly (pot grown plants that grow quickly and are indoors or outdoors) or at quarter the strength for feeding every time I water. I use this last rate and frequency for the African violets. It is a gentle feeding system that promotes growth without blasting them. When I use organic water soluble fertilizer, I use it full strength because the analysis is low all ready on pot grown plants either indoors or out once a week (tomatoes respond well to this). The quarter strength rate with feeding every time the plants are watered can be used for any of the rest except for cacti and bog plants. Bog plants should not be fed fertilizer. Cacti should only be fed during summer and active growth and then only once per month and dilute at that.

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