Choosing a Fertilizer for Your Indoor Plants

By Bill Marken, Suzanne DeJohn, and The Editors of the National Gardening Association

When you know what nutrients your plants need and what type of fertilizer you prefer, you can choose the fertilizer that’s best for your houseplants. A fertilizer’s guaranteed analysis (the amount of nitrogen, phosphate, and potassium) is one of the most important guidelines for choosing the right fertilizer, but there are other considerations, too. For example, do you want all-natural fertilizer, or are you willing to use a fertilizer that was made synthetically? Should the nutrients be released quickly or over a longer period of time? And would you rather apply it in liquid form or solid form?

Decoding numbers on a fertilizer label

When you buy a commercial fertilizer, the guaranteed analysis is listed on the label with three numbers. These three numbers tell you how much of each of the primary nutrients is in the fertilizer. The first number indicates the percentage of nitrogen; the second, the percentage of phosphate; and the third, the percentage of potassium, also known as potash. A 10-5-5 fertilizer is 10 percent nitrogen, 5 percent phosphate, and 5 percent potash by weight.

Do the math, and you find that a 100-pound bag of 10-5-5 fertilizer contains 10 pounds of nitrogen, 5 pounds of phosphorus, and 5 pounds of potash — a total of 20 pounds of usable nutrients. Although the remaining 80 pounds contain some useful nutrients (also listed on the label), most of the balance is either filler or carrier left over from manufacturing.

Debating organic versus synthetic fertilizer

Most organic fertilizers derive their nutrients from plants, animals, or minerals. Synthetic or chemical fertilizers are manufactured from mineral salts. Is one better than the other? That question is the subject of countless debates and philosophical face-offs. Some gardeners believe organic is better — for their health and the health of the planet. Others say, hey, the plants don’t know where their nitrogen is coming from.

Organic fertilizers usually contain familiar-sounding materials, such as fish emulsion and kelp (seaweed). They may also contain various composted animal manures, including cow, poultry, and horse manure, as well as slaughterhouse by-products such as bone, blood, and feather meal. Some fertilizers even contain added beneficial microbes. Individual ingredients may contain limited nutrients; for example, fish emulsion contains mostly nitrogen; bone meal mostly phosphorus. But many organic fertilizer formulas contain a wide range of nutrients, especially the micronutrients that may be lacking in synthetic formulas.

If you want to be sure you’re using a product appropriate for organic growing, look for the term OMRI-Listed on the label. The Organic Materials Review Institute (OMRI) is a nonprofit that evaluates products to see whether they conform to the standards established by the National Organic Program. If the label says OMRI-Listed, the product has been approved for use in certified organic farming.

Synthetic or chemical fertilizers are manufactured from mineral salts and usually have higher N-P-K ratings than organic ones. Sold as ready-to-apply or in concentrates requiring dilution, they are sterile and provide a precise dose of nutrients. They’re usually relatively inexpensive.