Though light and water may be the most basic components for a plant’s survival, it’s also important to make sure its soil remains healthy and able to provide the nutrients that plants need to thrive. Most potting mixes already have some compost and other soil amendments included in their makeup, which help ensure long-term soil quality, but it’s occasionally necessary to supplement these with more immediately available nutrients.
What’s In A Fertilizer?
You’ve probably noticed the three numbers on fertilizer labels, something like 2-2-2, for example. These numbers are the N-P-K ratio and indicate the percentage of the three main plant nutrients: N - Nitrogen (important for fuelling new foliar growth), P - Phosphorus (helpful for root growth and for moving energy from the root system throughout the rest of the plant), and K - Potassium (also helps move water, nutrients and carbohydrates through plant tissue in addition to promoting early growth, general hardiness, and resistance to pests and diseases).
But 2+2+2 equals only 6!, you might say. The other 94% of our example fertilizer would consist of trace amounts of magnesium, calcium, sulfur, iron, boron, and other filler ingredients to aid in dispersal of the nutrients.
You may have also noticed that the N-P-K numbers are always lower on organic fertilizers. This is because these numbers can only specify the amount of immediately available nutrients, and organic fertilizers generally contain materials that break down more slowly and feed your plant over a longer period of time. It does not necessarily mean they have less nutritional value than synthetic fertilizers.
Are Organic Fertilizers Better?
Organic fertilizers are made from unprocessed components such as bone meal or composted manure and naturally occurring mineral deposits. Generally not water soluble, they release their nutrients gradually over the course of months or even years, increasing the soil’s fertility, stimulating beneficial microorganisms, and optimizing the general soil structure. There are a few exceptions: liquid fish or seaweed emulsion and compost tea can be absorbed immediately upon application, if your plant is in need of a quick boost.
Synthetic fertilizers often start with organic materials, but are chemically processed to make them water soluble and therefore more immediately available to plants. Consider using these types of fertilizers if your plants are going through a sudden growth spurt. However, used exclusively, synthetic fertilizers don’t promote beneficial soil life or improve the soil’s fertility over time.
Whichever type of fertilizer you select, it's very important to read the instructions and not exceed the recommended application rate. This is especially true with strong synthetic fertilizers, such as Miracle-Gro, which can “burn” foliage, over-stimulate growth, possibly kill your plants, and even pollute local waterways with chemical runoff. When in doubt, consider using half the recommended amount.
When And How To Fertilize
Different plants will have different fertilizer requirements over the course of the season, so it’s best to do some research to discover the N-P-K needs of your specific plants, and also to read the labels on your fertilizer before using it.
Indoor Tropicals: A good rule of thumb is to fertilize your houseplants only when they’re actively growing. Most likely this will be during the spring and summer. Feeding them while they're dormant during the winter months can burn their foliage or even kill them. When you fertilize, don't overdo it. Too much can be more harmful than helpful, so always follow the directions on the label.
Outdoor Container Plants. Container plantings generally require more fertilizer than plants in the ground, as daily waterings tend to wash away many of the soil’s nutrients before the roots can absorb them. We recommend fertilizing your container plants with a water-soluble fertilizer at half strength, once a week during the growing season. Alternatively, a slow release fertilizer refreshed every couple months also works well.
Trees and Shrubs. When planting new trees and shrubs into the ground, it’s important to apply fertilizer directly into the root zone to provide the plants with a slow and steady supply of nutrients. Once established, it’s a good idea to feed your trees and shrubs annually, in early spring.
Perennials. As with trees and shrubs, it’s good to add fertilizer to the planting hole when putting your new perennials into the ground. An organic, all-purpose fertilizer is recommended to make sure the plants have early access to the phosphorus, potassium and other nutrients they need to get established. Once your perennial garden is established, an annual application of fertilizer in early spring should be sufficient.
Annuals. Most annual flowers and vegetable plants are fast-growing and therefore especially hungry for nutrients. Try using a granular all-purpose fertilizer at the time of planting and then apply a low nitrogen liquid fertilizer once or twice a month until the end of summer. (Lower nitrogen levels will help your plants focus on flower/fruit growth, as opposed to excess foliage.)