Science of Slow Release Fertilizers

Fast versus Slow Release Fertilizers

There are many choices you have for fertilizing.  In the end, there are two main categories: Slow Release Fertilizers and Quick Release Fertilizers. Fast release fertilizers are useful because their nutrients are immediately available to plants and have an immediate response. In the commercial world, Miracle Grow is the most common example and most well-known brand.  Fast release fertilizers, however are subject to rapid depletion from the soil due to leaching. If fast release fertilizers are applied too heavily, the plant can be damaged usually by burning.  Not only that, but the nutrients can leach out and hurt the water supply. The other type of fertilizer is a slow release fertilizer. Two types of slow release fertilizers are available. The first type is an organic fertilizer. Organic fertilizers are made up of organic materials that must first be broken down by microbial activity before the nutrients are available to the plants. In general, organic fertilizers take a long time to release nutrients and these nutrients may not be available when the plant needs them. For organic fertilizers to be effective the soil needs to be moist and warm enough to promote soil organism activity. The other types of slow-release fertilizers are those that are slowly soluble or coated. Soluble and coated fertilizers (typically in pellet or spike form) depend on soil moisture and temperature to release nutrients. Nutrients are released over a period of time, sometimes lasting up to 12 months. Fewer applications are needed with slow release fertilizers, but nutrients may not be available when the plant needs them.

Fertilizer Basics

Many people overlook the importance of fertilizing indoor plants. That’s unfortunate because feeding is essential to keeping healthy, beautiful plants. Unlike an outdoor garden, where nature provides rain and plants can send new roots searching for food, the nutrients available to a houseplant are strictly limited by the amount of dirt in the pot and whatever else you give it.

Think of fertilizer as the second half of your potting soil. When your potting soil is fresh, your plants won’t need much if any fertilizer. This is especially true of modern, fortified potting soils, which often have fertilizer and other enhancements mixed in. But after about two months, the plant will have consumed the nutrients in the soil, so you’ll have to fertilize if you want continued, healthy growth.

As a word of warning, always follow the label instructions on your fertilizer. Too much fertilizer can kill a plant or scorch its leaves, and there are environmental concerns to fertilizer overuse as these nutrient-rich solutions find their way into groundwater supplies.

Different Types of Fertilizer

Fertilizers come in several different varieties: liquid, sticks and tablets, and granular and slow-release forms. Of these, the two best suited for indoor use are liquid and slow-release fertilizers. Sticks and pills seem convenient, but they don’t distribute nutrients very well through soil and, once you’ve shoved a fertilizer stick into your pot, you have no control over its release. Granular fertilizers are really designed for outdoor use.

  • Liquid fertilizer. Liquid fertilizers are added to your watering can. Depending on label instructions, you might fertilize every time you water or every other time. There are literally dozens of liquid fertilizers on the market. The advantage to liquid fertilizer is a steady supply of nutrients that you control. It’s easy to suspend feeding when the plant is dormant during the winter months. The disadvantage is remembering to do it every time.
  • Slow-release fertilizers. These products have quickly become favorites for many gardeners and professional growers, both indoor and out. Slow-release fertilizers, like Dynamite and Osmocote, are coated in time-release shells that slowly leech nutrients into the soil. A single application of Dynamite can feed your plants for up to nine months, while Osmocote lasts about four months. Their main drawback is their price, but because they last so long, it evens out in the end.

Buying Fertilizer

All general-purpose fertilizers contain the basic macronutrients that plants need to grow, including nitrogen, phosphorous and potash. Each macronutrient has a special function:

  • Nitrogen encourages healthy foliage growth
  • Phosphorous encourages root growth
  • Potash encourages bigger, healthier blooms

Specialty fertilizers, such as African violet fertilizers, contain optimized proportions of these nutrients for particular kinds of plants.

In addition to these macronutrients, better quality fertilizers also contain micronutrients such as boron, magnesium and manganese that will encourage healthier growth.

(Many thanks to Jon VanZile for some of the above content)