Tips to Remember When Selecting a Plant Fertilizer


While many plants will do OK with little or no fertilizer they will only reach their full potential with the correct nutrition. However, more fertilizer is not necessarily better; it is possible to harm your plants by feeding them too heavily. Here are a few tips to remember when selecting a plant fertilizer.


All fertilizers are labeled with three numbers. These numbers represent the ratio of nitrogen (N), phosphate (P) and potassium (K). In basic terms, nitrogen promotes foliage growth; phosphate encourages healthy root growth and flower production while potassium promotes the overall health of the plant by building disease resistance.


There are three main types of fertilizer: water soluble, slow release and controlled release. Water soluble fertilizer packages will be filled with either a crystalline or liquid fertilizer. These substances should be mixed with water according to the directions on the package and added to plants every 7 to 14 days when you water. Look for a fertilizer package with a formula that has a fairly large N number (20 to 24), a smaller P number, about half of N, (10 to 12), and a K number that is equal to or slightly less than N (15 to 20). The fertilizer package would have the following formulas 20:10:20, 24:12:17 or 20:10:15. These fertilizer ratios should work best with a variety of plants. Follow the directions on the package for the correct usage of your specific fertilizer.


Slow release fertilizer looks like pellets, about a 1/3 of the size of a pea. A small amount of nutrients are released over a period of several months, but how much is released depends on microbial activity and soil temperature. There are microbes in the soil that are activated by warm temperatures; therefore more nutrients are released with warmer temperatures. Slow release fertilizers can last for several months but with temperature as a factor, it is difficult to predict how long the fertilizer will last.


Controlled or time release fertilizers are similar to slow release fertilizers, but rather than the amount of nutrients released being determined by microbial activity, they are directly controlled by temperature. If the temperatures are cool, not much fertilizer is released because little is needed due to low plant growth rates. Controlled release fertilizers will last about two months to more than a year, depending on the formula chosen. You should choose your specific formula based on the length of your growing season. In other words, annuals that will last for four months do not need an eight month fertilizer and shrubs and perennials would benefit from a longer term formula. The fertilizer package should tell you about how long the fertilizer will last.


If you are adding plants to a landscape, be sure to add and mix the appropriate amount of fertilizer to the soil at the bottom of hole before planting (consult the package for specific amounts). Roots can be burned by direct contact with slow release fertilizer, if you are applying fertilizer to an already established plant, top dress according to the directions on the fertilizer package.


Should I use a water soluble or controlled/slow release fertilizer? In general, most people are best served by using a controlled or slow release fertilizer. You would not have to mix fertilizer every week to add to your plants and you would only have to apply fertilizer once or twice a growing season.


Here are some cases when you should supplement your controlled release fertilizer with doses of a water soluble fertilizer.


1. If you have plants in pots that are "heavy" feeders (those that need a lot of nutrition) you may want to use a water soluble fertilizer every couple of weeks to boost nutrition levels.

2. If you have gone through a long rainy period or had a heavy rainfall, a dose of water soluble fertilizer will return some nutrition to your potting mix and help your plants rebound.

3. If your plants have grown very large, supplemental doses of water soluble fertilizer may help them maintain lush and continuous growth.





- Proven Winners

Back to Growing >