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Feed Me, Seymour

Fertilizing plants can be a bit bewildering but to get the most out of your plants, especially container plants, it is essential. This article will go over the basic types of fertilizers and give recommendations on when and how to apply those fertilizers.

Fertilizing plants can be a bit bewildering, but to get the most out of your plants, especially container plants, it is essential.  Have you ever wondered why some people and places seem to have larger, fuller plants?  The likely answer is regular fertilization and correct watering.  While many plants will do OK with little or no fertilizer, they will reach their full potential only with the correct nutrition.  However, fertilizer is one of those things where more isn’t necessarily better.  It is possible to harm your plants by feeding too heavily.

Fertilizing plants is an area that can be difficult to understand.  You stand in front of the fertilizer shelves at your garden center staring at the myriad options wondering, bloom booster, slow release, water soluble, N, P, K, micronutrients, bigger, more, better?  It’s enough to make a sane person, well, a little bit nuts. 

You can get really technical and bogged down in the little things when you get to talking fertilizer.  However, what most gardeners really want to know is what kind should I use and how often should I use it. 

The first question asked is often what are N, P and K?  ABC’s of Fertilizer gives a more in depth description of Nitrogen (N), Phosphorous (P) and Potassium (K).

The next question is what kind of fertilizer should I be using?  There are 3 main kinds of fertilizer: water soluble, slow release, and controlled release.  All 3 kinds have their positive and negative points.

Water soluble fertilizer is the oldest form of fertilizer.  The package you buy will be filled with either a crystalline or liquid fertilizer.  This substance is mixed with water, according to the directions on the package, and is normally applied every 7 to 14 days while you water.  Look for a formulation that has a fairly large N number (20 to 24 is good), a smaller P number - about half of N, (10 to 12 would be good), and a K number that is equal to or slightly less than N (15 to 20 is fine).  So look for a 20:10:20, 24:12:17, or 20:10:15.  This fertilizer ratio is a good bet for all types of fertilizer.  Follow the directions on the package for correct usage of your specific fertilizer.  Water soluble fertilizer is generally the least expensive form of fertilizer.

 

After water soluble fertilizer came slow release fertilizer which looks like pellets, about 1/3 of the size of a pea. Slow release fertilizers will slowly release a small amount of nutrients over a period of several months.  How much is released at a time is largely dependent on soil temperatures.  Microbes in the soil control how much slow release fertilizer is released at one time and the microbes are more active at warm temperatures than cool temperatures.  Slow release fertilizers can last for several months but since temperature has such a big impact it is difficult to predict exactly how long they will last.  Slow release fertilizers tend to be more expensive per package than water soluble, however, because you need to apply much less often they aren’t necessarily more expensive to use. 

The most recent addition to the fertilizer market are the controlled or time release fertilizers.  They are similar in appearance to slow release fertilizers but rather than working based on microbial activity in the soil they are largely controlled directly by temperature.  If temperatures are too cool no fertilizer is released.  However, when it is that cool plant growth rates are low so little fertilizer is necessary. Controlled release fertilizers are more exact and more expensive, than slow release fertilizers.  Controlled release fertilizers will last from 2 months to more than a year depending on the formulation you choose.  Choose your specific formulation based on how long your growing season is.  In other words annuals that will last for 4 months have no need for an 8 month fertilizer.  Shrubs and perennials would benefit from the longer term formulation.  The fertilizer package should tell you how long the fertilizer will last.

Slow and controlled release fertilizers are applied in a similar manner.  If you are planting in pots you can either mix the fertilizer into the potting mix as you add it to your container or you can top-dress.  Follow the package directions to determine how much to add to each pot.  Top-dressing  (photo, left) is when you add the desired amount of fertilizer to the top of an already planted pot.  It is best if you can mix the fertilizer into the top couple of inches of soil but it will be OK if you can’t.   

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

So should you use a water soluble or controlled/slow release fertilizer?  In general I think most people are best served using a controlled or slow release fertilizer.  You apply it once or in some cases twice a growing season and then just water as necessary.  You don’t have to mix up fertilizer every week or two and your plants should be perfectly happy.

There are times when it makes sense to supplement your controlled release fertilizer with an application or two of water soluble fertilizer.

  • If you have plants in pots that are “heavy” feeders (those that need a lot of nutrition), such as Supertunias®, you may want to use a water soluble fertilizer every couple of weeks to boost the nutrition level.  Heavy feeders planted in the soil are taking advantage of the native fertility of your soil and shouldn’t need the extra fertilizer.
  • If you have gone through a long rainy period or had a very heavy rainfall, an application of water soluble fertilizer will return some nutrition to your potting mix (what was there has likely washed away in the rain) and help your plants rebound.
  • If your plants have grown very large supplemental water soluble fertilizer may help them maintain lush growth.

Plants will only need fertilizer during active growth periods.  So if the plants are dormant don’t bother feeding.  If the plants are actively growing you should be fertilizing.  Be careful not to over fertilize in early spring (only a problem with water soluble fertilizers) when cooler temperatures mean plants aren’t growing as much.

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