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Understanding Fertilizers

November 2013

I think we have heard every question and comment about fertilizer, and that's no bull. It seems to be a mystery for many, so in this Plant Digest I will try to shed some light on this important topic so we can all give our plants what they need when they need it. 


Understanding Fertilizers That Help Our Plants Thrive


We all know that plants need fertilizer, but what kind do they need? When do they need them? Do plants need more than one feeding per year? How much per plant? Organic or Chemical? Can I hurt my plant with too much? Will it hurt the environment? What influences the production of flower buds? What is N-P-K? Should I use a dry granular or liquid fertilizer?  If I use a dry granular, do I have to scratch it in, or just sprinkle it about, and where should the fertilizer be applied? I have heard that I should use a controlled release type...should I use one that is temperature release or moisture release? Will any fertilizers be a health risk to children or pets? What is the difference and which is better...Osmacote, Nutricote, or Apex Polyon? What are some symptoms of fertilizer deficiency in plants? What about using compost as fertilizer?

Requirements of All Plants

Plants are living organisms that need soil, water, air, light, and room to grow and thrive.  

It is often believed that most all plants do not need to be given fertilizer, as it is supplied to them by the soil they are growing in. This can be true for plants growing out in nature, but for our garden plants to perform the way we would like, adding nutrients yearly is often necessary. It might be good at this time to discuss soil and how it works. 

The Make Up of Soil

What we walk on and what plants grow in is 50% soil, and 50% empty space. The soil part is 90-95% rock, broken up into tiny particles over time, and 5-10% organic material. The empty space is filled with water or air, both important for plant life. 

Plants drink the nutrients they need

The bigger molecules of nutrients necessary for plant growth are broken down into useable smaller molecules by physical, chemical, and biological processes so they can be taken into the plant via the roots. Sometimes soil fungi is involved in this process, and it has been documented that if these soil fungi, called mycorrhizal fungi, are present plants are healthier and have more vigor. Some bagged soil mixes and organic composts sold today boast that their products contain these valuable fungi.  

Periodic Elements of Soil

The 14 elements plants typically get from the soil, together with the symbols chemists use for them include: nitrogen (N), potassium (K), calcium (Ca), magnesium (Mg), phosphorus (P), sulfur (S), chlorine (Cl), iron (Fe), boron (B), manganese (Mn), zinc (Zn), copper (Cu), molybdenum (Mo), and nickel (Ni).     

Symptoms tell the tale

Some elements are found in abundance, while others may be scarce. Plant growth, or leaf and stem colors, can be indicators of elements missing and needed for good plant health.

The rhododendron foliage above has chlorotic foliage due to a lack of nitrogen availability. This could be rectified by fertilizing the plant, but this may also be due to the ph being too high or too low, thus preventing access to nitrogen. 


In general, for the best results, garden soils must be replenished of necessary elements yearly to insure that plants thrive.  A yearly application of a well-balanced fertilizer will generally do the trick.

Types of Fertilizers 

DEFINITION OF FERTILIZER: anything that's added to soil to help plants grow.    

 Natural or Organic Fertilizers come from

(a)Animal or plant parts like bone meal, blood meal, manures and urea, alfalfa meal, dried seaweed, or cottonseed meal.

(b)Rocks or minerals like Rock Phosphate, Dolomite, Greensand, or Sulfur.

Synthetic Fertilizers come from chemicals like petroleum, acids, or other chemicals.


Which is better?    

Often this is a matter of opinion. Natural or Organic fertilizers tend to be much lower in concentration, and may protect plants from some disease.

Scientists are just beginning to do research on this, and it could be a reason to use natural rather than synthetic fertilizers in some situations.

(University of Wisconsin).

These organic elements may also feed soil organisms that often contribute to building a bridge from nutrients into the plants. These types are considered to be "slow release".  These types are more desirable when growing food crops organically. 

Synthetic Fertilizers are more concentrated and may feed over a longer period of time, and may provide a wider range of elements that plants need. Mostly they are more convenient. One note of caution, many chemical fertilizers are quite strong and can damage plants if too much is applied. Always feed a little less initially and see what the results are before upping the application amounts. Some are classified as 'Controlled Release' synthetic fertilizers. These, like the  'slow release' organic fertilizers, dissolve over a set time period. These can be sold as an "8 month" or "12 month" feed, for example. Commercial growers use these predominantly  as they can count on a steady feed over a set amount of time. 

Control Release Chemical Fertilizers

These are prills, polyamer coated to cause an even release of the nutrients, thus reducing any chance of "burning". The most commonly used of these types are Apex Polyon, Osmocote, and Nitrocote. It has been documented that Osmocote releases the fastest and is more temperature influenced. Nitrocote releases very evenly over the release period, and Apex Polyon releases the slowest with very uniform results. Apex Polyon performs best in the cooler maritime climates. 

Safety of Fertilizers  

In general, plant fertilizers, both organic and chemical are completely safe around children and pets. Where there is concern is when using lawn food that contains herbicides like "weed and feed". These, I feel, should be used with caution as these herbicides can "gas off" over a few days time, and may be a health risk to children and pets.  If a plant fertilizer is saying it will also kill weeds and insect pests, I would consider it risky to use around children or pets. It's just common sense. 

Understanding the numbers

On every bag of fertilizer you should see 3 numbers, like 6-7-5 on this bag. These numbers represent the percentages in this bag of the three nutrients that plants need in the largest amounts (in order, these are nitrogen, or N, phosphorus, or P, and potassium or K). If, for example, the bag weighs 100 pounds and has the numbers 6-7-5, there would be 6 lbs of Nitrogen, 7 lbs of phosphorus, and 5 lbs of potassium. The remainder of the weight is made up of elements in lower concentrations, inert ingredients and coatings. This brand lists the ingredients Derived from: Blood Meal, Feather Meal, Polymer Coated Sulfer Coated Urea, Alfalfa Meal, Methylene Urea, Kelp Meal, Ammonium Phosphate, Treble Super Phosphate, Sulfate of Potash, Sulfate of Potash-Magnesium.

How much fertilizer per plant?

(Don't dump a pile of fertilizer by your plant)

Some plants are heavier feeders than others, and some plants require very little to no fertilizer to thrive. So, you will have to do a little research as to how much your plants need. For rhododendrons, we use the formula of 1 cup scattered around the drip line for a plant 3 ft. tall or less, and 1.5 -2 cups for plants larger than 3 ft. tall. This is a very general guideline, and if your rhododendron is huge, obviously it will take more fertilizer. Some might say "I see these huge old rhododendrons around that bloom profusely every year, and no one is feeding them. What's up with that?". This is very true about old established rhododendrons, but feeding them might offer some desirable benefits like generating new shoots and branches, thus filling in an old straggly plant.

How often do I need to fertilize? 

The following is for slow release granular types of fertilizers:

For rhododendrons, we recommend a late winter feeding, around Valentines Day, to give your plants the necessary energy for the upcoming bloom. This works well for us, but we are in a maritime climate that has mild winters. This schedule would not make sense if you have snow, so in that case I would feed just after blooming. Using dry granular slow release fertilizers is easy as well because you do not have to scratch it in...just sprinkle it on and let your watering or the rain dissolve it into the ground around the plant.

Late Spring Feeding

If you only plan on fertilizing your rhododendrons once a year, this is the time to do it, as this feeding will stimulate more flower buds to be produced, and this is the reason we are growing these plants, right? The fertilizer must be present in the biology of the plant when it is ready to start making flower buds, or you will not get the results. So, make sure you scatter your bloom fertilizer around the drip line just after your plant blooms. 

Using a Liquid Feed

What about water soluble types of fertilizers like Miracle Grow?

Water soluble fertilizers are absorbed by the plant either through the leaves or roots, and this is faster acting. A plant can be greened up quickly by dosing the leaves with a water soluble fertilizer, but the process may have to be repeated every few weeks through the growing season as plants use up this type of feeding a lot quicker. This approach works well for garden centers wanting to keep their stock looking "top notch", but for the home gardener, a slow release type of granular fertilizer is superior. 

Flower Bud Production

For many years it was thought that phosphorus was the main ingredient that influenced the production of flower buds, and this is true on many levels. However, it was found that nitrogen was a bigger part of flower bud creation. Harold Greer of Greer Gardens in Eugene, Oregon has this to say in regards to rhododendrons:

"I am glad to see you emphasize the need for nitrogen to promote flower buds. I have long said that and the "proof is in the pudding" as the old saying goes. Over the many years I have grown and have seen others grow rhododendrons, those with high nitrogen levels flower best. Oregon State University proved that years ago in developing the 10-6-4 fertilizer that I modified in structure some to become 20-12-8-8. After some 25 years of using this, it works. Most gardeners do not feed enough and while there is nothing wrong with 'organic', there usually is not enough nitrogen there for best results. But I still say if what you do works for you, that is fine." - Harold Greer

Using Compost as your Fertilizer 

The addition of compost to our gardens is excellent. Compost adds the necessary humus that plants need for soil biological activity. This biological activity makes nutrients in the soil more available to your plants. However, composts are not often high enough in nitrogen or phosphorus to promote flower bud development, and that is where the application of a stronger, granular fertilizer will do the trick. The combination of using composts and granular fertilizers will be the best possible option. Now, many communities have green waste composting services where one can buy pure humus. This is another excellent source of organic material that has lots of goodies to add to your garden. We have customers who use this Green Waste Humus to top dress their plants, along with some well balanced, slow release fertilizer, and I will have to say that these plants are the healthiest that I have seen to date. 







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