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What do the numbers mean?


GARDEN TIP: Fertilizers 101        

What do the numbers mean? Every package of fertilizer is required by law to carry the percentage by weight of the three major nutrients, Nitrogen, Phosphorus and Potassium, found in the product.  The percentages are represented as you see them on the packaging. Nutrients are always listed in this order but sometimes minor nutrients are listed as well.  For example, a 5-10-5 fertilizer  has:

                        5% Nitrogen (N)

                        10% Phosphorus (P)

                        5% Potassium (K)

Sometimes a 4th number will be present. This will be the percentage of Sulfur. The rest of the product in the bag is either other nutrients or filler. Each nutrient affects plant growth differently.

   Nitrogen, the first number listed, is the nutrient that contributes to green, leafy, vegetative growth. Plants will grow larger, faster, and greener with the application of Nitrogen. This is an important nutrient for lawns, corn, and leafy vegetables.

   Phosphorus aids in root development and increases flowering and fruiting ability. Obviously, when you want to encourage rapid root development and/or flowering, a good percentage of phosphorus in the fertilizer is important. N.B. Bone Meal is a good organic source of phosphorus but is NOT available to the plant until broken down by soil microorganisms into a form plants can use. This is why it is NOT a good choice for rapid root establishment.

   Potassium, too, is important to plant development. It guards against diseases, develops strong cell walls, aids in drought protection and is crucial in developing cold tolerance. 

    When you know how each of the nutrients is used in plant growth, you can determine which analysis (percentage) of fertilizer would be best in different applications.


            When you check the analysis of organic fertilizers as compared to synthetic fertilizers, you will quickly see that the organic formulations have very low percentages. For example, two commonly available products have the following analysis:                   

      Organic: 3-3-2                    Synthetic: 15-30-15

If you consider only the analysis, the synthetic fertilizer would appear to give a ‘lot more bang for the buck'. But each type works differently and the numbers don’t tell the whole story.


            Organic fertilizers were once alive: animal manures and bone-meal, for example. Inorganic fertilizers come from naturally occurring, but non-living, sources. Rock phosphate and garden sulfur are examples. Synthetic fertilizers are man-made, combining a variety of elements, ingredients and processes. It makes little difference to the plant whether the nutrients are provided in organic, inorganic, natural or synthetic form. 


            The real difference is in how they work. Inorganic and synthetic fertilizers are IMMEDIATELY available to the plant. When you apply a synthetic fertilizer such as Miracle Grow, which is a fast-acting soluble  fertilizer, the 15-30-15 is immediately available. But this also means that because it readily dissolves in water, it quickly washes through the soil. Its effects are short-lived and, in consequence, it must be continually re-applied throughout the growing season.


            Organic fertilizers must be converted by soil micro-organisms into a form that plants can use.  While the analysis may be low because only a small percentage of the nutrients are available at any one time, the effect continues all season-long (sometimes for years) as soil microbes continue to break down the organic material.  And because soil microbes are more active at warmer soil temperatures, organic fertilizers are particularly beneficial in that plants grow more quickly in warmer temperatures and consequently require more nutrients.  


            While I prefer to use organic fertilizers and will use them in most situations, there are times when the ‘fast-acting’ character of a synthetic fertilizer is a distinct advantage. You may have been told to add Bone Meal at planting time to encourage rapid root development. This is, in fact, not true. Because bone meal is organic, it must be broken down by soil micro-organisms into usable form before plants can access it.  It is, to all intents and purposes, inactive until at least mid-summer.  However, it is an important source of root-growth-stimulating phosphorus over a very long period of time.  So when immediate root growth is desired, a synthetic fertilizer that is high in phosphorus (the second number) so that the nutrient is immediately effective is the one to use. This is why I recommend using a Transplanter fertilizer with a 5-15-5 analysis (and also contains a rooting hormone) when planting.

            Applying fertilizers, particularly synthetic ones, when soils are cold and plants are not in active growth usually results in the nutrients washing through the soil before the plants will actually use them.  Not only is this a waste of product and money, it increases the nutrient load in ground water and contributes to pollution!

            Organic fertilizers are valuable, long-acting sources of nutrients for trees, shrubs, perennials and vines. They feed the micro-organisms which, in turn, contribute to the over-all health of the soil..  Continued fertilization with synthetic fertilizers does not enhance the health of the soil and eventually, when soil microbes have been depleted because there is nothing on which they can feed, plant growth will be negatively affected.

Organic matter is essential to a healthy soil.

            Two other terms you will encounter when choosing a fertilizer are ‘Slow-release’ and ‘Granular’. Slow release fertilizers are usually synthetic (but may be organic which, as explained earlier, are naturally slow-release) and, contrary to common belief, are not released with each watering. They are instead formulated so that nutrients are released with warmer temperatures and most will provide nutrients from 3 to 6 months depending on the formulation. Granular fertilizers can be from organic, inorganic or synthetic sources. ‘Granular’ is just a term that means that the products are in a form than can be easily applied by hand or with a spreader. 

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