Decoding Fertilizer Labels

On a recent visit to a garden center, a customer was overhead asking  about the prominent numbers on the front of the fertilizer package. What  does  13-25-12 mean?

The numbers correspond to the percentages of three different compounds: Nitrogen, Phosphorous and Potash (or Potassium). Each can contribute to plant health. Nitrogen, along with other nutrients, helps plant foliage grow strong. It is used in large amounts by plants. Ever notice your lawn has a growth spurt after fertilizing? That’s likely Nitrogen at work. High Nitrogen fertilizers  make for quick growth but weaker plants, which can more easily succumb to pests and diseases

Phosphorous helps plants develop strong roots and abundant flowers and is very beneficial to sandy soils, which are common in Colorado. Phosphorous does not leach out of our soil, so continual additions are not needed in the landscape.

Potash aides in overall plant health. Front Range soils have ample potash, so it is not advised to add more in our soils. It won’t help either your garden or your wallet.

So which is best?   There is no one-size-fits-all answer, but in general:

  • Lush, flowering garden pots benefit from weekly feeding of water-soluble fertilizer high in phosphorous.
  •  Houseplants can benefit from fertilizers with equal numbers of each nutrient.
  •  In the garden, it is best to get your soil tested. Information can be found at: http://www.soiltestinglab.colostate.edu/. This will greatly help you add the correct nutrients for your soil and growing needs.

A word of caution: More is not better. Always follow the application instructions and give the plant time to absorb the nourishment.

Lots more information on fertilizers and soil amendments can be found at:  http://www.ext.colostate.edu/mg/gardennotes/234.pdf

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