Answers to your gardening questions
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Sorting Out The Fertilizer Puzzle
Welcome through Fred's Garden Gate! During the past few weeks, as
houseplants across the northern half of the US gradually stretch from want of light, and weaken from hunger, several people have asked
about fertilizers. It seems that just about every gardening book or column has a different suggestion about formulas, frequency of
application, and specific brands.
It's not uncommon for us to receive mail with lists of
different, very complex formula fertilizers, usually accompanied by the question, "I read this in a gardening book. Please tell
me if I have listed all the different plant foods that it said I need." Sadly, those questions—and the wide variability of listed
formulas—point to apparent misinformation out there in relation to plant foods.
Yes, the subject of plant foods
may seem confusing; but fertilizers are really simple when looked at objectively—and the same principles of plant nutrition
that apply to houseplants during the winter months, apply just as effectively in the spring and summer garden. So this time, lets re-visit a very brief "short-course"
in fertilizers that should help clarify the subject for you.
Of the three numbers in a fertilizer formula, the first one is
always nitrogen (N), and is primarily for leaves—lush, green, fast-growing, and healthy foliage. Nitrogen in excess (out of balance),
however, makes plants grow very quickly, and they may become weak and susceptible to stresses like drought, heat, and disease.
The second is always phosphorus (P), and is generally required in
substantial quantity for the production of flowers and fruit (seeds). Phosphorus is the element that gives a plant the chemical signal
to bloom. Without adequate phosphorus, the plant produces flowers poorly and, once again, it becomes susceptible to environmental
The third number is always potassium (aka: potash—K), and is
necessary for strength, durability, toughness, winter survivability, disease resistance, insect tolerance—in short, health. Potassium
is important in our USDA growing zones, and should be in balance with the other two elements in the formula. "In balance" is a
A small "wrinkle" in the mix is the relationship
between phosphorus (for flowers) and soil pH. (Understanding
Soil pH.) If pH is below about 6.3 (moderately acidic), phosphorus is locked-away
("bound") by other mineral elements in the soil, so the plants are unable to access it. You can pour all the phosphorus in
town around a plant but, if the pH is too acidic, that plant won't be able to utilize it. Therefore, no flowers; well, not very many
Whittled down to it’s simplest terms, if the plant doesn’t
normally bloom (strictly foliage plants, whether indoors or out), go for a higher first number (N). If you want a plant to sport
handsome flowers, back off a bit on the nitrogen, and lean toward a higher middle number (P).
Having said that, Osmokote 14-14-14 is a great general-purpose
plant food for most houseplants—an almost perfect balance of primary nutrients that will be released over the space of about 3 months.
Nutrient release is "controlled" by soil temperature: little is released in cold soil; as the soil warms (and the plant
"wants" to grow faster), more is released. Perfect for corresponding with plant growth needs. This is the formula I use on
almost all of our houseplants and greenhouse containerized plants in pots, planters, or baskets, and we get loads of color.
Some Osmokote formulas lean more heavily in the direction of
nitrogen, and are used by many nurseries to stimulate rapid growth early in the season. Nurseries can't sell wimpy little plants; both
they and the plant-buying public want big, healthy looking specimens.
Other Osmokote mixes are heavily skewed toward
nitrogen and are generally used on woody plants like shrubs that aren't normally expected to bloom. There are also special formulas for
rhododendron, azalea, holly, mountain laurel, and others. Be very careful in your selection of Osmokote blends. Remember:
As the time for normal, seasonal blooming approaches for the types you've chosen to grow, I'd recommend you supplement with one
of the "blossom-boosters" like Peters 10-30-20 powder-for-mixing-with-water. Notice the high middle number: right on plan! A supplement once a week should have negligible impact on
still-active Osmokote remaining in the container. My personal preferences lean away from
Miracle-Gro—far too much nitrogen and
advertising hype for my taste.
So, unless you have orchids or very tender types like African Violets (both of which prefer their own special nutritional formula), the two mentioned above are about all you need. Leave all the others to the people with lengthy, complicated lists of a different special plant food for every single variety or specie they attempt to grow. Just remember: fertilizers don't have to be complicated. Keep it simple; enjoy healthy plants and abundant flowers.
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