Hill Gardens of Maine - Sorting Out The Fertilizer Puzzle

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Sorting Out The Fertilizer Puzzle
by Fred Davis, MG, Hill Gardens of Maine (To view other articles, click Archives)

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 stresses.

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 key phrase.

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 anyway!

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: balance.

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.

More about fertilizers: They don't have to be complicated. Understanding controlled-release fertilizers. July 4th in Northern-tier gardens (give 'em "lunch"!). A Special Report On Fertilizers (.pdf).

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