Plants get their energy from the sun, but they need 17 elements to grow. It’s pretty amazing when you think about it—only 17. They don’t use neon or helium (no squeaky little plant voices) or lead, for example. They get carbon, oxygen and hydrogen from the air or water, and the other fourteen they absorb from the soil. When you buy a packaged fertilizer, the container will tell you what minerals (compounds of elements) are in the mix. The three that plants use in the highest amounts are nitrogen, phosphorus and potassium, and the container tells you the amounts of these three in the mix as a ratio like “15-5-10.” The first number represents Nitrogen (N), which encourages green, leafy growth. The second number represents a Phosphorus compound (P); proper levels of phosphorus improve root growth and flowering, among other things. The last represents a Potassium compound (K); potassium improves heat and cold tolerance. Other elements typically included in the mix include magnesium, sulfur and iron.
There are different formulations of fertilizers for different uses, so you will find fertilizers labeled for use on lawns, or for tomatoes, or for orchids. For lawns you want a lush, green, leafy lawn. For tomato plants you want lots of tomatoes, for orchids you want flowers. The formulation/ratio of the ingredients in the particular fertilizer will vary to reflect the outcome desired. Use the proper amount: follow the package label instructions. The label is always the key to proper application. Too much fertilizer can cause weak, floppy growth, reduce flowering, or burn the roots–so you never want to overdo it.
Organic vs. inorganic: organic fertilizers are made from biological products such as composted manure or seaweed. Inorganic fertilizers are chemical compounds that come from a variety of sources including petroleum. Organic fertilizers are slow-acting and require that the soil be warm enough for bacteria to break them down into simpler forms that plants can use. The inorganic fertilizers may be slow-release or immediately available depending on the way they are made.
I have a shady garden of trees, shrubs and perennials, and for these I use an organic—Plant-tone. I scatter it by hand in the early fall so that the plants can absorb the nutrients in the fall or spring, for spring growth. I like the “Tone” fertilizers because it’s hard to overdo it with them. You can also apply in spring with the idea that they will start to work sometime in April as the ground warms up. I don’t use much because I let my oak leaves decay in place in my plant beds.
I really don’t care much about my lawn, and it shows. I’ve tried to make it as small as possible over the years because I hate mowing and I don’t like using lawn herbicides, so I have what might be delicately described as a mixed planting of lawn grass with a sprinkling of dandelions, wood violets and clover. I use inorganic (but slow release) Turf Trust in the fall for the lawn, and am rewarded for about two months in the spring with a nice lawn, until the stress of trying to grow in dry shade kicks in. For cool season grasses that stay green all year, the bulk of fertilizing is done in the fall. For warm season grasses (Zoysia) that turn brown in winter, the fertilizing is done in June when they are green. Click here to learn more about fertilizing your lawn in Maryland. Note that it is now illegal in Maryland to fertilize lawns in the winter to reduce runoff to the Chesapeake Bay. Fertilizer runoff, especially phosphorus, encourages unwanted algae growth. Because phosphorus is considered to be available in our soils in enough quantity for good growth, phosphorus has been removed from lawn fertilizer formulations in Maryland.
For houseplants I use the water-soluble non-organic Jack’s Best products. Houseplants require little fertilizer in the winter months, so I start up in March and fertilize until October, on a sort of when-I-think-about-it basis. I find that in the less-than-optimal conditions of a window sill or a table, your goal is more to keep the plant alive and green rather than encourage a lot of new growth, since light is often the limiting factor. So, I fertilize less frequently than recommended on the package. I use the Jack’s Classic because it’s easy, it doesn’t smell (some of the organics do), and there are specific formulations for some of the trickier plants like orchids, palms or citrus.
For outdoor containers, I use the Jack’s Best or slow-release Schultz All Purpose Extended Feed Plant Food, which only has to be applied once or twice a season. I don’t have annual or vegetable gardens (not enough sun). If I did, I would likely use an organic fertilizer such as Garden-tone.
Before starting a lawn fertilizing program, or a vegetable garden, it would be a good idea to get a soil test. You submit soil samples from your garden to a soil-testing lab and they test the soil and make recommendations for what fertilizer or other additives (such as lime) that you should add for optimum results. Residents of Maryland can submit to the University of Delaware. There is a fee. Click here to order a test kit.
by Larry Hurley, Behnke horticulturist