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The Iowa Gardener: Fertilizer 101
Know the basics to make your garden grow
Plants, like people, need nutrients to thrive and do their best.
In our yards, the soil already contains certain nutrients, but when we plant fast-growing plants that we want to flower and produce, they need supplemental nutrition or plants will grow small and weak. That’s where fertilizing comes in.
Not all plants need additional fertilizing. Trees, for example, usually do just fine with the nutrients already found in the soil of your yard. Large shrubs, those growing 5 or 6 feet or higher, also are usually fine with the nutrients in your yard.
Most other plants need some additional nutrition. And some plants are what are called “heavy feeders.” Without additional nutrients from the gardener, they won't perform their best. Turf grass is one of the hungriest, but many heavy producing vegetables, like tomatoes, and fast-growing annuals, like marigolds and impatiens, do best with additional nutrients.
Often, when new gardeners plants vegetables, flowers, and herbs in a new space they do very well the first year. That’s in part because the nutrients in the soil haven’t yet been depleted. But after a year or two or three in the same spot, they may produce less well as the nutrients are depleted.
Here are some strategies to keeping your smaller shrubs, flowers, vegetables, and herbs well-nourished and performing the best:
- More is not better. Follow any fertilizer package directions to the letter. If you over fertilize, you can burn the roots of your plants or encourage lots of green growth with few flowers or vegetables (like the 10 foot tomato plants that don't produce a lot of tomatoes).
- Know what’s in your fertilizer. Again, read the package directions. The three major nutrients are N (nitrogen), P (phosphorus), and K (potassium), always listed in that order. Each does something different, expressed with the ditty “Up, down, all around.” Nitrogen helps plants grow lots of tall green growth, phosphorus helps plant establish strong roots, and potassium helps overall health. For most outdoor plant fertilizers, look for a "balanced" fertilizer that has roughly equal percentages of each, like 10-10-10 or 10-15-10, or something similar.
- Not sure how often to fertilize? You can make it easy on yourself by simply scattering a granular slow-release fertilizer one time in spring, such as Osmocote or any other similar synthetic fertilizer. Some gardeners like to sprinkle bone meal (rich in phosphorus) or blood meal (rich in nitrogen) in the spring, but both are rather expensive for larger scale use and both can attract animals.
- Manure is an excellent, organic type of fertilizer, rich in nitrogen. However, it needs to have been “aged” about a year, just sitting in a pile, or it can burn plants when you apply it or work it into a soil. I think it’s best used when added to a compost pile, along with grass trimmings, leaves, and other yard waste.
- Choose from other organic fertilizers, such as fish emulsion or Milorganite, processed sewage sludge. Again, follow package directions exactly.
- Compost is just about the perfect fertilizer, which is why gardeners call it "black gold." It contains all the main essential nutrients, and lots of micronutrients and beneficial microbes. It also improves soil texture and attracts earthworms to further fertilize the soil with their castings. It's hard to use too much of it in your garden. It never burns. Compost is also organic and keeps waste like grass clippings, autumn leaves, and food waste out of landfills. And it's free, compared to synthetic fertilizers, which have shot up in price in recent years. Work it into the soil of your vegetable garden. Make it a practice to work in a spadeful or two into every planting hole. Spread it on the soil surface around plants, a practice called “top dressing.” I even spread a thin 1/4 inch layer on my lawn to smooth out bumps and feed the grass.
- Except for compost, don't apply fertilizer to plants after August or so. Fertilizer stimulates new growth, and can spark tender new growth that will get damaged by winter cold.
Veronica Lorson Fowler is co-publisher of the Iowa Gardener website at www.theiowagardener.com.