Choose the correct fertilizer for your plants

A task of utmost importance at this time of year is fertilization. Fertilizing perennials in the fall instead of spring is ideal because it can be in tandem with pruning. Also with perennials’ summer activity fresh on the mind, it is easy to assess which plants need what kind of help. As OSU FactSheet HYG 1002–96 instructs: “The best time to fertilize in the northern United States is autumn, generally after the first hard freeze in October and before the soil freezes in December.”

Choosing the correct fertilizer can be quite overwhelming and sometimes feels more like a leap of faith than an educated decision. After all we garden to relax and enjoy, not to flashback to high school chemistry class. 10–10-10? Fish emulsion? 10–20-30? Nitrogen? Manure … to grow food in? A brief scientific lesson to dissect the good, bad and ugly of fertilizer.

First and foremost: the numbers. Most products sold as “fertilizer” have three numbers somewhere on the packaging. The three numbers are divided by dashes. Most commonly is 10–10-10. 10 of what? Your chemistry teacher, or OSU FactSheet HYG 1002–96 would tell you, “The analysis or grade of a fertilizer refers to the minimum amounts of nitrogen (N), phosphorus (as P2O5), and potassium (as K2O) in the fertilizer, and is always printed on the bag, can, or bottle. A 10–10-10 fertilizer would represent 10 percent nitrogen, 10 percent P2O5, and 10 percent K2O. Therefore, in 50 pounds of 10–10-10, there are 5 pounds of N, 5 pounds of P2O5, and 5 pounds of K2O.”

Essentially a 10–10-10 fertilizer is like a multi-vitamin. It is designed to be balanced, supplement on a broad spectrum, and improve overall strength of a plant. Just as one takes a multivitamin in a preventative manner to boost the immune system, a 10–10-10 fertilizer gives a plant a general boost in health and vitality. That is why 10–10-10s are a good place to start with fertilizers. For plants that have performed well all summer, recently been pruned, or plants that seemed a little lackluster this past season but with no precise culprit, a 10–10-10 is probably the fertilizer to choose. For more specific problems, a more specific fertilizer is in order.

I have several hydrangeas and a common problem for them is vigorous leaf growth and little-to-no flowering. If the plant is getting enough sun, the problem is likely an abundance of nitrogen and not enough potassium and phosphorus. This is also a common problem with other flowering shrubs like roses, also fruits and vegetables. More blooms on a tomato plant means more tomatoes. Without being brand specific, my favorite fertilizer for a problem like this is an organic fish-emulsion and seaweed fertilizer. As you might imagine, it does not smell pleasant but it works exceptionally well. It is a 20–30-10. The 30 percent phosphorus helps boost blooms while still being a balanced fertilizer with 20 percent nitrogen and 10 percent potassium. Vigorous new leaf and green growth signifies there is ample nitrogen, but when none of that new green growth is flowering, the 30 percent phosphorus will help. It was with this discovery a few years ago where I learned of another question in fertilizer. Organic or inorganic? Not in the sense of organically grown apples vs. apples, but naturally occurring substances vs. synthetic substances. There are not mashed-up bananas in most fertilizers to supply potassium, instead there are often synthetic compounds that supply potassium and are designed for the plant to absorb rapidly. Essentially this is a matter of choice. Organic fertilizers sometimes take longer for the plants to absorb, but inorganic fertilizers can be more likely to cause burn to the leaves if over-applied.

Conversely, another common problem — plants that are abnormally small and diminutive, with little new leaf growth. This could be due to a lack of nitrogen. Often flowering plants from the nursery are loaded with bloom boosters of potassium and phosphorus creating an abnormal amount of blooms on a small, sometimes lacking nitrogen. In this instance the ideal fertilizer would be one where the first number (that represents nitrogen) is the highest.

If this all makes your head spin, there is far less guesswork or potential for error in just adding compost and manure. Compost and manure are natural, balanced and add a tremendous amount of health to the soil. It is a little bit more work to mix compost and manure into the soil around your perennials, but they will likely reward you next year with healthy, beautiful growth and blooms.

An easy way to get the benefits of compost with the ease of spreading or pouring liquid fertilizer is compost tea. Compost tea certainly sounds strange but gardeners who make and use it swear by its results. Compost tea is very easy to make, and just like the tea we drink is entirely customizable. And because everything is diluted by water it is fairly fool-proof. To make compost tea all you need is compost, a five gallon bucket, and water. The ratio is about 1 cup compost to 1 gallon water. So for a five-gallon bucket that is 5 cups of compost (about one shovel-full). Add water to the bucket and let steep anywhere from 24–72 hours. If you are looking to cut that time, add some boiling water to raise the overall temperature. The compost will steep more rapidly in hot water.

While you cannot be completely certain what your soil needs without a soil test, which OSU Extension offers for around $40, observing how plants behave can provide insight onto which fertilizer may be best. And the most important element to choosing fertilizer is knowing what all those numbers mean. If you would like to speak with a Master Gardener about navigating the world of fertilizer, please call our Help Line at 740–833-2030, email us at mgdelco@gmail.com or visit our blog at mgdelco.blogspot.com.

Stephen Jones is an OSU Extension Master Gardener volunteer.