Maggie Pritchard (front), 2, of Bangor fertilizes her family's garden at a plot in West Market Square with her sister Elena (back) as part of Downtown Bangor Partnership’s Beautification Committee's adopt-a-garden program in 2016. Credit: Micky Bedell / BDN

Gardening season is nearly upon us, and fertilizer is an important element to making sure the plants have everything they need in terms of nutrients in order to thrive. Choosing the fertilizer can be intimidating, though, and it can be difficult for first-time gardeners to determine what is right for their plots.

“Crops that don’t have proper nutrition won’t yield as well, they won’t grow as fast and they may be more susceptible to pests and diseases,” said Caleb Goossen, organic crop and conservation specialist at the Maine Organic Farmers and Gardeners Association.

Knowing which and how much fertilizer to apply is important for both plant growth and the health of the local environment.

“People can over-fertilize as well,” Goossen said. “Excessive nitrogen can make plants grow rapidly and weakly [which makes them] more susceptible to pests and diseases. Nitrogen or phosphorus run-off can lead to algae blooms which can deoxygenate waterways. It can really be an ecological issue.”

Plus, over-fertilizing can be a waste of money for gardeners.

“The plant will take up excess fertility [from the fertilizer] but you won’t necessarily see better growth,” Goossen said.

Contrary to common belief, Bruce Hoskins, a scientist at the University of Maine Soil Testing Lab, said that compost alone will not provide all the nutrients that your garden needs.

“When you need organic matter, compost is a good source, but it’s not a complete nutrient source,” Hoskins said. “[It’s] probably not going to supply enough nitrogen for fast growing crops, like annual crops you would grow in your vegetable garden. I see a lot of nitrogen deficiencies from people in the growing season using compost and nothing else.”

The importance of a soil test

No matter what your approach to fertilizer, experts agree that the best thing to do is to get a soil test. You can also get a home soil test, which Hoskins said works fairly well for spot checking nitrogen and pH of the soil, but the University of Maine Cooperative Extension soil test will be much more comprehensive and better help you figure out what fertilizer you need.

You can select a fertilizer based on the results, which will show you what nutrients you are lacking in your soil.

“Depending on the testing profile you will get two or three choices for a nitrogen source, one or two sources for phosphorus and one or two sources of potassium,” Hoskins said. “You can look at the recommendations, take the ratio, check the lines and they have a huge range of different blends, [or go to the University of Maine Cooperative Extension webpage for] blended natural fertilizers and take numbers in parenthesis and figure out a ratio and see which one is available at your garden center is a closest match.”

Types of fertilizers

If you look at a bag of store-bought fertilizer, you will notice a three-number ratio. That indicates the balance of three key nutrients that fertilizer aims to provide: nitrogen, phosphate, which contains phosphorus, and potash, which contains potassium.

“For example, 5-5-5 means 5 percent of the total weight of the product is supplying nitrogen, 5 percent of phosphate and 5 percent is potash,” Goossen said. “If you apply 100 pounds on a given area you have just applied 5 pounds of each of those. If you know how much you need of each element, sometimes it takes a minute to figure out the total weight of the fertilizer [you need].”

All these three factors are important to healthy plant growth — nitrogen makes for sturdy plants with dense leaves, potassium builds resilience to pests, disease and the elements, and phosphorus encourages root growth. All three will also deplete in the soil over time — hence, the need for fertilizer.

If you are purchasing fertilizer from the store, you will have the option of synthetic or organic fertilizers. Synthetic fertilizers are chemically manufactured, made to ensure the immediate release of nutrients by penetrating the soil quickly. Because they are so water-soluble, they often need to be reapplied, especially when it comes to maintaining nitrogen.

“If you put it all down at planting time, it will convert to the nitrate form, which is what most plants require, but plants aren’t ready to take up nitrogen for about three or four weeks after you plant,” Hoskins said. “If you put it all on at planting time and we get a big rainstorm you’re going to lose a lot of that. Split the nitrogen application — if possible put a little bit down at planting time, wait a few weeks and [reapply] right before that rapid growth stage.”

Also, synthetic fertilizers can run off into the local water systems and burn your plants if applied incorrectly.

Organic fertilizers, meanwhile, are made of once-living organisms that release more slowly. They don’t have the same immediate results as synthetic fertilizers — some are even best applied in the fall so they can break down over the course of the winter. For the long-term health of your garden, though, feeding your plants with organic fertilizers and compost will build soil that is rich in organic matter and teeming with microbial life.

You can even go one step further and use natural fertilizers instead of buying bags of blends that have the nutrients measured out for you. The University of Maine Cooperative Extension has a list of natural fertilizers and the amount of nutrients they will add to the soil. They can be a little more difficult to find, though.

“The non-chemical potassium sources are a little more difficult to come by,” Hoskins said. Some of the garden centers will carry those natural materials [but] the big box stores aren’t going to carry them.”

Long-term soil health

Over time, you can take a more holistic approach to soil health than simply applying fertilizer every year. Tom Roberts, owner of Snakeroot Organic Farm in Pittsfield, said that he uses a combination of compost, grass clippings, leaves and mulch, as well as a rotation of cover crops with nitrogen-fixing properties, to keep his soil healthy.

“Basically if you’re going to think about fertilizing your garden you have to ask yourself one basic question up front: is my garden a chemistry project or a biology project?” Roberts said. “Basically what you want to do is look at what nature does [and] try to imitate nature.”

Goossen also emphasized that, when it comes to giving your plants the best substrate to thrive, the fertility of the soil isn’t the only factor you are considering.

“Folks need to take a step back first and look at the overall soil health,” Goossen said. “Yes, the plants absolutely need fertility but there’s a few things to be addressed [like] soil texture, how well does the soil drain, soil structure, biological activity [and] soil pH. If the pH isn’t where we want it, it doesn’t matter if the fertility is there — it may not be accessible to the plants.”