Know what your plants need before fertilizing
July 7, 2022 | View PDF
In a perfect world, garden plants would feed themselves. As it is, we've got to help them along sometimes.
Plants take up nutrients from the soil but when those nutrients are missing, it's time for fertilizer.
"Plants pull out nutrients as they grow," said Weston Miller, a horticulturist with Oregon State University Extension Service. "Unless they're put back in, the plant suffers."
But knowing which fertilizer to choose, how much to apply and which plants to feed is not always obvious. Knowing the plants' needs makes all the difference.
"You need to know what plants you have and what their requirements are," Miller said. "Doing research up front really pays off."
Most landscape trees and shrubs and many perennials don't need fertilizer at all unless they're showing signs of stress like yellowing foliage.
"Prepare the soil by adding compost or other organic material well ahead of planting, you can have a fabulous landscape that needs very little fertilizer," he said.
Annuals are heavy feeders and should be hit weekly with a fertilizer that dissolves in water, especially if growing in containers.
Vegetables also need plenty of nutrients. At planting time, incorporate fertilizer as indicated on the label. Early in the season also use a water-soluble version to get plants off to a good start. For corn and garlic, feed two to three times with a fertilizer high in nitrogen (the N in the N-P-K on the label). Blood meal is a good choice for organic gardeners, Miller said. The rest of the time, watch for yellowing foliage. If the plant starts to look stressed, add some fertilizer.
Fruit trees and berries, especially blueberries, are heavy feeders and will be more productive if fertilizer is added at the right time. Each type of plant has specific guidelines. You can search the Extension catalog for publications about specific plants like Growing Strawberries in Your Home Garden and Growing Blueberries in Your Home Garden.
How you fertilize lawn depends on a couple of things. If you irrigate and want a green lawn in summer, feed at least twice a year. Use the "holiday" schedule of Memorial Day and Thanksgiving. Always use a spreader to get good coverage and avoid over fertilizing. If you mulch mow and allow the clippings, which are high in nitrogen, to stay on the lawn that is equivalent to one application. If you choose to let your lawn go dormant, one feeding a year can keep the lawn more competitive against weeds.
Some plants like roses and hydrangeas can benefit from more feeding than many other shrubs in the garden. You can find fertilizers labeled specifically for them, as you can for plants that like an acid (low pH) soil. These include blueberries, gardenias, rhododendrons and citrus.
Plants don't care if they get nutrients from organic or synthetic fertilizers, Miller noted. It's all the same to them. Organic fertilizer ingredients are made from plant, animal or mineral sources. Examples are alfalfa meal, kelp, blood and bone meal, soft rock phosphate and green sand. Inorganic are derived from a chemical manufacturing process.
All fertilizers have a ratio, which is the percentage by weight of N-P-K (nitrogen, phosphorous and potassium). Generally, nitrogen is used by plants to produce green growth, phosphorous for roots and potassium for flower and fruit development. A balanced fertilizer such as 10-10-10 is a good choice for vegetables and other high-nutrient plants.
If you're looking for an organic fertilizer check to see if they have OMRI on the label. For vegans and vegetarians, avoid fish, blood and bone meal products. For inorganic products, choose a slow-release fertilizer. If all the nutrients aren't needed by the plant, the fertilizer can percolate down into the ground water or run off into waterways causing pollution.
Lime is a soil amendment to raise the pH, an important consideration for vegetables. For those gardeners living west of the Cascades, applying lime is recommended by a soil test or based on the label of lime products.
Always water in fertilizer after applying and don't forget to read the instructions. Using too much can burn plants.
For additional information on fertilizing, refer to OSU Extension's Fertilizing Your Garden and Growing Your Own.