Fertilizing Lawns and Gardens in Western Washington

From Wahkiakum Master Gardeners

One of the most challenging tasks of gardening is properly fertilizing your plants. Aside from air and water, there are three primary nutrients, three secondary nutrients, and eight micronutrients that a plant absorbs from the soil to complete the plant’s life cycle. This article will focus on the primary nutrients (nitrogen, phosphorus, potassium) and the secondary nutrients (calcium, magnesium, sulfur).

With the high rainfall in western Washington, these six essential nutrients can leach down through the soil and become unavailable to the plants. Nitrogen deficiency is the most common nutrient deficiency in western Washington lawns and gardens, as it leaches more rapidly compared to the other nutrients. The type of plant and the stage of its maturity also affect the availability of nutrients in the soil. For example, green leafy plants and lawns absorb more nitrogen and phosphorus, root crops absorb more phosphorus, and plants setting fruit or flowers use more potassium.

The only accurate way to determine the availability of nutrients and soil pH is through soil samples sent to a soil testing laboratory. Home soil test kits only give you an approximation and are not reliable. Most gardeners are not geared for high production and do not want to pay for multiple soil sample tests. However, today’s commercially available organic and chemical fertilizers found at nurseries and garden centers are tailored towards the home gardener and can produce satisfactory results without rigorous soil testing.

On any bag of fertilizer, you will see three numbers such as 16-16-16 or 10-5-4. These numbers are the percentage of primary nutrients in the bag. They are always presented in the order: nitrogen (N) – phosphorus (P) – potassium (K). Fertilizer companies are packaging organic and chemical fertilizers aimed at providing nutrients for specific plants. By simply reading labels, a gardener can choose from a variety of fertilizers that fit their specific nutrient requirements. There are fertilizers labeled for flowers, rhododendrons, roses, vegetables, fruit trees, lawns, etc. It is still important to keep in mind the leaching rate and plant usage rate of the primary nutrients. An example, would be applying an all purpose fertilizer (16-16-16) on a lawn that has not been fertilized for a number of years and then applying a lawn food fertilizer (25-3-5) in follow up applications to prevent phosphorus and potassium buildup in the soil.

The secondary nutrients calcium, magnesium, and sulfur come in to play with the pH level of the soil. Since western Washington has high precipitation levels, our soils tend to be acidic in nature. Most garden variety plants prefer a near neutral pH level. A way to raise soil pH is through the application of dolomitic lime. Dolomitic lime is composed of calcium and magnesium carbonates. These components react in soil to neutralize the acidic nature of the soil and provide calcium and magnesium nutrients to the plants. There are some plants that prefer a more acidic soil such as rhododendrons, azaleas, blueberries and cranberries. The addition of sulfur to the soil will aid in lowering soil pH. Gardeners can use ammonium sulfate (21-0-0) to lower pH levels.

For those looking for an organic solution, use of pine needles.

as a mulch will lower soil pH but takes longer to have a noticeable effect.