Spruces, various species within the genus Picea, are coniferous evergreen shrubs or trees prized for a variety of available forms and the year-round interest they provide. Large, established spruce often require no fertilizer unless they demonstrate a nutrient deficiency and may suffer as a result of excessive or improper fertilizer application. The appropriate application of a fertilizer around young spruces or trees where soil fertility is low can encourage establishment and healthy growth.
Identifying Nutrient Deficiencies
The best way to determine if a nutrient deficiency is to blame for a problem with spruce and to calculate the type and amount of fertilizer required is to test the soil and needles. If sending samples to a lab is not feasible, ruling out environmental and cultural factors like flooding, drought stress or fertilizer burn and disease or insect injury and recognizing nutrient deficiencies by their characteristic symptoms aids in diagnosing a possible deficiency. A nitrogen deficiency appears as a yellowing of the entire plant that may first appear in older foliage. A phosphorous deficiency causes the development of a purplish cast, and yellowing of needle tips indicates inadequate magnesium.
Where a spruce or other conifer demonstrates a nutrient deficiency, a higher-than-ideal pH level could potentially be responsible; again, the best way to determine if soil pH is presenting problems is to have the soil tested. Spruce prefer an acidic soil and are less able to take up nutrients like iron as the pH increases. Properly applied elemental sulfur or ammonium sulfate effectively lowers a high pH and may eliminate the need for any additional fertilizer.
Fertilizers for Spruce
Spruce trees do not have very high phosphorous needs and benefit most form an acidic fertilizer designed for evergreens or a high-nitrogen fertilizer with a formula like 12-6-4 or 10-8-6. Organic fertilizer options include compost, fish emulsion, cottonseed meal and alfalfa meal. A 3- to 4-inch layer of organic material like well-rotted compost or shredded bark applied as a mulch regulates soil temperature, conserves moisture and contributes nutrients to the soil as it breaks down. Pine needles and shredded pine bark also lower the pH, as these materials are slightly acidic.
Fertilizer application is only warranted when a spruce demonstrates a nutrient deficiency or has a slow growth rate. To get the most benefits, apply fertilizer in the early spring. Provide a spruce with ample water before and after fertilizing. If the rate of nitrogen application is no higher than 2 pounds for every 1,000 square feet, it is possible to broadcast the fertilizer over the ground in the area under and slightly beyond the spruce branches using a drop spreader. Afterward, water the area thoroughly. A higher rate of application using this method could burn any nearby grass or other vegetation.
An alternative fertilizer application method suitable for fertilizing larger plants is to bore holes in the ground that are spaced 2 feet apart in concentric circles beginning 3 feet out from the trunk of larger spruce or slightly closer for young trees. Thoroughly soak the area around the tree and use an auger or metal rod to create holes about 2 inches across and 8 to 12 inches deep. Pour fertilizer into the holes at a rate proportionate to the fertilizer's nitrogen content or as directed by the label. With a 10-8-6 fertilizer. this means placing 2 1/2 tablespoons of fertilizer in each hole; if a 21-0-0 fertilizer is used, put only 1 1/4 tablespoons of fertilizer in each hole.
- Michigan State University Horticulture: Conifer Nutrition
- University of Illinois Extension: Yellowed Spruce
- University of Wisconsin, Extension: Evergreens: Planting and Care
- University of Minnesota Extension: Fertilizing Evergreens (Conifers)
- Growing Shrubs and Small Trees in Cold Climates; Debbie Lonnee et al
Angela Ryczkowski is a professional writer who has served as a greenhouse manager and certified wildland firefighter. She holds a Bachelor of Arts in urban and regional studies.