Fertilizing Conifers

By Website Editor

Dr. Bert Cregg of Michigan State University answers frequently asked questions on feeding your conifers.

Nutrient deficiencies in conifers are linked to site factors such as unfavorable soil pH
Nutrient deficiencies in conifers are linked to site factors such as unfavorable soil pH

Nutrient deficiencies are a common cause of reduced growth or poor appearance in many plants, and conifers are no exception. Unfortunately, the Internet and other sources are full of home remedies for nutrient deficiencies that are of dubious value as well as other misinformation about plant nutrition and proper fertilization. Below are some of the common questions which arise when dealing with nutritional issues in conifers.

What nutrient elements are needed for conifers?

Conifers, like all plants, require 16 elements for normal growth and development. Plants obtain three of these elements; carbon, hydrogen, and oxygen, from air and water. These three are not considered when discussing nutrients which must be obtained from the soil. The remaining elements are grouped based on the relative amounts contained in leaf or needle tissue.

Macronutrients are elements which occur in relatively large amounts, usually 0.1 to 2.5% of leaf dry weight. These are nitrogen, phosphorus, potassium, calcium, sulfur, and magnesium. Micronutrients are elements needed in relatively small amounts, sometimes as little as a per million of leaf dry weight or less. These elements are sometimes referred to as trace elements and include: iron, boron, manganese, molybdenum, copper, zinc, and chlorine. See table for abbreviated elements.

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Are there certain elements commonly deficient in conifers?

Conifers, especially evergreen conifers, typically have lower nutrient requirements than deciduous broadleaved trees since evergreens don’t have to produce an entire new canopy of leaves every year. The likelihood of encountering nutrient deficiencies depends on several factors including the type of conifer and soil conditions.

In general, micronutrient deficiencies are comparatively rare since plant need for these elements is low, and most soils can supply them in adequate amounts. Some exceptions are iron and manganese, which can occasionally become deficient as soil pH increases. Nitrogen can become deficient since it is the element plants need in the largest amounts. Also, nitrogen is very dynamic in soils and can be lost by a variety of ways such as leaching, volatilization, and denitrification.

Magnesium and potassium can sometimes be limited in sandy soils that have a low cation exchange capacity and, therefore, a low ability to retain these nutrients. Phosphorus availability in soils varies widely around the country and even between locations within a region. Because excessive P can contribute to surface water pollution, it is important to establish a need for P before applying P fertilizer. In fact, some states have banned P fertilizers for homeowners, or require a soil test before applying P fertilizer.

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How do soil properties influence plant nutrition?

Plant nutrient availability is inextricably linked to soil properties. Discussing all the soil factors which impact plant nutrition is beyond the scope of this article, but there a two key soil properties critical to dealing with plant nutrition; soil pH and soil texture.

For most plants, the optimum soil pH is around 6.5. This is because the availability of some elements decreases as pH goes above 6.5 while others decrease as pH goes below 6.5. For conifers, this “sweet spot” of soil pH is lower than for deciduous trees, usually 6.0 or even a little lower. Soil texture describes the relative proportion of sand, silt and clay particles in a soil. Ideally, soils should have a mixture of particle sizes since sand provides porosity and air space while silt and clay contribute to water holding capacity.

Clay particles, along with soil organic matter, also contribute to cation exchange capacity (CEC). CEC refers to the ability of a soil to act as a reservoir for important nutrients such as K, Mg, Fe, Mn, Cu and Zn. Conifers grown in very sandy soils with low organic matter have a potential to experience deficiencies of some of these elements.

How do I diagnose a suspected nutrient problem?

Diagnosing a suspected nutrient problem in conifers often requires some detective work. Visible symptoms expressed by a plant are usually the starting point. There is a common misconception that nutrient deficiencies can be diagnosed by simply matching the plant symptom to an image in an extension bulletin or website. In reality it’s rarely that simple. Several nutrient deficiencies can result in symptoms that look similar; N, Mg, and Fe deficiencies can all result in chlorotic (yellow) foliage.

It is also possible that symptoms may not be related to a nutrient problem at all. Drought, heat, insects, herbicides and other factors can produce symptoms that can be mistaken for nutrient problems, so it is important to eliminate other causes. A soil test that includes soil pH is a minimum requirement to adequately assess a nutrient problem. In many states, soil testing is available through university extension services, as well as through private labs.

Detailed instructions for collecting and handling samples are usually provided by most testing labs. The key step to remember is to collect a series of samples which are representative of the area where plants are having issues. Many university extension labs and private labs also perform foliar nutrient analyses. These will show the actual concentration of the essential nutrients in the leaf tissue. Again, detailed directions on sampling are available from most labs.

Foliar sampling is particularly useful in nurseries and large landscapes where it is possible to sample “good” and “bad” specimens of the same species or cultivar. By comparing the foliar test results of the two samples, nutrients that are deficient will often become apparent.

Foliar symptoms such as yellow (chlorotic) needles may indicate a nutrient problem but soil or foliar sampling are often needed to identify which element is limiting
Foliar symptoms such as yellow (chlorotic) needles may indicate a nutrient problem but soil or foliar sampling are often needed to identify which element is limiting

Should I fertilize my conifers?

With increased public concerns over the impacts of excessive fertilizer nutrients on our surface waters, the days of recreational fertilization are over. Fertilizers need to be applied with a purpose. This requires identifying a specific deficiency through visible symptoms, a soil test, a foliar test, or, preferably, a combination of at least two methods.

When should I fertilize conifers?

Fertilizer nutrients are most efficiently taken up when roots are actively growing. For most trees, including conifers, this usually means during the spring. Avoid fertilizing in the summer to reduce potential volatilization in hot weather. Fertilizer can also be applied in the fall after budset, but there is potential for leaching if using a nitrate-based N source.

If a soil test indicates that soils are deficient in potassium, muriate of potash (KCl) is a commonly-used source of K. This is a fertilizer which has a high salt index. It is often applied in the fall to reduce the potential for fertilizer burn and to allow excessive chloride to leach out with rainfall and snowmelt.

What is the best fertilizer to use?

The best fertilizer to use is one that meets plant needs based on a soil test or foliar test. Where possible, look to use a fertilizer which addresses more than one need. For example, if plants are N deficient and soil pH is above optimum, a fertilizer that contains ammonium sulfate can help to add nitrogen and reduce soil pH.

Avoid applying excess elements that are not needed. For example, if plants are deficient in N, but a soil test indicates other nutrient are sufficient, use a source such as coated urea rather than a complete fertilizer such as 10-10-10, which will provide excess phosphorus and potassium that are not needed.

How much should I fertilize?

Most soils labs will provide fertilizer recommendations along with soil test results. This will usually include a recommendation for N along with any soil element that is deficient. Labs that are accustomed to working with homeowners may report fertilizer recommendations in pounds per 1,000 sq. ft. of ground area. So, if you have a landscape bed that is 10’ x 25’ (250 sq. ft.), you would multiple the recommended amount by 250/1,000 or 0.25.

Many agricultural labs will provide recommendations in pounds per acre. The key number to remember is 43,560 - which is the number of sq. ft. in an acre. So, for our 250 sq. ft. bed, the conversion is 250/ 43,560 or 0.006. Also, an internet search of “area conversions” will link you to many useful calculators.

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How can I adjust soil pH?

If a soil test indicates that pH is lower or higher than the desired range, it may be possible to adjust pH either by adding lime to raise the pH, or applying sulfur to lower the pH. Most soil test reports will supply recommendations for lime rates to achieve a desired pH. In general soil pH adjustments will be easier to accomplish on coarse soils than on clay soils.

Liming is most effective when lime can be incorporated into the upper surface of the soil. For this reason, lime is often applied as a pre-plant adjustment. Surface application of lime after plants are established can be effective, but the effect will be much slower than if lime is incorporated. In agronomic crops applying sulfur to lower pH is less common than liming to raise soil pH, so soil testing labs may not provide recommendations for lowering pH.

The table provides some general guidelines for using elemental sulfur to lower pH. Soil pH can also be reduced by applying urea, ammonium sulfate or other ammonium-based fertilizers. Conifer gardeners may also apply products such as Holly-tone or Miracid to adjust pH. When using sulfur or fertilizers to lower pH, keep in mind that soil acidication is accomplished by soil microbes. So, it may take a year or longer to see the desired impact.

As with liming to raise pH, it is typically harder to affect a change on a clay soil than on a coarse soil. Lastly, the effect of the sulfur on pH is transitory and pH will drift back up over time, so be prepared to follow up with additional soil tests and re-adjust every three years or so.

Needle chlorosis in Mugo pine (Pinus mugo)
Needle chlorosis in Mugo pine (Pinus mugo)

What is a fertilizer analysis?

Fertilizer analysis (or grade) refers to the chemical composition of a fertilizer. By convention, fertilizers are classified by three numbers such as 10-10-10, which represent the amount of N, P, and K in the fertilizer. The first number is the % N in the fertilizer. Thus, if a soil test recommended 1.5 lbs. of N for a 1,000 sq. ft. bed, we would need to add 1.5 / 10% (1.5/0.10) = 15 lbs. of product.

For P and K the numbers are little more complicated. The second number is the amount of P as phosphate (P2O5), and third value is the amount of K as potassium oxide (K2O). Fortunately, most soil tests will provide a recommendation based on the amount of P2O5 and K2O, and many commercial fertilizer bag labels now express the analysis in both the traditional N-P2O5- K2O format as well as actual elemental concentration. And, if all else fails, a quick internet search of “fertilizer calculator” will link to a number of excellent university extension sites.

What about foliar fertilizer?

Foliar fertilization refers to the application of liquid fertilizer directly to the foliage of plants to remedy a nutrient deficiency. Growers apply foliar fertilizers in certain horticultural applications such as bedding plants in order to overcome specific deficiencies and prepare plants for sale. Most conifers are poor candidates for foliar fertilizer because the thick, waxy cuticle on their foliage is a barrier to nutrient uptake. In certain situations, micronutrient deficiencies in conifers may be addressed with foliar fertilization, but a better approach is to understand and address the underlying soil nutrient or pH issues.

What about organic fertilizers?

Organic fertilizers include a wide array of products that supply nutrients from living or once-living sources. These are in contrast to most standard inorganic fertilizers produced synthetically. Some examples of organic fertilizers are composted manures, fish emulsions, bone or blood meal, and Organic Materials Research Institute (OMRI) – approved pelletized organic products.

We have conducted trials growing conifers with OMRI-approved and conventional fertilizers at Michigan State University and, given the same amount of nutrients, trees grew similarly and had similar foliar nutrition with both types of products. Some factors to consider in using organic products include material handling (organic products usually have a relatively low analysis so more product needs to be applied) and odors and attractiveness to animals for products such as fish emulsion or blood-based products.

Summary

Most garden soils can provide adequate nutrients to grow quality conifers. When nutrient problems occur, try to identity the underlying cause, which usually requires a soil test including soil pH. If fertilization or soil pH adjustment is recommended, focus on addressing the principle issue and avoid applying fertilizer elements, especially P and N, if they are not deficient. This will help to keep your conifers looking healthy and protect the environment.

Text by Dr. Bert Cregg. Photographs by Petr Kapitola.

Dr. Bert Cregg is an Associate Professor in the Departments of Horticulture and Forestry at MSU.

This article was originally published in the Winter 2017 issue of Conifer Quarterly. Join the American Conifer Society to access our extensive library of conifer-related articles and connect to a nationwide group of plant lovers! Become a member for only $40 a year and get discounts with our growing list of participating nurseries in our Nursery Discount Program.

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