Acknowledgments: Technical assistance and review was provided by Tim Wilmot, University of Vermont Proctor Maple Research Center. Additional assistance was provided by Steve Horsley and Bob Long of the Northeastern Forest Experiment Station, Richard Barry of the University of Monkton, Lew Staats, of the Uihlein Sugar Maple Field Station, and Ron Kelley, Brian Stone, Ginger Anderson, and Brent Teillon from the Vermont Department of Forests, Parks, and Recreation. Funding was provided by the US Forest Service: Forest Health Protection.

Is Sugarbush Fertilization Worthwhile?

Fertilization of sugarbushes is not generally a recommended practice at this time. Research has involved a variety of fertilizer mixtures, rates, and types of application, resulting in some successes; improvement in tree condition and some failures; no change or some damage to trees. Sugarbushes differ in many ways which affect how fertilizer works. Consequently, it’s not yet possible to provide consistant recommendations. Given the complexity of forest ecosystems, it may never be possible. We do know that careless application can be harmful.

Under certain conditions, adding fertilizer, and/or lime, has improved sugar maple stands. A positive response to fertilization is more likely on nutrient-poor sites. If you evaluate your site and the nutrient status relative to the trees, and believe that fertility may be a problem, you may want to experiment with fertilization. The recommendations that follow summarize the current science on sugarbush fertilization, based on an extensive review of existing research. They are for people willing to risk spending time and money to improve tree growth, when these benefits are not assured.

Fertilization Is Not Recommended If:

•Plants are present which indicate that the site is already calcium-rich and nutrients are available. The best indicator plants include maidenhair fern, wild leek, blue cohosh, wild ginger, wild ginseng, and herb robert. Abundant nettles, butternut, hazelnut, white cedar, white ash or basswood also indicate a rich site.

•Vigorous condition indicates that trees are getting all the nutrients they can use. Trees are vigorous if they are closing tapholes in two years, if foliage is dark green, and if sky is visible through less than 5% of the leafy part of the crown.

•Fertilizer can’t be applied without wounding trees or driving over roots.

•Trees are too crowded. If tree branches don’t have room to grow on three sides, release crop trees by cutting their neighbors before considering fertilization.

•Trees are overzealously tapped. Follow tapping guidelines, and allow trees to recover before considering fertilization. Don’t exceed two taps per tree.

•The site is wet. Sugar maple will never grow well on these sites. Wet site plant indicators include ostrich fern, royal fern, sphagnum moss, and sedges.

•There is exposed bedrock near symptomatic trees. Shallow soil may be limiting moisture availability.

•Trees are very old. They may have deteriorated until the tree won’t be able to respond. The investment in fertilizer is most likely to pay off in younger trees with decades of production ahead of them.

•Trees are severely declining. If a tree has less than half of its branches alive, its odds of recovery are small.

•Trees have been recently stressed by defoliation, spring frost, storm breakage, extreme cold, or drought in the past three growing seasons. Allow several years for trees to recover before considering fertilization.

Fertilization May Be Beneficial If:

•The solid rock under the soil in the area is low in calcium. Bedrock geology maps are available from state and federal agencies.

•Leaves are generally light green, small, red, or tufted, and no other causes of stress can be identified.

•There is a lot of hay-scented fern, indicating a nutrient-poor site.

•Soil samples show that the pH is low. At low pH, many nutrients present in the soil are not available to plants.

•Foliar samples show essential nutrients are below threshold values.

Research Results: Soil Fertility and Maple Health

Sugar maple foliage from good sites has more base cations: calcium, magnesium, and potassium. These nutrients become unavailable to plants when soil pH is low. Where the bedrock is calcareous, soil pH is above 4.5, and these nutrients are available, sugar maple generally grows faster and has less twig dieback, greener and denser foliage, more fine roots, better taphole closure, and better ability to withstand defoliation by maple pests. Calcium is the nutrient that tends to vary most from site to site.

There is not as much information about regeneration, but in one study, liming was associated with an increase in the amount of maple flowers, seed production, and the survival of seedlings. Liming also increases the growth of competing vegetation, and may increase the palatability of maple seedlings to deer and other browsers.

Research Results: Soil Fertility and Sap Production

Few relationships have been found between nutrients and sap sweetness. Vigorous trees tend to produce more sap, and trees with more foliage tend to have sweeter sap. If trees with adequate nutrition are generally healthier, they should be better producers. Trees which grow faster will close tapholes more rapidly.

About Limiting Factors

If a plant is growing in shade, sunlight may be a limiting factor. Expose it, and it has all the sunlight it can use for the amount of water available to it. Water is now the limiting factor. Water it, and the amount of water and sun it can use are limited by the amount of roots it has to take up the water. Fertilization will be of benefit in the sugarbush if soil nutrients are a limiting factor to tree growth.

Steps to Take if You’re Considering Fertilization
• Analyze current nutrient status. Take soil samples to determine pH, and foliar samples to determine if nutrients may be deficient. Foliar sampling should be done in mid-late August. Before sampling, contact your local agricultural testing lab to find out procedures to follow.

•Calculate fertilizer needs to correct soil pH. To impact nutrient availability and to raise pH, if needed, use 1-2 tons of limestone per acre. Some researchers suggest as much as 4 tons.

•Base calculations on foliar analysis results. Dolomitic lime should be used if magnesium is below the threshold value. Otherwise use calcitic lime. If potassium is below the threshold value, add potassium sulfate at 400 lb/acre.

•Apply nitrogen only if you’re eager to experiment. Don’t apply nitrogen to stressed trees or late in the summer. If foliar analysis shows that nitrogen may be deficient, recommended rates range from 50 to 250 lbs of nitrogen per acre. Urea is the favored nitrogen source in forest fertilization.

•Calculate costs. Price out what you need in a fine powdered or pelletized formulation. Plan on treating 1-5 acres per day.

•Apply fertilizer when the soil is dry, without compacting soil around tree roots or wounding tree bark. This is usually in late summer or fall, if you are not applying nitrogen ( apply nitrogen after leaf drop or in early spring). Application will probably be from the ground with a hand or tractor-mounter spinner. Hand fertilization may be safely done in the spring. Don’t run equipment near tree trunks, and limit heavy traffic to woods roads.

•Think like a scientist. Sugarbush fertilization is being put to the test, and if you’re fertilizing a sugarbush, you’re part of the experiment. Try a small area first. Keep an untreated area for comparison. Keep good records. Label trees or mark the boundaries of the treated area.

•Evaluate results over several years. The greatest response should normally be within the first two years. Trees have improved if tapholes are closing more rapidly, tree foliage is dark green, and/or sky is visible through less than 5% of the leafy part of the tree crown.

Evaluating Nutrient Status: Foliar Sampling

Foliar sampling should be done during mid-late August. In each stand, put together one or more samples containing foliage from four to five trees. Use trees of average to good health which are at least 100 feet apart.

Sample leaves should come from “mid-crown” branches which are exposed to sunlight. It’s best if they come from the south side of the tree. You can use a shotgun, pole pruners, or climbers to get leaves to the ground. Whatever method you use, be careful.

Select at least thirty leaves with little or no insect feeding or brown edges. Don’t get soil on the leaves. Send samples in labeled paper bags to your local agricultural testing laboratory. Request testing for nitrogen, phosphorus, potassium, magnesium, calcium, and aluminum. The cost will be approximately $20 per sample.

The lab results you receive will tell you how much of each nutrient is present, compared to the weight of the leaves when they are dried (oven dry weight). Usually it is given in percent, but may be in terms of milligrams per kilogram (mg/kg), milligrams per gram (mg/g), or parts per million (ppm). Use the following table to determine whether the nutrient levels in your leaves are above or below the threshold value at which the nutrient may be limiting to the growth of sugar maple.

How to Interpret Foliar Analysis to Determine Whether Nutients are Limiting

Nutrient Threshold Value at Which Nutrient may be Limiting to the Growth of Sugar Maple, compared to the Oven Dried Weight (ODW) of Foliage expressed as:

Evaluating Nutrient Status: Soil Sampling

The main purpose of soil sampling is to determine pH. Sample at a consistent time, preferably late summer/early fall. Take the samples when the soil is moderately dry.

The upper layers of the soil, are known as the O and A horizons. This is the feeding zone of fine roots. To take a soil sample, remove any loose surface litter, such as leaves, twigs, etc. Use a trowel to remove a block of soil about 4" on a side, and about three inches deep. Look for an abrupt color change towards the bottom of the sample between the dark upper layer(s), and a dull, lighter, sometimes greyish lower layer. Remove only the darker upper layer for sampling.

Take 3-5 blocks of soil within a radius of three feet. Move to another area of the stand, and collect 3-5 more blocks. Combine soil from at least five areas throughout the stand to make one sample, but avoid any spot that looks atypical . Atypical spots include small depressions that collect organic matter, or the thin coating of soil on top of a rock. Once all the soil blocks have been taken, mix them together thoroughly. Remove sticks, stones, leaves, and other coarse material.

You can use a pH test kit from a farm or garden supplier. Alternatively, you can send a sample to a soil testing lab Bag about a cup of soil into a tightly sealed plastic bag, and refrigerate until sent. The cost of a basic analysis will be approximately $10 per sample. You will receive a pH and a measure of macronutrients. The lab may analyze soil pH “in water” or “in salt”. The same soil will have a lower pH rating in salt than in water, so you can tolerate a pH several tenths lower than the threshold values given if the pH reported by your lab is determined in salt.

All forest soils are acidic. A soil pH of 4.5 or greater is considered acceptable for maple growth. Soils below 4.1 may be too acidic.

What About Nitrogen?

Of the nutrients a plant obtains from the soil, nitrogen is the one it requires in the largest supply. However, experimental results with nitrogen are the most inconsistent. Some studies have shown improved growth, while others have shown no effects or negative effects.

Nitrogen has the largest chance of doing harm. When nitrogen is added to the soil, trees use up carbohydrates to absorb and utilize it; if trees are unhealthy, adding nitrogen will further deplete low food reserves. With excess nitrogen, shoots may grow out of balance with the roots, and become more susceptible to damage by weather and insects.

Once you have the lab results from your sampling, if you have questions about applying nitrogen or any of the other nutrients discussed here, contact an Extension Specialist or Forest Protection Specialist for advice on application rates.

Additional Source of Information on Sugarbush Fertilization:

Barry, R. and L. Roy. 1998. Effects of maple fertilization on sugar production. Research Report. Ecole de sciences forestieres, Universite de Moncton, Edmundston, NB E3V 2S8. 37 pp.

Cote, B., I. O’Halloran, W.H. Hendershot, and H. Spankie. 1995. Possible interference of fertilization in the natural recovery of a declining sugar maple stand in southern Quebec. Plant and Soil. 471-480.

Hendershot, W.H. and A.R.C. Jones. 1989. Maple decline in Quebec: A discussion of possible causes and the use of fertilizers to limit damage. The Forestry Chronicle. pp 280-287

Long, R.P.; Horsley, S.B.; Lilja, P.R. 1996. Nutrient status and sugar maple health: liming effects on growth, vigor, seed crops, regeneration, and foliage chemistry in northcentral Pennsylvania. In: Forest management impacts on ecosystem processes: 14th North American forest biology workshop; 1996 June 16-20; Quebec City, PQ. Quebec City, PQ: Universite Lavel: 74. Abstract.

Wilmot, T.R., D.S. Ellsworth, and M.T. Tyree. 1995. Relationships among crown condition, growth, and stand nutrition in seven northern Vermont sugarbushes. Canadian Journal of Forest Research. 386-397.

Wilmot, T.R., D.S. Ellsworth, and M.T. Tyree. 1996. Base cation fertilization and liming effects on nutrition and growth of Vermont sugar maple stands. Forest Ecology and Management. pp 123-134.