Seasonal care of trees & shrubs: Fertilizing

April 4, 2008 at 6:46 am 3 comments


From an environmental quality and tree health perspective, all fertilizing of trees is recommended only in a case-by-case situation in the landscape. Fertilizing is not recommended unless nutrients are proven deficient in a soil sample tested by a laboratory or by a trained eye observing a deficiency in a plant’s foliage.

The recommendations in this chart refer only to nitrogen applications.  Before fertilizing your landscape with a complete fertilizer (nitrogen-phosphorus-potassium), contact a soil testing laboratory for a basic soil test [For MN only: U of MN Soil Testing Labratory]. A basic soil test will provide you with readings on organic matter, pH, cation exchange capacity, macronutrients and micronutrients (Smiley, 2003).  Soil testing laboratories may offer timing and quantity recommendations for complete fertilizers (N-P-K).

For information on selecting a soils lab for testing visit Guidelines for Choosing a Soil-Testing Labratory. Once criteria have been determined for selecting an appropriate soil lab, visit University Related Plant Disease and Soil Testing Services (.pdf) to find the lab nearest you.

Nitrogen needs are not usually highlighted on soils test.  Standard soil tests do not include total nitrogen readings since nitrogen is present in so many forms and is so mobile. According Dr. Kim Coder, adding (aka. “dumping”) too much nitrogen at one time can increase the plant’s susceptibility to insects and diseases, decrease drought tolerance, and can pollute waterways (1997).  It has been determined that high nitrogen use efficiency is achieved when trees are leafed out.

Application rate is critical: NEVER exceed package recommendations.  Do not apply on hot, dry and/or frozen soils.  Avoid nitrogen applications from leaf drop to bud break because uptake is minimal and pollution may occur.

For more information : Tree Nutrition Series: Nitrogen Perscription for Trees

More information on each step:

Authored by Rebecca Koetter, Gary R. Johnson, and Dave Hanson: University of Minnesota
Funded in part by USDA Forest Service: Northeastern Area
Chart designed by Andrew Rose:

Download & print your own poster or magnet copy of the “Seasonal Care for Trees and Shrubs in Northern U.S. Climates”

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