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New Mexico State University

Issue: May 8

Trees may need nitrogen fertilizer

Q. Could you suggest a good fertilizer to green up my globe willows and lawn? I also have some ash and mulberry trees nearby the willows.

Ben H.

A. Nitrogen is the nutrient needed most by plants to stimulate growth. It is also the nutrient most often deficient in New Mexico soils. Phosphorus and potassium are also needed, but are less likely to be deficient. A soil test is the only way to know for sure what is needed. However, experience shows that your trees will benefit most from either a nitrogen fertilizer (ammonium sulfate) or a complete fertilizer that is high in nitrogen. Lawn fertilizer without herbicide will work well. Herbicides with the fertilizer (weed and feed products) will harm the trees. Water well after applying the fertilizer. Applying such a fertilizer will be beneficial to the trees immediately, but the recommendation to have your soil tested remains good advice. Your local NMSU Cooperative Extension Service office can provide information regarding soil testing. The soil test can give additional information to allow you to optimize nutrient applications for your trees. It may also identify other needed nutrients deficient in your soil, such as iron.

Some trees benefit from chelated iron treatments

Q. My willow tree was yellow by the time summer ended last year. For the first few years, it looked good, but now it yellows early and the leaves turn brown. I was told that the trees have iron chlorosis. What can I do to help my willow tree?


Roswell, NM

A. Willow trees and other trees often exhibit iron deficiency. The symptom of iron deficiency is yellowing of the leaves between the veins and browning in extreme cases. This is called iron chlorosis. The solution is to provide iron that is available to the tree. The best way to make iron available to the tree is to apply it as an iron chelate. This is a nutrient source in which the iron is bound to an organic compound (called a chelate) that prevents the high pH of our New Mexico soils making the iron unavailable to the tree (making the iron insoluble) so that the tree cannot extract it from the soil. A soil test in which micronutrient levels are tested will help you determine the best treatment for your tree.

For more gardening information, visit the NMSU Extension publications World Wide Web site at or

Send your gardening questions to:

Yard and Garden, ATTN: Dr. Curtis Smith
NMSU Cooperative Extension Service
9301 Indian School Road, NE, Suite 112
Albuquerque, NM 87112

Curtis W. Smith, Ph.D., is an Extension Horticulture Specialist with New Mexico State University's Cooperative Extension Service. New Mexico State University is an equal opportunity/affirmative action employer and educator.