Fertilizing Trees and Shrubs

Curtis E. Swift, Ph.D., Colorado State University, Area Extension Agent, Horticulture

Nitrogen is the key nutrient needed by trees and shrubs and should be applied yearly beginning the year after the tree or shrub is planted. Nitrogen should not be applied the first year of planting. The application of all other nutrients should be based on a soil test.

How much nitrogen should you apply?

If the tree is planted in the lawn and the lawn is being fertilized on a regular basis, apply one pound of nitrogen per 1000 square foot area surrounding the tree or shrub.

If the tree is not in the lawn, apply two pounds of nitrogen per 1000 square foot area surrounding the tree or shrub.

To determine the amount of area to fertilize, assume the roots of trees extend out to a distance of 3 times the height of the tree. Shrubs roots will typically extend out 3 times the diameter of the canopy of the shrub. Fertilizer should be applied on all sides of the tree or shrub between the time the plant starts growth in the spring until about mid-June.

Ammonium sulfate is recommended. This is 20% nitrogen and contains no phosphorus or potassium. Five pounds of this fertilizer contains one pound of nitrogen.

What do you do if you have the results of a soil test?

If you have the results of a recent soil test the following table should be used to determine the nutrients that need to be applied. Soil Test Levels (ppm)
Recommendations based on a soil test report (Ammonium Bicarbonate DPTA technique)

 

NO3- N (nitrate nitrogen)

 

Level of sufficiency

 

Requirement [1]

0- 13 ppm

low to moderately low

Apply from 1 to 2 lbs N/1000 sq ft area; apply the highest amount for the lowest ppm

14

sufficient

0

> 50

excessive

0 Likely a drainage problem.

P (phosphorus) [2] & [4]

Level of sufficiency

Requirement: P2O5/1000 sq ft area

0- 6 ppm

very low

No more than 1 lb

7

sufficient [3]

0

K (potassium) [2]

Level of sufficiency

Requirement: K20/1000 sq ft

0- 60 ppm

very low

up to 8.5 lbs

61- 120

low

up to 5 lbs

120- 150

medium low

up to 1.75 lb

> 151

sufficient

0

Fe (iron) [2]

Level of sufficiency

Fe recommendation/1000 sq ft area

< 5 ppm

very low

1 lb/1000 sq ft

5- 10

low

0.5 lb/1000 sq ft

> 10

sufficient

0

 


[1] Do not apply Nitrogen to newly planted trees and shrubs; apply to established trees and shrubs only.

[4]The extraction method used to determine the P in the sample needs to considered with using this fertility chart. Conversion information based on the extraction method used is provided at Phosphorus_analysis_techniques.html

[2] Must be applied and worked into the soil prior to planting.

[3] Excess levels of P can increase iron chlorosis problems. Levels above 50 ppm may kill mycorrhiza.

 

Placed on the Internet Tuesday, September 21, 2004
Updated on June 16, 2009