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Aug 142012
 

Recommended rates of fertilizer are calculated using the ground area under the tree canopy. The amount of fertilizer to add depends on the fertilizer composition and is usually calculated using the desired N rates. Nitrogen rates range from 0.2 to 0.4 pounds per 100 ft². Excess nitrogen can be detrimental to plant growth.


Quantities of common fertilizers, incorporated into the soil, needed to provide equivalent amounts of N.
Applications rates expressed pounds of fertilizer material per 100 ft² of ground area.
Fertilizer Rate
Fertilizer analysis
0.4 lb N per 100 ft²
0.25 lb N per 100 ft²
2-3-2
20 lbs
12.5 lbs
4-10-4
10
6.2
6-10-4
6.8
4.2
8-10-8
4.8
3.0
10-10-5
4.0
2.5
12-12-5
3.2
2.0
14-10-0
2.4
1.5
16-20-0
2.2
1.4
20-16-0
2.0
1.3
21-0-0
2.0
1.3
33-0-0
1.2
0.8
45-0-0
0.8
0.5

from 1997 Bulletin CIS 1068. Fertilizing Landscape Trees


Sample Fertilizer Application Problem

If a 10-year-old tree has a canopy that is 20 feet wide, a trunk that is 5 inches in diameter, and roots that extend 35 feet from (one side of) the trunk, how much 18-6-12 fertilizer should be applied via the broadcast method? Use a rate of 3.5 lb. of nitrogen (N)/1000 ft²

First, figure the surface area occupied by the tree’s roots. Area =π*r² where π = 3.14 and r = 35 ft.
= 3.14 x 352
= 3846.5 ft2

Second, calculate the number of pounds of nitrogen needed to cover the root zone.
Pounds of Nitrogen = 3.5 lb. N x 3846.5 ft²
Needed 1000 ft²
= 13.5 lb. N

Third, calculate the number of pounds of fertilizer needed to cover the root zone.
1. Important Relationship
Pounds of Fertilizer x Percent Nutrient = Pounds of Nutrient

2. Calculation for
Pounds of Fertilizer 13.5 lb. N
Needed    = 0.18 N     = 75 lb

 August 14, 2012