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Fertilizing

The type of soil that a tree or shrub grows in can affect its nutrient needs. Soil texture and soil structure influence the amount of water, air, and nutrients held in the soil for plant use. Clay soils can be nutrient rich, but have a large amount of fine particles that tend to compact and restrict water and air movement. Sandy soils drain well, but contain many coarse particles that have little capacity for storing water, air and nutrients. Organic material can be thoroughly mixed into soils with high clay or sand contents to help improve soil structure. Repeated applications may be needed depending on the amount applied and the stage of decomposition or type of organic matter used. Organic material should be mixed into the soil up to several years before trees are installed to obtain maximum benefit.

No single symptom tells you that trees or shrubs need additional fertilization. Some nutrient deficiency symptoms can be similar to symptoms of cultural problems or diseases. Slow growth rate, small leaves, fewer flowers, smaller fruit, and pale green or yellow (chlorotic) foliage with mottling between the leaf veins may all be signs of nutrient deficiency.

Two methods of determining nutrient deficiencies include:

Soil Testing

Advantages – provides soil pH, levels of K, P, organic matter content and minor nutrients such as iron or zinc.
Disadvantage – does not provide reliable information on N because N is rapidly lost through leaching or removed by plants

Plant Analysis

There are two methods of determining nutrient deficiencies through plant analysis:

Visual symptoms – include length of shoot growth, leaf color, leaf size, and color pattern and timing of leaf drop

Advantages – N and Fe are often the easiest visual symptoms to identify
Disadvantage- symptoms can be deceiving and/or nonspecific

Foliar tissue analysis – provides the concentrations of specific elements in plant foliage (usually leaves)

Advantages – when combined with soil tests it can provide a good picture of nutrient problem(s) – deficiency or toxicity
Disadvantage- nutritional needs for many landscape plants is unknown

Trees should be fertilized in early spring or mid-fall as long as the soil temperature is above 40º F two inches below the soil surface. Soil should also be moist. Avoid fertilizing in late summer and early fall as a nutrient application at this time could cause unwanted succulent growth that may fail to harden off before fall frosts hit.

Fertilizer Application Methods

Broadcast or topdress – fertilizer is added directly to the soil surface. This method is good for N, which moves readily through the soil, but poor for P and K that move slowly through the soil. Fertilizer should be applied to the drip line and at several foot intervals out from the drip line for mature trees.

Soil incorporated – dry or liquid fertilizer is added to holes in the soil beneath the canopy and extended beyond the drip line and provide a long lasting effect. Holes should be up to 12 inches deep and 1 to 2 inches in diameter and made in concentric circles 2 feet apart around the tree trunk with the first circle no closer than 3 feet from the trunk.

Foliar sprays – best for supply nutrients for plant use in only trace amounts, such as Zn, Mn and Fe.

Tree spikes are a dry soil injection method, with a hardened column or cylinder of fertilizer hammered into the soil.

Controlled release pellets are typically broadcast on the soil surface, but they can also be placed in holes augured into the soil.

Tree spikes and slow release pellets may delay the development of winter hardiness so it is best to use them in late fall or early spring.

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

University of Idaho Extension

Physical Address:
E. J. Iddings Agricultural Science Laboratory, Room 52
606 S Rayburn St.
Moscow, ID

Mailing Address:
University of Idaho Extension
875 Perimeter Drive MS 2338
Moscow, ID 83844-2338

Phone: 208-885-5883

Fax: 208-885-6654

Email: extension@uidaho.edu

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Barbara Petty