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| HOME: TREES:: FERTILIZING LANDSCAPE TREES & SHRUBS |
Three Categories of Plant Problems Common Plant Problems UI Publications on Plant Problems Why Prune? When to Prune? Two Basic Types of Pruning Cuts Location of Pruning Cuts Pruning Tools More Information Time of Year to Plant Special Planting Needs Planting Instructions More Information Instruction for Specific Plants Why Fertilize Landscape Trees Soil Types Diagnosing Nutrient Problems When to Fertilize Determining Fertilizer Quantities More Information Insects Diseases Physiological |
Why Fertilize Landscape Trees Under natural forest conditions, the annual decomposition of leaves, needles and twigs provide a fresh resource of minerals for tree and shrub use. Landscape trees grown in lawns and driveways usually do not have this nutrient source and are in need of additional minerals since landscape debris is routinely hauled away. Sixteen chemical elements are known to be important to a plant's growth and survival. The first of these are carbon (C), hydrogen (H) and oxygen (O), which plants acquire in sufficient quantities from the air and water. The other 13 mineral nutrients, are acquired by plant roots, which absorb soil minerals dissolved in water. The required mineral nutrients are divided into two groups: macronutrients and micronutrients. The primary macronutrients are nitrogen (N), phosphorus (P), and potassium (K). These major nutrients are usually lacking from the soil first because plants use large amounts for their growth and survival. The secondary macronutrients are calcium (Ca), magnesium (Mg), and sulfur (S). Fertilization with these nutrients is not always needed. Micronutrients are nutrients needed needed in only very small quantities . The micronutrients are boron (B), copper (Cu), iron (Fe), chloride (Cl), manganese (Mn), molybdenum (Mo) and zinc (Zn). Southern Idaho soils can be deficient in S, Fe, Mn, and Zn. Soil Types
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 - two methods 1)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 Disadvantages - symptoms can be deceiving and/or nonspecific 2)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 Disadvantages - 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. Determining Fertilizer Quantities 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.
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)/1000ft2 First, figure the surface area occupied by the tree's roots. Area =pie*r2 where pie = 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 3846.5ft2 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 For additional details on fertilizing trees, please see: Fertilizing
Gardens
Other websites that present fertilizing information for trees and shrubs are: Texas A&M: |
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