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More on fertilization and transplanting

Type: In most cases, nitrogen is the only element added to the soil that is associated with a shoot or root growth response in trees (Chadwick et al., 1957; Davey, 1930; Neely, et al. 1970; van de Werken, 1981; Watson, 1994). Phosphorous and potassium applications have seldom showed a benefit to established trees (Neely et al., 1970; van de Werken, 1981), although there are exceptions in the forestry literature (Klein et al, 1988).

There are numerous reports of recently transplanted trees (Gilman and Yeager, 1990; Wright and Hale, 1983; Ponder et al., 1984; Khatamian et al., 1984; Whitcomb, 1981; Shoup et al., 1981; Smith, 1975; van de Werken, 1981) not responding to nitrogen applications although there are two reports of trees responding to nitrogen the first year after transplanting (Hensley et al, 1988; Schulte and Whitcomb, 1975). Established trees have responded to nitrogen applications (Neely, et al., 1970; van de Werken, 1984) or not responded (Perry and Hickman, 1998).

Rate: Trees generally have not responded to applications in excess of 14-24 g N/m2 (3 to 5 lbs N/1000 ft2)/year (Conover and Joiner, 1974; Gilman and Yeager, 1990; Ingram et al,.1998; van de Werken, 1984), although there are examples of increased tree trunk and shoot growth with nitrogen rates up to 49 g N/m2 (10 lbs N/1000 ft2)/year (Klein et al, 1989), shrubs producing darker green foliage with 49 g N/m2 (10 lbs) compared to 24.3 g N/m2 (5 lbs N/1000 ft2 )/year (Gilman, 1987), and shrubs increasing shoot growth as rates increased up to 73 g N/m2 (15 lbs N/1000 ft2)/year (Gilman, 1988).

Root growth increased with light applications of nitrogen (Watson, 1994) but was suppressed at very high nitrogen rates (Warren, 1993). There are no reports of increasing landscape tree growth to applications of potassium or phosphorous alone (Neely et. al., 1970; Watson, 1994).

Source of nitrogen may not influence tree response in field soil. Fertilizers listed as organic, slow release, inorganic, liquid or granular gave about the same tree growth response in clay, loam, or sand because they all contained nitrogen (Chadwick et al., 1957; Conover and Joiner, 1974; Corley et al., 1988; Gilman, 1987; Gilman and Yeager, 1990; Ingram et al., 1998; Smith, 1965). One study showed that slow release nitrogen sources produced taller trees with larger trunks and broader canopies than trees receiving ammonium nitrate (van de Werken, 1981). Another showed that slow release nitrogen sources produced larger plants than urea (Gilman, 1987).

Where to place it: Two studies showed that tree growth was enhanced more by broadcast application than placing fertilizer below the soil surface (Chadwick et al., 1957; van de Werken, 1984); whereas others showed that method of application had no effect on tree or shrub growth (Davey, 1930; Gilman, 1989; Hensley et al, 1988, Neely, 1970). No studies showed more growth from subsurface application than from broadcast surface application.