EFFECTS OF AMENDMENTS, SOIL ADDITIVES, AND IRRIGATION ON TREE SURVIVAL AND GROWTH

Journal of Arboriculture, Sep 2004 by Gilman, Edward F

Abstract. During the first and second year after transplanting into good soil, there was no apparent benefit from adding amendments to the backfill soil around newly planted 5 cm (2 in.) caliper live oaks (Quercus virginiana Mill.), nor was there a benefit from applying liquid additives over the root ball. Top growth in the first 2 years after planting was not enhanced on trees with amendments added to the backfill soil compared to the control. In the first growing season following planting, trees irrigated frequently (3 limes per week for 38 weeks) grew about twice as fast as those irrigated only during the first 3 months after planting. There was no lingering effect of irrigation on top growth rates once irrigation ceased. Trunk diameter and height growth on trees treated with paclobutrazol (Profile(TM) 2SC) was less than on trees in all other treatments due to the growth-regulating effect of paclobutrazol. Root growth in the backfill soil was not affected by soil amendments added to the backfill soil at planting and was not affected by irrigation after planting in 5 of the 7 treatments.

Key Words. Backfill amendments; transplanting; planting; root growth; landscape; paclobutrazol; hydrogels; humic substances.

The influence of incorporating soil amendments into the backfill soil around newly planted trees and shrubs has been studied in several climates. Numerous soil types, plant species and sizes, and nursery production methods have been tested with varying results. Addition of hydrophilic gel, peat, and slow-release fertilizer in soil inside fabric field-grown container seedling green ash (Fraxinus pennsylvanica) had little effect on growth in the nursery (Henderson-Cole and Hensley 1992).

No benefit to silver maple (Acer saccharinum L.) was derived from the use of soil amendments in backfill soil either with a clay loam soil or very poor silt loam subsoil (Schulte and Whitcomb 1975). In fact, pine bark as a soil amendment was detrimental to growth unless fertilizer was applied to offset the nitrogen tie-up by microorganisms. Trees growing in soils amended with peat moss developed a more fibrous root system than trees in all other treatments. However, the fact that the roots did not develop into the surrounding soil, as was the case with all other treatments, may mean the trees would be more susceptible to drought. These data combined with that from other studies (Pellet 1971; Corley 1984; Kelting et al. 1.998; Ferrini and Nicese 2002) do not support use of soil amendments or liquid additives in backfill soil in the establishment and growth of newly planted trees, at least in good soil.

Smalley and Wood (1995) also found increased root density within the backfill soil after amending it when planting balled-and-burlapped red maple (A. rubmm) into clay soil. But aged pine bark mixed 50:50 with backfill soil induced nitrogen deficiency symptoms and reduced shoot growth during the first year. Shoot growth on trees with amended soil was similar to growth on trees backfilled with native soil 2 years after planting. Authors concluded that the data support use of native soil in backfilling planting holes.

There are several studies on amending backfill soil around shrub species. Root growth of container-grown Pittospomm tobira Thunb. 6 months after transplanting into a landscape with a sandy soil was greater when the backfill was amended with peat than when amended with colloidal phosphate or not amended. Neither backfill affected top or root dry weight compared to controls after 12 months (Ingram et al. 1981). Top dry weight of Juniperus chinensis L. 'Hetzii' increased with backfill amendments after 12 months but not when fertilizer was added (Ingram et al. 1981). Root and top growth on azalea (Rhododendron spp.) was significantly improved when organic amendments were added to an entire landscape bed in sandy soil (Beeson and Keller 1998). Azalea survival was dramatically improved by incorporating 5 or 10 cm (2 or 4 in.) of pine bark into a clay loam soil planting bed (Bir et al. 1995). Other research supports incorporating organic amendments into large planting beds to improve shrub growth and survival (Banko 1986; Bir and Ranney 1991). Inorganic soil amendments incorporated into the entire root zone have been shown to improve turf performance, but only during drought and only when the amendment comprised more than at least 40% of the soil volume (Wehtje et al. 2003).

The objective of this research was to test the effectiveness of backfill amendments, commercial products, and irrigation in minimizing water stress and increasing survival and growth after planting in the landscape.

MATERIALS AND METHODS

Three meter tall (10 ft), acorn-propagated live oak (Quercus virginiana Mill.) with an average caliper of 5.1 cm (2 in.) in #15 [41 cm (16 in.) top diameter], smooth-sided plastic containers were planted between January 5 and 11, 1995, into a Millhopper fine sand (loamy, silicaceous, hyperthermic Grossarenic Paleudulis) with less than 2% organic matter. The site was a tree nursery for about 10 years prior to this test, so the soil was not compacted [bulk density 1.37 g/cc (0.8 oz/in^sup 3^) in the top 15 cm (6 in.)]. Holes were dug 76 cm (30 in.) wide and as deep as the root ball with a soil auger and adjusted with a shovel. Holes were spaced 3 m (10 ft) apart within rows and rows were 3.7m (12 ft) apart.