Compost-Containing Substrates and Their Effect on Posttransplant Growth of Containerized Tree Seedlings

Chemical Properties of Media

An important chemical property of any soil or growing medium is its pH, a factor that influences the availability of essential mineral nutrients. The pH of CT and CT-containing substrates in these studies was consistently and significantly higher than it was for composted media without CT (Table 1). Although the pH of media containing mixtures of CT and MM360 averaged 7.3, the pH of CT-amended soils (GPT and SP) and soilless composts (CM and MM560) averaged 6.6 and 5.2, respectively. Although the pH of CT was approximately neutral (7.4), it should be noted that the hydrogen ion concentration of this product (like with other MSW-containing products) may vary considerably from batch to batch, depending on the biosolid content and length of the composting process. In certain instances, it may be necessary to leach some MSW-containing products with water to bring the pH within a more acceptable range for planting.

Although plants can be grown in solution culture over a wide pH range (assuming precautions are taken to keep the essential elements in solution), in nature, pH is more critical because it affects the solubility of essential mineral elements. In higher pH (alkaline) media, iron and phosphorus deficiencies can sometimes become a problem, whereas in lower pH (acidic) media, calcium and magnesium are frequently deficient (Kramer and Kozlowski 1979). In the current study, most of the media tested were within a pH range expected to yield adequate nutrients for plant growth; however, MM560, at pH 4.8, could affect nutrient availability if large quantities of this product were used exclusively as a backfill amendment.

Conductivity measurements in this study ranged from <1 to >14 dS/m (Table 1). As a general rule, EC values of saturated soil/media extracts greater than 4 dS/m are considered high (Lal and Shukla 2004) and thus may not support good growth for some plant species. According to these guidelines, 3 of the media tested in this study (GPT, SP, and MM560) may need to be leached with water to reduce potentially high salt buildup. However, the ion content contributed by the addition of a relatively small amount of compost would normally be overwhelmed by the ion content of a proportionately larger volume of backfill soil.

The MSW component of many composts can contain heavy metals that could be phytotoxic (Chaney 1973). An independent laboratory analysis of the CT used in these studies by the Ohio Cooperative Extension Service revealed the following heavy metal content (ppm): cadmium = 14; chromium = 176; copper = 249; lead = 209; nickel = 66; and zinc = 1531. In the United States, federal regulations regarding the use of sewage sludge have recently been adopted under section 503 of the Clean Water Act, but there are no federal laws or regulations that apply specifically to solid waste composting (Harrison and Richard 2005). Although there are many similarities between sewage sludges and MSW-derived composts, there are also important differences (e.g., bioavailability of metals, ratio between different metals). However, considering the ‘risk-based’ approach for regulating compost quality, the heavy metal content of CT used in these studies was within recommended guidelines.

Physical Properties of Media

Sustainable use of a soil or other planting medium cannot be achieved unless the physical conditions of the medium are maintained at a satisfactory level (Lal and Shukla 2004). The addition of fertilizer alone will not likely preserve productivity of a planting substrate if physical conditions are not maintained above some minimum threshold level or if significant deterioration of physical quality occurs.

Bulk density, sometimes referred to as apparent density, is the ratio of oven-dried soil or medium per unit volume (Gerrard 2000). Thus, a dense medium has more solids per unit volume than a porous medium. As expected, bulk density values for the soil-containing composts used in the current study (GPT and SP) were significantly higher than the bulk densities of those media without any soil (Table 1). Similarly, for substrates containing a biosolid component, bulk density averaged ≈0.4 g/cm³, whereas for substrates without any biosolids, bulk density was significantly lower (0.1 g/cm³). Bulk density values for soils acceptable for plant growth range from 0.7 to 1.8 g/cm³ (Lal and Shukla 2004), but for media containing organic compost, bulk density would likely be lower.

Particle density is defined as the mass per unit volume of soil particles (Brady and Weil 1999). The particle densities of inorganic soils normally range from 2.6 to 2.8 g/cm³, whereas those for organic matter are usually about half that amount (Lal and Shukla 2004). The particle densities of media used in the current study varied from 1.6 g/cm³ for SP to 0.3 g/cm³ for MM360 and 1CT:3MM (Table 1), all of which are lower than expected for unamended backfill soils. Like with bulk density, the particle density of compost-amended soils (GPT and SP) was always significantly higher than for media without soil. As expected, the higher the amount of organic matter in the medium, the lower the particle density.

Air-filled porosity (AFP) is a measure of the relative proportion of air-filled pores in a soil or planting substrate. In relation to plant growth, media with AFP values of 7% or lower are most likely to have problems of poor drainage (Bragg and Chambers 1988). However, plants can be grown successfully in low AFP media provided that water management is carefully controlled. In the current study, AFP values for MM360 and substrates containing MM360 were significantly lower then those for media without MM360 (Table 1). For MM360 alone and for mixtures of MM360 and CT, porosity measurements were sometimes below the 7% threshold level, suggesting that anaerobiosis could be a concern.

Container capacity represents the total per cent water, by volume, held by a saturated soil or planting substrate in the absence of evapotranspiration. Almost any soil, amended or nonamended, when placed in a container will retain a larger volume of available water than will the same soil under field conditions (White and Mastalerz 1966). Thus, the use of containers changes the normal physical relationship between roots and substrate for plants growing under such conditions (Bunt 1961). This is an important consideration not only for container-grown plant material in greenhouse studies such as reported here, but also for planting pits and above-ground containers in which growing space can be severely restricted. In the current study, container capacities were greatest in composted substrates containing soil (GPT, SP) and significantly lower for soilless substrates, especially those without any CT (MM360, MM560, CM) (Table 1).

Pore space is an expression of the volume of pores to the total volume of soil or planting medium and is usually determined from the relationship between bulk density and particle density (Gerrard 2000). For soils or planting media with the same particle density, the lower the bulk density, the higher the percent pore space. A medium-textured, well-granulated soil or planting medium in good condition for plant growth would have pore space of ≈50% (Brady and Weil, 1999). However, porosity can range from as low as 25% in compacted soils/media to more than 60% in well-aggregated, high organic matter content soils/media. The pore space of media used in the present investigation ranged from 46.7% for CT to 78.3% for MM560 (Table 1). For compost-amended soils (GPT and SP), porosity was 51%, a pore space considered “ideal” for plant growth. For composted media containing biosolids, with or without soil, percent pore space was always significantly lower than it was for a composted medium without any biosolids (MM560). With the possible exception of MM560, all media tested in this study were considered to have porosity values expected to support good root growth.

Part 1: Growth of Jiffy Plug Seedlings in MSW Compost (CT) and Noncomposted (MM360) Substrates

For Jiffy Plug red maple seedlings (simulated balled-and-burlapped plant material), total biomass was significantly greater in substrates containing only noncomposted media (MM360) than in substrates containing only MSW compost (CT) (Table 2). For substrates containing mixtures of CT and MM360, overall growth was significantly better in a medium with greater quantities of MM360 (1CT:3MM) than in a medium with greater quantities of CT (3CT:1MM). In fact, maple seedlings planted in 3CT:1MM never really became established as evidenced by the absence of new shoot growth, lower total plant biomass, and a significantly higher root: shoot ratio (Table 2). Shoot (stem and leaf) dry weight of seedlings grown in noncomposted substrate only (MM360) was always significantly greater than for comparable seedlings grown in MSW compost only (CT) (Table 2); and leaf, stem, and root dry weights were consistently higher for seedlings in substrates containing more noncomposted substrate (MM360 only; 1CT:3MM).

In one of the few backfill amendment studies involving balled-and-burlapped Acer rubrum, Smalley and Wood (1995) reported no significant difference in either root or shoot growth of red maple trees (3 cm [1.2 in] caliper) planted in 60 cm (24 in) diameter by 30 cm (12 in) deep planting holes backfilled with a proprietary product containing composted poultry litter. The current study, although involving plant material considerably smaller in size, also suggests that red maples planted in backfill amendments containing appreciable quantities of solid waste compost (CT in this instance) may not grow as well as similar maples planted in backfill soils amended with a noncomposted substrate such as MM360.

For Jiffy Plug green ash seedlings, growth (percent height increase and total biomass) was significantly better when seedlings were planted in MM360, a noncompost-containing substrate (Table 2). Leaf, stem, and root biomass were also consistently greater for seedlings grown in the noncomposted substrate; however, no significant differences in root:shoot ratio were observed between any of the treatments. In mixtures of CT and MM360, green ash grew best in a medium containing equal parts of both substrates (1CT:1MM). These results are different from those observed for red maple, in which seedlings grew best in a mixture containing more MM360 (1CT:3MM). One possible explanation may be the difference in physical properties between the two media. Bulk density, particle density, air-filled porosity, and container capacity of 1CT:1MM were all significantly higher than for 1CT:3MM (Table 1). Because the growth rate of green ash normally exceeds that of red maple (Nowak et al. 2002), one might expect a faster-growing species to outperform a moderately growing one, especially in short-term studies such as those reported here.

Watson et al. (1992) have reported that root density (after 1 year) and terminal growth and trunk diameter (after 2 years) of Fraxinus pennsylvanica ‘Summit’ trees (5 to 7 cm [2 to 2.8 in] in diameter) were not significantly different when planted in unamended backfill soil, in soil amended with sand and composted organic matter, or in friable topsoil. The authors concluded that although there is no reason not to use amended backfill soils when transplanting trees of this size, there may be no significant benefit in terms of increased growth. In the current study, the only appreciable growth advantage noted for transplanted green ash seedlings was when a noncomposted substrate (MM360) was used exclusively as the growing medium. Under these circumstances, height growth, total biomass, and leaf area were all significantly greater than for most of the remaining treatments (Table 2).

Part 2: Growth of Bare-Root Seedlings in Biosolid Compost (GPT; SP; CM), in Compost Without Biosolids (MM560) and in Noncomposted Substrate (MM360)

There seemed to be no particular advantage in growing bare-root red maple seedlings in any of the composted media, with or without biosolids, versus growing them in a noncomposted substrate (Table 3). Within the compost treatments used in this investigation, seedlings grown in media containing biosolids (GPT, SP, CM) generally grew better than seedlings planted in a medium without biosolids (MM560), but only occasional significant differences were noted. The root:shoot ratio of compost-grown maples was generally higher than for maples planted in a noncomposted medium (Table 3), suggesting that root growth might be stimulated to a greater degree than shoot growth in these treatments; however, the variability between replicates was so great that no definite conclusions could be drawn from these data.

In a study of posttransplant growth of 1-year-old, bare-root red maple seedlings, Kelting et al. (1998) reported that a treatment consisting of three parts native backfill soil to one part yard waste compost (by volume) did not increase height, stem diameter, top dry weight, or root length after 2 years when compared with a treatment of native backfill soil only. In the current study, the compost treatment most closely approximating that used by Kelting et al. was the SP treatment (60% soil:20% compost:20% sand). As already noted, red maple seedlings grown in SP did not show any significant increase in height growth when compared with the other media tested, and there were no significant differences in any of the growth parameters measured between SP and the other soil-containing substrate (GPT) used in these studies (Table 3).

For bare-root Acer saccharum, height growth of seedlings in MM560, a soilless compost without any biosolids, was significantly greater than it was in any of the other compost-containing media (GPT, SP, or CM) used in this study (Table 3). Although total biomass of MM560-grown seedlings was not significantly different than it was for any of the other treatments, leaf area was significantly greater than for at least one other compost-containing substrate (CM), and leaf dry weight for maples grown in MM560 was appreciably greater than it was in the other media tested, except for the control.

Although no studies could be found in the literature dealing specifically with the effects of soil amendments on posttransplant growth of sugar maple, investigators working with other Acer species have reported a variety of responses. Schulte and Whitcomb (1975) found no particular benefit when silver maples (Acer saccharinum L.) were planted in backfill soil amended with bark and peat moss and, as already mentioned, Kelting et al. (1998) found no increase in the growth of bare-root Acer rubrum seedlings planted in native backfill soil amended with composted yard waste. On the other hand, Smalley and Wood (1995) reported increased fibrous root growth in balled-and-burlapped red maples 2 years after planting in backfill amended with topsoil, pine humus, granite sand, crushed granite, expanded shale, and composted poultry litter. The results with 2-year-old bare-root sugar maple seedlings in the current study suggest that there was no particular advantage when seedlings of this species were grown in a variety of compost-containing media for 12 weeks.