292 Roberts: Compost-Containing Substrates maintained at a satisfactory level (Lal and Shukla 2004). The addition of fertilizer alone will not likely preserve productiv- ity of a planting substrate if physical conditions are not main- tained 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 (Ger- rard 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 bio- solids, 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 with- out 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 pro- portion 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 manage- ment is carefully controlled. In the current study, AFP values for MM360 and substrates containing MM360 were signifi- cantly lower then those for media without MM360 (Table 1). For MM360 alone and for mixtures of MM360 and CT, po- rosity 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 con- tainers 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 ©2006 International Society of Arboriculture 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 signifi- cantly 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 deter- mined 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 com- pacted 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 with- out 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 me- dium 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 seed- lings grown in MSW compost only (CT) (Table 2); and leaf, stem, and root dry weights were consistently higher for seed- lings 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
November 2006
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