|Organic and Compost-Based Growing Media for Tree Seedling Nurseries (World Bank, 1995)|
There is a large amount of information available pertaining to the composting of wood products and their use in potting media. This annex presents an overview of some of this published information.
Bark's high C/N ratio requires it be lowered for composting by addition of nitrogen. Urea or anhydrous ammonia speeds decomposition and raises the pH into the desired range of 6.5-8.5 (pH of fresh bark is 4.0-5.5) (Hoitink and Poole, 1980). Pine and hardwood barks require slightly different conditions (moisture and nitrogen) for optimal composting because of differences in their cellulose content (i.e. readily decomposable carbon) (Hoitink and Poole, 1980). As a result, additional nitrogen is necessary for composting bark with lower cellulose contents (<5%). The recommended nitrogen amendment (urea) for softwood bark is approximately 0.5-0.75% (the lower range is for wet bark) and for hardwood bark it is higher between 1-1.2% (Cappaert et al., 1976a). One source recommends the addition of 0.6-1.0 kg N/m³ to produce finished compost in 6 weeks for pine and 2-3 times that to complete composting in 10 weeks for hardwood species (Self, 1978). In addition to nitrogen, 0.3 kg P2O5/m³ (0.4% P as superphosphate) has been recommended to increase composting rates (Hoitink, 1980).
Fresh bark has a moisture content around 40%, so a thorough wetting is necessary to increase the moisture content (between 50-65%) (Hoitink, 1980). This may take several days because raw bark tends to be hydrophobic. The moisture content also affects the amount of nitrogen needed for composting, so addition of 0.5% N is recommended for fresh bark (40% moisture) and 0.8% N for wetted bark (Cappeart et al., 1975). Bark from younger trees and bark which contains wood wastes also tend to need more nitrogen for composting.
Sources of additional nitrogen may include piggery and poultry slurries added at a ratio of 1:3 (by volume) slurry-bark mixtures (Verdonck, 1983). Addition of manures can also improve the physical properties over composting bark alone, but only if they do not have excessively high moisture contents (e.g. poultry slurry) which would reduce the porosity of the compost.
Size reduction prior to composting is accomplished through shredding or hammermilling. Either before or after composting, the bark should be sieved to remove particles larger than 2.5 cm and one source recommends that 25-33% of the bark be less than 0.5 mm in size (Landis et al., 1990). Some barks may not be utilized in growing media due to their texture after processing. For example in South Africa, Eucalyptus bark was reported to resemble coconut coir dust after hammermilling and was unusable in containers less than 90 mm in diameter (Hodgson, 1980).
The time required for bark to reach the end of active composting is dependent on species, additives and other physical conditions as discussed above. Since use in nurseries require maturity, then one can assume that up to 10 months will be required. If mixed with peat, less mature compost may be used with increasing quantities of peat.
When composted, pine bark has a low bulk density, a moderate CEC, significant (but small) quantities of macro- and micronutrients, a desirable pH range, and is easy to handle (see Annex I, Table 2). Raw bark contains toxic levels of volatile monoterpenes which vary between species, age, geographic location, and the season of collection. Composting will reduce levels of monoterpenes to below toxic levels and kill off pathogenic fungi.
Utilizing pine or hardwood bark as a potting mix provides added protection to seedlings by suppressing some disease causing pathogens. The suppression of pathogens however, differs between hardwood and pine barks. Composted barks of both types suppress Phytophthera and Pythium root rots, but only hardwood bark compost suppresses Rhizoctonia-induced damping-off (Hoitink and Poole, 1980). Inclusion of wood chips in a potting mix will reduce the bark's suppressive capabilities.
Bark-based medias have been used successfully for a variety of nursery seedling species. Composted pine bark (occasionally mixed with peat) has been used in South Africa in forest nurseries to raise wattle (Acacia spp.), Eucalypt spp., and pines (mostly P. patula) (Donald, 1986), and P. radiata bark has been used in New Zealand (Josiah and Jones, 1992). Seedling establishment trials in Australia used containers filled with a composted, nutrient-rich milled pine wood and bark media mixed with 2 or 4 parts (by volume) fine sand (Dalton, 1987). Four months after outplanting, the seedlings (Eucalyptus fasciculosa, E. condinensis, Banksia burdettii, Grevillea, and Melaleuca elachiophylla) all had doubled in height regardless of the potting media in which they were raised.
In South Africa, growth comparisons were made of E. grandis, P. patula, and Acacia mearnsii grown in pure composted pine bark and mixtures using the composted bark with worm digested slaughterhouse wastes. The pine bark proved to be the best substrate for E. grandis and A. mearnsii provided that a 3-2- 1 NPK fertilizer was added, and a 2-3-2 fertilizer application was made to P. patula (Donald and Visser, 1989).
Uncomposted pine bark can be used as a growing medium, but additives are required for successful growth. This may not be practical for all seedling programs because of fresh bark's toxicity to some species of plants. Milled and screened (with no sawdust or wood chips), fresh bark has been used at 3:1:1 (bark-sand-shale) and treated with dolomitic lime, trace elements, and 3.5 kg/m³ nitrogen (Van Landingham, 1978).
Sawdust has also been widely used as a growing media component. Sawdust generally has a high C : N ratio and poor water holding capacity, both of which are improved upon composting (Cull, 1981). It has been observed in South Africa that Eucalyptus sawdust tends to shrink when it dries and can impede drainage. The addition of bark to sawdust can be used to raise the CEC of the media (Hodgson, 1980).
Australian studies have shown that 3:1 eucalyptus sawdust-bark mixtures give satisfactory results as a medium for raising E. ficifolia and P. radiata seedlings (Hodgson, 1980). Sawdust of a variety of species composted together was studied as a growth medium in the Philippines (Eusebio et al., 1984). Compost was produced in 45 days both with and without inoculation with a mycelial enzyme solution from Lenzites striata, a brown rot fungus. Better growth was obtained using a 1:1 inoculated sawdust-garden soil media compared to a 100% garden soil for Pterocarpus indicus and Paraserianthes falcataria.
Growth trials in the Philippines using decomposed sawdust medias in poly-bags gave varying results dependent on the species of tree and the mixture of components. Based on average height and diameter growth, decomposed sawdust alone was found to be an acceptable media for the growth of kamachile (Pithecelobium dulce) and kakauate (Gliricidia septum) seedlings. A 1:1:1 humus-decomposed sawdust-coirdust media was also acceptable for kakauate. Another study in the Philippines concluded that decomposed sawdust provided acceptable growth to Benguet pine (Pinus kesiya) although not as good as sphagnum moss or topsoil.
In Puerto Rico, trials comparing potting medias showed that a 1:1 mixture of vermiculite-mahogany sawdust (aged I year) was an acceptable growing medium for Honduras pine (P. caribaea var. hondurensis) (Marrero, 1965). Seedlings were grown in poly-bags (11.5 x 24.35 cm) filled with media soaked in a 20-20-20 NPK chemical fertilizer and fertilized monthly thereafter. Height growth and average stem diameter were not significantly different (but were slightly less) than trials using sphagnum peat moss. Further experimentation using mahogany sawdust at 100% and 3:1 or 1: I sawdust coconut husk (coco-peat) showed sawdust to be an acceptable media with performance improved by increasing quantities of coco-peat.