Potting Substrate

The supply of potting substrate in adequate quantity and quality is a major problem in many large nurseries, which can require huge amounts. Several substrates have been used for raising A. mangium planting stock, including topsoil, vermiculite, perlite, different types of compost, and tropical peat. These

substrates can be used as a mixture or in pure form. Vermiculite or perlite should be mixed with a less expensive substrate (for example, bagasse).

A medium for container-grown seedlings should drain well and at the same time have good water-holding capacity, aeration, and firmness. The optimum pH of the nursery substrate is 5.5-7.0 (FAO 1987). A. mangium seedlings do not seem to be particularly sensitive to low pH-values of the nursery substrate; in trials these have been successfully grown in peat substrates with a pH as low as 3.1 (Adjers et al. 1988). In the field, A. mangium tolerates moderately acidic soils with pH as low as 4.0 (FAO 1987).

In Indonesia, tropical peat in different mixtures has been used extensively. Tropical peat is usually highly humified and its aeration is poor. By mixing the peat with a suitable material, aeration can be increased (Adjers 1987). Rice husk is widely used with peat in large-scale production of A. mangium (Plate 5.4), employing a proportion of 70-80% peat with 20-30% rice husk (Supriadi and Valli 1988; Suomela et al. 1988; Hendromono 1988). Sumatran experience indicates that the proportion of rice husk can be lower (10-30%)), depending on the peat (Valli 1993, pers. communication).

The pH of tropical peat substrate can be quite low. Low pH is usually corrected by liming. Also, the pH of peat increases in storage. In South Kalimantan, the peat is lifted during the dry season and stored for at least 3 months (Supriadi and Valli 1988). During storage, the pH increases by 1 to 1.5 units. Drying in the dry season is carried out in open fields. During the rainy season, the peat should be covered with canvas or plastic to prevent soaking and kill weeds.

With A. mangium. Adjers et al. (1988) found high liming doses (15 kg/m?) to have a negative effect with three peat sources in South Kalimantan. The pH varied from 3.1 to 6.3 in the trial after application of 8 and 15 kg of dolomite lime. Liming increased the pH relatively little.

The most common nursery substrate is topsoil. Sandy loams are the best, but heavier soils can be modified by adding sand, peat, vermiculite, perlite, shredded bark, sawdust, wood shavings, coconut coir, or rice husk, in proper proportion (FAO 1987). Even when using top soil, for uniform growth of A.

mangium, soil collection should be standard- Plate 5.4. A mangium grown in

, . ., ., , a medium of ironical neat and nee lzed to get more umlorm and fertile soils, rather hUSk Note new root development. Photo: Enso

than haphazard collection from different locations (Poole 1987).

Different types of compost have also been used as seedling media. Trials in South Kalimantan tested 13 different compost substrates with A. mangium. Compost of rice straw, maize stems, and leaves, mixed with 50% or 30% topsoil, produced the best growth and survival. Bagasse yielded good height but poor diameter growth. Pure compost media (maize and rice straw), excluding bagasse, yielded poor survival. In the same trial, peat and rice husk (70:30) yielded moderate growth but highest survival (Rusmana 1993, pers. communication). Newman (1989a) reported the results of using sawdust as germination medium for A. mangium at SAFODA's nursery. Two seedlots of this species were germinated outside in trays in the shade in three media: soil/sand (2:1). old (mixed species) sawdust, and fresh (mixed species) sawdust. Control seeds were germinated in petri dishes in a germinating cabinet. Germination reached 807 in the control and in the soil/sand and old sawdust media but did not occur at all in fresh sawdust.

Tropical peat and compost have the additional advantage of light weight for transport and planting.

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