The poor growth of young Eucalyptus regnans seedlings in undried soil from the mature forest of E. regnans can be overcome by previously air-drying the soil or by adding sufficient amounts of complete soluble fertilizer or equivalent concentrations of P (as NaH2PO4) and N (as NaNO3). A factorial pot experiment in which phosphate and nitrate were added to undried soil indicated that P was the primary deficiency for young seedlings and that response to N did not occur until this lack was satisfied. In dried soil, seedlings also responded to additions of complete fertilizer but most of this effect was due to N rather than P. Field trials in the mature forest also indicated greater growth in dried soil than undried soil and confirmed a response of young seedlings to superphosphate. In pot experiments, the concentration of P and N per g plant dry weight after four months was relatively constant irrespective of the final size of the plant.

The growth of Eucalyptus regnans seedlings in forest soil is enhanced when it has been air-dried. In undried forest soil seedlings grow poorly and develop purple coloration in the foliage, indicating P deficiency. This paper reports the results of pot experiments designed to investigate the relationship between growth and P acquisition, ectomycorrhizal infection and age of seedlings grown in air-dried and undried soil. The effect on seedling growth of their inoculation with air-dried or undried soil or with ectomycorrhizal roots from plants growing in air-dried or undried soil was also investigated. Ectomycorrhizal root tips were detected in 3-week-old E. regnans seedlings in both air-dried and undried soil, and from then on the frequency of ectomycorrhizal root tips increased rapidly. In air-dried soil, seedlings were fully ectomycorrhizal at 9 weeks, and the occurrence of maximum ectomycorrhizal infection coincided with enhanced P acquisition and the initiation of rapid seedling growth.

The growth of Eucalyptus regnans F.Muell. (mountain ash) seedlings is poor in natural forest soil, where purple coloration of the foliage indicates P deficiency and where the fungus Cylindrocarpon destructans (Zinsm.) Scholten is commonly isolated from the roots of the seedlings. When forest soil is air-dried, P acquisiton and growth of seedlings are markedly improved, although the degree of growth stimulation varies considerably at different times, as does the frequency of occurrence of C. destructans on the roots. C. destructans has been implicated as a possible reason for suppressed growth of seedlings in undried natural soil. To find out whether C. destructans contributes to growth inhibition of E. regnans seedlings in undried forest soil, the effect of three isolates of C. destructans on the root growth of E. regnans seedlings was tested in Petri dish experiments in vitro and the effect of C. destructans inoculation on seedling growth both in air-dried and undried forest soil was tested in pot experiments. The frequency of occurrence of C. destructans on the roots varied at different times, and was not consistently higher in undried than in air-dried soil, even though the growth of the seedlings was always poor in undried soil compared with that in air-dried soil. In vitro, C. destructans decreased the root growth significantly and caused blackening of root tips. This effect was removed by adding natural air-dried or undried soil. In pot experiments using undried forest soil, there was no evidence of either direct toxic effect or any other adverse effect on the roots when soil was inoculated with this fungus, even when the growth of the seedlings was reduced to ~1/2 of that in uninoculated undried soil. In air-dried soil, inoculation with the fungus did not significantly reduce seedling growth. Although potentially pathogenic and able to cause blackening of root tips, C. destructans is unlikely to be the main reason for poor seedling growth in undried forest soil. It appears to be antagonistic rather than pathogenic, suppressing seedling growth only under unfavourable conditions, such as in undried soil, possibly by competing for limited nutrients, or by suppressing other beneficial micro-organisms. The results are discussed in the context of field conditions.

Abstract. The growth rate of Eucalyptus regnans seedlings in their first year can be much increased if the soil is first dried and then rewetted. The ratio of growth on predried soil to growth on undried soil (the Growth Ratio or GR) reaches a maximum at air‐dryness (pF 6.0–6.4). In E. regnans forest soil, GR is greatest in humus‐rich topsoil and declines with depth. The effect of air‐drying persists for several months after rewetting when soil is stored under glasshouse conditions. It is largely unaffected by repeated drying and wetting, by the rate of drying or by the season of collection. The mixing of dried and undried soil or the placement of a layer of dried soil above undried soil produces an enhancement of growth proportional to the amount of dried soil added. Firing of a litter layer above soil at wilting point increases subsequent seedling growth to that in air‐dried soil. The addition of ash from a litter fire to undried soil produces an increase in growth approximately equal to that caused by air‐drying The drying effect is most pronounced in soils from mature E. regnans forest and nearby brackenland and is less in dense younger forest, frost‐hollow grasslands and old grassy gaps in the mature forest. The effect is restored by the inoculation of E. regnans mycorrhizal roots from both dried and undried soil. The effect varies along an gradient from 500 to 1500 m a.s.l. and is a maximum in the wet E. regnans climatic zone and a minimum in zones or local aspects where forests are normally subject to frequent drying. The stimulatory effect on seedling growth in soils of the E. regnans zone may have an effect on the outcome of competition during regeneration in large gaps. Part of the growth responses previously ascribed to the ‘ash‐bed’ effect may be due to the desiccation effect in these soils.