Sphagnum moss in growing media : water retention and plant performance

Weakly decomposed peat has become the most used growing medium in soilless plant production. Extracting it, however, results in several harmful effects to peatland ecosystems. In the continuing search for an abundant substitute for horticultural peat, this thesis examines the physical properties of undecomposed and partly living Sphagnum moss when used for growing plants. The responses of plant growth to the replacement of horticultural peat with Sphagnum moss are discussed through the changes observed in the water retention properties of the media and by using modeling-based calculations for water availability. In the first study, the physical properties of undecomposed Sphagnum moss, dominated by Sphagnum fuscum (Schimp.) Klinggr., were investigated in the laboratory both before and after preprocessing by cutting and compacting. The results were compared with light (H2) and dark (H5) Sphagnum peat of Finnish origin. Water retention curves and saturated hydraulic conductivities were measured for samples of S. fuscum-dominated moss with natural structure as well as for samples that were cut to selected fiber lengths or compacted to different bulk densities. The six-parameter form of the double-porosity van Genuchten equation was fitted separately for each given bulk density, to model the complete water retention curve of moss. In the second study, two types of preprocessed Sphagnum moss were added to peat-based growing media. Three experiments were undertaken, two with sweet basil (Ocimum basilicum L.) and one with verbena (Verbena × hybrida Groenl. and Rümpler) plants that were first grown under optimal watering, then subordinated to severe drought. The occurrence time of water stress symptoms of wilting sweet basil plants was further studied in the growth chamber using thermal imaging. In the third study, a new water retention function covering the bulk densities from 40 to 80 kg m-3 was formulated for the preprocessed S. fuscum moss that was used in both earlier studies. This was done using a modification of the multimodal water retention function by Durner (1994). The original parameters of the bimodal equation were substituted with low-order polynomials of bulk density. The results underline that on a dry mass basis, Sphagnum moss retains more water than light or dark Sphagnum peat under equal matric potential. Thus, enhanced water retention was the primary effect of adding Sphagnum moss to peat in laboratory conditions. Compacting the preprocessed moss increased its water retention by reducing the volume of the largest pores. The saturated hydraulic conductivity of the samples decreased steadily with the increasing bulk density of moss. In the experiments with plants, substituting peat with undecomposed Sphagnum moss accelerated plant growth. The pore system of Sphagnum creates a larger body of nutrient solution that can supply more available water and plant nutrients to plants compared with horticultural peat. Under drought conditions, substituting at least 50% of the peat with Sphagnum delivers significant tolerance against wilting. Compared with horticultural peat, this larger volume of water in the media contributes positively to plant growth and drought tolerance by buffering unbeneficial concentration changes between waterings. As bulk density largely governs the volume of retained water in Sphagnum moss it consequently also determines the extent of the effects above. The modeling of water retention properties suggests that bulk density predicts well the water desorption curves of undecomposed and preprocessed S. fuscum-dominated growing media. The practical advantage of the new model type includes quick adaption to a wide range of BD, without the need for re-fitting for each curve separately and easy calculations of several important horticultural parameters for hydrology, such as air-filled porosity (AFP) and easily available water (EAW) of the media as a function of bulk density. In conclusion, this study has shown that the water stored into the hyaline cells of Sphagnum moss-based media is usable for horticultural plants under drought. Sphagnum moss promotes the growth of plants and increases their tolerance against wilting if added to peat-based media but also when used alone as a growing medium. Compaction of the moss for growing media purposes causes systematic changes in the pore size distribution (air–water relation). Thus, when using Sphagnum for growing plants, the supply to plants of two important growth factors, namely water (H2O) and oxygen (O2), appear to be controlled by compaction. To fully benefit from the exceptional water retention properties of compacted Sphagnum moss, the densities of the growing media should be optimized to the level where AFP still allows for sufficient ventilation to facilitate fast growth, but the manipulated pore size distribution still ensures efficient uptake of water by roots.