Development and experimental comparison of peat-free potting media for organic horticulture

Summary

The use of peat as a basis for growing substrates has allowed the horticultural sector to become increasingly efficient and industrialized, which nowadays makes peat an indispensable external input for a majority of horticultural businesses. As peat is considered a nonrenewable resource, whose extraction is recognized to have negative impacts on climate change, wetland ecosystems biodiversity and water regulating mechanisms, the organic regulations towards the use of peat are becoming stricter and the need to develop alternatives emerged. So far, alternatives have mainly been tested by substituting peat with one other material to different extends, but a single material which is able to substitute peat completely has not been found. This research is a study looking into the potential for peat substitutes using a mixture of materials. It was hypothesized that similar physical- and chemical characteristics to a commonly used peat based substrate have better effect on plant development and quality. Physical and chemical characteristics of nine different materials (wagram compost, vermicompost, wood fiber, coconut coir dust, lavasand, quarz sand, biochar, bark humus and horse manure substrate) are analyzed and according to the results four compositions created. In a pilot experiment, plant productivity of tomato and lettuce after six weeks of growth in the four compositions as well as on a commonly used peat based substrate “Bio Potgrond” and an already existing peat free substrate “Bio Erde” by Vermigrand is compared. Diverse parameters such as plant height and biomass, flower production and nutrient uptake were measured and compared using one-way ANOVA and post-hoc tests. Transplants grown on the peat substrate performed significantly better than those on the peat-free substrates. Plant productivity amongst peat free substrates was highest on substrates containing coconut coir instead of wood fiber, also significantly higher than on the peat free control “Bio Erde”, indicating that the physical characteristics of coconut coir, which are similar to those of peat, have a major effect on plant growth. After the experiment plant nutrient contents were measured and nitrogen recovery calculated with results respective to the outcomes of the experiment, showing that next to the physical characteristics the nitrogen mobilization and availability of the substrates plays a major role. In a follow-up experiment these results were taken into account, and an additional seven substrates were composed, which were on the one hand additionally fertilized with horn meal or vermicompost, and on the other hand contained different wood fiber brands, in order to find one, which is able to compete with coconut coir. Due to small sample sizes in the follow up experiment, the results were not analyzed with statistical tests, and should therefor merely be used as an indication. The tomato transplants grown in the peat based Bio Potgrond were still performing the best, closely followed by a mixture with coconut coir and additional horn meal, as well as a mixture to which vermicompost was added. No wood fiber could be identified which led to better results than the one used in the pilot experiment. The results of the lettuce transplants in the follow-up experiment were to some extend controversial. The peat free Bio Erde performed better than the peat based Bio Potgrond, which is remarkable as in the pilot experiment this substrate performed significantly worse. External factors such as radiation and temperature may thus have a big effect on plant development and growth. Although none of the substrates is proven to compete with the peat based substrate, this research provided a first approach for the development of peat-free substrates, and revealed the importance of more in depth research when using mixture substrates.