Assessing the Ratio of Worm Castings to Peat-based Potting Mix for Greenhouse-grown Organic Basil

Guest Authors: Anisah Madden, Summer Research Intern and Mehdi Sharifi, Assistant Professor/Canada Research Chair in Sustainable Agriculture, Trent University

There is evidence that the use of worm castings as an amendment have the potential to supply sufficient nutrients to leafy vegetables during their growth period. A greenhouse experiment was recently conducted at Sustainable Agriculture Program in Trent University to access the optimum ratio of worm castings to growing media for organic basil (Ocimum basilicum).

Worm casting treatments were combined with an organic peat-based potting mix1 in different percentages (0, 15 and 30% weight basis) to study the effects on basil plant growth and nutrient availability. The total concentrations of carbon, nitrogen and sulphur in the worm casting were 13%, 1.05 % and 0.24% respectively on a dry weight basis. The moisture content of the worm casting amendment was between 20 to 22%.

Worm castings, (also called vermicompost), are the end product of the breakdown (digestion) of organic matter by earthworms. Vermicompost is rich in nutrients, beneficial bacteria and enzymes, and has been shown to improve soil structure, aeration, porosity, drainage, and moisture holding capacity, while exhibiting a positive effect on plant growth and health.2

The results of this project indicated that a mixture of 15% worm castings to 85% peat-based potting mix provided sufficient nutrients for basil plants during their growing cycle (8 weeks), significantly enhancing the majority of basil traits including plant height, biomass, leaf area, and number of branches compared with other treatments - without any need for supplemental nutrient application (Table 1).

Worm castings applied to the potting mix at the higher rate of 30% did not further enhance the growth traits observed, and only showed a marginal increase in nitrogen and sulphur uptake by the plants compared with the 15% treatment rate.

In addition, the study looked at the effects of adding biochar 3 and mycorrhiza 4 to the three different worm casting/potting soil treatments.

Biochar is the name for charcoal used as a soil amendment, and is produced by the pyrolysis (thermophilic decomposition of organic matter under high temperatures, in the absence of oxygen) of biomass (crop residues, or wood). Biochar is being researched for its carbon sequestration potential as well as its ability to improve soil fertility and agricultural productivity.

A mycorrhiza is a symbiotic association between a fungus and the roots of a vascular plant. This beneficial relationship improves the plant’s ability to uptake water and nutrients, and additionally protects the plant roots from infection by pathogenic organisms.

In this study, mycorrhizal inoculation showed a positive significant effect on some basil plant growth parameters (shoot dry weight, leaf/stem ratio), and mycorrhizal dependency, and also improved nitrogen and sulphur uptake by plants; however, the addition of biochar did not show a significant effect on measured parameters.

This research project will help organic greenhouse producers to use the optimal amount of worm castings as an amendment for their growing media mix. The results of this project are also relevant to growers using integrated production methods as well as conventional growers who wish to minimise their synthetic fertilizer use, thereby reducing the risk of environmental damage while improving the sustainability of their production systems.

Table 1: Effect of worm casting ratio to peat-based potting mix in growing media on organic basil growth parameters.
Worm Casting Rate (%) Shoot DWg/
Stem Diametre (mm) Leaf DWg/
Total Leaf area/
Number of Branches/
Mycorrhizal Dependency Chlorophyll Content

Values in each column with the same letter are not significantly different at 0.05 probability level using LSD test

Figure 1: Dr Reza M. Ardakani evaluating basil plants grown in a worm casting and peat-based media for organic greenhouse production systems

Figure 1: Dr Reza M. Ardakani evaluating basil plants grown in a worm casting and peat-based media for organic greenhouse production systems.

1 Sunshine #4 Natural & Organic Peat


3 Blue Leaf ; 10% weight

4 +MYKE® PRO PS3, Glomusintraradices

Acknowledgements: This research was made possible by support and the generous donation of worm castings by Greenscience Technologies Inc., plant donations from Johnny’s Seeds, Biochar donation from Blue Leaf, and funding from the NSERC Engage program.

Credits: Dr. Mehdi Sharifi, Canada Research Chair in Sustainable Agriculture, Trent University; Dr M. Reza Ardakani, Research Associate in Sustainable Agriculture Program at Trent University; Dr Tom Hutchinson, Professor Emeritus in Environmental and Resource Science/Studies at Trent University, Anisah Madden, Sustainable Agriculture and Food Systems (SAFS) student at Trent University.

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