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Does biochar in seedling mix help pepper and broccoli seedlings?
Four kinds of biochar mixed into a peat seedling mix gave different results in a Polish greenhouse trial. Pepper shoots did best with pine sawdust biochar, which raised shoot dry weight by 37.2%, while broccoli shoots did not differ statistically with any of them.
Does it matter what a biochar is made from? Biochar is made by heating biomass with little or no oxygen, and it can be added to growing mixes. A team at Bydgoszcz University of Science and Technology made biochars from four leftovers (chestnut seeds, compost from a wastewater treatment plant, pine bark and pine sawdust), mixed each into a standard peat substrate, and raised pepper and broccoli seedlings in them for five weeks (Tymoszuk et al., 2026).
What this means for your grow
In this trial, the answer depended on both the biochar and the crop. None of the four biochars changed the broccoli seedlings' leaf number, shoot length or shoot weight, and the authors say adding them to broccoli seedling mix at this rate does not seem justified. In peppers, pine sawdust biochar gave the heaviest shoots and chestnut seed biochar the biggest roots, but compost biochar gave fewer leaves and shorter shoots than the plain mix. So a bag labelled "biochar" is not one thing, and this trial tested just one rate (4.4% by dry weight), in one greenhouse, over five weeks, without fertiliser.
What did the team do?
The four feedstocks were compost from a municipal wastewater treatment plant, chestnut seeds from green-area maintenance, pine bark, and pine sawdust from a local sawmill. Each was air-dried and turned into biochar at 400 °C for one hour in a furnace, then ground in a ball mill. The biochars differed a lot: pine sawdust biochar had a pH of 3.86, while chestnut seed biochar had a pH of 8.24 and the most potassium of the four.
Each biochar was mixed into a universal peat substrate at 4.4% (dry weight), with plain substrate as the control. Seeds of pepper 'Marta Polka' and broccoli 'Cezar' were sown one per 0.25 L pot, 10 plants per treatment, in a greenhouse at the university. After five weeks the team measured the shoots, scanned the leaves and roots, and read leaf pigments and chlorophyll fluorescence with a portable pigment meter and a chlorophyll fluorimeter.
What did they find?
- Broccoli shoots: leaf number, shoot length and shoot fresh and dry weight did not differ statistically between any of the mixes.
- Broccoli leaves and roots: pine sawdust biochar gave a larger leaf perimeter than the control (26.12 cm against 20.23 cm). Compost biochar cut root diameter by more than 38% compared with the control, gave the most root tips, and gave the lowest anthocyanin, chlorophyll and SPAD index readings, though these were significantly lower only than with pine bark biochar, not than the control.
- Pepper shoots: pine sawdust biochar gave the highest shoot dry weight, 254.98 mg, a 37.2% increase on the control (185.80 mg). Its shoot fresh weight (3.46 g) was not significantly different from the control (2.68 g).
- Pepper leaves: with pine sawdust biochar, leaves had almost 38% more surface area than the control.
- Pepper roots: chestnut seed biochar gave the longest root systems (155.40 cm), 45.4% longer than with pine sawdust biochar, and more root tips than any other mix.
- Pepper leaf colour: chestnut seed biochar gave the highest chlorophyll reading (8.63, against 7.32 for the control) and SPAD index (30.63, against 27.95).
- The poor performer: in peppers, compost biochar gave fewer leaves (6.00 against 7.50) and shorter shoots than the plain mix.
What could it mean?
The authors conclude that different biochars affect shoot and root growth differently, and that the plant species matters. They suggest pine sawdust and chestnut seed biochars may be a useful addition to peat substrates for producing pepper seedlings, and that the right biochar should be chosen for each species. For broccoli, they found no benefit at this rate and suggest higher concentrations could be tested. The authors have filed a Polish patent application for using pine sawdust or chestnut seed biochar in pepper seedling production. These are findings for seedling producers and researchers to follow up; the biochars were made in the lab, so this is not a test of products you can buy.
What were the limitations?
Only one rate of biochar (4.4%) was tested, no fertiliser was given, and the seedlings were assessed after five weeks, before transplanting. For broccoli, the authors suggest higher concentrations might have positive effects and are worth testing. They call for future work on more vegetable species and cultivars, combinations of biochars at higher concentrations, and the microbial and biochemical activity of biochar-amended substrates.
Trial details
- Where: a greenhouse at the Faculty of Agriculture and Biotechnology, Bydgoszcz University of Science and Technology, Bydgoszcz, Poland (53°07'12.0" N 18°00'29.4" E). Facility type: automated greenhouse.
- Who: Bydgoszcz University of Science and Technology, with the Institute of Plant Protection, National Research Institute, Poznań.
- Crops: pepper (Capsicum annuum) 'Marta Polka' and broccoli (Brassica oleracea var. italica) 'Cezar', from seed to five-week-old seedlings.
- How long: 1 July to 5 August 2024.
- System and substrate: one seed per 0.25 L plastic pot, sown 2 cm deep, in universal peat substrate (Hartmann Polska) with or without biochar at 4.4% (dry weight). Biochars made at 400 °C for 1 hour from chestnut seeds, compost from a municipal wastewater treatment plant, pine bark and pine sawdust, then ball-milled. Substrate moisture kept at 70 to 80%.
- Biochar and peat properties (before mixing): pH 8.24 (chestnut seed biochar), 7.35 (compost biochar), 4.72 (pine bark biochar), 3.86 (pine sawdust biochar) and 6.49 (peat substrate); EC 777.8, 632.1, 127.95, 108.05 and 356.7 µS·cm-1 in the same order.
- Lighting: no artificial lighting; natural 16-hour photoperiod and natural light intensity.
- Climate: air temperature 20 °C ± 2 °C; air humidity 80%.
- Nutrition: no extra fertiliser or fertigation.
- Design: five treatments (control plus four biochars) for each crop, completely randomised design, 10 plants per treatment. Measured after five weeks: leaf number, shoot length, shoot fresh and dry weight, leaf size (scanner and WinFOLIA), root length, area, volume, diameter and tip number (scanner and WinRHIZO), flavonols, anthocyanins, chlorophyll and SPAD index (MPM-100 meter), and chlorophyll fluorescence (OS-30p+). One-way ANOVA with Fisher's LSD at α = 0.05.
- Results (pepper, mean ± SD): shoot dry weight 185.80 ± 83.76 mg (control), 254.98 ± 33.11 mg (pine sawdust), 188.81 ± 34.84 mg (chestnut seed), 179.50 ± 43.90 mg (pine bark), 137.56 ± 62.30 mg (compost); leaf number 7.50, 8.17, 7.43, 6.38 and 6.00; shoot length 11.21, 12.70, 11.19, 11.28 and 9.46 cm. Broccoli shoot traits did not differ statistically between treatments.
- Funding and conflicts: the research did not receive external funding. A Polish patent application (P.455823) has been filed for the method of producing pepper seedlings with pine sawdust or chestnut seed biochar.
- Publication: Acta Scientiarum Polonorum Hortorum Cultus 25(5), 47 to 61, issue published 30 October 2026, https://doi.org/10.24326/asphc.2026.5716. Licence: CC BY 4.0.
- Not reported in the paper: a conflict of interest statement, light intensity (PPFD or DLI), CO2, the substrate's EC and pH after mixing, and how the seedlings grew on after transplanting.
Links
- The study: Effects of different biochar types on the growth and biochemical characteristics of broccoli and pepper seedlings under greenhouse conditions
- The institution: Bydgoszcz University of Science and Technology
References
- Tymoszuk, A., Kulus, D., Wojewódzki, P., Kuklicz, A., Mikołajczak, A., & Antkowiak, M. (2026). Effects of different biochar types on the growth and biochemical characteristics of broccoli and pepper seedlings under greenhouse conditions. Acta Scientiarum Polonorum Hortorum Cultus, 25(5), 47 to 61. https://doi.org/10.24326/asphc.2026.5716