Cash in the Ash

Cash in the Ash: Using waste-stream products as sustainable growing media for horticultural crops

By: Poonam Singh.
Professor, Horticulture Science
School of Agriculture and Environment
Assiniboine Community College, Brandon, Manitoba

The interest in biomass use as a renewable fuel for heat and energy production is growing worldwide to decrease dependence on fossil fuels and reduce environmental impacts. Consumption of biomass in energy applications results in production of biomass ash. Currently, most of the biomass ashes are disposed off in landfills, which is non-sustainable, both economically and environmentally. Biomass ash carries important macronutrients, and can be recycled to soils, thus preventing the loss of valuable resources.

Numerous greenhouses in Canada burn several tons of agricultural biomass like flax shives, wood chips and other waste of landscape and timber industry as boiler fuel to heat their greenhouse, producing biomass ash as a by-product. One such example is Vanderveen’s Greenhouse Ltd. (the largest wholesale greenhouse of Manitoba) in Carman, Manitoba, where they use 6000 tons of flax shives annually as a boiler fuel resulting in 120 tons of ash as a waste product. Assiniboine Community College (ACC), collaborated with Vanderveen’s to explore the use of biomass ash in soilless media for producing horticultural crops in nurseries/greenhouses.

Biomass ash affects soil chemistry and biology in complex ways, due to primary effects, such as elevated pH and nutrient addition, and to derived effects such as changes in nutrient availability and interaction with biological processes (Bardgett, 2005). We expected biomass ash addition to increase pH and nutrient (mainly K and P) levels in the peat-based growing media. Peat is naturally acidic (pH≤4.5); commercial peat-based mixes need applying liming material, often finely ground calcitic limestone or dolomitic limestone. We hypothesized that biomass ash (owing to its high pH) would act as a liming material and a fertilizer for improving the growth of greenhouse crops.

During spring of 2022, we conducted an experiment at the Assiniboine Community College’s sustainable greenhouse to study effects of biomass (flax shives) ash on plant growth parameters. Calendula plants were grown in a greenhouse on peat-perlite media mixes (3:1 volume/volume) amended with ash at different concentrations (1%, 2%, 3%, 4%, 5% by volume). Untreated media mix was amended with dolomite lime to bring the pH to appropriate range (6.0-6.5). No fertilizers were added in this treatment. Another treatment of media mix amended with dolomite lime and fertilized twice weekly with standard fertilizer solution of 10N:10P:10K was included. Day and night temperatures inside the greenhouse were main¬tained at 22 °C and 18 °C respectively.

Plant growth (plant height, spread, number of leaves) was better at 1 and 2 % ash treatments as compared to plants grown with 3-5% ash and those grown without the ash in the media mix (Fig 1 and 2).

Fig. 1:  Plant growth parameters in ash amended peat media mix
Fig. 2:  Plant growth response in ash amended peat media mix

Plants at lower ash concentrations (1-2%) had greener leaves with higher total chlorophyll contents as compared with plants grown in higher ash concentrations (3-5 %) (Fig 3). Growth at 1-2% ash treatments was very similar to plants treated with fertilizer mix. There was no difference observed in flowering of the plants 0, 1, and 2 %. Plants at 4% and 5% died after approximately 45 days after transplanting (DAT). 3% ash treated plants died after 68 days of 90 day lifecycle after transplanting.

Fig. 3:  Total chlorophyll content (mg/cm2) of lettuce plants grown in ash amended peat media mix on 40 DAT

The ash treatment at 1% resulted in 50.84% increase in fresh weight of above ground marketable parts over control plants, and that of 2% ash treatment accounted for 53.55 % increase in fresh weight over control plants. At 1-2% ash concentration, the roots also developed better with greater number and denser mass of roots than that observed for ash treatments of 3, 4, and 5%, and for untreated control. Plant and root growth was poorer at ash levels of 3-5%. These growth parameters were similar to those obtained with use of standard fertilizer in the media mix (Figures 4 and 5). It is worth noting that the ash-induced increases in plant biomass was achieved without addition of any fertilizers.

Fig. 4:  Fresh weight (g) at harvest of plants grown in an ash amended peat media mix on 40 DAT


Figure 5:  Dry weight (g) at harvest of plants grown in ash amended peat media mix on 40 DAT

Figures 6 and 7 show that ash added nutrients especially nitrogen, phosphorus and potassium also raised the pH of media quite substantially.

Fig.6: Nutrients in the pour through leachate obtained from ash amended peat media mix after 30 days of planting

The availability of nutrients in the growing media is highly affected by pH. The desirable pH range for optimum plant growth varies among crops; generally, soil pH 6.0-6.5 is acceptable for most greenhouse plants. In our study, 3-5% ash raised the pH of the media above 8 making it alkaline (Fig. 7). Most nutrients (phosphorous and micronutrients especially) tend to be less available when soil pH is above 7.5. The exact mechanisms responsible for reducing availability differ for each nutrient, but may include formation of low solubility compounds, greater retention by soil particles and conversion of soluble forms to insoluble ions that plants cannot absorb. This is evident from the reduced plant growth at high pH values obtained with ≥3% ash in current study (Fig. 1 and 2).

Fig. 7:  Average of weekly readings of pH by pour through method for ash amended peat media mix

Electrical conductivity (EC) is a measure of the dissolved soluble salts concentration in a solution. High EC observed at 3%( 2.76 mS/cm), 4% (3.33 mS/cm) and 5% (3.46 mS/cm) ash concentrations was due to salt accumulation (especially phosphates, potassium, and sodium) in the growing media (Fig. 8). Alkaline pH of ash (≥3%) largely influenced the plants’ ability to take up nutrients resulting in poorer plant growth. Due to the differences in nutrient uptake at different pH, the composition of different elements in the leachate solution was drastically different.

Fig. 8:  Average of weekly readings of EC by pour through method for ash amended peat media mix

Overall, biomass ash had a profound effect on our potting mix. We suggest that biomass ash (1-2% v/v) could be used in greenhouse horticultural applications as a low analysis fertilizer and/or a liming agent for floricultural crops. However, too high a concentration of ash (>2%) had negative effects on plant growth and may represent a limiting factor for growing crops. Further studies on soil application of biomass ash should be explored in order to find more environmentally sound practices than present landfilling practices.

Bardgett R (2005) The Biology of Soil: A Community and Ecosystem Approach. Oxford University Press, Oxford.
Ingram, Dewayne L. (2014). Understanding Soilless Media Test Results and Their Implications on Nursery and Greenhouse Crop Management. Agriculture and Natural Resources Publications. 161.

The author is thankful to the Natural Sciences and Engineering Research Council of Canada for providing funding for this project.

Photos by: Dr. Singh
Published:  January 2023