The organic greenhouse sector continues to be a nexus of debate and advancement. Lighting, nutrient sources, soil substrate, energy use and effluent management have been the subject of much discussion and research. Alongside these production considerations, consumer demand for organically produced vegetables is steadily rising. From 2014 to 2018, sales in the greenhouse vegetable industry grew by 5% per year, and much of this demand is not met by Canadian production.
There are some barriers to upscaling production. In the fine-tuned, high-yielding environment of greenhouses, the yield gap between organic and conventional production can be particularly pronounced. At the same time, the organic environmental ethos raises questions around groundwater pollution and fossil fuel use for heat in northern climates. Organic greenhouse growers are looking to scientific research for ways to increase yields and meet consumer expectations with more intensive production.
Spearheading scientific investigation into these issues is Université Laval’s Dr. Martine Dorais, a renowned figure in organic greenhouse research. She and her research team, students and collaborators have played a key role in propelling organic greenhouse vegetable production toward more efficient systems. This important work has been supported by all three Organic Science Clusters since 2009.
For Dr. Dorais, achieving the twin goals of productivity and sustainability in organic greenhouses depends on:
- Improving soil nutrition to better nourish plants by managing the root environment (i.e., choice of substrate, irrigation methods, fertilization regime, nutrient sources)
- Optimizing use of water and nutrients, and recycling drainage water.
MANAGING THE ROOT ENVIRONMENT
One of the major issues for organic greenhouse production is the management of nitrogen – supply and timing of availability. Hydroponic greenhouses can manage nitrogen supply comparatively easily to meet the plants’ changing requirements as it grows. Organic producers don’t have the same maneuverability, given the complex exchanges between soil particles, roots and soil life.
This contrast is the primary source of the large and variable yield gap between organic and conventional crops in both fruit and vegetables. Dr. Dorais’ work has shown, however, that organic yields can be similar or superior to conventional crops, including hydroponic ones. The key is mastering the cornerstone of organic greenhouse systems – the root environment – no matter the soil type or growing media. The environmental impact is another important consideration. Groundwater pollution and emission of greenhouse gases from nutrient leaching and volatilization (nutrient loss to the air) are major issues that go hand-in-hand with fertility management. Dr. Dorais and her collaborators demonstrated the importance of:
- Choosing the best soil to incorporate into the growing medium if a container system is used (muck soil was found to perform well)
- Using the same growing medium for a number of years to develop biological activity and nutrient cycling • Minimizing long-term salt accumulation
- Ensuring oxygenation in the root zone; this is essential for water and nutrient absorption and crop health
- Providing a balanced supply of nutrients; this takes into account the plant requirements and losses to the environment
- Adapting irrigation management (frequency and quantity) for different soil types to stimulate biological activity in the soil while ensuring an adequate water supply for the plant
A set of tensiometers can be a magic bullet for managing the root environment. Essentially, a tensiometer is a water-filled tube with a vacuum gauge and a ceramic tip that is inserted in the soil. It measures the amount of water tension, or the suction it takes to pull water out of the soil, which shows how much water is accessible to the plant roots. The vacuum gauge can be read by a person, or by a high-tech data logging system, which allows the irrigation to be set to automatically maintain a specific water tension. The tensiometer system accounts for multiple factors such as air-drying capacity (moisture deficit in the air, vapour pressure deficit), air velocity, soil electrical conductivity and plant stress (e.g., due to high temperature, soil disease). Tightly regulated irrigation is a primary way to reduce leaching of nutrients into the groundwater.
Synchronizing nutrient availability with the crop’s needs helps address the dual problem of yield limitations and nutrient discharge to the environment. Dr. Dorais’ work suggests that a good strategy is to use dehydrated (pelletized) poultry manure for rapid supply of nutrients, and alfalfa meal, manure or compost for a slower nutrient release.
Researchers found that fertilizers with higher carbon to nitrogen ratios (e.g., alfalfa meal, shrimp meal and pelletized poultry manure) had lower nitrogen mineralization rates, meaning that less of the nitrogen was available for the plants. The higher carbon content, however, promoted greater microbial diversity. Composted manure or compost have even more carbon and less readily available nitrogen than alfalfa meal. Feather meal and blood meal, which have lower carbon to nitrogen ratios, had more readily available nitrogen, but didn’t stimulate the soil microbiome. Dr. Dorais recommends using a mix of different inputs, like the organic fertilizers described above, to stimulate an active and biodiverse soil microbial community, to provide nitrogen at the right time, and to contribute other important nutrients (e.g., phosphorus, potassium and calcium).
A promising way to optimize the root environment is to add biochar. It can increase the soil’s ability to retain nutrients in a similar way to clay, without the challenges of managing moisture in clay soil. It has also been widely touted as a way to decrease emissions of nitrous oxide, a potent greenhouse gas, from soil. Biochar is similar to charcoal, but produced through the process of biomass pyrolysis: the decomposition of organic material (wood) in the presence of very high heat and absence of oxygen. It is usually alkaline and can therefore increase soil pH.
Biochar varies widely in its characteristics based on biomass source, temperature, speed of temperature increase, and how thoroughly oxygen was excluded. Quality is important – if it leaves your hands black, it’s not a good sign! Dr. Dorais’ team looked at five different biochar products as an amendment to peat-based growing media (Table 1, previous page). 5-15% biochar that was produced at or above 550°F reduced leaching of N, P, Mg and Ca, and substantially improved water and nutrient use efficiency. The biochar produced at temperatures lower than 550°F either reduced root zone aeration due to the small size of biochar particles (with the willow chips), or partially tied up nitrogen (with the maple bark). As a result, these are less suitable as amendments for a peat-based growing medium.
Another exciting research result for organic greenhouse producers is the use of wollastonite (an alkaline calcium-silicate rock) to protect against powdery mildew. The addition of 8 g/L of wollastonite to a peat-based growing medium improved plant growth by 6.5%.
MANAGING GREENHOUSE EFFLUENT
Greenhouse effluent management strategies are aimed primarily at reducing the pollutant load of irrigation wastewater (particularly nitrates, phosphates and sulphates). If the goal is to reuse the irrigation water, it is also important to eliminate pathogens and other compounds that can be toxic to plants. Dr. Dorais and her team have concentrated on constructed wetland technologies to manage effluent, due mainly to their low cost and their capacity to reduce pathogen levels.
Biofiltration is a procedure comprising a precise sequence of physical, chemical and biological reactions. A series of distinct ecological niches must be set up in a specific order to support microbial activity. The microbes will then be able to carry out the reaction chains required to eliminate pollutants and pathogens. The ecological niches include low and high pH environments, aerobic and anaerobic zones, and, specifically for greenhouse effluent, must include diverse sources of carbon to allow microorganisms to break down the pollutants.
According to Dr. Dorais, one of the best choices for constructed wetland biofiltration is a horizontal subsurface flow system with an extra carbon source in addition to the plant roots growing in gravel (Figure 1). The researchers added either sugar (a simple carbon) or compost (a more complex carbon source). This system meets environmental criteria by significantly reducing phosphate, sulphate and nitrate levels in effluent and greatly reducing nitrous oxide emissions. A constructed wetland as small as 10% of the greenhouse surface area is required to purify heavy summer runoff, making this system a practical and economical option.
Both pozzolana and biochar are very effective filtering media that can be added to the gravel bed of a constructed wetland to increase the efficiency of the system. Pozzolana is a porous volcanic mineral commonly used as part of cement. In the constructed wetland, the researchers used pieces of pozzolana 10-15 mm in diameter to replace all or part of the gravel; note that pozzolana can be an expensive material.
This research also shows that treated greenhouse wastewater can be reused from the horizontal subsurface flow biofiltration systems. Plant pathogens, such as Pythium ultimum, which causes root rot, were nearly completely eliminated (99.99%) from the wastewater.
More than ten years of progressive organic greenhouse research by Dr. Martine Dorais, under the Organic Science Clusters and beyond, has provided the sector with new, effective production methods. Her research continues, working to untangle the many factors at play. The organic greenhouse industry is heading toward a bright and responsible future.
(OSC3) Originally published in Organic Science Canada magazine, Issue #3, Spring 2021
