Article written by: Jason Danaher Ph.D.
Using the correct substrate for seedling and transplant production is one of the important management considerations to make sure your aquaponic facility has strong and healthy plants ready to enter the production system (Figure 1). The substrate provides seedlings and transplants with support, aeration, water retention, and in some cases nutrients. If the substrate does not provide the proper physical and chemical parameters necessary for growth, then the entire production timeline from transplant to harvest can be compromised; thus, affecting the operation plan and viability of the aquaponic business.
Figure 1. Basil (Ocimum spp.) transplants growing in a soilless substrate mix.
Importance Of A Substrate’s Physical and Chemical Parameters
The substrate provides physical and chemical parameters important for germination and transplant growth. Important physical parameters include total porosity, container capacity, air space, and bulk density. The substrate should allow water to drain while retaining sufficient moisture, which can be a precarious struggle at times for the grower. Saturated media should be avoided because it leads to root rot and disease.
Important chemical parameters include pH and electrical conductivity and directly impact the solubility and uptake of essential macro- and micronutrients. Managing the chemical parameters of a substrate is essential for successful germination and transplant production. Chemical parameters can be directly affected by the combination of substrate and the prescribed fertigation program. Mastering and awareness of a specific substrate’s physical and chemical parameters allows producers to modify the growing environment to meet their specific plant needs and optimize their growth potential.
Categories and Types of Substrate for Transplant Production
There are two categories of sterilized, horticulture substrates used for seed germination and transplant production: soil-based substrate and soilless substrate. Soil-based substrates consist of a mixture of soil, sand, and organic matter. Soil-based substrates are commonly used in horticulture for growing a variety of plants, but should be generally avoided in aquaponics to prevent the potential introduction of pests, diseases, and weed seeds in a controlled environment system.
A soilless substrate does not contain soil. It can be a standalone product or mixture of organic and inorganic materials (Figure 2). Common soilless substrates used for seedling and transplant production include organic materials like peat moss, coconut coir, tree bark, wood fiber, and rice hulls. The aquaponic community tends to avoid peat moss as a substrate component because it is a known vector for root fungi diseases like Pythium spp. The aforementioned products are often combined with the proceeding inorganic materials to enhance the mixed substrate’s physical properties. Inorganic materials like perlite, vermiculite and sand, are commonly mixed with the aforementioned organic materials. They improve root zone aeration and optimize the balance between maintaining root zone moisture levels and drainage properties.
Additionally, inorganic materials like clay pebbles, rockwool, and foam are typically used as standalone substrates for transplant production. Rockwool and foam are a popular choice for aquaponic systems because they offer superior water retention and root zone aeration properties. Rockwool and foam are available in various forms, including blocks, slabs, and cubes.
Figure 2. Commercially available soilless substrate for container plant production containing both organic and inorganic materials.
Occasionally growers mix self-created soilless substrate using locally available resources. For example, a grower may decide to utilize coconut coir, perlite, and vermiculite at a volume ratio of 2:1:1, respectively. However, caution should be exercised when mixing two or more soilless substrate components because there is no guarantee a direct mean value of the original component’s physical characteristics will be achieved. The self-created mixture will often have its own unique physical properties, which can be vastly different from the original components… and that is alright! Just keep in mind, self-created soilless substrates should generally provide the following range of physical characteristics and chemical parameters for seed germination and transplant production:
1) Total porosity (50% to 85%)
2) Container capacity (45% to 65%)
3) Air space (10% to 30%)
4) Bulk density (0.2 to 0.5 g/cm3 or 12.0 to 32.0 pounds/ft3)
5) pH (5.5 – 6.5)
6) Electrical conductivity (0.9 - 1.9 mS/cm)
Private laboratories are capable of measuring physical and chemical parameters of container substrates. However, farmers can also quantify physical and chemical parameters themselves with simple and controlled methods using conventional appliances and test kits. See the two publications below for do-it-yourself procedures.
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Procedures For Determining Physical & Hydraulic Properties Of Horticultural Substrates
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The Pour-Through Procedure for Monitoring Container Substrate Chemical Properties: A Review
In conclusion, choosing the correct soilless substrate is critical to help growers plan and execute production plans for successful seed germination and transplant production at aquaponic facilities. Whether you decide to use an individual soilless substrate, purchase a premade soilless mix, or create your own unique and specialized soilless substrate, understanding the physical and chemical characteristics is essential to make an informed decision. You will increase your probability of growing transplants successfully by using an appropriate soilless substrate that provides an optimal plant growing environment. Feel free to call Aquatic Equipment & Design, Inc. staff at 407-995-6490 or email info@aquaticed.com if you have additional questions or concerns about your options for soilless substrate for aquaponics.
References
Altland, J.E. 2021. The Pour-Through Procedure for Monitoring Container Substrate Chemical Properties: A Review. Horticulturae 7: 536. https://doi.org/10.3390/horticulturae7120536
Fonteno W.C., C.T. Harden and J.P Brewster. 2003. Procedures for determining physical & hydraulic properties of horticultural substrates. Hort. Substrates Lab. NCSU, Raleigh, NC.