By Gerardo H. Nunez
Blueberry production in containers filled with soilless substrates is rapidly expanding throughout the world and in the Southeast. Blueberry bushes are notorious for their strict soil requirements. They prefer well-drained, acidic soils with high organic matter.
These soil characteristics are not common in many parts of the world. Thus, blueberries have traditionally been grown with soil amendments such as pine bark and elemental sulfur. The use of soilless substrates is a natural evolution in this process. Soilless substrates are composed of organic fibers and coarse materials that are mixed to create ideal rhizosphere conditions for plants, including optimal water and nutrient availability, and absence of soilborne pathogens.
The nursery industry has been using soilless substrates to produce young plants for decades. The new trend is for high-value crops, such as small fruits, to rely on soilless substrates for the entire production cycle. However, nursery substrates and small fruit substrates are different. Nursery substrates generally have high water-holding capacity and relatively low porosity. These substrate characteristics favor young plants with root systems that are small and still developing.
Additionally, nursery substrates are not meant to be used for long-term production. Some nursery crops spend as little as 21 days in their substrate before transplant, whereas blueberry bushes can take up to four years to reach peak productivity. Nursery management practices are very different from management practices in a fruiting field.
CONTAINER SELECTION
Container size and shape are important decisions for growers interested in using soilless substrates. The first generation of substrate-based blueberry farms used large nursery pots with 10- or 13-gallon capacity. These large containers were costly to fill, and the large root volume did not improve plant growth or productivity.
Nowadays, 5- or 6-gallon containers with abundant drainage are becoming more popular. Drainage holes or grills at the bottom and sides of the pots increase substrate aeration and promote root health. Pot “legs” that lift the plant from the subtending soil are also used for these means.
Many growers also use bags instead of pots. Substrate bags can be purchased prefilled with substrate, which reduces the amount of labor needed to start a field. Whether in pots or bags, the small rooting volume allows for plants to be spaced very close to one another, achieving plant densities up to 2,900 plants per acre.
SUBSTRATE COMPOSITION
The largest components (by volume) in soilless substrates are organic fibers such as sphagnum peat moss, pine bark and coconut coir. These fibers give the substrate its water- and nutrient-holding capacity. The water-holding capacity refers to the substrates’ ability to retain water in microscopic pores where plant roots can access it.
Since substrate-based production systems usually use fertigation, the ability to hold water directly impacts the availability of dissolved nutrients for plant uptake. Coarse materials like perlite, coconut pit and biochar are also used in soilless substrates. These materials usually have large particle sizes (> 5 millimeters), which increase substrate aeration and drainage. The relative contents of fibers to coarse materials determines the substrate air-filled porosity. Substrates for blueberry production generally have >30% air-filled porosity, compared with nursery substrates where the target is 10% to 20%.
Substrate composition can also affect the native pH of the substrate. Blueberry bushes prefer substrate pH in the range of 5.5 to 6.5. The pH is lowest in substrates composed of pine bark, highest in substrates composed of coconut coir and intermediate in substrates composed of sphagnum peat moss. However, soilless substrates do not have high pH buffering capacity. So, their pH can change rapidly according to the pH of irrigation water or fertigation solution and plant nutrient uptake. Water carbonate and bicarbonate concentrations (collectively called alkalinity) should also be considered when managing substrate pH.
DECOMPOSITION CONCERN
A current concern among blueberry growers who use soilless substrates is decomposition. Organic materials decompose rapidly in the warm and moist conditions inside the pot or bag. Decomposing fibers and coarse materials consume nitrogen, creating competition for this nutrient with the plant. Additionally, particles get smaller as they decompose, changing substrate water-holding capacity, porosity and drainage as the substrate gets older.
Decomposition rates vary depending on the initial status of each organic material. Growers overseas (where substrate-based production started earlier than in the Southeast) usually keep plants in the same substrate for up to five years, but there is a lot of interest in researching practices to “rejuvenate” the substrate.
MANAGEMENT AND PRODUCTIVITY
Management is key to achieve the high plant vigor that characterizes substrate-based blueberry production. Most substrate farms use two or more emitters per container and aim to maintain drainage at 10% of the total water input to achieve uniform substrate moisture.
The solution that drains from the containers (leachate) provides a good representation of the substrate chemical conditions. Therefore, periodic leachate collection and analysis is used to monitor substrate pH and electrical conductivity. Where water salinity content is high or heavy fertigation regimens are used, salinity buildup in the substrate can be a problem. Thus, the ability to cycle between irrigation and fertigation events is useful to percolate salts out of the containers when necessary.
Appropriately managed soilless substrates provide the plant with optimal conditions to grow. In recently published University of Florida research (see doi.org/10.1016/j.scienta.2022.111149), plants were observed that reach “mature” size and productivity 10 months after planting. Year one and two yields varied per variety, but they surpassed field production expectations both on a per-plant and a per-acre basis. Berry quality was not affected by this significant increase in productivity.
Ultimately, while blueberry cultivation in soilless substrates requires investment and intensive management, research results and the experience of many growers in the Southeast suggest that this system is a viable alternative.
Gerardo H. Nunez is an assistant professor of horticulture at the University of Florida in Gainesville.