Strawberry plants are very sensitive to root zone environment (physical and chemical environment). Their roots do not grow well when inappropriate substrate is used or moisture content is not managed well. Strawberry fruit yield is therefore largely affected by substrates. Physical characteristics interact with container geometries. This means that substrate optimization/selection must take a whole system approach.
Optimizing substrate physical, chemical, and biological properties can increase the overall yield of strawberry by ~30% per our recent study at the Ohio State. There are pre-mixed commercial substrates available from various horticultural media suppliers. When you select pre-mixed substrates, please keep in mind that optimum substrates are depending on the climate conditions (potential transpiration), container systems (column height and drainage design) and source water quality (e.g., alkalinity). The following properties are keys for selecting and managing the substrate for strawberry.
High porosity (greater access to oxygen). Strawberry reportedly requires high amounts of oxygen in their root zones. Therefore, it is generally recommended that strawberry grow in substrates with high porosity. As chemical and physical properties of bio-based substrates (such as coconut coir) could vary with different sources, we recommend a small test to evaluate the materials before introducing them at a large scale.
An example substrate mix we used in our greenhouses is a mix of 50% perlite, 25-30% coconut coir and 20-25% peat. This provides ~15% air porosity and ~60% water holding capacity at a standard measurement column height (10 cm). The addition of peat is adjusted based on the resulting substrate pH. We also use a commercial mix designed for growing strawberries.
Substrate pH (5.5-6.0). The substrate pH should be in the range (5.5 – 6.0) to maintain the root zone pH at around 6.0 – 6.5 after fertigation.
When root zone pH exceeds 7.0, young leaves turn yellow-green, a typical symptom of iron deficiency, which restricts overall growth of plants. Because of the relatively small amount of irrigation requirement per plant (<500 mL per day) compared to the substrate volume requirement (2 or more liters per plant), the substrate pH has a strong influence on the root zone pH. Addition of ammonium nitrogen (NH4-N) helps to stabilize the root zone pH, but addition of NH4-N should be carefully done as too much may make the plants more vegetative (producing more shoots and runners and less flowers). Our strawberry nutrient solution used at OSU Kubota Lab therefore has 10% of the total N in the form of NH4-N. [For propagation of runners, we typically increase total N as well as NH4-N ratio]
Actively growing roots in hydroponics take up hydrogen ion (H+) together with NO3-N uptake. This increases root-zone pH, to become more basic. However, when roots are old and not functioning, we see pH go down mainly due to oxidation and deterioration of roots.
Substrate EC (0.4 dS/m or less). EC is electrical conductivity and is an indicator of the total concentration of ions dissolved in the solution. Many experts of greenhouse hydroponic strawberry suggest that strawberries are sensitive to salts accumulated in the root zone and the root zone EC should not exceed 1.2 dS/m, which is a very low level compared to other crops grown hydroponically. Coconut coir typically contains high salts and needs to be washed with fresh water to bring the EC down to the acceptable level before planting.
Volume (>2 liter per plant). Substrate volume recommended is a minimum of 2 liters (a half gallons) per plant. Smaller volume means limited root growth, and thereby reduction of growth and productivity. At the same substrate volume, taller containers work better than short ones as taller containers have better drainage (enhancing aeration).
Microbiological additives. Various reports show potential productivity increase for strawberry via inoculation of beneficial fungi (such as AMF – arbuscular mycorrhizal fungi) or bacteria (such as Bacillus spp.). There are pre-inoculated commercial substrate mixes and granular inoculums available.