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. The following 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.  Based on this understanding, we conducted a small trial in 2008, examining different substrates and substrate mix (granulated rockwool, 100% coconut coir, 50% coco + 50% perlite, 70% coco + 30% perlite).  The growth and yield of strawberry plants were restricted when 100% coconut coir was used as the substrate but was greater when rockwool and the highest percent perlite was used, supporting our hypothesis of the importance of oxygen availability. 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.  The current substrate we use in our greenhouse is a mix of 50% perlite, 25% coconut coir and 25% peat.  The addition of peat is for better pH management as our source water has high pH. 

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.  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 UA CEAC therefore has 10% of the total N in the form of NH4-N.         

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.

EC (1.0 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 1.0 dS/m, which is a very low level compared to other crops grown hydroponically. When the root-zone EC exceeds 1.2 dS/m, the growers often flush the root-zone with water to wash off the excessively accumulated salts in the root zone. However, this practice is suggested by Japanese hydroponics consultant and is something about which we have not yet reached a conclusion out of the study at the University of Arizona as higher EC levels may be acceptable (depending on cultivars).

Volume/container height. Substrate volume recommended is 2 liters per plant (Nobunkyo 2008), but growers may find a better plant response when a larger volume of substrate is used.  At the same substrate volume, taller containers work better than short ones as taller containers have better drainage (enhancing aeration).

(Updated 8/8/13)