The practical way to reduce salt concentration in a substrate is to increase the amount of solution that leaches out of the substrate. As a plant transpires, it removes water and leaves behind salts. Applying excess fertilizer solution each time the plants are watered ensures that these salts are washed out. If the electrical conductivity (EC) in the substrate is high, you may want to leach up to 50% of the volume you add each time you irrigate. If the EC in the substrate is close to what you are feeding, then you may want to leach less.
As a rule of thumb, irrigation should be short and frequent in order to maintain the solution level in the substrate at “field capacity”. For Grodan rockwool, this may be up to 20 times per day. The number of irrigations per day is dependent upon the environment the plant is exposed to. On a hot, sunny day or on a day where the relative humidity is low, the plant will require a high number of irrigations. Under cloudy or high humidity conditions, conditions where the plant is not transpiring at a great rate, the number of irrigations may be as low as 5 or even once a day.
The duration of each irrigation will determine how much leachate, and therefore the EC in the slab. Under normal conditions, the EC in the slab should be 0.2 to 0.5 mmhos/cm higher than that which is applied to the slab. Leaching 20% each irrigation will usually result in the proper level of fertilizer in the substrate, but again, this depends on how much the plants are transpiring. The amount of leachate is increased to decrease the EC in the slab, while a small amount of leachate will usually result in higher EC’s in the slab. Proper control of water and fertilizer availability is critical to maximizing yield.
