Higher ECs within the medium than in the runoff
I have been growing tomatoes in rockwool slabs for over three years. Recently, my plants have been looking stressed and I have had some blossom end rot.
Friends have suggested that it might be high EC in the slabs, but my run-off EC (electrical conductivity) is usually below 3 (mS/cm).
At their suggestion I recently bought a WET (Water, EC, Temperature) meter to measure properties within the slab. I now find that I am sometimes getting much higher EC readings within the slab than in the drain. Is this normal, and what should I do?
Answer by RICK DONNAN
Distribution within a medium
In the extreme, some growers think that what they feed down the dripper line is what is in the medium in the bag and uniformly distributed. This is totally wrong. In your case, you have been measuring the properties of the run-off solution (also called the “drain”) so that you already knew that there is a difference, usually a significant rise, in EC between the drip and the drain solutions.
What you have now discovered through using the WET sensor is that there is also a distribution pattern of EC within the medium. Although it is possible in rockwool (only) to syringe a solution form the medium, collecting a representative sample is too difficult because every point sampled probably has a different EC due to the distribution pattern. Consequently, the solution used for measuring root-zone solution EC (and also pH and nutrient balance) is the run-off solution, collected over the 24-hour period. This avoids sampling errors and gives consistency regardless of the medium used.
In broad outline the EC pattern has the following major influences: In the direct route from dripper to drain the EC tends to be closer to the drip EC, but in more remote zones, solution movement is lower and consequently, the EC there tends to be higher. In all cases the pattern is erratic and cannot be accurately predicted.
Once irrigation ceases for the day, usually an hour or two before sunset, there will be a continuing uptake of water and nutrients overnight. At this point the medium is at saturation, but from then the water content of the medium will steadily reduce overnight as the plant continues to take up nutrients and also water, although at a lesser rate because of lower transpiration. Consequently, the water content in the medium will drop overnight.
Depending upon how low the grower allows the water content to drop, irrigation is usually started about one to two hours after sunrise. If conditions are very overcast, it may need to be delayed until the pants are working.
There are different ways of triggering an irrigation cycle. Sometimes used is a simple timer, but this gives poor control because of not allowing for differing plant uptake. Better methods are using a solar integrator to trigger an irrigation cycle (counts a set number of Joules of light, then resets) or using the water content measured either by a WET sensor or weigh scales. Normally, a set amount of irrigation is given each cycle, what the Dutch call a ‘gift’. The best approach to effective control of irrigation is to give “little and often”.
Between the frequency of irrigation and the gift, the overall rate of water addition needs to be slightly more than the plant is taking up. This leads to the water content eventually getting back to saturation, which usually happens about late morning. Subsequent irrigations replace what the plant has taken and the surplus beyond saturation runs off as drain. Then irrigation is stopped and the daily cycle starts over again.
Solution movement within the medium
In general active roots are reasonably well spread throughout the medium (sometimes zones at the top of the medium have fewer active roots). Between irrigations roots take up water (and nutrients) from throughout the medium. In general the plants take up more water relative to nutrient through the day due to radiation driven transpiration. Removal of water brings more air into the medium. Through the influence of gravity, the solution in the upper layers moves lower down leaving more air spaces higher up.
When the next irrigation happens, the solution spreads into a limited volume under the dripper, fills part of the air spaces and displaces some older solution towards the drain. In an ideal situation the hydraulic pressure from dripper gift is enough to overcome the hydraulic resistance within the medium and displace some old solution into the drain. This will have a relatively high EC, that is, higher than most other readings within the slab.
What can go wrong is if much more solution feed is dripped onto the slab than can all overcome the hydraulic resistance through the slab. In this case the surplus new feed finds paths of least resistance and ends up at the drain, what I term a ‘short circuit’. In the simplest case it flows across the top of the slab and down to the drain.
This is sometimes called ‘false drain’ and the symptom is relatively low EC in the drain but high EC within the medium, adversely impacting on the plants, such as inhibiting water uptake. The cause is usually that the quantity of gift is too high. One or both of the following can cause this: Too long between irrigation cycles so the gift has to be large, and/or too high a flow rate from the dripper. The recommended dripper rate for media-based hydroponic systems is 2 litres/hour. Some systems use 4 litres/hour drippers, but this can be too high.
If you have these conditions, to break out of this cycle you need to delete one irrigation to regain control and then substantially reduce the quantity of your dripper gift. If possible, this should be done on a permanent basis by increasing the irrigation frequency. RD
September 2017 / Issue 183