Thanks for your answer highlighting the importance of measuring the run-off solution from media-based systems (Reader Inquiries April 2017) and your detailed description of how to do this. Are there other management issues for media-based systems that hydroponic growers need to be aware of?
by RICK DONNAN
As described last month there will be a distribution pattern within the medium in a container, progressing from dripper input to container outlet(s). Properties that change in different zones are measurements of EC, pH, temperature, nutrient balance and water content.
With a mature crop for feed solution to get from dripper to running off from the drain takes an average of about one to two days, in particular dependent upon the volume of root zone solution held per plant.
Each portion of feed solution from one irrigation cycle (which the Dutch term a ‘gift’) works its way further through the medium with each successive irrigation. Chemical engineers term this ‘plug flow’ and in a perfect system each plug remains as an individual unit. In the real world within the medium the boundaries of each plug get progressively blurred. Some of the plug spreads into zones including dead spots and other zones where short-circuiting occurs.
Last month I discussed sampling and analysing the run-off solution as a management tool. There are sensors, such as a WET sensor (Water – that is, water content, EC, Temperature), which give continuous measurements from within the slab. These are very useful for ongoing real time management, however, it pays to remember that they only measure one spot in a single container. Move the probe and you can get a different reading.
There are several types of containers and these can influence the solution patterns within the container. These are:
Pillow bags. With these a medium is packed into long relatively narrow bags. Typical media are rockwool (slabs), cocopeat, and perlite. These minimise evaporation from the medium and allow a number of plants to share the bag. Where there are hanging gutters, pillow bags are almost universally used because of their stability.
Open topped bags. These will lose more water by direct evaporation from the medium. Depending upon the medium being used, especially sawdust; the coning of nutrient solution down from the dripper can leave significant areas ,which remain relatively dry and unavailable for root growth.
Normally, there would be one dripper per plant in each of these bags.
Open beds. These are not as common now. The medium used needs to drain well and be deep enough to have good aeration at the top surface. These would normally use in-line dripper line or be sub-irrigated and usually operated as flood and drain (also known as ‘ebb and flow’).
How you set up containers and initially wet them can be critically important. For example, if dry rockwool is laid on a wet surface it will only pick up water to about 30mm high and above that remains dry. Hence, when using rockwool, typically in pillow bags, the bags must be left without drainage slits. They must then be fully filled with nutrient feed so that the entire rockwool slab is saturated. The wetting up nutrient solution to be used is that which gives the recommended root zone solution. When the drainage slits are cut later the entire slab retains its capillary action and remains wet. Slits should be at the bottom of the bag. For a single drain, cut it at the lower end of the bag, and for multiple drains at the side midway between drippers. Never cut slits underneath the bag.
Coco peat is often supplied in a compressed form (to reduce transport and storage costs). Some come already in growing bags. In this case, they need to be wet up and allowed to fully expand before planting. Occasionally the product does not expand properly and in this case do not use them because aeration in the medium could be dangerously low. Compressed product for use in cell trays or bags must be fully wet, expanded and well mixed.
Circumstances can arise where short-circuiting of feed solution can lead to much lower ECs in the run-off solution than the solution measured in the medium. If this is not discovered and corrected it can lead to stressed plants although the drain analysis indicates good control. Dutch advisers term this ‘false drain’.
It is most likely to happen in a system with relatively high flow drippers (for example, 4 litre/hour instead of the more standard 2 litre/hour). If this is combined with a high volume ‘gift’ per irrigation, once the medium is saturated any excess feed tends to short-circuit direct to the drain. For short-term correction allow the medium water content to drop and significantly reduce the volume of the gift. To maintain the required feed input you will probably need to also increase the irrigation frequency. Longer term consider changing to a lower flow dripper.
An example of possible problems with propagation media:
A lettuce grower I visited was having problems with stressed seedlings. They were in small rockwool blocks under shade cloth, but out in the open and often exposed to hot, dry winds.
“They look stressed by high EC”, I said.
“Impossible – I feed them at 0.9 EC (mS/cm)” was the reply.
“What is the EC in the blocks?”
“0.9 of course”.
When we sampled a block the EC was 3.3 mS/cm – much too high for lettuce.
In another case nursery rooted cuttings were dying. The cause was an EC of 8 mS/cm in the medium.
What happens is that small seedlings take up only tiny amounts of both water and nutrient, but there can be considerable evaporation of water from the medium, leading directly to a rise in EC, especially if there is no significant drainage from the propagating medium.
Watering seedlings must be regular and result in some run off. Check the EC of the run-off to avoid EC problems. Growers often use flood and drain tables to water seedlings. For modest sized operations the solution EC is often not automatically controlled. In this case it is important to regularly check the solution EC and not assume that it will remain unchanged. RD
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PH&G May 2017 / Issue 179