The positioning of the drainage slits in combination with the right irrigation dose size influences directly the distribution of water and EC in the slab to produce the minimum variation of water content and EC laterally and vertically in the slab. This is a job that has to be done only once but will influence the slab for the rest of the season, so it is very important that it is done correctly.

Additional drainage slits should never be made in the slab at a later date as they will change the drainage pattern in the slab which may be detrimental to the roots in some areas of the slab.

Ideally there should be the same number of drippers in every cube with no extra drippers put directly into the slab.

Additional tips for making the drainage slits:

• make drainage slits at both ends of the slabs at the lowest points;
• make additional drainage slits exactly between the cubes. This creates the same “cone” or drainage pattern for every dripper reducing the standard deviation of EC and more efficient flushing and water efficiency;
• this can also be done on both sides of the slab on hanging gutters to make sure that all the slabs drain evenly when the gutters are not quite level;
• make sure that all the drainage slits are right to the bottom of the slab and are wide enough to prevent the roots slowing down the drain water (3 to 4 cm);
• drain the slabs 24 hours before planting;
• do not make additional drainage slits during the crop as this will change the drainage pattern in the slab and can cause root death;
• never make a drainage slit under a dripper as this can give “false drain”.

In the Season Dynamics (Smart Root Zone Management App) we state that it is important to have a target EC for each of the different periods during the crop.

To control the EC to the target levels it is important to use the proper measuring strategy so that you can make small and frequent changes to your irrigation strategy. It is always easier and safer to make frequent small changes than to make big changes when the EC has got too high.

The important factors for good EC control are:

• good initial root distribution after planting (first 6 to 8 weeks);
• the correct dose size per dripper;
• correct use of the Daily Dynamics (Smart Root Zone Management App).

Period 1 in the Daily Dynamics is very important to re-fill the slabs as evenly as possible with the correct dose size. This will not only give better EC control later in the day but also reduce the variation in EC around the glasshouse.

To reduce EC it is necessary to flush the slab more. This can be done by:

• increasing the frequency (by light or time);
• increasing the dose size (within the recommended range).

Other points of attention:

• So look for a higher percentage of drain especially in period 2 and period 3 of the Daily Dynamics.
• Take care if reducing the drip EC that the Delta EC does not get too big (>3).

Mistakes happen and sometimes the slab can dry out with insufficient water especially the top of the slab.

To re-wet the slab it is best to use small and frequent waterings. This will help keep the water you apply in the slab and at the top. Large dose will push the water to the bottom of the slab and quicker to the drain point.

So it is important to use a dose size factor of 20 or lower if possible within the limitations of your system.

It is also important that you apply more water than the plant is using, so watering at night or before plant activity in the morning is a good time for this but take care of the Delta EC to avoid problems with fruit splitting (tomatoes) etc.

Light intensity

The intensity of light is measured in watts/m2. This is an instant reading of the brightness of light at the moment of measurement.

Light sum
The light sum is measured in joules/cm2. This is the total amount of light received over the period. So over a period of one hour the total amount of light received in joules/cm2 will depend on the intensity of the light during this period.

Light triggered irrigation starts
In glasshouses plant water use is measured per m2. A general figure used in Northern European conditions is 2.5 to 3.0 mls per joule per square meter as a setting for the computer with a target figure of 3 mls per joule for the total on a bright day.This is based on a full crop canopy.

Plants use more water per joule of light at low light intensities which is why it is important to have an irrigation trigger on time and not just light.

The best day of the year would be about 3,000 joules.

• 3 cc per joule on 3,000 joules = 9 lts/m2 per day

The highest light intensity in Northern European conditions would be around 1,000 watts/m2.

• 1,000 watts for 1 hour = 360 joules/cm2
• 3 cc per joule on 360 joules = 1.08 lts/m2 per hour
• The dose size per irrigation round is calculated by the dose size calculator in relation to the volume of the substrate

For example with a calculated dose size of 100ml with 3 drippers per m2

• 3 cc per joule on 3,000 joules = 9 lts/m2 per day
• 1 irrigation round is 3 x 100mls = 300 cc per m2
• 9 lts/m2 per day / 300 cc per round = 30 irrigations per day
• 3 cc per joule on 360 joules = 1.08 lts/m2 per hour
• 1 irrigation round is 3 x 100 cc = 300 cc per m2
• 1.08 lts/m2 per hour / 300 cc per round = 3.6 irrigations per hour which is every 16 minutes

In hotter countries especially where air humidity is low (or HD is high) the cc per joule requirement can be 4 or higher. This would mean that the time between irrigations would need to be less.

This is why the volume of the substrate per m2 and its relation to the dose size and the necessary frequency of the subsequent irrigations during the brightest part of the day need to be match to the limitations of the nursery irrigation system.

The spacing of the dippers has a big effect on the even distribution of water and nutrients in the slab. The flow of this water is spread evenly by the stone wool to refresh the nutrient solution around the roots. This is what we call the Cone. The Cone starts in the cube and spreads down and sideways.

The cube will increase the width of the cone and the volume refreshed by the dripper giving more efficient water and fertilizer use. This is vital to give good refreshment and even water content, EC control and minimizing drain.

When changing plant density or crop there can be more drippers than cubes to put them in. In this case:

• check if you can remove or block off these drippers and still have enough irrigation capacity;
• if not try and put the same number off drippers into every slab;
• make sure that you do not put the dripper above a drainage slit;
• try and put the extra dripper at the high end of the slab;
• take care when measuring with a hand meter that you do not measure too close to this dripper as the EC level will be closer to the drip EC;
• make sure that this extra dripper is not giving you “false drainage”.

The optimal dose size for our slabs can be calculated using the Dose size Calculator or manually as below.

Example:

Slab with volume = 100 x 15 x 7.5 cm = 11.25 liter
3 cubes volume = 3 x (10 x 10 x 6.5 cm) = 3 x 0.65 liter = 1.95 liter

Total stone wool volume = 13.2 liter

Dose size:

• 3 drippers / slab = 13.2 / 3 = 4.4 liter / dripper
• Dose size = 25 x 4.4 = 110 cc

This is the total volume of substrate (cube and slab) supplied per dripper multiplied by the DSF (Dose Size Factor). So the larger the volume of substrate per dripper the larger the dose size.