Slab calculator

The water content of the substrate provides the reservoir of the water and nutrients for the plant. Changing the water content is a usefull tool in controlling the plant balance and also maintaining root heath (How to change the water content).

In the Season Dynamics for each crop we suggest target levels for the water content for each of the critical periods. This is done for specific reasons in each period:

• in long periods of low light levels it is better to run a lower water content as the plant is less active. This makes the slab more reactive and dry back faster to allow more oxgen into the slab and also give a generative action;
• in high light levels with high transpiration levels it is best to run a higher water content to give more buffer for the higher water use by the plant. This also gives more safety in case of a breakdown.

The range of water contents used would be from 45% to 70%. The volume of substrate used is also a key factor (How much substrate do I use?)

Measure you slab in an organised way using a WET meter and set target water content objectives for the different periods.

“To control this water buffer to realize the potential of the slab in controlling the plant balance.”

The distribution of the water and nutrients in substrate is important to get even root distribution. Good nutrient availability for the roots is vital to get optimal quality and yield. Changing the EC is also a usefull tool in controlling the plant balance (How do I control my slab EC?).

In the Season Dynamics for each crop we suggest target levels for the EC for each of the critical periods. This is done for specific reasons in each period:

• in long periods of low light levels it is better to run a higher EC to steer the plant in a generative way;
• in high light levels with high transpiration levels it is best to run a lower EC to help uptake for transpiration.

Sometimes like with cucumbers for example a high uptake of nutrients in a low volume of substrate can cause low levels of some nutrients so irrigation is necessary to replace these. The volume of substrate used is a key factor (How much substrate do I use?).

Measure your slab in an organised way using a WET meter and set target EC objectives for the different periods.

“To control the EC to realize the potential of the slab in controlling the plant balance.”

The ability of the plant to take up the nutrients it requires is affected by the pH of the nutrient solution. This means that it is very important to have the correct pH around the root sytem. The picture below shows the uptake of different elements at different pH levels.

The optimal pH for most plants is between 5.5 and 6.5 and this should be the target level.  The pH in slabs can change because of selective root uptake. When NH4 is transported into the roots, the pH will drop. When NO3 is transported into the roots, pH will rise. Also NH4 and NO3 can be used by micro organisms, that will also influence the pH in the substrate. The change of concentration of the fertilizers NH4 and NO3 can influence the pH in the substrate.

So it is important that the pH of the source water is corrected to the right pH at the irrigation rig. Take care:

• if the bicarbonate levels are very low (no buffer) in the source water it may be necessary to add bicarbonate to give some buffer;
• with low bicarbonate levels it may be necessary to dilute the acid used for pH correction to avoid a sudden decrease in pH to below 5.0;
• with amonium also with low bicarbonate levels which can cause a sudden decrease in the pH.

Always check the pH of the source water in the storage tank, at the irrigation rig and at the dripper. pH levels of below 5.0 will damage the roots. Remember the plant gets the water that is coming out of the dripper so that has to be right!

All parts of the plant need oxygen, leaves, stems and roots. In the slab, roots need oxygen for cell respiration. When there is enough oxygen at the roots, the root system can function optimally.

Oxygen is also used by micro organisms, fungi and bacteria. In stonewool mainly bacteria, which are present in large numbers a few hours after wetting up the stonewool. When there are circumstances with low oxygen levels, the micro organisms can compete with the roots for the use of oxygen.

For Oxygen availability

Oxygen supply in the substrate is important for good plant growth. In the practice of growing plants, this means that the water content of the wettest part (bottom layer of substrate) need refreshment and air. When there is 80% water content at the bottom layer of a slab, the air canals make sure that oxygen can get to the water and plant roots (diffusion). At high temperatures (32 ^oC), less oxygen can solve in water, and roots at the most wet parts of the substrate can have problems with to lower oxygen levels in the irrigation water.

“To control the O2 to realize the potential of the slab.”

The temperature of the root zone is not always measured or controllable. It is however very important and root temperatures should be kept between 18 oC and 25 oC.

For Oxygen availability

As the temperature of the root zone increase the availabily of oxygen from the irrigation water decreases. At root temperatures of over 25 ^oC especially if root distribution is poor this combined with competition between roots can lead to root death. This opens up the possibility of secondary infection by root pathogens such as Pythium.

Measurement is knowledge so measure to know what is happening with the slab temperature.