What are the salt deposits on the side of my slab?

What you see is most likely precipitated calcium phosphate and calcium sulphate which is not nice to see, but it is not harmful to the roots. Drip solutions in substrates can precipitate more easily in the following conditions:

• higher EC in drip and slab;
• higher concentrations of Ca, PO4 and SO4 (not directly linked to higher EC);
• higher pH (mistakes or sensor failures);
• lower temperatures;
• less drain.

One or more of the mentioned conditions can cause precipitation. This is less of a problem at companies who always have large drain percentages and good pH control.

• Good pH control: pH 5.5 to 5.6 in the drip and the use of sufficient NH4 concentration (sometimes next to NH4 also urea is used).
• Also check the pH at the dripper or in the tubes, at the first drips of the day. This should be between pH 5.2 to 5.8.
• Calcium phosphate can be dissolved with very low pH (for an analyses in the laboratory, you can solve the precipitation in 38% nitric acid).
• Calcium Sulphate cannot be dissolved even in acid.

The precipitation will not go back into solution quickly, or not at all in the slab, so there is no danger that this will cause a nutrient imbalance in the rooting zone.