you are watering wrong. i can tell by the description of your actions.
stop trying to give a volume of water you choose.. that's not what is needed. 1/2 liter of feed won't saturate the entire pot... so all you are doing is piling up minerals in the substrate.
soikl -- water until saturated - minimal runoff can ensure this... no dry pockets. adjust frequency/concentration of ferts as needed, but always saturate entire pot.. if it's too much you can either reduce concentration or reduce frequency of fertigation.. it all adds up the same in the end as far as how much of any molecule is needed relative to your garden's variables. Wait for top 1" to dry and repeat. (weight of pot is best indicator and will result in a consistent volume needed... dictated by substrate characteristics and size of pot... not something you randomly choose.)
soiless. water with fertilizer every time with a 1.3-1.5 ec concetration of well-balanced mix with 10% or more runoff... the runoff ensures conssitency of concentration in soilless substrate. (if soil is completely devoid of original amendments, you can treat it this way too, otherwise you have to consider what it already has... soil is more about supplementation and soilless is about a 100% fulfillment of diet.)
Probably a combination of bad watering habits and aslightly off nute mix... get the watering habits worked out first, then see what problems remain.