Irrigation on different substrates
Cultivating in NFT the inflow of water must reach all the roots to each irrigation. 
Although some leave always on the pump is recommended a program at intervals to ensure 10-20 irrigations per hour during daylight hours, to be reduced during the dark.
If the medium is perlite is necessary to make frequent watering, every 2-3 hours.
If the solution is administered from the top make sure it is well distributed. This also applies to the clay , especially if the size is quite large.
The water however, tends to create "channels" within the substrate and particular mainly through them: this is to be avoided in order to reach all the roots.
The substrates that retain much moisture as compost , coir and wool tend to imbibirsi and should be paid special attention not to overdo irrigation, while ensuring an excellent drainage.
Better irrigation during daylight hours, when evaporation is high, and control the humidity (the higher, the lower the demand for water).
Quite long intervals between administration and the other will allow a better aeration of the medium.
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