Re: How often should i replace a Hoagland solution in my hydroponic system?

There are several approaches:

1. You can replace the nutrient solution every week or every two weeks whether 
it needs it or not.  Replace the volume lost by transpiration and evaporation 
between replacements with distilled water. It is hard to recommend an exact 
replacement frequency without knowing the environmental conditions and how 
fast the plants are growing. If the plants are growing slowly, the time 
between replacement can be longer.

2. Don't change it at all and just add fresh solution to replace the volume 
lost by transpiration and evaporation. The potential problem with that 
approach is that the concentration may increase over time.

3. Use an electrical conductivity (EC) meter to monitor the solution. Replace 
the volume lost by transpiration between replacements with distilled water.  
When the EC is substantially less than the initial EC, replace the solution. 
This target EC could be anywhere from 10% to 25% of the initial EC.

4.  When the nutrient solution volume has declined significantly, measure the 
EC. Add distilled water if the EC has gone up significantly and add fresh 
solution if the EC has stayed about the same or gone down.

If it is a mineral nutrient deficiency experiment, you probably don't have to 
change the solution at all because symptoms usually develop in three to four 
weeks. Plants deprived completely of one macronutrient (nitrogen, phosphorus, 
potassium, sulfur, calcium, magnesium) will grow poorly so will not need 
normal amounts of other mineral nutrients. 

The second reference shows a catalog page of about the least inexpensive EC 
meter available at $62. It would be very satisfactory for monitoring the 
strength of a hydroponic solution.

References


Re: What is electrical conductivity in hydroponics and how to measure it ?


EC meter