My general practice for topping off versus adding nutrients is based on just measuring the numbers I am seeing in the reservoir. I like to do a little math to figure it out. So let’s say we’re using a 5 gallon bucket with a 6" net pot lid, so we’re only putting in say 3 gallons of water (with the size of the net pot and the requisite air gap taking up the rest of the space in the bucket).
Let’s say that we have a plant we’re feeding 1000ppms, so we set the reservoir at 1000ppms.
In two days let’s say that our plant drinks a gallon of water, so we have 2 gallons left in the bucket. If we measure our PPMs and they are pretty close to 1000ppm, then it’s my totally unscientific and unsupported belief that indicates it’s eating and drinking optimally. If the ppms are higher - which is normally the case - let’s say 1100ppm for the 2 gallons of water left, it probably indicates we could lower our nutrient level since it’s not eating more than it’s drinking. Likewise, if we measure it and it’s actually lower at 2 gallons than it was when we set it at 3 gallons, say you measure it at 900ppm, we might consider raising our PPMs.
So the way I approach top offs is like this:
- If the PPMs are higher before the top off than they were when I set up the reservoir, I just add plain water (this is the case 90% of the time). I then measure the PPMs I have after I top them off, and it’s usually lower than where I started when I setup the bucket. That’s where I’ll probably set the bucket on the next change. So for example:
- Start with 1000ppm
- Before first top off, 1100ppm
- Top of the bucket, 950ppm
Next full reservoir change I’d aim for 950ppm.
- If PPMs are lower than where I started
- Started at 1000ppm
- Measured at 900ppm prior to topping off
- In this case I added nutes mixed exactly like the starting much at 1000ppm and top it off with that
- If PPMs are equal to where I started
- I almost always just add water until they get down to a point where I see the plants want food
And my general rule of thumb is to reset the whole reservoir at the point where I think I’ve replaced all the original nutrients. So in the above scenario, if we have three gallons of water in the bucket and the plant is drinking a gallon a week, I’d give a new bucket every three weeks.
In my current situation I have a plant in a 5 gallon bucket drinking 3 gallons a week, so I am giving it complete bucket changes every week and only topping it off with water during mid-week.