I actually really like the title- it's why I opened the thread.

That's about it- you add water to lower EC/PPM ( see footnote) and add nute to raise it. If the plants are drinking more than they're eating, ya give them more water. If they're eating more than they're drinking, ya give them more food.

It's just about impossibly complicated to determine the relative ratios of nutrients in your solution- Large scale hydro produce ( lettuce, basil, tomatos) will either have a lab on site or, more commonly, sub out their weekly testing to a lab that serves a pretty good region.

The common practice is to maintain a consistant EC/PPM and replace the nutrient solution perioically. The larger your (res volume)/biomass ratio is, the slower nute ratios will drift, and therefore the longer you can go between complete changes. IE- you might get away with 14 days w/ 12 6"plants, but you'd need to go once a week for 12 12" plants, given a res volume of X.

If I saw little variance in periodic ( maybe daily?) EC/PPM measurements, I'd feel more comfortable going longer periods. If the EC/PPM was slamming around, I'd wanna swap out more often. Yer gonna have to see what ya see. 18g will bounce around a good bit once they get going.

If yer flying without a meter, then it's usually safest to use straight water or maybe a 25% mix, and swap nute more often.
If you're doing a lot of topping off, get a meter- it'll pay for itself in nute in six months.

Hey, it just occurred to me- you say " drip irrigation"- is the excess going back into the res, or just going to waste? If yer just going to waste, ya just mix according to label and set your timer to give at least 10% waste- IE, an 18 gallon res should end up with at least 1.8 gallons of waste draining from the media. 25 to 30% would be safer. Ya go thru a shitload of nutrient, but ya don't have to deal with any of this adjustment crap- you're just flushing the used shit down the drain. Even at 25%, I'd run a couple of cycles of just water through at res re-fill.

Hope this helped.

Footnote- "ppm" meters actually measure the electrical conductivity( EC) of a solution, which is then expressed in millisiemens (mS). A chip on the "ppm" meter then determines what the concentration of total dissolved salt ( TDS) would produce the same conductivity ( EC) and expresses it in parts salt per million parts water (PPM) and displays that number. As things which are not sodium chloride will have differant conductivities than salt, it's a pretty relative number.

Further complicating matters, theres debate about what the correct scale is- an EC of 1.0 mS can be expressed as either 500 ppm or 700 ppm, depending on who made the chip. It doesn't matter, as long as you know which one you have, and which one anybody that you're comparing notes with has.

Also, you'll sometimes see things expressed in conductivity factor, which is just EC(in mS) x 10. I guess that they just thought that decimal points were too expensive to print.

You see stuff in EC more and more often- I really prefer it to the ppm convention, if only because ya don't have to worry about the whole 500/700 conversion thing.
rhizome Reviewed by rhizome on . Six accounts of murder... This is a question regarding PPM and top-offs: Vegetative Setup T5 Fluorescent Drip Irrigation 18 Gallon Reservoir 18/6 Light Cycle 10 min. Feeds 3x Daily Lets say I start off with PPM of 550; after 1 week it goes up to 800. I know to bring it down you simply add water. This is a "top-off," correct? How would I measure the amount of nutrients it needs? Will there be a time when the PPM goes down? Finally, as the plant grows it will need more nutrients per top off? Is there a set Rating: 5