Ok writing this cause i see this issue often and rather have a thread to link than explain this each time. This is not my work but a collection of info from many members here and I have put into practice myself.
So first off COCO is NOT soil. For best results we want to keep it saturated between 90-100% that does not mean you can't go below this but you may see slower growth or nutrient issues over time if your consistently letting COCO get to dry.
Coco has an amazing gas exchange and air holding capacity when compared to soil. For this reason it is next to impossible to over water. Over watering is NOT to much water... its lack of o2. You will see this in soil with less drainage but because of coco's high drainage, air holding capacity and gas exchange its highly unlikely to see this in coco.
I do suggest if growing in more than 1 gal pots to add perlite if you are seeking faster grow rates. However if you want to water less often then you can eliminate the perlite as this will lower the drainage. As a rule of thumb:
1gal no perlite
2gal 20% perlite
3gal 30% perlite
4gal 40% perlite
5+gal 50% perlite
You can do more or less but it will affect the frequency of watering required. More perlite the more frequent you will have to feed. Personally I feel the better the drainage the faster the growth but its a balancing act and the above ratios i would say are a great place to start.
Coco has a low CEC (cation exchange capacity) so it does not hold onto nutrients as well as something like soil so we feed with every watering to keep a balanced nutrient ratio, ph and ppm in the coco. To do so we also need to flush out some of the last feed with each feeding to prevent a nutrient buildup which can affect all of these. THINK OF COCO AS SOILESS... much closer to hydro and very similar to rockwool.
I won't get into ppm in detail as thing will change quite a bit from grow to grow depending on many factors. But generally speaking start at around 300-400ppm and work up to around 800ppm. Now some may need to alter that as it will depend on your specific grow conditions.
HOW OFTEN DO WE FEED!!!
Ah the always debated subject.. 2 times a day, 1 time every 2 days, 10 times a day????
Forget that nonsense... like I say each grow is different so we need to use a better method. Let the plants tell you!!!!
Ok here is how we calculate it simply to your specific grow conditions, stage of growth, pot size and every other variable. You will never need to wonder am I feeding to much or to little again.
The formula:
1.Feed 5% of the pot size as your nutrient solution.
2. Of that we want 10-20% to come out as run off.
3. If you get more you can lower the frequency.
4. If we get less we need to increase the frequency and for that feed we need to add more to get our run off.
So I will do the calculations up to 5gal below to save you all some time. Feed the amount listed and check to make sure you get runoff of the amount listed from there adjust to dial in the feedings as they will change as the plants grow.
REMEMBER THIS IS A GUIDLINE AND NOT A HARD RULE. Our goal is to get as close as reasonably possible. It's not gonna kill your plants if its not exact.
I'm gonna round up.
1gal. Feed 250ml get 25-50ml of runoff
2gal. Feed 500ml get 50-100ml of runoff.
3gal. Feed 750ml get 75-150ml of runoff.
5 gal Feed 1litre get 100-200ml of runoff.
By doing this your plant will tell you how often you should be watering for best results.
Plant transpiration happens during photosynthesis so we only need to water during the lights on period. But in smaller containers you may find that you need to fertigate once in the middle of lights out if fertigation events are higher. Its unlikely but if your coco is dry before the first feed its not a bad idea to toss one in. Generally feed a couple hrs before lights out for the last time but you want to split the events evenly throughout lights on.
It may sound like you will be watering way to much but when you calculate how much you are feeding and then the frequency its not much more than most already do, just supplied in a manner that provides much greater stability. You will be using much smaller amounts of nutrient solution and by feeding much more frequently it doesn't take much to get your runoff while keeping the rootzone ph, ppm and moisture content ideal.
