Different pH & PPM levels with every runoff !?

Vagician said:

If your runoff ppm is dropping over time then those nutrients are being temporarily stored in the soil for the plant to use later. The higher initial reading is the nutrient solution that drained quickly and hasn't had time to bond with the soil. Water takes the easiest route through the pot so in dry-ish soil it is simply running through the soil column before attaching to the soil particles. The lower readings later on mean that the nutrients are attaching to the soil particles. As the soil gets moist it is easier for the ions to bond to the soil, reducing the amount of nutrients in solution, then the water will pass/filter through and be measured at a lower ppm than your 600. If you were to keep watering with nutrients until your runoff is 600 ppm to match your 600 ppm in then you are wasting nutrients. The matching ppm in your runoff means that the nutrients you are applying are going straight through the soil and not attaching to it. Nutrients attach to the soil, then the root hairs pull the nutrients off of the soil. Very little nutrients actually go directly from the water to the roots skipping the soil step. It is simply an issue of dry soil not absorbing nutrients as well as moistened soil. If you want to go down the rabbit hole then read about gravitational water, capillary water, and hygroscopic water. Cation exchange capacity (CEC) is also important to know about when feeding your plants.

The rising pH is related but separate. Try hitting them with 5.5~5.7 to see if that helps. I wouldn't go lower than 5.5. Sometimes a climbing pH means you need to adjust your nitrogen because your plants are consuming so much of it but that is more complicated.

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Terpz719 said:

Thanks so much for posting this! I recently posted about a similar problem, and this goes a long way toward explaining it.

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Vagician said:

Yes indeedy, no problem.

The pH and nitrogen levels are cool because is basically the plant/soil telling you to increase or reduce the nitrogen input. The further your runoff pH is from your input pH, the more nitrogen hungry your plant is. If your pH in matches you runoff pH (or is at least very close to it) then you have found the nitrogen sweet spot. This is assuming ALL other factors are stable.

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Vagician said:

Generally, but that is not an absolute. pH sway upwards is tied to nitrogen use but N is not the only thing messing with the pH. Rising pH is simply an indicator that the plant can potentially uptake more N. When adjusting your nitrogen input to correct for a higher pH you also need to keep a very close eye on N toxicity. It also works the other way, if your pH is going too low then you need to back off of the nitrogen while keeping an eye on N deficiency. Just remember that the pH is an indicator of how much nitrogen you can throw at a plant, but you need to keep an eye on all other factors.

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Terpz719 said:

Awesome! Thanks so much. I had researched growing a ton before I started, but hadn't run across that info.

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Vagician said:

No Worries! I went to the INDO Expo in Portland a few years ago and they had some really good presentations on growing.

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