Your 4.2g of Mega Crop added 409ppm. My Mega Crop adds 760ppm per 5g, or 638ppm for 4.2g. So there's a pretty substantial difference (229ppm). You're running the old product right? That or yours has absorbed a bunch of water weight. There has to be something to explain that big of a difference.I'll give you some of my numbers from a feeding yesterday with 4L of tap water.
Tap ppm: 164 EC(us/cm) 348
With .4g of Kelp Extract added: 182 ppm EC 387
With 4.2g of MC added to above: 591 ppm EC 1289
Without adjusting pH it brought it all to 6.3
In case you wanted something to compare against.
I heard back from Mr ProMix! His suggestion was (obviously) to always grow with new media. But he did give good reasons for doing so, in spite of the commercial interest he has.I sent an email to my ProMix guy about what to add for a second run but haven't heard back.
My current Faux Mix has no mycos. More like the standard Pro Mix HP than the Pro Mix HP with Mycorrhizae. It also has no wetting agent. As far as lime, while browsing around trying to find how much to add, I ran into an experiment. Someone did 8 batches of soil, starting at 2 Tbsp per gallon and increasing 2Tbsp each batch until the last batch had 16 Tbsp in it. From 2-14 Tbsp, the soil wasn't out of range. It took 16 Tbsp per gallon before things became too base. So there is a big fudge factor there. I'd say 2 Tbsp, which is on the lower end and what I did. The 2 Tbsp number came up in a number of articles I found on building your own mix. 6.75 pounds of Espoma Garden Lime from Home Depot is a whopping $5. There's probably enough in a bag to do a good hundred gallons of soil.I heard back from Mr ProMix! His suggestion was (obviously) to always grow with new media. But he did give good reasons for doing so, in spite of the commercial interest he has.
He basically said that it's doable to refurbish the used ProMix with mycorrhizae, limestone, and wetting agent, but figuring out how much of each to use would be an issue, along with the fact that there are leftover nutrients and pathogens/insects. Not to mention that the pH of the medium has most likely drifted with all the nitrogen and alkalinity of the water poured through.
"When using new growing media, the product is formulated the same each time and there is less variability and you will not inherit cultural conditions from previous crops."
The question is what is the cost of the lime, wetting agent, and mycorrhizae needed to bring it back to spec, and how close can I get it to what I've been using previously. At $25 for 2.2 cf bag the stuff ain't cheap, though neither was the Dr Earth Pot of Gold I started with, which I had to add loads of perlite to .