Yes you can use it in soil. It's no different than using any other nutrient that you mix into a solution. People use the same cannabis specific nutrients in both soil and hydro grows. The only difference with Jacks is that it's in water soluble granular form instead of prediluted in a bottle. Same raw ingredients made at the same fertilizer factories. I would not use it full strength as the soil is already going to contain nutrients.
On a side note, it seems that Jacks is getting more popular as people are getting tired of paying insane prices for some designer nutrient that doesn't grow anything better. Jacks is apparently having trouble keeping up with demand.
This message on their website:
"Due to the large volume of recent order our turnaround times are delayed longer than usual. We are working around the clock to get your plants the nutrients they need. Your FedEx tracking # will be emailed to you once your order has shipped. Thank you for your interest in getting your plants fed with the blue juice!"