The reason for not re-using soil is simple, you can state you re-use it all you want but in reality if your flushing out soil your only using a bi-product of your original mix and adding fillers to it, and while this May give some people great results it In fact is not as guaranteed to provide you with as standardized results. A new mix will contain exactly what you put in it every time, and nothing more, if you research anything about sustainability you will find that soil degradation while amended by adding organic matter is often not a long term solution (as in less standardized results, if you can state that every single one of your crops you did the same exact thing with everything else and nutes, grew the same plant under the same light, you will come back to your soil affecting your overall weight, and while this may be higher on some odd runs it is often based on other factors contributing to a grow, not primarily your soil).
Potted soil is not technically soil at all. It is a close approximation of the real deal comprised of lightweight fillers to aid drainage, and organic matter that provides nutrition and water retention. This substitution is necessary as soil from the garden will become compacted in pots and rot your plants’ roots. Nutrients and organic matter leach out of a pot much faster than they do in the ground or a raised bed. By year’s end what’s left in the pot is primarily filler with very little nutrition if any at all.
Reuse
In the springtime, just before planting:
Replenish depleted potted soil with organic matter such as compost or manure. Add no more than 25% organic matter to 75% old soil. Compost is dense, so using too much can lead to that compaction you are trying to avoid. Vermicompost, aka worm poop, is a lightweight, nutritionally balanced additive (approx. equal (N) nitrogen, (P) phosphorous, and (K) potassium) that I prefer for containers. Start a worm bin and make your own!
Add more fillers such as coir (coconut husk fiber), grit, or perlite if the mix feels too heavy.
Slow-release fertilizers are also good additives that won’t weigh down the mix. I always add kelp meal by the handful to help plants deal with the stresses of container life as well as crushed eggshells, which add both calcium and act as grit, and fishmeal for nitrogen and healthy leaf growth.
Diseases can carry over so do use fresh soil with disease-prone crops like tomatoes, or in pots where disease was previously a problem. To avoid spreading disease I practice crop rotation between pots, using the old mix for tough herbs that are better suited to poor soil.
- See more at:
http://www.hgtvgardens.com/garden-basics/can-i-reuse-old-potting-soil#sthash.OAQXwSMi.dpuf
cannabis in itself is prone to diseases, some strains in particular, so you can state I'm stupid all you want but most people I presume would prefer to have standardized results and prevent a level of disease retention entirely. I'm sure if you completely flush your mix and leave it composting for a month before reuse as is advisable your soil will be sufficiently effective, but if you were growing on a commercial scale or doing a perpetual system within a small space that might not be an option