Don't be so cheap. Go buy some fresh dirt. You're just asking for problems That are avoidable.
Has nothing to do with being "cheap", but there can certainly be cost savings associated with recycling mix (esp. the larger the soil grow). You are saving not just in soil costs, but should also be saving in fertilizer/amendment costs if you do it right. What is inherently wrong with that?
There is a proper way to do this which should not cause any problems at all. I would say there is no such thing as "fresh dirt". Real soil; and quality soil like old growth forest soil is the result of thousands of years of growth, death, decay and regrowth. Couple that with the weathering of minerals, etc. which is also a long-term process. More specifically "soil" is a mixture of some proportion humus (thoroughly decayed organic matter), clay, silt and sand along with pebbles, rocks and undecayed/decaying debris.
Most "potting soils" differ quite a bit from real soil, as they are typically made to be significantly lighter and more porous for container growing. Most are sphagnum peat moss based, while many [newer] potting mixes contain some amount of coco coir. Both materials are rather light and absorb several times their weight in water, while retaining some air (coir is better at this). Pulverized limestone, or oyster shell (forms of calcium carbonate) are added to make the pH acceptable as peat in particular is rather acidic. Very commonly, perlite- an inert material is added to improve drainage.
In order to make it a "potting soil", instead of a strictly "soilless potting mix" one or more forms of humus will be added; including compost, forest humus or earthworm castings. Humus holds water, provides and retains nutrients and also adds micro-organisms to the mix. Microbes are very important to plants, more-so than many growers could realize. Humus, clay, coir and sphagnum all feature microscopic particles sufficiently small as to act as "exchange sites" for ions. These tend to be predominately negatively charged and so able to attract cations, or positively charged ions which plants use as nutrients (NH4+, Ca++, Mg++, K+ etc). This is referred to as cation exchange capacity which can be very helpful to understand.
Both potting soil and actual soil are designed to maintain an environment conducive to life, of both plants and other organisms which live in the soil (microbes, microarthopods). With potting soils we often amp this up with more minerals, microbes, organic matter sources, etc.
Many soil amendments release minerals long term: including bone meals, rock/granite dusts, Azomite, greensand, crab meal, etc. Longer than the roots are in the container for. Besides this the roots, fan leaves, components of the potting soil: all of this is organic matter. All of that will physically be broken down into the smaller, elemental/ionic pieces which plants require by microbes and this entire process will yield additional benefits for the plant related to the symbiotic nature of the microbes and the myriad of compounds they produce (enzymes, for example, which solubilize/release more nutrients). It will yield more humus, further improving soil characteristics and quality. Humic substances are also involved in nutrient chelatation, complexing and availablity (they increase nutrient availability and transport).
Used soil may work. But, do you re-use the water in the pot after you boil pasta?
Worst. analogy. ever. What soil\potting soil does for plants is nothing like boiling pasta. The soil itself should only get better; provided you actually understand some things about soil and how to recharge/recycle it over time. This is an organic process. Best results are obtained with methods of the organic/living organic paradigm. The point here is to foster microbial activity within the potting soil; which will increase the breakdown of the roots and debris from the previous grow. This can be
thermophilic, or in order words release a substantial amount of heat.
First of all, as someone mentioned the containers from the previous grow should be emptied out, cleaned, etc. Break the root balls up, collect the mix in a tote, kiddie pool, on a tarp, etc. and remove any large roots you can find (or sieve), break up the smaller roots. If desired you can dry the larger portions of the rootball, along with the fans leaves, grind it all up and amend the soil with it. It shouldn't be extremely dry, if it is moisten it, I recommend a pump sprayer.
Next up you amend it with any of various ingredients: castings, meals, rock dusts, guano, azomite and other minerals etc. or you could try an all purpose organic dry fertilizer, (e.g. Espoma Garden-Tone). Numerous 'recipes' and methods are described in the Organics forum. Importantly, you'll most likely want to add more dolomitic limestone, and/or oyster or egg shell flour (1-2 tbsp per gallon mix). After amending you mix very well, moisten (DO NOT saturate) and cover. If you use a plastic tote or something, drill holes in the bottom and on the sides for drainage\air exchange.
You can mix up to a tablespoon of blackstrap molasses per gallon of water to moisten the mix with, or spray with an actively aerated compost tea. Usually you let this do its thing for about 4 weeks, mixing it periodically (weekly or whenever).
Coco coir and sphagnum themselves take years to break down completely, but you can opt to add a little bit more of either after re-using a few times, or add a bit more perlite each time. Another option is to use, for example, half recycled mix and half bagged soil/potting mix at planting, or a layer of recycled "supersoil" on say the bottom 1/3rd of the container.
