The amount of refuse that your garden creates can be reduced if the growing media is reused. In fact, in the case of organics, reusing the growing media is my preferred method. Also, nutrients that remain in the plant waste material can be then recovered and reused.
Reusing garden media is not always the best choice, however, as certain growing mediums—such as rockwool—are not very suitable for repeated use. Any plant material or media that has been exposed to a pathogen should also be removed from the system in order to prevent the disease from spreading.
For example, a single plant infected with the dreaded tomato mosaic virus can infect growing media and future crops grown in it for years; hence why infected plant material and media should be isolated and destroyed.
Another concern to be taken into consideration is that several chemical sources of nutrients can leave heavy metal and salts behind after macronutrients have been used, and this can result in slowly rising amounts of both toxins over a period of years. This problem is faced by soil fields where long-term heavy use of chemical fertilizers has damaged the natural microflora-based nutrition cycles.
Still, even with the above considerations in mind, there are many times when reusing media and leftover plant material not only makes sense from an economic standpoint, but from a nutritional one as well.
During growth, plants distribute the nutrients they absorb throughout the plant, not just in the harvested portions. As such, there is a quantity of valuable nutrients still locked in the plant material and roots, and the media attached to the roots. These nutrients can be reclaimed from the leftover plant material through composting.
A hot compost pile will produce compost in a matter of weeks, but even a cold compost pile will often be ready for use in a matter of months—just in time the following planting season. (Note that while any moist plant material will eventually decompose, making an actual compost pile helps to speed the process and to contain the mess.)
The resulting compost is a valuable garden resource that, in soil gardens, can be used as an improving amendment to add to the growing media or as the basis for new potting mix. Hydroponic gardeners can use the compost for teas.
Also, keep in mind that many organic nutrient sources can take months or even years to become processed by microfauna into available nutrients. As such, throwing away growing media before it has completed processing the available nutrients is like collecting the dinner plates halfway through the meal—you’re throwing away nutrients that have been absorbed by the media.
Reusing media is important to maximizing nutrient extraction and although reused hydroponic media may not appear as pretty as new media, it might in fact be a superior growing medium that improves with use. Remember, however, to rinse well any media that might contain high levels of salt or other chemical residue before reusing. In my own garden, the media I use is a combination of mostly perlite, with a smattering of clay balls and grow stones that I repeatedly reuse (I add fresh new media as needed).
For composting, start with the plant material and root balls. Plant material grown with either organic or chemical fertilizers can be composted to reclaim nutrients. Indeed, chemically fertilized plants tend to be larger than their organically grown counterparts and therefore can offer additional value when composting techniques are used.
During fall harvest, there is not only a plethora of garden waste to deal with, but often leaves from trees. Combining even amounts of “greens” (high-nitrogen carbon waste like leafy plant leaves) with “browns” (low-nitrogen carbon waste like tree leaves) in a pile will allow this garden trash to become compost.
Potting soil is also reusable and compostable, and while durable hydroponic media does not compost in the traditional way, it can be run through the process alongside organic material to form a pre-amended compost material.
Even some questionable media can be salvaged though a process known as soil solarization—gnats in particular can be successfully treated with this method. Place the media in closed black plastic garbage bag and cover with a clear plastic tarp tent. Allow to sit in the summer sun for several days. Internal temperatures can be checked with a compost thermometer.
Peak temperatures of over 120ºF for a couple of weeks is hot enough to kill many weed seeds and garden pests. Higher temperatures (160ºF is often easily obtained in sunny areas) will semi-sterilize the media within a few hours, killing most friendly and pathogenic microflora alike. Media semi-sterilized in this method should have beneficial bacteria and fungi reintroduced after treatment, or mixed with inoculated soil before use.
Indeed, adding additional nutrients and other enhancements to the composed media can improve the quality of the media and reduce the amount of additional nutrients that will have to be added during the growing season. Meals, manures, worm castings, kelp and granite dust are commonly added to replace lost nutrients. Peat moss, coir and garden soil are sometimes also added to improve structure.
While we are considering reuse of resources, recirculating systems waste less fertilizer than drain-to-waste systems. If a drain-to-waste system is implemented, it should drain not to waste, but to an additional vegetable or flower garden plot. This would both maximize the use of purchased nutrients and minimize nutrient pollution leaving the garden.
Gardens cannot be closed systems. Harvested material is removed from the garden, however, that does not mean the amount of additional resources required to enter the garden and the amount of non-harvest resources leaving the garden cannot be minimized to reduce both gardening expenses and carbon impact.
In natural systems, both macronutrients and micronutrients grow plants, which then fall and decompose to become available for new plants to grow. Natural growth does not replace its growing media each year; it reconditions and improves the existing media over a period of years.
Gardeners often think of gardens as an event, with a beginning in the spring, a summer in the middle and ends in the fall; but in natural settings, gardens are a self-sustaining cycle. Proper additive application should seek to improve the media over time, not damage it. Nutrients and additives must be added to the garden to replace that which is removed as harvests, but there is little reason to have to start from scratch each season.