Organic and sustainable are two unrelated concepts. A grower can be totally "organic" and, at the same time, not be sustainable and visa versa.
In my way of thinking, 'sustainable' means that a farm or garden can "stand alone" without any outside inputs. To put it another way, a "sustainable grower does not purchase or import any fertilizers, nutrients or organic matter from outside sources. In other words, how can a grower be "sustainable" if he or she relies on outside sources of fertilzer imports? It makes no difference if the grower is "organic" or not. If you purchase outside inputs, you are not "sustainable."
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Said it well.
My understanding goes along with his. Sustainable means you dont require outside inputs. Its basicly a closed loop system. You can provide for yourself. Seeds would come from seed savings, water could come from rain catchment systems, compost would come from shredded newspaper, food scraps, weeds, leaves, and manure from farm animals, everything comes from the farm and nothing leaves the farm. There is very little waste because everything is used in some other form. This is the most ideal form of sustainability. Obviously there are different levels of sustainability and some of us are more sustainable than others. Many people think organic and sustainable are the same thing obviously they are not, You can be very wasteful and still be organic. You can be ustainable but not organic. This is why our government stepped in (like it or not) to set a definition of the word organic. While I personally dont approve of all the goings on in the USDA and FDA and NOP standards at least they drew a line and said here is a definition and guidelines. The term sustainability is up in the air to be whatever you make of it.