Soil and location will make a differences in how completely someone will be able to or not be able to flush when growing outdoors but it is wrong to look at the earth as a sealed container that nothing put into it ever escapes from it or vanishes deep within it.
If you have ever owned a home you might have noticed that lawns will at times become completely void of nutrients and need to be fertilized. If you owned a home and it had it's own well you might have pondered a time or two about how when it rains the rain water works its way down through the soil to the depths of your well, roughly 400-feet in my case, and that on it's way down the rain water brings various things with it, sometimes unwanted things, and they eventually end up in your well alone with the rain water.
Do you believe that when you fertilize outdoor plants that the fertilizer, regardless of type, magically and mystically suspends itself at the general depth of a root-ball and it defies what takes other things deep below the surface? Did you ever consider that when someone waters their outdoor plants or it rains, as the water passes down through the soil, much farther down than the lowest point of a plant's root-ball, that nutrients already in the soil might possibly travel downward with the water, as happens when someone flushes pots, and it eventually works it's way below where a plant's roots would ever reach?
What happens if there happens to be trees in the area, trees with roots that spread out beneath an area where someone plants a crop? Do you believe that the tree roots would be gentlemanly and not 'eat' any of the plant's 'food?' What about other root systems that might be nearby? Do you believe they would not tap into nutrients in the soil simply because the nutrients were not intended for their use?
Nutrients in soil can and at times will vanish, for various reasons, faster than some might ever believe.
