Tomasinho if you are growing for personal use the amount of fert you dump in your soil wouldn't even make it to the ocean...if you are concerned about a build up of nutes just spread the nutes out around your lawn and other plant sites fairly evenly and you should be fine...

And to comment on the farmers...they are growing fields and fields of plants so yes when you are spraying 100,000 gallons of fertilized water a week I'm sure it could have an affect but for the isolated case of dumping a 20gallon res, it just isn't enough to damage anything...most likely the nutes you dump in will be used in the general area you dumped them by the plants that are already growing...

Peace and happy cultivating