Instead, crops are sent everywhere on earth. People eat them and then human waste is sent to a wastewater treatment plant. There it is treated to remove bacteria and the remaining nutrient rich sludge is usually sent to a landfill instead of back to farmers' fields. To replace all the missing nitrates from the cycle, farmers must add fertilizers to their fields. If more is added than the plants need, extra nitrates and ammonia can be washed into waterways by rain.
Nitrates can also wash off over-fertilized lawns or pastures where animals waste has accumulated.
Extra nitrates in our waterways from over-fertilization, soil erosion, and sewage inputs can disrupt the stream ecosystem. Aquatic weeds can clog waterways making boating and swimming difficult. Most importantly, these plants cannot live forever. Eventually they die and are decomposed by bacteria, a process that pulls oxygen out of the water. This process is called eutrophication.