The benefits are obvious , with taller , healthier plants giving a higher yield from crops , and therefore cheaper , more plentiful food . Disadvantages begin when heavy rainfall runs from the surface of the soil , taking fertiliser into streams and rivers . Too much fertiliser is added to the crop . Fertiliser in the river promotes plant growth just as it does on land .
Bacteria feeding on the dead plant material use up the oxygen in the water .
Fish may then die through lack of oxygen . Also, too high a level of nitrates in drinking water is a health hazard , particularly for infants . Nitrates can interfere with oxygen transport in the blood .