Fertilizer Drawbacks: Though fertilizers have their benefits, there are also downsides to their use, particularly with regard to synthetic fertilizers. Phosphorus from fertilizer can cause algae to accumulate in lakes and ponds, killing fish by robbing them of oxygen. Fertilizers can also contaminate water with an overabundance of phosphates and nitrates, making it unsafe for consumption. Oxidized nitrogen, a byproduct of synthetic fertilizers, also increases smog, which may be related to higher incidence of respiratory illness and asthma. The Upside of Fertilizer: Fertilizer comes in many forms, from chemical products to old grass clippings. By providing nutrients like nitrogen, fertilizers help plants thrive despite the threat of weeds and disease. Organic fertilizers, like manure, improve the fertility of soil by feeding microorganisms in the soil, reducing erosion and keeping soil well hydrated. Grass clippings, scattered on the lawn, are a form of fertilizer that provides valuable phosphorus, nitrogen and potassium and can be had for free by mowing the lawn.