These are substances which mainly contain phosphates, nitrates, ammonium and potassium salts that are applied to plants to enhance their growth. They're also a source of heavy metals like Hg, Cd, As, Pb, Cu, Ni, and Cu.
Advantages of this type of fertilizer include being immediately available to the plants, such that the effect is usually direct and fast, and having enough nutrient content so only relatively small amounts are required for crop growth, also increasing the cost advantage per crop for farmers.
For instance, maize is the most produced crop worldwide (822,712,527 tons yearly) and the most fertilizer-hungry one, consuming approximately 9 tons/ha.
Unfortunately, the great drawback is the environmental impact these products have. Soil, water, and air pollution, in addition to eutrophication, are only some examples of the negative consequences of the use of chemical fertilizers.
Fertilizers with high levels of sodium and potassium can eventually change the soil's pH and promote the accumulation of toxic chemicals which can then expand to the food chain.
Nitrate is the most common nitrogen contaminant as it dissolved in water easily, potentially polluting drinking water and rivers. It has been estimated that in ideal conditions, about 2-10% of the fertilizers interfere with the surface and ground water.
Arid and semi-arid areas are specially vulnerable given their high evaporation rate. Chemical fertilizers can be transformed into other dangerous compounds like carbon dioxide, methane, hydrogen sulfide (H2S) with chloro-fluoro hydrocarbons, such as halon gases.
Our parents always tell us to eat fruits and veggies because they contain useful nutrients for our bodies. They've also told us though, that everything in excess is bad, and that is precisely the case for water bodies.
Eutrophication occurs when too many nutrients, mainly nitrogen (N) and phosphorus (P), are added to bodies of water and cause excessive growth of algae. This is a serious problem, since it can ultimately lead to the whole depletion of these environments, but just before going too far, let's look at the root cause of the problem.
Chemical fertilizers currently used to nourish plants and increase crop yields are unsustainable in many different ways. First of all, their manufacturing process produces Green House Gases (GHGs), contributing to 3% of worldwide emissions. They pollute underground and surface water, and require non-renewable resources. Additionally, their use actually reduces soil fertility in the long term, plants can only take up about 50% of the fertilizer they're applied, AND they pollute water bodies when they run off. Then using them doesn't make sense!
But how though? Well, when nitrogen and phosphorus aren't fully utilized by plants, they can be lost from the farm fields (runoff) and start polluting adjacent water bodies. That's when eutrophication starts.
Initially, these nutrients do what your parents and I would expect: they cause growth of certain species. However, it's a disproportionate growth. Some plants, for example, will start growing like Popeye and block the light to others that aren't growing as much.
Apart from that, an entailed problem of eutrophication is hypoxia: the lack of oxygen. This can be caused by the excessive amounts of nutrients, the decay of algae and seagrass, and the bacterial degradation of the excess biomass. This whole lack of oxygen can lead to the deaths of inhabiting animals.
According to the National Oceanic and Atmospheric Administration (NOAA), 65% of studied estuaries and coastal waters in the contiguous United States have experienced eutrophication. One famous example is the dead zone in the northern Gulf of Mexico, which measured 8,776 square miles in 2017.
Going back to nutrition, we were wondering why plants even need fertilizers in the first place. Isn't nature supposed to take care of that? To answer that question, we need to take a step back and review some basic biology.
Plants (just like us) require nutrients to grow. Unfortunately, they cannot obtain them on their own. Despite nutrients being apparently available in the soil, their chemical composition doesn't allow for plant uptake.
One of the most essential elements that living things need is nitrogen. Again, Earth’s atmosphere contains a huuuuge pool of nitrogen gas (N2), but this gaseous form cannot be used by plants without undergoing a transformation. We then asked: so what do plants do?