Organic and inorganic fertilizers form the backbone of a productive global agricultural system. They replenish the soil with essential elements needed for healthy crop growth. However, like pesticides, indiscriminate and excessive use, as well as improper disposal of fertilizers can release various contaminants into the soil. Organic fertilizers are increasingly being used due to a rising interest in sustainable, organic food production methods such as through recycling of wastes in agriculture. However, these manures can become a source of soil pollution when used in excess.
Inorganic fertilizers
As you might know, N, P, and K are the most commonly used inorganic fertilizers. Inorganic nutrients like nitrogen, phosphate, and potassium can become pollutants when applied to agricultural soils as fertilizers, or in areas of intensive livestock production that transfer to animal manures or leach into the soil. 65% of the 115 million tonnes of nitrogen applied by farmers globally run off into the environment (Ritchie, 2021). This excess use of nitrogen fertilizer can lead to the following effects on soil, and therefore other pollution types:
(1) Loss of nitrogen from soil can occur through denitrification, volatilization, and surface runoff. Eutrophication can be caused by the leaching of nitrogen from soils into groundwater.
(2) Nitrogen can be transformed by microbial activity into nitrous oxide, a greenhouse gas contributing to climate change, as well as to the depletion of stratospheric ozone.
(3) Nitrogen pollution can influence soil microbial community composition and activities.
Figure 1. Nitrogen cycling in soil. Source: NCSU
Additionally, phosphate fertilizers can become a source of harmful heavy metals (HMs) including arsenic, cadmium, chromium, lead, and radionuclides like uranium, as they are mobilized during the phosphate mining process.
Organic fertilizers
Organic fertilizers include animal manure, compost, and sewage sludge. These fertilizers improve soil health by enriching organic carbon in soils and allowing the slow release of nutrients from the soil. However, toxic HMs can leach from these organic manures and pollute the soil. In intensive livestock production, animal feeds (such as poultry and dairy production) contain HMs such as lead, arsenic, and chromium to maintain production efficiency.
Source: 123rf
Impact on soil properties and human health
Continuous and long-term application of organic and inorganic fertilizers can lead to the accumulation of heavy metals in soil which can then influence the chemical, biological and physical properties of soil. The extent of the impact that HMs and excess fertilizers have on soils depends on the soil type (i.e., buffering capacity), and the intrinsic properties of the materials. For example, excess nitrogen fertilizer can lower soil pH by converting ammonium (NH4+) to NO3- through oxidation, generating H+ ions. Soils with lower pH can then lead to a reduction in soil microbe populations. Accumulation of heavy metals in soils can also adversely affect living organisms, including microorganisms and plants.
Heavy metals can transfer into groundwater due to high build-up and dilution from the soil solution system, instead of binding with clay and organic matter. The entry of heavy metals into the food chain may occur through crops or water. Exposure to HMs over the long term can lead to negative human health outcomes, including respiratory illnesses and cancer.
References
FAO and UNEP. (2021). Global assessment of soil pollution: Report. Chapter 3: Sources of Soil Pollution.
Rashmi, I., Roy, T., Kartika, K. S., Pal, R., Coumar, V., Kala, S., & Shinoji, K. C. (2020). Organic and Inorganic Fertilizer Contaminants in Agriculture: Impact on Soil and Water Resources. In M. Naeem, A. A. Ansari, & S. S. Gill (Eds.), Contaminants in Agriculture: Sources, Impacts, and Management (pp. 3–41). Springer International Publishing. https://doi.org/10.1007/978-3-030-41552-5_1
Rodrguez Eugenio, N., McLaughlin, M. J., Pennock, D., & Global Soil Partnership. (2018). Soil pollution: A hidden reality.