The inception of chemical fertilizer has provided numerous benefits to agricultural growers. These fertilizers are predictable, reliable, and easy to use. They can be customized to fit specific crop needs and add immediately available nutrients to even poor-quality soil. These fertilizers are historically more inexpensive than many organic fertilizers and have been a major factor in increasing farm yields and profitability. Since the creation of chemical fertilizers, they have become a heavily relied upon component in modern agriculture. While there are various benefits to chemical fertilizers there is, over time, a trade off in terms of soil health as the long-term application of chemical fertilizers negatively impacts the soils’ biological and physicochemical properties. Soil fertility and vegetation are highly dependent upon a sufficient supply of essential minerals and nutrients. The excessive use of nutrients causes a soil nutrient supply imbalance which lead to deterioration of stable soil. Several soil health risks include soil compaction, acidification, erosion, contamination, salinization, and decline in organic matter, which can affect P and N losses to water and air.9

Organic Matter, Leaching and Runoff

Long-term research evaluating soil organic carbon (SOC) from 1904-2005 showed a steady increase in SOC for the first several decades when manure was the primary method of fertilization, however since 1967 showed a steady decrease in SOC when synthetic nitrogen became the predominant fertilizer.4,7 This decline was attributed to the application of nitrogen fertilizer which stimulates soil microbes that eat and breakdown organic matter at a higher rate. Over time, the amplified appetite of the microbes was breaking down organic matter faster than what was being returned to the returned to the soil in the form of crop residue. As organic matter declines so does a soils’ ability to store nutrients and water which results in:

• The leaching of nitrates into the groundwater and emission of nitrous oxide (N2O) into the atmosphere
• Greater additions of N are required in subsequent years to maintain production, contributing to further soil degradation.4,7
• There is increased potential for phosphorus runoff which pollutes waterbodies and is the main cause of eutrophication.

Significant loss of organic matter also reduces a soils ability to buffer against increased salinity, acidification, and compaction.

Salinity, Acidification and Compaction

Salts are a major component in chemical fertilizers and are believed to be damaging to the soil and plants, especially fertilizers with a high salt index.10Application of highly soluble fertilizers increases the concentration of soluble salts in the soils increasing soil salinity. Increased soil salinity causes:

• Nutritional imbalances
• Seedling injury
• Stunted root growth
• Blocks plant uptake of essential nutrients
• Reduced microbial activity

The loss of organic matter and the accumulation of mineral salts from fertilizer applications leads to compaction layer and soil degradation in the long-term. Soil compaction causes other problems such as reduced aeration, poor drainage, erosion, runoff1, reduced permeability, hydraulic conductivity, and groundwater recharge.2,6 Crop yields are reduced because of limited root and plant growth are also inhibited due to reduced water and nutrient uptake.5 Issues caused by salinity are especially amplified during times of drought.

In addition to increased salinity, chemical fertilizer application causes an accumulation of acids which creates harmful impacts on the soil such as:

• Disrupting soil structure by dissolving soil crumbs formed from decomposed material such as dead leaves and humus with clay resulting in highly compacted soils.
• Increased acidity also reduces the concentration of base cations such as Ca and Mg and increases the presence of aluminum (Al3+) which can often reach harmful levels causing nutrient deficiencies in plants.9
• Loss of beneficial microbial communities and soil enzymes making crops more susceptible to pests and disease.

Conclusion

Having a healthy soil gives plants greater resiliency against unfavorable growing conditions and is directly correlated with the nutrient content of food crops.3 Healthy soil therefore is needed to produce nutritious crops. According to the USDA there has been a significant drop in the amounts of protein, calcium, phosphorus, iron, riboflavin and vitamin C in conventionally grown produce over the past fifty-years.3 When maintaining soil fertility, it is important to consider fertilizer sources that provide benefits to the soils’ long-term health in addition to nutrients.

References

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3. Ewing-Chow, D. (2020). Earth’s rapidly degrading soil is bad news for human health. Forbes. Retrieved November 29, 2022, from https://www.forbes.com/sites/daphneewingchow/2020/06/24/earths-rapidly-degrading-soil-is-bad-news-for-human-health/?sh=1ff13ffe7865
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