[Fertilizer Myth] The price of increasing yield? Chemical fertilizers are quietly harming the soil and our tables!

Although long-term use of chemical fertilizers can increase yields, it may lead to soil acidification, nutrient imbalances and accumulation of heavy metals, which not only endangers crop health, but also threatens human food safety and environmental sustainability.

Chemical fertilizers play a pivotal role in modern agriculture. Their main function is to significantly increase crop yields to meet the growing global food demand.1. Study shows direct correlation between chemical fertilizer use and significant increase in crop yields1. However, over time, the scientific community and the public have become increasingly concerned about the environmental and human health problems that may result from the long-term and excessive administration of these synthetic substances.1. Currently, people’s focus has shifted from simply pursuing yield to taking into account both agricultural sustainability and food safety. This report aims to provide an in-depth analysis of how certain chemical fertilizers, based on academic research and credible website content (excluding news reports, private company information, and Chinese-language information), ultimately cause potential harm to crops and human health through multiple pathways including a decrease in soil pH, nutrient imbalances, and the release and accumulation of heavy metals.

Long-term and frequent application of certain chemical fertilizers, especially those with high nitrogen content such as ammonium nitrogen fertilizers (e.g. ammonium sulfate, monoammonium phosphate) and urea, is one of the main causes of soil acidification in agricultural ecosystems1. Soil acidification is a complex process in which nitrification plays a key role25. Nitrification refers to the process in which soil microorganisms, mainly nitrifying bacteria, gradually oxidize ammonium ions (NH₄⁺) into nitrite (NO₂⁻) and nitrate (NO₃⁻). It is worth noting that these two steps of oxidation reactions will release hydrogen ions (H⁺) into the soil, which directly leads to a reduction in soil pH, thereby causing soil acidification. For example, the nitrification of diammonium phosphate (DAP) releases three hydrogen ions per molecule, clearly explaining the chemical basis for this type of fertilizer to acidify soil.25。

Different types of nitrogen fertilizers have different acidification potentials26. Ammonium nitrogen fertilizers exhibit the highest acidification potential because they directly provide ammonium ions for nitrification.26. Although urea consumes a hydrogen ion during the initial conversion to ammonium, the subsequently formed ammonium will still cause soil acidification through nitrification, and the degree of acidification is similar to that of other ammonium nitrogen fertilizers.26. In contrast, nitrate nitrogen fertilizers have little or no effect on soil pH because plants release hydroxide ions when they absorb nitrate ions, which help neutralize soil acidity.26。

Long-term field trials and meta-analyses provide compelling evidence that long-term application of nitrogen fertilizers causes significant decreases in soil pH16. A meta-analysis of 115 studies in China, spanning 1980 to 2024, found that long-term application of nitrogen fertilizer led to an average decrease in soil pH of 15.27%, underscoring the breadth and severity of the problem19. Research also shows that excessive application of nitrogen fertilizer can lead to a significant reduction in the number of bacterial operational taxonomic units (OTUs) by changing soil pH.16。

Phosphate fertilizers generally have less impact on soil acidity than nitrogen fertilizers, primarily because they are typically applied at lower rates and have a lower acidifying capacity per unit of phosphorus.26. However, phosphoric acid is the most acidifying phosphate fertilizer26, and some long-term studies have also observed that long-term application of phosphate fertilizer will indeed lead to a decrease in soil pH.

Table 1: Acidification potential of common nitrogen fertilizers

Fertilizer type	chemical formula	Lime equivalent when nitrate is absorbed by plants (kg lime/kg nitrogen)	Lime equivalent during nitrate leaching (kg lime/kg nitrogen)
Urea	CO(NH₂)₂	0	3.6
Ammonium sulfate	(NH₄)₂SO₄	3.6	7.1
Ammonium nitrate	NH₄NO₃	0	3.6
sodium nitrate	NaNO₃	-3.6	0
monoammonium phosphate	NH₄H₂PO₄	7.1 (pH < 6.7) / 10.7 (pH > 7.7)	10.7 (pH < 6.7) / 14.3 (pH > 7.7)
Diammonium phosphate	(NH₄)₂HPO₄	3.6 (pH < 7.2) / 7.1 (pH > 7.2)	7.1 (pH < 7.2) / 10.7 (pH > 7.2)
liquid ammonia	NH₃	0	3.6
calcium ammonium nitrate	5Ca(NO₃)₂·NH₄NO₃·10H₂O	-0.5	3.1
Potassium nitrate	KNO₃	-2.6	1.0
Potassium dihydrogen phosphate	KH₂PO₄	-1.3	2.3
superphosphate	Ca(H₂PO₄)₂·H₂O	0	0
triple superphosphate	Ca(H₂PO₄)₂	0	0

Soil pH is critical to the effectiveness of plants in absorbing essential nutrients35. Even with adequate fertilizer applications, plants may suffer from nutrient deficiencies if the soil pH is not within the optimal range for plants to absorb nutrients. Low soil pH (<6.0) caused by long-term application of nitrogen fertilizers can significantly reduce phosphorus (P) availability36. Under acidic conditions, phosphorus easily reacts with aluminum and iron ions in the soil to form insoluble compounds (aluminum phosphate and iron phosphate), which effectively fixes the phosphorus and prevents absorption by plant roots.36. Phosphorus availability is usually optimal between pH 6.5 and 7.5, deviations from this range will result in reduced phosphorus uptake36。

Likewise, low pH can affect the availability of potassium (K)36. In acidic soils, high concentrations of hydrogen ions (H⁺) and soluble aluminum ions (Al) compete with positively charged potassium ions (K⁺) for binding sites on the soil’s cation exchange capacity (CEC)36. This competition may cause potassium to be displaced from soil particles into the soil solution, making it more susceptible to leaching, especially in sandy or low-CEC soils.36. Potassium availability is usually highest at pH levels above 6.036。

Soil acidity also has complex effects on trace element availability36. While the solubility of certain essential trace elements (such as iron, manganese, zinc, and copper) typically increases at lower soil pH, potentially making them more readily available for plant uptake, this increased availability can sometimes reach toxic levels, disrupting the overall nutrient balance within the plant.36. In general, the optimal pH range for overall nutrient availability is usually slightly acidic to neutral (approximately 6.0 to 7.0), and any significant deviation from this range may result in nutrient imbalances.36. All in all, soil acidification caused by chemical fertilizers fundamentally disrupts the fine balance of nutrient availability in the soil, leading to deficiencies in key macronutrients required for plant growth, such as phosphorus and potassium, and the potential for toxic levels of certain trace elements, ultimately affecting plant health and productivity.20。

Chemical fertilizers, especially phosphate fertilizers, are considered to be one of the important sources of heavy metal pollution in agricultural soils.2. Phosphate rock, the main raw material of phosphate fertilizer, naturally contains trace amounts of various heavy metals, including impurities such as cadmium (Cd), lead (Pb) and arsenic (As).39. Research data shows that there are differences in the average content of heavy metals in phosphate fertilizers in different regions39. For example, the average cadmium content of European phosphate fertilizers is approximately 7.4 mg/kg39。

With the continuous application of phosphate fertilizers, these trace amounts of heavy metals will gradually accumulate in the soil.45. If the amount of heavy metals input into the soil through fertilization exceeds the amount exported through crop uptake, leaching (leaching of many heavy metals is limited), erosion, or other loss pathways, then there will be a net accumulation of these contaminants in the soil.45。

It is worth noting that soil pH (usually reduced by long-term use of nitrogen fertilizers, as discussed in Section 2) plays a crucial role in affecting the mobility and bioavailability of heavy metals in soil40. In general, the solubility and mobility of many heavy metals, including cadmium and lead, two of the most concerning contaminants, increase under acidic soil conditions40. Lower pH helps release heavy metals adsorbed on soil particles (e.g. clay minerals, organic matter) into the soil solution40. The increase in the concentration of heavy metals in the soil solution directly increases the possibility of plant roots absorbing these heavy metals.

In addition, nitrogen fertilizer may also indirectly affect the absorption of heavy metals47. Although nitrogen fertilizer itself is not a direct source of heavy metals, its effect in lowering soil pH (as mentioned above) can indirectly enhance the activation and plant uptake of heavy metals already present in the soil (usually derived from phosphate fertilizers or other sources).

Table 2: Average content of heavy metals in phosphate fertilizers (example data)

heavy metal	European average phosphate fertilizer content (mg/kg)	US phosphate fertilizer content range (mg/kg)	Brazilian phosphate fertilizer content range (mg/kg)	Phosphate fertilizer content range in China (mg/kg)
cadmium	7.4	5 – 41	0.14 – 51	Not checked out – 27
chromium	90	–	–	–
zinc	166	–	–	–
nickel	15	–	–	–
lead	2.9	–	–	–

The accumulation of heavy metals in soil, especially those exacerbated by fertilizer use and soil acidification, has significant negative effects on crop health, development and yields39。

Once cadmium (Cd) is absorbed by plants (mainly through roots), it disrupts a variety of important physiological processes50. This includes interference with nitrogen metabolism, resulting in reduced protein synthesis and overall growth inhibition50. Accumulation of cadmium may also lead to imbalances in the absorption and distribution of other essential trace elements in plants.50. In addition, exposure to cadmium often triggers the overproduction of reactive oxygen species (ROS) in plants, leading to oxidative stress and damage to plant cells and tissues, ultimately reducing biomass and yield.

Lead (Pb) is mainly absorbed through plant roots, but its transport to aboveground parts (stems, leaves, fruits) is usually restricted46. However, even with limited transport, accumulation of lead in roots can hinder root growth and function, affecting the plant’s ability to absorb water and other nutrients.46. Lead may also interfere with key plant processes such as photosynthesis and negatively impact soil microbial communities, which are critical for nutrient cycling, thereby indirectly affecting plant health and productivity46。

Arsenic (As) contamination of soil can seriously inhibit plant growth and development51. Research shows that exposure to arsenic significantly shortens the length of seedlings’ roots and stems, with root growth generally more severely affected.51. Arsenic can also cause changes in the anatomy of plant roots, such as the reduction or complete absence of root hairs, which are essential for the uptake of water and nutrients.51. In addition, arsenic can damage epidermal and cortical cells in the roots, further impairing their function51。

Overall, heavy metals interfere with essential plant physiological processes such as photosynthesis and nutrient uptake39。

Consumption of crops grown in heavy metal-contaminated soil poses serious risks to human health, primarily due to the bioaccumulation and toxicity of heavy metals2. Bioaccumulation refers to the process of heavy metals accumulating in organisms over time, while biomagnification refers to the gradual increase in the concentration of heavy metals in the food chain.40。

Long-term, low-dose intake of cadmium (Cd) through food can cause serious health problems, primarily affecting the kidneys and bones. Cadmium can impair kidney function, increase the risk of chronic kidney disease, and interfere with calcium metabolism, leading to osteoporosis and increased bone fragility. In addition, cadmium exposure has been linked to an increased risk of certain cancers.

Lead (Pb) exposure can have widespread negative health effects, with particular concern for children46. Even low-level lead exposure can cause irreversible neurological damage in children, affecting their learning, behavior, attention, and overall cognitive development52. In adults, lead exposure may cause kidney dysfunction, high blood pressure, anemia, and reproductive problems. Lead can also pass through the placenta, posing risks to fetal development.

Arsenic (As) is a known human carcinogen, both organic and inorganic arsenic poses health risks51. Long-term exposure to arsenic through contaminated food (including rice grown in arsenic-contaminated soil) and water can lead to a variety of health problems, including skin lesions, cardiovascular disease, various cancers (e.g., lung, bladder, skin), neurological disorders, and diabetes51。

In addition, nutrient imbalances in crops (which may be exacerbated by soil problems associated with fertilizer use) may also have indirect negative effects on human health by reducing the nutritional value of food.

Analysis in this report shows that long-term use of certain chemical fertilizers can lead to soil acidification, nutrient imbalances, and the release and accumulation of heavy metals, posing threats to crop health and human health. To mitigate these negative impacts, the following more sustainable agricultural practices are recommended:

• Use chemical fertilizers carefully and in a balanced manner based on soil testing and plant needs to avoid over-fertilizing58。
• Where agriculturally feasible, preferentially use nitrate nitrogen fertilizers over ammonium or urea nitrogen fertilizers to minimize soil acidification.
• Carefully choose phosphate fertilizers that are low in heavy metals and regularly monitor heavy metal levels in fertilizers and soil60。
• Incorporate organic fertilizers (e.g., manure, compost, biochar) and soil amendments into agricultural systems to improve soil health, enhance its buffering capacity against pH changes, and potentially reduce the bioavailability of heavy metals5. Emphasis on the benefits of organic matter to soil structure and microbial diversity13。
• Promote the use of sustainable agricultural practices such as crop rotation and green manuring to improve soil structure, enhance nutrient cycling, and reduce reliance on synthetic fertilizers11。
• Consider phosphorus-soluble biofertilizers as a more sustainable alternative to traditional phosphate fertilizers to reduce heavy metal inputs and increase phosphorus availability through natural microbial processes13。
• Calls for continued and expanded research into the long-term effects of different fertilization practices, including on soil health, crop quality (including nutrient content and heavy metal accumulation), and ultimately human health outcomes.
• It is recommended that stricter national and international regulations be developed and strictly enforced to regulate the allowable levels of heavy metals in chemical fertilizers to protect agricultural soils and food supplies.

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