Katy Dynarski, PhD - - Soil Nutrients

Preventing Over-Fertilization for Better Crop Quality and Yield

Fertilizers can be an important source of nutrients to support crop growth. Applying fertilizer in excess of crop requirements, though, can actually harm crops and soil as well as damage environmental and human health.

Studies have shown that U.S. growers overspend on fertilizer by as much as $6 billion per year, an economic drain that has surprising impacts both on and off the farm. Applying too much fertilizer can impair farm soil health, reduce crop quality, and even cause crop yields to decline. Excess nutrients can also become pollutants that damage water quality, cause emissions of greenhouse gases, and contribute to smog formation—costing billions more across multiple economic sectors every year.

Sustainable nutrient management strategies, which aim to reduce nutrient loss to the environment and maximize uptake by crops, can help growers improve farm health, save money on fertilizer costs, and contribute to environmental and human health. These strategies can be as simple as applying the appropriate fertilizers for specific crop needs, lending numerous benefits for growers like greater crop yields, better bottom lines, and a healthier planet.

What are the signs of over-fertilization on a farm?

When more fertilizer is applied than plants can take up, the surplus nutrients, particularly nitrogen, can be lost to the environment. Unused nitrogen fertilizer can leach downward into groundwater, enter nearby surface waters through runoff, or be released into the atmosphere as gases.

One easy-to-spot sign of over-fertilization is heightened nitrogen concentrations in waterways adjacent to fields. Long-term over-application of ammonium-containing nitrogen fertilizers can also cause soils to become more acidic, so if soil pH is declining, it may be a sign that not all of the nitrogen fertilizer is being taken up by crops.

Over-fertilization also introduces excess salts to soils, which diminishes crops’ ability to take up water. This can cause drought stress even when water is readily available, damage roots, cause leaves to wilt, and eventually even kill crops. Drought stress in well-watered crops can, therefore, be another indicator of over-fertilization.

Can over-fertilization impact crop quality?

Yes. In addition to excess salts from fertilizer diminishing the crops’ ability to take up water, over-fertilized crops may take up more nitrogen than they need, which disrupts the balance of nutrients in plant tissue. The result is that crops will be deficient in those other necessary nutrients, such as sulfur and zinc, reducing crop quality.

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Can over-fertilization impact crop yield?

Above a certain threshold, any nutrient, including nitrogen, ceases to promote plant growth and can actually be toxic. Below the toxicity threshold, deficiencies in other nutrients will prevent crops from responding effectively to the application of any one nutrient. For example, sulfur is necessary for plants to metabolize nitrogen, but isn’t included in typical superphosphate fertilizers.

“You can apply 300 pounds of nitrogen to a field, but, without sulfur, the plants won’t be able to use it,” explains Farm Journal Field Agronomist Ken Ferrie. These interactions between nutrients mean that past a certain rate of application, crop yields will plateau even when more fertilizer is applied.

How does fertilizer application produce gas emissions?

Fertilizer leads to gaseous emissions via two main pathways:
Microbial denitrification. When soils are saturated with water, oxygen availability plummets because soil pore spaces are filled with water instead of air. Under these conditions, certain soil microbes that can use nitrate instead of oxygen in their metabolism will transform accumulated nitrate into a gas—this process is called denitrification. A typical agricultural soil will lose about 15 percent of its nitrogen this way; the more excess fertilizer nitrogen in the soil, the more gas produced. This rate can be even higher in soils in prolonged waterlogged conditions.
Ammonia volatilization. This can happen when nitrogen is applied as urea or manure on the soil surface, especially in alkaline (pH > 8) soils. Urea is easily converted into ammonia gas, which then escapes to the atmosphere.

How do gas emissions from over-fertilization negatively impact the environment?

The process of microbial denitrification can produce two polluting gases:

Nitrous oxide (N2O). This is a potent greenhouse gas that has a global warming potential almost 300 times that of carbon dioxide and can persist in the atmosphere for a millennium. Today, about half of the man-made N2O in the atmosphere originated from agricultural soils.
NOx. Oxides of nitrogen—like nitric oxide (NO) and nitrogen dioxide (NO2)—are pollutants that contribute to smog and acid rain. They can also cause respiratory illness, cancer, and cardiovascular disease. Recent research in California and the Midwest estimates that that 25 to 40 percent of NOx emissions come from agricultural soils.

The process of ammonia volatilization produces ammonia gas. Ammonia is not itself harmful to human health, but it reacts with nitrogen and sulfur oxides in the atmosphere to produce PM 2.5, tiny airborne pollutants that contribute to smog formation and can damage human health by aggravating respiratory and cardiovascular illnesses. Ammonia can also hurt aquatic life if too much reaches lakes and streams.

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Does fertilizer application impact the environment if applied in the correct amounts?

Applying the correct amount of fertilizer can still harm the environment, if it is not applied in the correct form, at the right time, or in the right location. These four factors—rate, source, time, and location—are known as the 4 Rs of sustainable nutrient management. Following these principles, growers apply fertilizers in a way that maximizes crop nutrition without over-fertilizing:

Right source. Providing fertilizers in crop-available forms that are appropriate for soil conditions, and contain the right balance of nutrients for the particular crop.
Right rate. Applying fertilizer in amounts that correspond to crop nutrient demand, taking into account the nutrients already present in soil, and nutrients in other amendments being applied.
Right time. Supplying fertilizer at times when plants need it.
Right place. Providing fertilizers in a location where plant roots can access the nutrients, and considering spatial variation in soil nutrient availability across fields.

All of these factors are also dependent on the local environment and soil conditions. Because of these interactions, what is “right” will vary by farm and by crop type.

How can over-fertilization be reversed?

One strategy to remediate soils that have been over-fertilized is to plant cover crops, which can mine the soil for excess nutrients. Deeply rooted cover crops such as tillage radish are especially beneficial, as they can recapture nutrients that have leached down to lower depths.

To prevent excess nutrients from polluting waterways, planting riparian buffer zones along streams and sloughs bordering croplands can help take up fertilizer runoff before it reaches the water.

How can farmers prevent over-fertilization without losing revenue?

Applying nutrients in a sustainable way helps growers increase crop production and save money.

More fertilizer doesn’t always mean more yield. When fertilizer application rates are above the economic optimum, reducing fertilizer application can maintain (and even improve) yield at a lower cost, increasing farm revenue. For example, a case study by researchers at the University of Nebraska showed that maize growers in the Midwest could cut their fertilizer application by nearly half without any loss of yield, representing significant potential economic savings.

By managing nutrients sustainably, growers can apply only what is necessary while still maximizing their yields. Incorporating additional soil conservation strategies will help reduce fertilizer loss while also improving soil quality—so fewer fertilizer inputs are needed in the future. Precision agricultural technologies that help growers control the exact timing and placement of fertilizers can also help with nutrient use efficiency and minimize losses to the environment.

Sustainable nutrient management strategies have been highly successful in the European Union (EU), where over-fertilization used to consistently threaten drinking water safety. By implementing new nutrient management techniques and cutting fertilizer use by more than half since 1987, growers in the EU have reduced over-fertilization without reductions in crop yield, resulting in safer drinking water and a 47-reduction in ammonia emissions.

Ultimately, applying nutrients in a sustainable way helps growers increase crop production and save money while contributing to the health of both their farm and the surrounding environment. Dr. Maya Almaraz, a researcher studying NOx emissions from agriculture, notes that by implementing nutrient stewardship strategies, “growers can produce food more efficiently, increasing their bottom line and improving environmental health.”