The fundamental relationship between improved soil quality and increased crop yields has been understood since the practice of agriculture became an essential part of virtually all human societies. The earliest fertilizers were readily available animal and human wastes that were spread over the soil. Through natural decomposition, these organic materials released their available macronutrients and micronutrients directly into the soil. These substances also contributed to soil density essential to moisture retention and the reduced risk of runoff that could damage farm fields. In addition, the biodegradation of these organic waste products introduces microorganisms into the soil that contribute to plant growth.
In the twentieth century, fertilizer science evolved from the localized use of organic waste methods to the sophisticated manufacture and processing of inorganic fertilizers capable of delivering larger amounts of nutrients to the soil in a shorter period than achieved through organic methods. Inorganic fertilizers usually are manufactured through ammonia synthesis processes that alter the chemical characteristics of nitrogen to render it reactive with its immediate environment. Synthetic controlled-release nitrogen fertilizers are used throughout the world to increase crop yields; slower nitrogen release rates also reduce the contamination of rivers, lakes, and subsurface water tables often caused by inorganic fertilizer use.
The resolution of the serious environmental concerns associated with inorganic fertilizer use has emerged as a key twenty-first century agricultural science research and development objective. As the world population continues to grow at a rate of approximately 1.5 percent per year, the need to increase crop yields to sustain human populations is a global imperative. The production of inorganic fertilizers involves significant amounts of non-renewal resources, including natural gas required for ammonia production, and mined micronutrient additives such as zinc.
Historical Background and Scientific Foundations
The objective of all fertilizer applications is increased crop yield and harvest quality. The positive effect on plant growth in soils that have been mixed with organic wastes has been observed since ancient times. Emigrants who settled near the Chesapeake Bay region of America in the early eighteenth century adopted the indigenous practice of adding menhaden fish to their fields to improve soil quality.
Terra preta, the dark soil found in the Amazon basin where charcoal from cooking fires significantly increased its fertility, is an example of a soil improved from an anthropogenic (human-made) source.
Crop rotation is a related agricultural practice that improves soil quality. Farmers grew different crops on the same fields because they appreciated that the pests and fungal diseases associated with a specific crop were less likely to reoccur if a different crop followed. In the mid-eighteenth century, the four crop rotation methods promoted by British scientist Charles Townshend (1674–1738) became the accepted agricultural standard in Northern Europe. The rotation of the field crops clover, barley, wheat, and turnips achieved healthier, more nitrogen-rich and productive soils. Clover later became known as green manure, because when it is plowed into the soil, clover will convert atmospheric nitrogen into ammonia that is subsequently broken down into soil nutrients.
The increased demand for cereal crops worldwide prompted the development of inorganic fertilizers. Ammonia is the essential component of the Haber-Bosch process developed in the early twentieth century. Methane extracted from natural gas is the crucial raw material; repeated exposure of the gas to an iron catalyst permits its component nitrogen and hydrogen molecules to form ammonia, because the process reduces the usual intermolecular strength of nitrogen atoms that renders the element inert in the atmosphere.
The first controlled-release, nitrogen-based fertilizers were sold in 1955. These were designed to prolong the nutritional benefits achieved through the exposure of nitrogen to depleted soils. Sulfur was the typical ingredient used to coat the fertilizer for slow nitrogen release; sulfur provides an additional macronutrient. Ammonia-based fertilizers have become the most widely used variety in the world.
Impacts and Issues
The development of synthetic fertilizers demonstrates how two important food sustainability objectives can come into direct conflict. The ability of agricultural science to develop technologies that promote increased crop yields is of paramount importance to the survival of many global populations. The accumulated pressure imposed on food supplies is generated by the combined effects of these factors:
- Steady increases in the global population from 2 billion persons in 1930 to 7.5 billion in 2017.
- Lands removed from agricultural production and converted to housing or industrial uses.
- Impact of droughts, frosts, and other extreme weather attributed to climate change; all impact the length and quality of crop growing seasons.
- Traditional agricultural regions where the soils have been nutrient-depleted. Overworked soils suffer from macronutrient depletion due to poor agricultural practices such as the failure to rotate crops.
- Insect infestation and fungal diseases that impair or destroy crop harvest quality.
The positive impacts of synthetic fertilizers on world agriculture are considerable. It is doubtful that there are sufficient sources of organic fertilizers available to sustain current and projected global agricultural needs. Modern slow-release nitrogen products, especially those that are further enhanced with additional micronutrients, contribute to increased yields for corn, wheat, and sorghum, three key crops in the food supply for the developing world.
The ease of handling and the powerful impact on crop yields realized through the use of synthetic fertilizers also carries a range of significant environmental risks. When soils are overexposed to nitrogen-based fertilizers, the soils will tend to become acidic and less capable of supporting crop growth. The runoff experienced in the ecosystems adjacent to fields where fertilizers are used extensively often leads to eutrophication of adjacent watercourses, where water quality and the diversity of aquatic life are diminished. Further, the manufacture of synthetic fertilizers requires the consumption of non-renewable fossil fuels, particularly natural gas, and minerals such as zinc and calcium that are mined from open pits using significant energy resources.
Organic fertilizers are recognized as having far fewer negative environmental impacts than the inorganic synthetic products. The principal drawbacks associated with these natural sources are their greater bulk than the synthetics, which poses additional handling and transportation costs. Further, organic wastes that are used as fertilizers may contain pathogens than can pose threats to human and animal health.
Biotechnology's role in modern fertilizers is a limited one in the early 2010s, but biotechnology likely will change the way fertilizers are used in agriculture in the future. Scientists are working to enable cereal plants to fix their own nitrogen levels, lessening the need for fertilizers, although this development is many years away, because nitrogen fixation requires the input of more than a dozen genes in most organisms. Genetically-engineered microorganisms within fertilizers that efficiently deliver nitrogen to crops and help them retain it are likely to happen sooner, which would also reduce the need for fertilizers. For the near future, biotechnology is having a quick impact in heartier species of plants that require less water or fewer insecticides, or have other qualities that make them desirable crops. Farmers are likely to justify increased use of fertilizers on these premium crops.
Primary Source Connection
Crops need certain chemical forms of nitrogen (called reactive nitrogen) found in Earth's atmosphere to thrive and grow. Reactive nitrogen is not always found at optimum levels in soil for productivity; therefore, it is often necessary to add it to soil. The added nitrogen is well known as a major source of pollution to the environment, and as such, the practice raises concerns about resulting water and air pollution secondary to crop fertilization. Farmers seek to grow abundant, healthy crops while at the same time reduce the impact of nitrogen pollution. Options to achieve this goal are discussed.
No Sure Fix: Prospects for Reducing Nitrogen Fertilizer Pollution through Genetic Engineering
Nitrogen is essential for life. It is the most common element in Earth's atmosphere and a primary component of crucial biological molecules, including proteins and nucleic acids such as DNA and RNA—the building blocks of life.
Crops need large amounts of nitrogen in order to thrive and grow, but only certain chemical forms collectively referred to as reactive nitrogen can be readily used by most organisms, including crops. And because soils frequently do not contain enough reactive nitrogen (especially ammonia and nitrate) to attain maximum productivity, many farmers add substantial quantities to their soils, often in the form of chemical fertilizer.
Unfortunately, this added nitrogen is a major source of global pollution. Current agricultural practices aimed at producing high crop yields often result in excess reactive nitrogen because of the difficulty in matching fertilizer application rates and timing to the needs of a given crop. The excess reactive nitrogen, which is mobile in air and water, can escape from the farm and enter the global nitrogen cycle—a complex web in which nitrogen is exchanged between organisms and the physical environment—becoming one of the world's major sources of water and air pollution.
The challenge facing farmers and farm policy makers is therefore to attain a level of crop productivity high enough to feed a growing world population while reducing the enormous impact of nitrogen pollution. Crop genetic engineering has been proposed as a means of reducing the loss of reactive nitrogen from agriculture. This report represents a first step in evaluating the prospects of genetic engineering to achieve this goal while increasing crop productivity, in comparison with other methods such as traditional crop breeding, precision farming, and the use of cover crops that supply reactive nitrogen to the soil naturally.
The Importance of Nitrogen Use Efficiency (NUE)
Crops vary in their ability to absorb nitrogen, but none absorb all of the nitrogen supplied to them. The degree to which crops utilize nitrogen is called nitrogen use efficiency (NUE), which can be measured in the form of crop yield per unit of added nitrogen. NUE is affected by how much nitrogen is added as fertilizer, since excess added nitrogen results in lower NUE. Some agricultural practices are aimed at optimizing the nitrogen applied to match the needs of the crop; other practices, such as planting cover crops, can actually remove excess reactive nitrogen from the soil.
In the United States, where large volumes of chemical fertilizers are used, NUE is typically below 50 percent for corn and other major crops— in other words, more than half of all added reactive nitrogen is lost from farms. This lost nitrogen is the largest contributor to the “dead zone” in the Gulf of Mexico—an area the size of Connecticut and Delaware combined, in which excess nutrients have caused microbial populations to boom, robbing the water of oxygen needed by fish and shellfish. Furthermore, nitrogen in the form of nitrate seeps into drinking water, where it can become a health risk (especially to pregnant women and children), and nitrogen entering the air as ammonia contributes to smog and respiratory disease as well as to acid rain that damages forests and other habitats. Agriculture is also the largest human-caused domestic source of nitrous oxide, another reactive form of nitrogen that contributes to global warming and reduces the stratospheric ozone that protects us from ultraviolet radiation.
Nitrogen is therefore a key threat to our global environment. A recent scientific assessment of nine global environmental challenges that may make the earth unfavorable for continued human development identified nitrogen pollution as one of only three—along with climate change and loss of bio-diversity— that have already crossed a boundary that could result in disastrous consequences if not corrected. One important strategy for avoiding this outcome is to improve crop NUE, thereby reducing pollution from reactive nitrogen.
Can Genetic Engineering Increase NUE?
Genetic engineering (GE) is the laboratory-based insertion of genes into the genetic material of organisms that may be unrelated to the source of the genes. Several genes involved in nitrogen metabolism in plants are currently being used in GE crops in an attempt to improve NUE. Our study of these efforts found that:
- Approval has been given for approximately 125 field trials of NUE GE crops in the United States (primarily corn, soybeans, and canola), mostly in the last 10 years. This compares with several thousand field trials each for insect resistance and herbicide tolerance.
- About half a dozen genes (or variants of these genes) appear to be of primary interest. The exact number of NUE genes is impossible to determine because the genes under consideration by companies are often not revealed to the public.
- No GE NUE crop has been approved by regulatory agencies in any country or commercialized, although at least one gene (and probably more) has been in field trials for about eight years.
- Improvements in NUE for experimental GE crops, mostly in controlled environments, have typically ranged from about 10 to 50 percent for grain crops, with some higher values. There have been few reports of values from the field, which may differ considerably from lab-based performance.
- By comparison, improvement of corn NUE through currently available methods has been estimated at roughly 36 percent over the past few decades in the United States. Japan has improved rice NUE by an estimated 32 percent and the United Kingdom has improved cereal grain NUE by 23 percent.
- Similarly, estimates for wheat from France show an NUE increase from traditional breeding of about 29 percent over 35 years, and Mexico has improved wheat NUE by about 42 percent over 35 years.
Available information about the crops and genes in development for improved NUE suggests that these genes interact with plant genes in complex ways, such that a single engineered NUE gene may affect the function of many other genes. For example:
- In one of the most advanced GE NUE crops, the function of several unrelated genes that help protect the plant against disease has been reduced.
- Another NUE gene unexpectedly altered the output of tobacco genes that could change the plant's toxicological properties.
Many unexpected changes in the function of plant genes will not prove harmful, but some may make it difficult for the crops to gain regulatory approval due to potential harm to the environment or human health, or may present agricultural drawbacks even if they improve NUE. For the most advanced of the genes in the research pipeline, commercialization will probably not occur until at least 2012, and it will likely take longer for most of these genes to achieve commercialization—if they prove effective at improving NUE. At this point, the prospects for GE contributing substantially to improved NUE are uncertain.
Other Methods for Reducing Nitrogen Pollution
Traditional or enhanced breeding techniques can use many of the same or similar genes that are being used in GE, and these methods are likely to be as quick, or quicker, than GE in many cases. Traditional breeding may have advantages in combining several NUE genes at once.
Precision farming— the careful matching of nitrogen supply to crop needs over the course of the growing season—has shown the ability to increase NUE in experimental trials. Some of these practices are already improving NUE, but adoption of some of the more technologically sophisticated and precise methods has been slow.
Cover crops are planted to cover and protect the soil during those months when a cash crop such as corn is not growing, often as a component of an organic or similar farming system. Some can supply nitrogen to crops in lieu of synthetic fertilizers, and can remove excess nitrogen from the soil; in several studies, cover crops reduced nitrogen losses into groundwater by about 40 to 70 percent.
Cover crops and other “low-external-input” methods (i.e., those that limit use of synthetic fertilizers and pesticides) may also offer other benefits such as improving soil water retention (and drought tolerance) and increasing soil organic matter. An increase in organic matter that contains nitrogen can reduce the need for externally supplied nitrogen over time.
With the help of increased public investment, these methods should be developed and evaluated fully, using an ecosystem approach that is best suited to determine how reactive nitrogen is lost from the farm and how NUE can be improved in a comprehensive way. Crop breeding or GE alone is not sufficient because they do not fully address the nitrogen cycle on real farms, where nitrogen loss varies over time and space, such as those times when crops—conventional or GE—are not growing.
GE crops now being developed for NUE may eventually enter the marketplace, but such crops are not uniquely beneficial or easy to produce. There is already sufficient genetic variety for NUE traits in crops, and probably in close relatives of important crops, for traditional breeding to build on its successful track record and develop more efficient varieties.
Other methods such as the use of cover crops and precision farming can also improve NUE and reduce nitrogen pollution substantially.
The challenge of optimizing nitrogen use in a hungry world is far too important to rely on any one approach or technology as a solution. We therefore recommend that research on improving crop NUE continue. For traditional breeding to succeed, public research support is essential and should be increased in proportion to this method's substantial potential.
We also recommend that system-based approaches to increasing NUE—cover crops, precision application of fertilizer, and organic or similar farming methods—should be vigorously pursued and supported. These approaches are complementary to crop improvement because each addresses a different aspect of nitrogen use. For example, while breeding for NUE reduces the amount of nitrogen needed by crops, precision farming reduces the amount of nitrogen applied. Cover crops remove excess nitrogen and may supply nitrogen to cash crops in a more manageable form.
Along with adequate public funding, incentives that lead farmers to adopt these practices are also needed. Although the private sector does explore traditional breeding along with its heavy investment in the development of GE crops, it is not likely to provide adequate support for the development of non-GE varieties, crops that can better use nitrogen from organic sources, or improved cover crops that remove excess nitrogen from soil. We must ensure that broad societal goals are addressed and important options are pursued nevertheless.
In short, there are considerable opportunities to address the problems caused by our current overuse of synthetic nitrogen in agriculture if we are willing to make the necessary investments. The global impact of excess reactive nitrogen will worsen as our need to produce more food increases, so strong actions—including significant investments in technologies and methods now largely ignored by industrial agriculture—will be required to lessen the impact.
Gurian-Sherman, Doug., and Noel Gurwick “No Sure Fix: Prospects for Reducing Nitrogen Fertilizer Pollution through Genetic Engineering.” Union of Concerned Scientists (December 2009): 1–4.