Why Are Nitrogen Fertilizers Not Added To Legumes
- November 2, 2021
(PhysOrg.com) -- Adding legumes to a crop rotation has many benefits, including reducing the need for external nitrogen input.However, planting legumes rather than fallow can have several benefits.Therefore, they do not need nitrogen fertilization, and can even add nitrogen to the soil.These benefits generally add up to higher yields and protein in wheat planted after an annual legume rather than a cereal.An annual legume crop can also be harvested as grain for an immediate economic benefit.Properly managed legumes in rotation can increase crop income by providing a legume forage or grain crop, or improving wheat yields after a legume green manure.Legumes improve soil health, especially compared to fallow, by adding nitrogen and organic matter and reducing potential erosion and leaching loss.Legumes may reduce the energy footprint of cropping systems by reducing the need for nitrogen fertilizer, and improve the stability and health of agro-ecosystems. .
Nitrogen - Nutrient Management
Nitrogen is so vital because it is a major component of chlorophyll, the compound by which plants use sunlight energy to produce sugars from water and carbon dioxide (i.e., photosynthesis).Finally, nitrogen is a significant component of nucleic acids such as DNA, the genetic material that allows cells (and eventually whole plants) to grow and reproduce.On soils containing large quantities of NH₄⁺-rich clays (either naturally occurring or developed by fixation of NH₄⁺ added as fertilizer), however, nitrogen supplied by the mineral fraction may be significant in some years.The quantity of nitrogen added to the soil in this manner is directly related to thunderstorm activity, but most areas probably receive no more than 20 lb nitrogen/acre per year from this source.Bacteria such as Rhizobia that infect (nodulate) the roots of, and receive much food energy from, legume plants can fix much more nitrogen per year (some well over 100 lb nitrogen/acre).When the quantity of nitrogen fixed by Rhizobia exceeds that needed by the microbes themselves, it is released for use by the host legume plant.These transformations are often grouped into a system called the nitrogen cycle, which can be presented in varying degrees of complexity.When these organisms die and are decomposed by others, excess NH₄⁺ can be released back to the inorganic pool in a process called mineralization.Immobilization and mineralization are conducted by most microorganisms, and are most rapid when soils are warm and moist, but not saturated with water.Significant loss mechanisms include leaching, denitrification, volatilization and crop removal.The nitrate form of nitrogen is so soluble that it leaches easily when excess water percolates through the soil.In this process, called denitrification, NO₃⁻ is converted to gaseous oxides of nitrogen or to N₂ gas, both unavailable to plants.Denitrification can cause major losses of nitrogen when soils are warm and remain saturated for more than a few days.The nitrogen in crop residues is recycled back into the system, and is better thought of as immobilized rather than removed.Once inside the plant, NO₃⁻ is reduced to an NH₂ form and is assimilated to produce more complex compounds.Because plants require very large quantities of nitrogen, an extensive root system is essential to allowing unrestricted uptake.A plant supplied with adequate nitrogen grows rapidly and produces large amounts of succulent, green foliage.Providing adequate nitrogen allows an annual crop, such as corn, to grow to full maturity, rather than delaying it.A nitrogen-deficient plant is generally small and develops slowly because it lacks the nitrogen necessary to manufacture adequate structural and genetic materials.On the other hand, some plants may grow so rapidly when supplied with excessive nitrogen that they develop protoplasm faster than they can build sufficient supporting material in cell walls.Rates needed to achieve different yields with different crops vary by region, and such decisions are usually based on local recommendations and experience.In recent years, there has been some interest in testing cornfields for NO₃⁻ in the more humid regions of the eastern United States and Canada, utilizing samples taken in late spring, after crop emergence, rather than before planting.Placement decisions should maximize availability of nitrogen to crops and minimize potential losses.Injecting side-dressed UAN may produce higher corn yields than surface application in years when dry weather follows side-dressing.Applying small amounts of "starter" nitrogen as UAN in herbicide sprays, however, is usually of little concern.Fall applications for corn can be used on well-drained soils, particularly if the nitrogen is applied as anhydrous ammonia amended with N-Serve®; however, fall applications should be avoided on poorly drained soils, due to an almost unavoidable potential for significant denitrification losses.Nitrogen fertilizers containing NO₃⁻ but no NH₄⁺ make the soil slightly less acidic over time, but are generally used in much lesser quantities than the others.*A minus sign indicates the number of pounds of calcium carbonate equivalent needed to neutralize the acid formed when 1 ton of the material is added to the soil.This delayed nitrification protects the fertilizer from losses due to denitrification and leaching in seasons when excessive rainfall occurs during the period of inhibition.Like N-Serve®, it might be viewed as an insurance policy that will reduce potential nitrogen losses in seasons when cultivation or rain does not incorporate the urea into the soil soon after application.They can be used as part of the 4R Nutrient Stewardship strategy to keep nitrogen in its proper place at the time the plant needs it. .
Nitrogen is the only nutrient that has no common soil mineral source.Nitrogen mineralization is the process where organic nitrogen is converted to ammonium ( NH 4 + ), which can then be nitrified to nitrate ( NO 3 − ).Plants can take up both these forms of N, but some prefer one source to the other.To maintain an adequate supply of N to plants, inorganic N in soil needs to be replaced either by mineralization of the soil organic pool or by the addition of mineral N from external sources, i.e. fertilizers or atmospheric inputs (Smethurst 2007).In the forms useful to plants, nitrogen is probably the nutrient most universally limiting to plant growth.Typical estimates of rates of symbiotic fixation in eucalyptus forests are in the region of 10 kg ha− 1 yr− 1 (Baker and Attiwill 1981).Symbiotic relationships occur in trees of the legume family, in which specialized nodules on the roots provide favourable environments for N-fixing bacteria: the bacteria have access to carbohydrates and the trees are provided with reduced nitrogen for use in the synthesis of amino acids and proteins.Anyone concerned with the establishment of trees, particularly in areas where the species they are interested in has not been grown before, should try to obtain any information available about microorganism associations with that species.Nutrient uptake by trees and its transfer to their component parts (stems, branches, foliage, …) are important facets of the biogeochemical cycle, but because uptake is the fundamental process underlying tree nutrition we defer consideration of uptake until after the following brief discussion on nutrient losses. .
What Is the Nitrogen Cycle and Why Is It Key to Life? · Frontiers for
I was born on Earth Day, so naturally I love being outside and look for any excuse to spend time with animals of all shapes and sizes!Farmers can add nitrogen fertilizer to produce better crops, but too much can hurt plants and animals, and pollute our aquatic systems.Without nitrogen fertilizers, scientists estimate that we would lose up to one third of the crops we rely on for food and other types of agriculture.Nitrogen is a key element in the nucleic acids DNA and RNA, which are the most important of all biological molecules and crucial for all living things.With too much nitrogen, plants produce excess biomass, or organic matter, such as stalks and leaves, but not enough root structure.In extreme cases, plants with very high levels of nitrogen absorbed from soils can poison farm animals that eat them .Excess nitrogen can also leach—or drain—from the soil into underground water sources, or it can enter aquatic systems as above ground runoff.Eutrophication happens when too much nitrogen enriches the water, causing excessive growth of plants and algae.Too much nitrogen can even cause a lake to turn bright green or other colors, with a “bloom” of smelly algae called phytoplankton (see Figure 1)!These dead zones can happen in freshwater lakes and also in coastal environments where rivers full of nutrients from agricultural runoff (fertilizer overflow) flow into oceans .Figure 2 shows the stages of Eutrophication (open access Wikimedia Commons image from https://commons.m.wikimedia.org/wiki/File:Eutrophicationmodel.svg).(8) The decomposition process causes the water to have reduced oxygen, leading to “dead zones.” Bigger life forms like fish cannot breathe and die.They can re-reroute excess nutrients away from lakes and vulnerable costal zones, use herbicides (chemicals used to kill unwanted plant growth) or algaecides (chemicals used to kill algae) to stop the algal blooms, and reduce the quantities or combinations of nutrients used in agricultural fertilizers, among other techniques .Algaecides can be expensive, and they also do not correct the source of the problem: the excess nitrogen or other nutrients that caused the algae bloom in the first place!Another potential solution is called bioremediation, which is the process of purposefully changing the food web in an aquatic ecosystem to reduce or control the amount of phytoplankton.There are five stages in the nitrogen cycle, and we will now discuss each of them in turn: fixation or volatilization, mineralization, nitrification, immobilization, and denitrification.Leaching is where certain forms of nitrogen (such as nitrate, or NO 3 ) becomes dissolved in water and leaks out of the soil, potentially polluting waterways.During nitrification the ammonia in the soils, produced during mineralization, is converted into compounds called nitrites, NO 2 −, and nitrates, NO 3 −.Knowledge of the nitrogen cycle can also help us reduce pollution caused by adding too much fertilizer to soils.For example, a study done by Haycock and Pinay  showed that poplar trees (Populus italica) used as a buffer held on to 99% of the nitrate entering the underground water flow during winter, while a riverbank zone covered with a specific grass (Lolium perenne L.) held up to 84% of the nitrate, preventing it from entering the river.Pollution of our water sources by surplus nitrogen and other nutrients is a huge problem, as marine life is being suffocated from decomposition of dead algae blooms.Farmers and communities need to work to improve the uptake of added nutrients by crops and treat animal manure waste properly.We also need to protect the natural plant buffer zones that can take up nitrogen runoff before it reaches water bodies.But, our current patterns of clearing trees to build roads and other construction worsen this problem, because there are fewer plants left to uptake excess nutrients.We need to do further research to determine which plant species are best to grow in coastal areas to take up excess nitrogen.We also need to find other ways to fix or avoid the problem of excess nitrogen spilling over into aquatic ecosystems.DNA: ↑ Deoxyribonucleic acid, a self-replicating material which is present in nearly all living organisms as the main component of chromosomes, and carrier of genetic information.Eutrophication: ↑ Excessive amount of nutrients (such as nitrogen) in a lake or other body of water, which causes a dense growth of aquatic plant life, such as algae.Phytoplankton: ↑ Tiny, microscopic marine algae (also known as microalgae) that require sunlight in order to grow.Leaching: ↑ When a mineral or chemical (such as nitrate, or NO 3 ) drains away from soil or other ground material and leaks into surrounding area.The author declares that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. .
Biological Nitrogen Fixation
Plants are (left to right), uninoculated with Bradyrhizobium, inoculated with Bradyrhibium, non-nodulating mutant peanut inoculated with Bradyrhizobium, and non-nodulating mutant peanut uninoculated with Bradyrhizobium.Theorbacteria colonize the host plant’s root system and cause the roots to form nodules to house the bacteria (Figure 4).The resulting plants are typically chlorotic, low in nitrogen content, and yield very little seed (Figure 5 and 6).The nodulation process illustrates an orchestrated interaction between the bacteria and host plant (Napoli & Hubbell 1975, Kamst.The process begins when the rhizobia are attracted to flavonoids released by the host legume’s roots.For legumes like alfalfa, clover, and soybeans (others like lupines and peanuts form nodules in other ways) the bacteria then begin to attach themselves to extensions of root epidermal cells called root hairs.This host specificity is referred to cross inoculation group cell signaling between the bacteria and the legume host. .
The chemical era really began when nitrogen (Chapter 4) and phosphorus (Chapter 5) fertilizers became readily available and increased yields of newly available hybrid corn.Fertilizer Use in the United States Decade Tons of Fertilizer Used/Year × 1000 Corn Yield bushel/acre 1900–1909 3738 27.3 1910–1919 6117 25.9 1920–1929 6846 26.5 1930–1939 6600 24.2 1940–1949 13,590 31.2 1950–1959 22,341 44.1 1960–1969 32,374 70.4 1970–1979 43,644 86.6 1980–1989 47,411 105.9 1990–1999 21,486 123.3 2000–2009 21,405 132.4 2010 20,843 152.8 2013 21,753 158.8 Source: http://quickstats.nass.usda.gov/results/18B3211B-14D5-330B-B775-FEF0058719C6.When nitrogen fertilizer was combined with the new hybrid corn varieties, first experimented with by Henry A. Wallace5 in 1913, yield and fertilizer use went up rapidly (Table 2.2).He began large-scale production of hybrid seed in 1931.Iowa’s corn yield has been increasing on an average of 2 bushel/acre/year/and many view 300 bushel/acre as possible.7 After 1945, when pesticides were developed and became widely available, yields continued to increase.The chemical era of agriculture developed rapidly after 1945.Until the 1950s, arsenic-based insecticides and arsenical herbicides as manufactured by the C.B.Later, salt was used as an herbicide in England.In 1855, sulfuric acid was used in Germany for selective weed control in cereals and onions.Farmers, eager for a way to control the potato beetle, diluted it with flour and dusted it on their potatoes or mixed it in water and sprayed it.If arsenic killed potato beetles, what else would it kill?If Paris green worked for potato beetles, would other chemicals work for other agricultural problems?Thus, spraying potatoes with Paris green and copper sulfate diminished, but did not solve, the beetle or the blight problem.Nearly concurrently, in the United States, Bolley (1908) studied iron sulfate, copper sulfate, copper nitrate, and sodium arsenite for selective control of broadleaved weeds in cereal grains.The corrosive sublimate treatment for potato scab developed by Bolley became known around the world.He is acknowledged as the first in the United States to report on selective use of salts of heavy metals as herbicides to eradicate weeds in cereal grains.He began the first US studies with iron sulfate, copper sulfate, copper nitrate, sodium arsenite, and salt for selective control of broadleaved weeds in cereal grains.Experiments initiated in 1896 were so successful that many states and European countries quickly began spraying cereal crops with inorganic chemicals to increase production.In North Dakota Agricultural Experiment Station Bulletin 80, published in 1908, Bolley described his view of the future of selective chemical control of weeds.He was a pioneer who recognized the potential benefits of selective chemical weed control in the latter part of the nineteenth century, before farmers were ready to adopt it.Concurrent work in Europe observed the selective herbicidal effects of metallic salt solutions or acids in cereal crops (Zimdahl, 1995). .