Farmers may add fertilizers containing nitrogen to their crops, to increase crop growth. 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. But we need to know how much nitrogen is necessary for plant growth, because too much can pollute waterways, hurting aquatic life. 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.
DNA carries the genetic information, which means the instructions for how to make up a life form. When plants do not get enough nitrogen, they are unable to produce amino acids substances that contain nitrogen and hydrogen and make up many of living cells, muscles and tissue. Without amino acids, plants cannot make the special proteins that the plant cells need to grow.
Without enough nitrogen, plant growth is affected negatively. 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 [ 3 ]. Excess nitrogen can also leach—or drain—from the soil into underground water sources, or it can enter aquatic systems as above ground runoff.
This excess nitrogen can build up, leading to a process called eutrophication. Eutrophication happens when too much nitrogen enriches the water, causing excessive growth of plants and algae. When the phytoplankton dies, microbes in the water decompose them. Organisms in the dead zone die from lack of oxygen. 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 [ 4 ].
Can eutrophication be prevented? People who manage water resources can use different strategies to reduce the harmful effects of algal blooms and eutrophication of water surfaces. 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 [ 5 ].
But, it can often be hard to find the origin of the excess nitrogen and other nutrients. Once a lake has undergone eutrophication, it is even harder to do damage control. 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. For example, water managers can introduce organisms that eat phytoplankton, and these organisms can help reduce the amounts of phytoplankton, by eating them!
The nitrogen cycle is a repeating cycle of processes during which nitrogen moves through both living and non-living things: the atmosphere, soil, water, plants, animals and bacteria. In order to move through the different parts of the cycle, nitrogen must change forms. In the atmosphere, nitrogen exists as a gas N 2 , but in the soils it exists as nitrogen oxide, NO, and nitrogen dioxide, NO 2 , and when used as a fertilizer, can be found in other forms, such as ammonia, NH 3 , which can be processed even further into a different fertilizer, ammonium nitrate, or NH 4 NO 3.
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. In this image, microbes in the soil turn nitrogen gas N 2 into what is called volatile ammonia NH 3 , so the fixation process is called volatilization. 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.
In this stage, nitrogen moves from the atmosphere into the soil. To be used by plants, the N 2 must be transformed through a process called nitrogen fixation. Fixation converts nitrogen in the atmosphere into forms that plants can absorb through their root systems. A small amount of nitrogen can be fixed when lightning provides the energy needed for N 2 to react with oxygen, producing nitrogen oxide, NO, and nitrogen dioxide, NO 2. These forms of nitrogen then enter soils through rain or snow.
Nitrogen can also be fixed through the industrial process that creates fertilizer. Organic nitrogen becomes available when soil organic matter is decomposed by soil organisms. Most of the nitrogen in commercially available fertilizer is derived by combining atmospheric N 2 with H 2 to form ammonia NH 3 , which can be used as fertilizer anhydrous ammonia or further processed into other dry or liquid nitrogen fertilizers such as urea, ammonium sulfate or polymer coated nitrogen fertilizers such as ESN.
The most common nitrogen fertilizer loss is by the removal of crop portions containing nitrogen during harvest, but losses can also occur by leaching , denitrification , volatilization , or as a result of runoff and erosion. By understanding these forms of nitrogen loss, growers can utilize farming practices, like 4R nutrient stewardship , to minimize the loss of nutrients and ROI. Leaching: The nitrate form of nitrogen NO 3 is very soluble and leaches easily when excess water moves through the soil.
Denitrification : When finer-textured soil becomes saturated, some organisms look for oxygen by decomposing NO 3 — a process called denitrification.
The NO 3 is converted to gases that are unavailable to plants, escape from the soil, and can cause significant losses of nitrogen when soil is warm and remains saturated for even short periods.
Volatilization: This happens when ammonia gas NH 3 is lost to the atmosphere from anhydrous ammonia, urea, or liquid nitrogen fertilizer sources. Erosion refers to the erosion of soil particles that are carried away by runoff from rain or irrigation, by wind, and by ice.
This condition is called leaching. When nutrients leach into the soil, they are not available for plants to use. You might not need much fertilizer in your garden. You may just need to liberate the nutrients already present in your soil with beneficial soil organisms, proper soil aeration, soil drainage, and re-mineralization. Without proper soil aeration, mineral nutrients, and other factors, your plants may not be able to absorb phosphorous and potassium anyway, so loading up your soil with high levels of phosphorous and potassium may not make much difference with the health of your plants.
Nitrogen is typically available to the soil without additional fertilizers. The trick is having healthy soil full of beneficial microorganisms that can make use of the nitrogen that is available in the air.
Earthworm castings and properly prepared compost are teeming with these beneficial organisms. Other excellent natural sources of nutrients are fish meal, kelp, alfalfa meal, and bat guano. Remember, if you have too little nitrogen leaves become yellow-green, the oldest showing yellowing first.
Since the plant can move nitrogen, when it is low it takes it from older growth and gives it to newer growth. Growth is reduced, there will be less and smaller fruits.
If the nitrogen is too high then fruits take longer to ripen. Fruit will be soft and have short storage life. Too much nitrogen also hurts root growth and water efficiency of plants. It also will make your plants less tolerant to cold spells. Testing your garden soil is the best way to evaluate the fertility status of your field, garden or high tunnel before planting.
A soil test is useful in the diagnosis of plant culture problems, and it is also an important tool in improving the soil nutritional balance.
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