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Nutrient Cycles

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Biology

There is limited availability of usable ions in the environment. Therefore, important elements such as carbon, oxygen and phosphorus need to be recycled. This happens in a nutrient cycle.

A nutrient cycle can be defined as the movement and exchange of organic and inorganic material back into the production of living matter. Inorganic material will be used by the producers and converted into organic matter that can be ingested by the primary consumers.

What are the main types of nutrient cycles?

The main nutrient cycles include water, carbon, nitrogen, oxygen, and phosphorus cycles. Primary producers, such as plants in the ecosystem, take up inorganic nutrients from the non-living environment. These inorganic nutrients can be transformed into biomass. The nutrient cycles work in balance with each other. They are responsible for replenishing the ecosystem with nutrients and getting rid of waste.

Biomass is defined as organic matter. Energy travels between organisms through biomass.

The water cycle

The water cycle can also be referred to as the hydrological cycle . The hydrological cycle is important in determining the weather and climate. Water is needed by producers (such as plants) to grow and therefore, it is critical for all life.

Water moves via:

  • Evaporation - most water evaporates from the oceans. Some evaporation will also occur from lakes, rivers, and streams.
  • Transpiration - water loss from the plant stomata.
  • Condensation - water vapor is converted back into liquid. Condensation will form clouds.
  • Precipitation - water is returned to the Earth by rain, snow and ice. Most of the runoff will return to lakes and rivers, but some will sink into the ground by the infiltration process.
  • Melting and freezing - water can be locked up in ice by freezing, in bodies such as glaciers. Water can be returned by melting these glaciers.

Stomata are small pores found on the surface of plant leaves and stems by which substances, such as water and oxygen, can flow through.

The infiltration process describes the flow of water into soil and rock.

The oxygen cycle

Oxygen plays an important role in respiration in all living organisms and is recycled between living organisms and the air. Breathing and respiration are processes that result in the removal of oxygen from the environment. Meanwhile, photosynthesis, which is complementary to respiration, results in the addition of oxygen.

The carbon cycle

This is a series of processes by which carbon compounds are interchanged in the environment. This involves the incorporation of carbon dioxide into living tissue by photosynthesis, its return to the atmosphere through respiration, the decay of dead organisms, and the burning of fossil fuels.

Carbon moves via a series of steps:

  • Combustion - the burning of fossil fuels releases carbon back into the atmosphere as carbon dioxide.
  • Photosynthesis - carbon dioxide is taken up by plants and converted into organic molecules, such as glucose.
  • Cellular respiration - organic molecules, such as glucose, are broken down during cellular respiration to release energy, water and carbon dioxide.
  • Precipitation - carbon dioxide can precipitate from the atmosphere as carbonate in the ocean sediment.
  • Decomposition - carbon can be released back into the atmosphere as carbon dioxide when an organism dies and enters the decay process.

Nutrient Cycles [+] the carbon cycle [+] StudySmarter

Figure 2. The carbon cycle. Source: NASA, Public domain, via Wikimedia Commons.

The nitrogen cycle

Nitrogen is the most abundant gas in the atmosphere. It is used in the production of amino acids, proteins and nucleic acids (DNA and RNA). Although abundant, nitrogen is tricky, in that the gaseous form present in the air is not available to all organisms. Hence, it has to be converted to more readily available forms. Only a few single-celled organisms, such as bacteria, are able to take up nitrogen in its gaseous form.

Plants can take up nitrogen in other forms such as nitrates and ammonia. These substances are produced during nitrogen fixation which describes the conversion of atmospheric nitrogen into ammonium ions, carried out by nitrogen-fixing bacteria. You can learn more about this process in our Nitrogen Cycle article.

Nutrient Cycles [+] the nitrogen cycle [+] StudySmarterFigure 3. The nitrogen cycle. Source: Wikimedia Commons.

The phosphorus cycle

Phosphorus is an important component of ATP, phospholipids, nucleic acids and other substances. Unlike the other nutrients we have talked about earlier in this article, phosphorous lacks the gaseous phase in the atmosphere. Phosphorus reservoirs mainly lay in mineral form - as a phosphate ions () in sedimentary rock deposits.

The main steps in the phosphorus cycle include:

  • Weathering - rocks erode, due to rain and other weather events. Phosphorus is washed into the soil.
  • Absorption by plants and animals - microorganisms are able to take up phosphorous from the soil. Plants can also absorb phosphorous directly from the soil and when they are consumed, animals also gain the nutrient. Animals can also gain phosphorous from drinking water.
  • Decomposition - decay results in the release of phosphorus back into the environment, and the cycle of being absorbed by plants and animals can be repeated.

Microorganisms in nutrient cycles

Microorganisms, such as fungi and bacteria, can form mutual symbiotic associations with plants.

You might have noticed that sometimes viruses are included in microorganisms. However, viruses should be referred to as microbes rather than microorganisms, because they are non-living!

Microorganisms and plant symbiosis

Mutual symbiotic relationships refer to an instance where two organisms live in close physical proximity and they both benefit from the association. The relationship between nitrogen-fixing bacteria and plants is one example.

In mutualistic relationships between plants and microorganisms, microorganisms receive organic compounds, such as sugars and amino acids, from the plants. This helps them perform metabolic processes, such as photosynthesis and protein synthesis. In return, microorganisms increase the total surface area of the plant for water and mineral absorption.

Mutualism between plant roots and fungi is a mycorrhizae association.

If plants lost their associations, they would become more susceptible to droughts and fewer nutrients would be available for the plants to take up. Less susectibility to draughts comes from the fungal ability to temporarily absorb water and store it for the future that plant can utilize when needed.

Examples of plant and microorganism associations

  • Legumes and Rhizobium - Rhizobium colonize the legume plant's root cells, where they fix nitrogen. In return, they receive sugars.
  • Endophytic fungi and plants - endophytic fungi colonize plant tissue without damaging it and release toxins to repel herbivores. They can also increase resistance to infections from other microorganisms.
  • Azotobacter - Aztobacter are a group of free-living nitrogen-fixing bacteria. They do not directly form a relationship with the plant. However, aztobacter will increase the soil fertility by fixing nitrogen.

Human impact on the nutrient cycles

What impact do we have on nutrient cycles? We will cover a few examples below.

Agriculture

When crops are harvested, nutrients are removed from the soil. Human population growth and increased agriculture have caused significant changes to the nutrient cycles.

Agriculture can also accelerate land erosion, leading to more nutrients draining off. Nutrient runoff will enter rivers and oceans and will lead to eutrophication, which describes increased algae growth due to nutrient excess. Uncontrolled algae growth can form dead zones, ie, a lack of oxygen for other organisms.

The addition of fertilisers will increase levels of macronutrients and cause imbalances. This can cause a decrease in biodiversity and can have other impacts.

deforestation

Similar to agriculture, felling trees removes stored nutrients vital for plant growth. Trees are major carbon dioxide consumers. Less carbon dioxide will be taken up from the environment and will lead to an increase in carbon dioxide concentration in the atmosphere. The build-up of carbon dioxide could lead to global warming, as it is the main greenhouse gas.

You have most definitely heard about global warming mentioned before. This is for a good reason! Most of what you may have heard is about raising awareness on this issue b ecause we are the main cause of it. By burning fossil fuels, there has been an increase in greenhosue gasses, ie gasses that trap heat in our atmosphere. This includes carbon dioxide and others such as nitrous oxide.

The heat trapped in the atmosphere raises our Earth's temperature which affects not only us but species diversity. In the areas where the temperatures has risen more than a species can tolerate, consequences such as suitable habitat loss and even death follow. A good example is bleaching in the coral reefs due to temperature rises.

Burning of fossil fuels

This affects the amount of nitrogen in the atmosphere. Fossil fuel releases nitrogen oxides due to combustion and consequently, they can combine with other elements, such as sulfur oxides. This leads to acid rain and smog. Excess nutrients will also wash into nearby water bodies such as rivers, causing eutrophication.

Nutrient Cycles - Key takeaways

  • The nutrient cycle is the movement and exchange of organic and inorganic material back into the production of living matter. Energy and matter are transferred between living organisms and the non-living environment.
  • The main nutrient cycles include the water cycle, oxygen cycle, carbon cycle, nitrogen cycle and phosphorus cycle.
  • Plants and fungi form symbiotic associations, known as mycorrhizae . Fungi will absorb water and minerals, allowing plants to resist drought better and absorb nutrients more readily. Meanwhile, fungi receive organic compounds such as sugars and amino acids from the plants.
  • Human activity, such as the combustion of fossil fuels, deforestation and agriculture, alter nutrient cycles.

Nutrient Cycles

A nutrient cycle can be defined as the movement and exchange of organic and inorganic material back into the production of living matter.

Energy and matter are transferred between living organisms and a non-living environment. There is a movement and exchange of organic and inorganic material back into the production of living matter.

Removing trees from a forest subtracts stored nutrients vital for plant growth. In addition, fewer trees mean less carbon dioxide is removed from the atmosphere. This contributes to global warming as carbon dioxide is the main greenhouse gas.

Nutrients are recycled within the system. There is limited availability of usable ions in the environment. Therefore, elements such as carbon, oxygen, and phosphorus need to be recycled.

The main nutrient cycles include the water cycle, oxygen cycle, carbon cycle, nitrogen cycle, and phosphorus cycle.

Final Nutrient Cycles Quiz

Question

What is the main difference between nitrogen fixation and nitrification in the nitrogen cycle?

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Answer

Nitrogen fixation is the process of converting atmospheric nitrogen into ammonium ions. Nitrification is the process of converting ammonium ions into nitrite or nitrate ions.

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Can you name two ways in which water is introduced into water bodies in the hydrological cycle?

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Answer

Precipitation and ice-melt.

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Saprobiontic microorganisms will take in inorganic ions and produce organic matter. True or false?


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Answer

False.

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Only a few organisms can access nitrogen in its gaseous form. True or false?

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Answer

True.

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Fill in the blanks about the phosphorus cycle. The main source of phosphorus is _________, the process in which breaking down of rocks occurs. Phosphorus is __________ by organisms including plants and animals. Phosphorus is released back into the atmosphere by ____________. The cycle can then be repeated.

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Answer

The main source of phosphorus is weathering, the process in which breaking down of rocks occurs. Phosphorus is absorbed by organisms including plants and animals. Phosphorus is released back into the atmosphere by decomposition. The cycle can then be repeated.

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What is the main form of phosphorus in sedimentary rock deposits?

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Answer

Phosphate ions (PO4^3-).

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What would happen to plants if they did not form mycorrhizae associations with fungi?


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Plants would become more susceptible to droughts, and less nutrients would be available for the plants to take up.

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What are three ways in which agriculture affects the environment?


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Answer

  • Nutrients are removed from the soil during harvesting.
  • Agriculture can accelerate land erosion and increase nutrient run-off, leading to eutrophication in bodies of water.
  • Addition of fertilisers will increase the number of macronutrients and cause imbalance within the nutrient cycles.


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Question

How can nitrates form during lightning?


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When lightning strikes, the energy separates gaseous nitrogen and releases free nitrogen atoms. These have an opportunity to bind with oxygen molecules and form nitrates.

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What are the two main processes of the oxygen cycle?


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Breathing and respiration, and photosynthesis. They are complementary to each other.

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During which cycle does evaporation occur?


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Answer

Water cycle.

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What is combustion in the carbon cycle?


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Burning of fossil fuels, which releases carbon into the atmosphere as carbon dioxide.

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What is nitrogen used for when taken up by the primary producer?


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Production of proteins, amino acids, and nucleic acids.

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What are the two main groups of microorganisms (highest classification)?

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Answer

Prokaryotes and eukaryotes.

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What is Rhizobium and why is it important to legume plants?

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Answer

Rhizobium, a nitrogen-fixing bacteria, colonise the legume plant’s root cells, where they fix nitrogen. They will receive sugars in return.

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What is meant by the nitrogen cycle and why is it important?

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The nitrogen cycle is a biogeochemical cycle during which organic and inorganic nitrogen in different chemical forms will circle through the atmosphere, terrestrial and marine environments. Nitrogen cycle is important because nitrogen is an important building block in all living organisms. Nitrogen is used to make amino acids, proteins and DNA.

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What is the difference between nitrification and denitrification?

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Nitrification refers to the oxidation of nitrogen compounds. Denitrification is  the reduction of oxidised nitrogen compounds.

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What group of the living organisms can access nitrogen in its gaseous form (N2)?


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Nitrogen gas is only available to nitrogen-fixing microorganisms.

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Name a primary producer in the marine nitrogen cycle.


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Answer

Phytoplankton.

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Question

Fill in the blanks about splitting of the nitrogen molecule during lightning. Nitrogen atoms in the atmospheric nitrogen molecule are held by ________ bonds. This means that the atoms share three pairs of _______. These bonds can be _____ by lightning which carries enough energy to split the atoms. When the nitrogen molecule splits, it bonds to the _______ atoms, forming nitrogen oxides (NOx).

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Answer

Nitrogen atoms in the atmospheric nitrogen molecule are held by covalent bonds. This means that the atoms share three pairs of electrons. These bonds can be broken by lightning which carries enough energy to split the atoms. When the nitrogen molecule splits, it bonds to the oxygen atoms, forming nitrogen oxides (NOx).

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What happens during the Haber-Bosch process?


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The Haber-Bosch process is an artificial way to produce ammonia. During the process, nitrogen gas is combined with hydrogen gas at a high temperature and a high-pressure environment.

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Mutualistic nitrogen-fixing bacteria will form a symbiotic relationship with the plant roots to exchange material. True or False?


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Answer

True.

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 Assimilation can be defined as the process where plants and animals use nitrate ions and ammonia formed by denitrification to make amino acids. True or False?


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False.

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Fill in the blanks about the denitrification step in the nitrogen cycle. Denitrification refers to the removal of _______ from the nitrate ions to ________ gas by anaerobic bacteria. Denitrification happens in _______ conditions. Environments such as wetlands are ________, they are important in reducing excess _______ levels.

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Denitrification refers to the removal of oxygen from the nitrate ions to nitrogen gas by anaerobic bacteria. Denitrification happens in anoxic/anaerobic conditions. Environments such as wetlands are waterlogged, they are important in reducing excess nitrogen levels.

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Oxygen is an electron acceptor in aerobic respiration. What is the electron acceptor in anaerobic respiration of specialised microbes? What is this process called?


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Microbes respiring anaerobically will use organic nitrate as electron acceptors. This is the DNRA (nitrate/nitrite ammonification) process.

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Nitrobacter bacteria facilitates the oxidation of the ammonium ions. True or False?


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False.

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Name three human impacts on the nitrogen cycle.


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Answer

Leaching, eutrophication and reduced species diversity.

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Why is nitrogen monitoring important?


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In agriculture, nutrient levels need to be monitored to maximise soil productivity. It is also important to monitor excess ammonia and nitrites in the soil, as these in excess amounts are toxic. Monitoring can also prevent nutrient runoff into the water bodies. This will prevent eutrophication.

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How can eating less meat contribute to a reduced nitrogen footprint?


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Answer

A large amount of crop feed is used to feed cattle. The crop feed will require large amounts of fertiliser to grow which could lead to soil imbalances and have other negative effects on the environment. 

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Fill in the blanks about the nitrogen cycle disruption. Disruption of the nitrogen cycle leads to the _______ within the ecosystem. Soil containing excess nitrogen will have a _______ pH compared to the soil that has a normal concentration of nitrogen. Too much nitrogen in the oceans will lead to _______. _______ is one of the main causes of nutrient ______ into the water bodies.


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Answer

Disruption of the nitrogen cycle leads to the imbalances within the ecosystem. Soil containing excess nitrogen will have a lower pH compared to the soil that has a normal concentration of nitrogen. Too much nitrogen in the oceans will lead to eutrophication. Leaching is one of the main causes of nutrient runoff into the water bodies.

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What are the two main groups of fertilisers?

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Answer

Natural and artificial.

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Why are artificial fertilisers important in agriculture?

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Plants use inorganic nutrients for growth. Fertilisers will add essential nutrients back into the soil. Artificial fertilisers will be quickly released as they only need to dissolve in water.

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What are the three groups of natural fertilisers?

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Plant-based, animal-based and mineral-based.

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Why would you want to add crushed egg shells to the soil?


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Answer

Egg shells contain calcium. Calcium is essential in the structural growth of the cells. This includes the growth of the cell wall and the membrane.

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Epsom salts contain calcium and sulfur. True or False?


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False.

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Why is manure often mixed with plant compost?


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Answer

Manure is extremely rich in nutrients. Adding only manure can cause nutrient burns on a plant from over-fertilisation.

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Fill in the blanks about the differences between the natural and artificial fertilisers. 


Natural fertilisers consist of _________ ________ from living organisms, while artificial fertilisers are a __________ industry by-products. Natural fertilisers contain a ________ concentration of nutrients than synthetic fertilisers. Natural fertilisers are ________ release, while synthetic compounds need only to ________ in water to be accessed by plants. Artificial fertilisers are likely to cause leaching, nutrient _______ and nutrient _______. Organic fertiliser also improves the soil _________.

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Natural fertilisers consist of organic matter from living organisms, while artificial fertilisers are a petroleum industry by-product. Natural fertilisers contain a lower concentration of nutrients than synthetic fertilisers. Natural fertilisers are slow-release, while synthetic compounds need only to dissolve in water to be accessed by plants. Artificial fertilisers are likely to cause leaching, nutrient runoff and nutrient burns. Organic fertiliser also improves the soil structure.

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What is soil conditioning and why is plant-based fertiliser important for soil conditioning?

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Answer

Soil conditioning is the improvement of soil’s physical qualities. Plant-based fertilisers will add natural humus to the soil which helps to improve soil’s porosity and improves moisture retention.

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Why can adding urine as a fertiliser to the soil have negative effects?


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Answer

Urine contains urea which contains a high concentration of nitrogen. Too much nitrogen can cause nitrogen burns on the plant leaves.

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Define fertilisation in the context of agriculture.


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Fertilisation refers to the action of applying fertilisers to the soil or land.

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What are the three different processes of fertilisation in the soil?


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Natural fertilisation, synthetic fertilisers, soil’s self-renewal.

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Fill in the blanks about humus. 


Humus is a dark ________ matter that forms in the soil when the plant and animal _______. It is broken down and _________. Humus will be broken down by _________. Humus is important in soil ________ maintenance and prevents nutrient _________. Humus can act as a _________ to retain nutrients that can be accessed by the ________.

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Answer

Humus is a dark organic matter that forms in the soil when the plant and animal decomposes. It is broken down and stable. Humus will be broken down by microorganisms. Humus is important in soil porosity maintenance and prevents nutrient runoff. Humus can act as a sponge to retain nutrients that can be accessed by the plants.

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The presence of the microorganisms is required when the chemical minerals are used to improve the plant growth. True or False?

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Answer

False.

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Eutrophication is a process during which the nutrient levels in the water decrease. True or False?

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Answer

False.

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What is an algal bloom?


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Algal bloom is a rapid growth of algae, resulting in an algae layer forming on the upper layers of the surface.

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Oxygen will be widely available in the water during the algal bloom. True or False.


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False.

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