Nutrient cycling
This section contains 10 questions:
- What are the major processes in the nitrogen cycle?
- What are the organisms involved in nitrification and what are the soil conditions where it occurs?
- What organisms are involved in denitrification and what are the soil conditions where it occurs?
- What is biological soil nitrogen supply?
- How can measures of soil nitrogen supply be incorporated into split applications and variable rate applications of nitrogen?
- How does nitrification lead to nitrogen leaching?
- How can nitrate leaching be managed?
- What forms of phosphorus are there in soil and and what are they composed of.
- Why is organic phosphorus important?
Question 1. What main nutrients to plants require and where do crop plants obtain them?
The main plant nutrients are shown below in decreasing order of how much of each plants require. The sources of these nutrients in agriculture are also shown.
- Carbon, hydrogen and oxygen - water
- Nitrogen, potassium and phosphorus - inorganic fertiliser and organic matter
- Calcium, Magnesium, sulphur and chlorine - gypsum, limestone, dolomite
Nitrogen
Question 2. What are the major processes in the nitrogen cycle?
The nitrogen cycle is a way to describe how the different forms of nitrogen in the air, soil, water and living organisms are interconnected. It is described as a cycle because the nitrogen is never lost completely, it just changes form and is held in different places. There are 5 major processes involved:
1. Ammonification
Ammonification is the conversion of organic forms of nitrogen (for example, nitrogen in proteins in dead plants and soil animals) to ammonium. Ammonium is an inorganic form of nitrogen that has the symbol NH4+ . The ammonification process is carried out by a wide range of soil organisms. Many different types of bacteria and fungi are involved.
2. Nitrification
Nitrification is the conversion of ammonium to nitrate (another inorganic form of nitrogen) by a specific group of bacteria. Nitrate is shown by the symbol NO3-.
3. Denitrification
Denitrification is the conversion of different forms of nitrogen in soil to forms of gaseous nitrogen. One example is nitrous oxide and another is nitrogen gas, which is the form of nitrogen most common in the atmosphere.
4. Nitrogen fixation
Nitrogen fixation is the conversion of nitrogen gas (N2) to ammonium - either by free living bacteria in soil or water, or by bacteria in symbiotic association with plants (eg legume symbiosis).
5. Nitrogen immobilization
This is the process whereby nitrogen is taken up by soil organisms and retained in the 'microbial pool' of nitrogen.
Question 3. What are the organisms involved in nitrification and what are the soil conditions where it occurs?
Nitrification is the process where ammonium is converted to nitrite and then to nitrate. Only a few specific groups of microorganisms are capable of transforming ammonium ions to nitrate and they are autotrophic. This means that unlike many other soil microrganisms, nitrifying bacteria are not dependent on organic matter to meet their need for carbon. They obtain their carbon from carbon dioxide, as plants do.
The most studied of all soil nitrate makers are the nitrifying bacteria. In the soil nitrate making process, nitrifying bacteria work as a team. Some nitrifying bacteria oxidise ammonium to nitrite and are called ammonium oxidisers. Nitrite is then converted to nitrate by another group of nitrifying bacteria known as nitrite oxidisers. The most commonly known genus of ammonium oxidizing bacteria is Nitrosomonas and Nitrobacter is a common genus of nitrite oxidising bacteria.
Question 4. What organisms are involved in denitrification and what are the soil conditions where it occurs?
Denitification is the process where oxides of nitrogen (nitrate and nitrite) are converted into gaseous nitrogen and are removed from the soil system. Many species are capable of denitrification of soil and it occurs mostly when there is little or no oxygen in the soil such as when the soil is waterlogged.
Question 5. What is biological soil nitrogen supply?
Microorganisms decompose crop residues and organic matter in soil to release plant available nitrogen. This is called soil nitrogen supply. In our agricultural soils 40-80% of the crop nitrogen requirements are met through soil nitrogen supply. The remaining nitrogen requirement is met through fertiliser applications. To improve nitrogen fertiliser management it is important to know the timing and location of soil nitrogen supply, so that fertiliser nitrogen is only applied when and where it is necessary.
Question 6. How can measures of soil nitrogen supply be incorporated into split applications and variable rate applications of nitrogen?
Splitting applications of fertiliser nitrogen at strategic plant growth stages is becoming common. This maximises the opportunity for crop uptake at the right time and minimises the risk of nutrient leaching. Growers can also spatially adjust fertiliser rates in the field using information gained from yield mapping, through knowledge of best/worst performing areas of a paddock, and by soil type.
The next extension of this approach is to utilise spatial soil maps that tell us about the soils capacity for biological nitrogen supply together with information on the chemical and physical fertility of the soil. This allows soil constraints limiting crop production to be identified.
For example where biological soil nitrogen supply is high, less fertiliser nitrogen may be needed to achieve optimal yields. Alternatively where biological soil nitrogen supply is low greater reliance on fertiliser nitrogen is required for adequate crop growth. Variable rate applications of fertiliser nitrogen across a field could thus be achieved based on knowledge of biological soil nitrogen supply. This would have an economic and environmental (minimising leaching) benefit to growers.
Question 7. How does nitrification lead to nitrogen leaching?
Nitrification can lead to nitrogen leaching because nitrate is more mobile in soil than ammonium. Ammonium is positively charged and is attracted to clay particles, because clay particles have a negative charge. Nitrate however, is negatively charged and is not attracted to clay particles. Therefore nitrate can easily be leached away from the root zone and find its way into ground water and rivers. Nitrate leaching has been estimated to cost the Australian wheat industry millions of dollars annually in lost production.
Question 8. How can nitrate leaching be managed?
Organic matter in soil can decrease nitrate leaching. The degraded residues of plants and animals contain positively charged ions (humic compounds) that can attract nitrate. Many tropical soils contain high levels of organic matter compared to temperate soils. The higher organic matter level gives the soil an overall positive charge that can bind nitrate and prevent it from leaching.
Nitrate leaching can also be decreased by plant uptake. Plants are key to the uptake of nitrogen in soil and prevention of build up of nitrate. For example, a summer crop can be used in a mediterranean environment (providing there is sufficient moisture) to use the nitrogen released by mineralisation of organic matter. If there is no summer crop, the nitrogen is likely to be lost due to nitrate leaching.
Some commercial pesticides can inhibit soil nitrate production. They do this mainly by either stopping or slowing down the activity of ammonium oxidising bacteria.
Phosphorus
Question 9. What forms of phosphorus are there in soil and and what are they composed of?
Phosphorus in soil can be divided into three main types in soil.
- Organic phosphorus is approximately 85-90% of the total phosphorus in soil.
- Inorganic phosphorus is generally less than 10% of the total phosphorus in soil.
- Microbial phosphorus is generally less than 5% of the total phosphorus in soil.
Only inorganic phosphorus is available for plant uptake, therefore even though it is a small proportion of the total phosphorus in soil it has been the focus of more research.
Question 10. Why is organic phosphorus important?
Organic phosphorus is important for several reasons:
- Through biochemical processes organic phosphorus can be transformed into inorganic phosphorus which is readily available for plant uptake.
- Many plants can establish a mutual symbiosis with mycorrhizal fungi or produce root exudates called phosphatases. Both these activities result in the change of organic phosphorus to inorganic phosphorus.
- The phoshorus in soil organic matter can become an important pool for plant growth with the use of management techniques such as residue application and mulching.
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