Fertilizer Practices: What a Plant Needs and Why
Nutrition and weather are perhaps the two most essential influencers on plant growth and development. However, the weather conditions are close to impossible for farmers to control in their outdoor fields. Hence nutrition remains the most significant factor in farmers control to ensure that their crops can develop to their fullest potential. However, meeting the nutritional requirements of crops is not an easy task. How much of what nutrient is required depends on a plethora of factors that even vary throughout the growing season, such as:
- Soil moisture
- Growth stage
- The crop grown the previous season (crop rotation)
- Nutrition already available in the soil (soil nitrogen supply)
- The actual nitrogen demand of the crop
The success of plant nutrition is also dependent on the nutrients themselves. This can be best illustrated and explained through the Law of the Minimum. Justus von Liebig, an organic chemist in the 1800-hundreds, took a particular interest in plant and soil nutrition. He found that the development of plants and their yields are proportional to the lowest nutrient value. Example:
If a crop has the optimal nitrogen and phosphorous levels but lacks potassium, its development will still be stunted. Crop development is only as successful as the lowest nutrient allows it to be. That is why farmers need to take extra care and monitor all the required nutrients.
What is the most important plant nutrient? As mentioned in our introduction post, nitrogen, potassium and phosphorus are the three essential nutrients that plants cannot be without. So let’s take a look at each in more detail and what happens when plants don’t get enough.
All living things on our planet need nitrogen to live, and as we have established, this includes plants. Nitrogen is a versatile nutrient and molecule found throughout the plant, for example, in proteins. There are many types of proteins that all carry out different functions in the plant cells they belong to. The role and impact of these proteins depend on their make-up. Proteins are large molecules that are made up of chains of amino acids. The amino acids, in turn, are made out of different atoms. One of these atoms that can bind to form the long amino chains that become proteins is nitrogen. If the order of amino acids in a chain is altered or an, e.g. nitrogen atom is missing altogether, the protein can no longer function and fulfil its purpose. This can lead to all functions that require the protein in question to shut down. However, nitrogen not only helps to make up protein but other important molecule structures. Let’s look at an example.
Photosynthesis is how plants make the energy that becomes their food and is essentially a continuous chemical reaction. When plants photosynthesise, they combine molecules from water and carbon dioxide through the help of sunlight to create the plant sugar glucose (and oxygen as a by-product). This process takes place in the plant leaves, more specifically in the tiny pigment molecules called chlorophyll. Chlorophyll is what gives the plant its green appearance. Hence if a plant starts to look less green, e.g. contains brown patches on the leaves, we know that those areas lack chlorophyll. This means that the plant produces less food in that location, and if it loses too much chlorophyll, it cannot sustain itself for long before dying. One cause that leads to the decrease of chlorophyll and photosynthesis is nitrogen. More specifically, a lack of it. Like all other organisms, plants need to produce and replace their cells, including those in the leaves. This means that the plant has to create new chlorophyll molecules continuously, and this requires nitrogen. Each chlorophyll is made out of several small molecules, and in a ring around the centre of its structure are four nitrogen atoms. Without the nitrogen atoms, the plant cannot build the chlorophyll molecules. That is why farmers take extra care to fertilise with nitrogen during periods where they know rapid plant growth occurs. As a plant grows, it essentially adds more and more cells to its body, e.g. by adding leaves. So in order to build the internal structures of the leaves, it needs nitrogen.
In other words, nitrogen to plants is like protein to humans. When we try to grow and build muscle, we need protein to do so.
Phosphorus is another building block required for all life and is a fundamental component of cellular energy in all organisms. This cellular energy is stored in the format of the molecule called Adenosine Triphosphate (ATP). ATP in plants is the point created when plants photosynthesise, and it is impossible to be made without phosphorus. When ATP is produced, three phosphate molecules attach themselves to other molecules of the so-called adenosine structure. Alike with the chlorophyll molecules, plants can’t create ATP molecules without the three phosphate atoms. The plant can then hold on to the ATP until the cells need it to perform its function, e.g. when the plant absorbs water through its roots. However, ATP in its stored format is not directly useful on its own but has to be converted to create energy that the cells can use. When a plant needs more energy, one of the phosphate groups of the ATP molecule is separated from the other two. Due to the very strong so-called bond between the phosphate molecules, the separation of the atom releases a lot of energy. Hence phosphorus is detrimental for the plant to have enough energy to perform all the processes it needs.
However, phosphorus also has another structural importance in plants. Plant cells contain many different parts, from organelles, proteins and lipids. To function correctly, the cells need to restrict what substances move in and out of the cell. To help protect itself, the out layer of plant cells have a cell wall. The cell wall allows the plant to withstand physical damages, e.g. osmotic pressure from too high water intake.
Underneath the cell wall is the cell membrane, and it is the gatekeeper of what substances are allowed in and out of the cell (through the help of built-in protein channels and carriers). The cell membrane is made up of two layers of lipid molecules. Each lipid molecule contains one circular phospholipid head (which contains phosphorus) and a hydrophobic tail—as with any plant growth and cell formation, building a cell membrane is essential. However, without enough phosphate molecules available for the plant to absorb, it cannot successfully make new cells.
To summarise, phosphorus is the foundation of cell formation and energy usage in plants.
Potassium is the third in the trinity of essential plant nutrients. Similarly for nitrogen and phosphorus, its use-cases are many, including transpiration. Transpiration is the loss or evaporation of water through small tiny openings in the plants. These tiny openings are called stomata and are essentially the lungs of the plant. When the stomata open, they allow the exchange of carbon dioxide (taken into the plant) and oxygen (transpired out of the plant). For example, when the sun shines onto the leaf of a plant, photosynthesis begins, and as mentioned above, this requires carbon dioxide. To take in the carbon dioxide, the plant has to open its stomata, which is controlled through water intake. To open the stomata, the cells (vacuoles in the cells) around the opening fill up with water and expand. As they expand and separate, the tiny opening becomes unblocked. When the transpiration should stop, the water exits the cells, which become small again. Transpiration in plants serves an important function, and the nutrient that allows it to occur is potassium. How? Osmosis!
Osmosis is the movement of molecules from one side of the membrane to another, for example, water into and out of a cell. When the values of the cells around the stomata are to fill with water, the first to enter are the potassium molecules, and after that, the water molecules follow.
Each nutrition serves a critical function in the cell, which would not be possible without it. If we return to the Law of the Minimum, the interaction and implication of nutrition according to this law become reasonably clear. When a plant has sufficient nitrogen, it can undergo photosynthesis. However, if there is a lack of phosphorous or potassium, this would not be possible after all. The missing values of the other nutrients hold it back. Without the influx of carbon dioxide for which potassium is needed or the energy in the form of ATP thanks to phosphorous, it doesn’t matter how much nitrogen the plant has. However, if the plant has the correct nutrition levels, its well-being goes beyond fulfilling the utmost needed processes. For example, research shows that plants have been found to become more resilient to both biotic (e.g. pest and fungi) and abiotic stressors (weather and environment ). More specifically, it has been found that the correct levels of potassium help crops survive better in the case of drought through the relationship of potassium and water ions.
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