In order to grow, plants need water, light, carbon, oxygen and minerals. The air supplies oxygen and carbon dioxide, a source of carbon, which the plant takes in through photosynthesis. The soil acts as a reserve for water and minerals to feed the plant; it is essentially a bioreactor that houses a complex ecosystem. The soil recycles organic materials in minerals, so they can be used again by the plants to provide farmers with a good crop yield.
An example: 1 m2 of cereal on a plot of land would need the following in order to grow:
- 660 g of oxygen, 630 g of carbon, 90 g of hydrogen +
- 20 g of nitrogen (N), 8 g of phosphorus (P2O5), 25 g of potassium (K2O) +
- 8 g calcium (CaO), 6 g of sulfur (SO3), 4 g of magnesium (MgO) +
- Trace elements: 0.15 g of iron, 0.05 g of manganese, 0.05 g of zinc +
- 01 g copper, 0.006 g of boron, 0.001 g of molybdenum +
- Several million bacteria and fungi, worms, crustaceans, etc.
The needs of a plant will evolve throughout its life. At each stage of its development it will need to find certain necessary elements, in an assimilable form in the soil solution (water + minerals). So if fertilizers provide the soil with nutrients, what do plants need?
1 - The soil
Soil is made up of:
- organic matter;
- fine and highly reactive elements (clay);
- siliceous or chalky coarse elements;
- iron, aluminum, calcium;
- water and air (50% of its volume).
Soil plays an essential role in the nutrition of plants in agriculture, as it:
- retains the soil solution;
- stores certain nutrients;
- houses microorganisms that contribute to the conversion of non-assimilable elements into elements that can be absorbed directly by the plants.
Each area of soil is a unique biotope with its own physical, biological and chemical properties. It is essential for the availability of nutrients.
2 - The role of nitrogen (N)
Nitrogen is an essential element for photosynthesis that enables mineral matter to be converted into plant tissue. Nitrogen is found in the air, but plants, with the exception of leguminous crops (alfalfa, clover, peas, etc.), cannot absorb it in gaseous form. In soil, nitrogen is found in organic or mineral form (ammonium NH4+, nitrate NO3-). Organic nitrogen (residue from previous harvests, organic fertilizer, etc.) needs to be converted into nitrates by microorganisms in the soil, so it can be used by the plants. This process is called mineralization. It is essentially nitrates that provide plants with nitrogen nutrition. The nitrogen cycle is dependent on weather conditions and the microorganisms present in the soil. Nitrates are not retained in soil very well, so they will need to be added, or mineralization will need to be encouraged when the plant is ready to absorb them, to prevent them from leaching into the groundwater. Nitrogen is both a growth and a quality factor that influences the level of proteins in plants.
3 - The role of phosphorus (P)
Phosphorus is necessary for plant growth. It is found in the soil in the form of phosphates, either dissolved in water, attached to soil particles, contained in minerals or even present in organic form. As the roots take up the phosphate dissolved in water, the fixed molecules are gradually released. The phosphorus in organic form is mineralized slowly. But these exchanges are very slow. The phosphorus cycle is highly dependent on the physical and chemical properties of the soil. Some of the plants with the highest phosphorus requirements include potatoes, vegetables and beets. Short-term bioavailability is very often the limiting factor, as phosphorus ages very quickly in soil. In acid soils rich in iron and free aluminum, the soluble phosphorus binds very quickly, as is the case with soils rich in calcium and/or magnesium.
4 - The role of potassium (K)
Potassium plays a fundamental role in the formation and storage of sugars. It also helps the plant to resist cold temperatures, drought and disease. The potassium in the soil solution is retained by the humus or the clay, while the potassium contained in minerals will only be released very slowly. As with phosphorus, the potassium cycle is dependent on the physical and chemical properties of the soil, although it always remains bioavailable. Not all plants have the same potassium requirements: potatoes, vegetables and beets are more demanding than cereals, for example. Potassium is generally supplied before planting.
5 - The functions of the other elements: Calcium, Magnesium, Sulfur, trace elements
Calcium and magnesium are generally used to improve the structure of the soil, and they are supplied in the form of amendments. Sulfur is necessary for protein synthesis and is provided in the form of sulfates in certain types of fertilizers. The plants with the highest sulfur requirements include cruciferous vegetables (cabbage, rapeseed, etc.), garlic, onions and leeks. It can also be supplied in other less oxidized forms. This will produce additional agronomic properties. Trace elements (copper, manganese, zinc, boron, molybdenum, iron, etc.) contribute towards plant nutrition in very small doses. However, a shortage of any one of these elements could cause problems for plants. Such deficiencies could be caused either by an insufficient concentration or unavailability of an element.
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