The mechanisms of absorption of nutrient ions into the root cells continue to be a subject of research by plant nutritionists. Nutrients move into the cortex free space of the root by the diffusion process.
Meanwhile, this cannot account for the high concentration of ions in plant cells compare to the very low quantity of the same ions in soil solution. For example, the normal concentration of potassium in soil solution is about 5-10mg kg-1, plant content of this element is between 1-5%.
Several factors influence the growth and performance of field crops. Some of the growth factors are genetic, that is, the inherent capability of the crop beyond which there is no level of environmental factor that could make the crop perform better.
Environmental factors, on the other hand, could be controlled and made favorable to increase the performance of crops. Both genetic and environmental factors such as water, temperature, or nutrient elements in the soil are referred to as limiting factors because plant growth and yields are constrained (limited) by the deficiency of either one or a combination of the limiting factors.
Thorough knowledge of the growth factors as they affect crop growth is a sine qua non for those who intend to make farming a profitable enterprise. Like human beings, crops must be supplied with balanced diets – the growth factors supplied in adequate quantity and quality at appropriate times during the growth period of the crops.
The rhizosphere is defined as the immediate soil area surrounding the root. It is about 2mm from the root surface. The rhizosphere has both biological and chemical properties such as pH, nutrient concentration, microbial population, and organic materials that are different from the bulk soil.
Nutrient elements in the bulk soil move to the root surface through any one or combination of three mechanisms, namely, mass flow, diffusion, and root interception.
However, the movement of nutrient ions from the root surface or rhizospheric soil solution into the root cells is probably affected mainly by organic carriers in form of an active transportation system.
Plant roots have both positively and negatively charged surfaces. The ions attached to the root surface charges often could be exchanged for those in the soil solution depending on the requirement of the plant.
This phenomenon is referred to as ion Exchange. For example, the H+ ion on the root surface may be exchanged for the K+ ion in soil solution. However, the carrier Hypothesis (active transport) is the most acceptable mechanism by which nutrient ions are taken up by plants.
Within the plants, there are carriers, organic compounds, which react with ions to form carrier-nutrient complexes which can pass through the membrane into the cell. By means of the organic carriers, plants can have selective absorption of certain elements to the exclusion of others.
The most probable theory is that all three mechanisms diffusion, ion exchange, and carrier hypothesis are employed in nutrient uptake but the carrier hypothesis is used to explain absorption against a nutrient concentration gradient.
In conclusion, several factors influence the growth and performance of field crops. Some of the growth factors are genetic, that is, the inherent capability of the crop. There is no level of environmental factors that could make the crop perform beyond the level of genetic factors.
Environmental factors, on the other hand, could be controlled and made favorable to increase the performance of crops. Both genetic and environmental factors, such as water, temperature, or nutrient elements in the soil are referred to as limiting factors. This is because plant growth and yields are constrained (limited) by the deficiency of either one or a combination of the limiting factors.