Factors influencing nutrient availability and uptake from soils
Mechanisms of ion transport to plant roots
In order to understand the nutrient uptake of crops we must know how nutrients reach the root surface. There three known mechanisms of this. The first one is called root interception, which is a physical contact resulted by root growth. The next mechanism is the mass flow, which involves the transport of nutrients to the root as a result of transpiration. Finally, there is also the mechanism of diffusion movement, which is caused by differences in concentration.
Table 11 Rates of Root interception, Mass Flow and Diffusion in Ion Transport to Corn Roots
Nutrient uptake by plants (crops)
Nutrient uptake of crops
Nutrient absorption by roots is a process of ion exchange at the surface.
The ion uptake of plants is characterized by the following three facts. Selectivity in ion uptake means that certain ions (elements) are taken up preferentially. Another important factor is accumulation, which means that the concentration of elements in the plant cell sap can be much higher than in the external solution. And ion uptake is also characterized by the genotype. There are considerable differences between plant species in their ion uptake characteristics.
Ion uptake has a passive part as well as an active part.
In the passive part we can observe the movement of low-molecular-weight solutes (e.g. mostly ions, organic (amino) acids, sugars) from the external solution into the cell walls of roots. This process is driven by diffusion or mass flow.
In the active part ion uptake takes place due to the movement of ions from the soil solution into the plant root against a concentration gradient. This is followed by the solute transport across membranes.
The carrier and ion pump systems
Carriers are the specific molecules that carry on ions across the cell membrane.
It is an interesting scientific fact that carriers have not been completely identified yet.
The process of carrier and ion pump systems can be described in the following way:
First the ion is attached to a carrier. After that the combined unit is transported from the root surface into the root. Then the ion is deposited inside the root with the carrier moving back across the cell membrane to repeat the process with another ion.
There is also another concept according to which: it is an ion pump that assists in the transport of ions across the cell membrane.
IMPORTANT:
• However, it is important to note that energy, which is derived from root respiration, is required for both systems to work.
Factors influencing crop nutrition
There are numerous factors that influence the nutrition in crops. These factors can be internal or genetic factors on the one hand, and external factors on the other hand. Both types play significant roles in the nutrition processes that we can observe in crops. For successful nutrient management we need to know these factors well.
As for the first class of these factors, for example, we must consider the nutrition characteristics of the species and varieties to be grown. These special features include the morphological characteristics of plants, the ratios of their shoots and roots and also the characteristics of their root development. Other factors that we must not overlook are the specific nutrient requirements and nutrient dynamics. Different plants also differ in their
temperature requirements, which naturally also affect their nutrition processes. Another similarly important factor is the pH tolerance and/or salt tolerance of the given crop. It is self-evident that soil environments with different pH values behave rather differently as far as the supply of soil nutrients is concerned.
The external factors that influence crop nutrition also fall in two sub-categories. In the first group we can find the environmental factors. It is quite clear that climatic and weather conditions will play a profound role as well as the water supply concerning both the quantity and quality of water. We must not forget about air (components) and light conditions (radiance) either, since these are crucial for all forms of plant life on earth.
Besides the above environmental factors, one has to consider also the properties of the given soil in which the particular crops are actually planted. One of the important soil properties is – naturally – the nutrient supply.
However, there are a number of other factors as well. The soil atmosphere, the moisture requirement and the ratio of air and water are crucial, as are the soil pH and texture of the soil, too. Organic matter content of the soil as well as the microorganisms have outstanding importance, whose roles in the ecosystem of the soil and consequently in plant life are far too great to be ignored.
Table 12 pH tolerance of several crops
Table 13 Optimum pH ranges of different crops
Nutrient uptake of crops
Table 14 Relative yield of crops as affected by soil pH (in percentages of maximum yield)
Table 15 Relative salt tolerance of crops
Table 16 Principal soil conditions resulting Mineral Stresses on Plants (Epstein & Bloom, 2005)
In the table below, a brief information is summarized on typical soil conditions resulting nutrient shortages and therefore inducing nutrient deficiencies for several crops. Generally, increased leaching, soil erosion, progressing soil acidification and too intensive cropping are among the most important factors enhancing nutrient depletion in soils. The extent of these processes will affect the appearance of nutrient deficiencies. On the other hand, plant responses are also variable, mostly depending on genetic characteristics of the crop (including differences in species and varieties).
Table 17 Soil Conditions Resulting Shortages of Available Nutrients and Inducing Nutrient Deficiencies for Crops
Nutrient uptake of crops
Availability ranges of nutrient elements depending on soil pH
• Maximum availability for the majority of nutrients: at pH = 6.5 i.e. under slightly acidic conditions
• Availability of metal cations (mostly microelements) increases with acidity, with the exception of Molybdenum.