Nutrients absorption by crops also known as nutrient uptake by crops can vary from less than 50 kg/ha to more than 1 000 kg/ha depending on the crop, variety, the nutrient, its availability, growth conditions and the biomass produced. Major nutrients constitute the bulk of the nutrients taken up.
For example, the total amount of nutrients absorbed by wheat and rice (paddy) per tonne of grain production is about 82 kg and 74 kg, respectively. Out of this, N and K2O alone account for about 75 percent.
On an element basis, S uptake is generally similar to P uptake. The six micronutrients taken together add up to about 1 kg/ha.
How Nutrients Get to the Root Surface into Plant Cell
Nutrient elements in form of dissolved ions in soil solution have to come in contact with the plant roots for uptake to take place. This contact is effected mainly by three mechanisms: mass flow, diffusion and root interception.
The three mechanisms may occur simultaneously, but one mechanism or another is usually favoured by a particular nutrient element.
For example, calcium moves to the root surface mainly by mass flow and root interception, whereas diffusion accounts for phosphorus supply to plant roots because phosphorus is very low in soil solution.
1. Mass–flow
Mass flow is the movement of plant nutrients in flowing soil solution towards a root that is actively drawing water from the soil. There is some amount of nutrients transported to the root surface in the water used for transportation or by movement due to water potential gradients.
For example, maize uses 500gm water per gram of dry matter accumulation. As the plant takes in this water, there are plant nutrients dissolved in the water for the plant to utilize.
As the water moves pass a root from high water concentration gradient to a low water concentration, nutrients are carried along for the plant to utilize.
2. Diffusion
Diffusion is a continual process in the soil whereby plant nutrients move from area of higher concentration towards the areas of lower concentration around the root surface.
The amount of an element moved to the root surface by diffusion depends on the path followed by the movement of water, on soil acidity, the amount of organic matter and the nature of the element itself.
Other factors that have been found to affect rate of diffusion of an ion to the root surface are the nature of the plant root system (tap or fibrous) which dictates the absorbing area of root surface, the soil type (clay, loam, sand, etc.) and the difference between the concentration of the nutrient at root surface and in the bulk soil solution.
3. Root Interception
This is the contact made between the growing roots and nutrient ions in soil solution. The roots grow into new soil zone where there are pools of nutrients in solution.
There is a direct contact of the root with the nutrient and results in a direct nutrient exchange between the root and the soil thus the term contact feeding used to describe this method of nutrient absorption.
Mechanisms of Nutrient Uptake in to Plant Cell
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. But 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%. 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, 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 the three mechanisms– diffusion, ion exchange and carrier hypothesis are employed in nutrient uptake but carrier hypothesis is used to explain absorption against nutrient concentration gradient.
Movement of Ions into the Crops Root system
The movement of nutrient ions from root surface into the root can be described by two processes: Passive movement/transport and the Active movement/transport.
Passive movement or transport of ions Passive transport of ions occurs in the outer or free spaces in the wall of epidermal and cortical cells of roots and is controlled by ion concentration (diffusion) and electrical (ion exchange) gradient.
The concentration of ions in the apparent free space is normally less than the bulk solution concentration and therefore, diffusion occurs with concentration gradient, from high to low concentration.
Passive transport is non-selective process and does not require energy from the metabolic activities of the plant. Active movement or transport of ions.
Active transport of ions is the movement of an ion against its concentration gradient using energy i.e. when the cell uses energy to pump a solute across the membrane against a concentration gradient.
The process of nutrient entry known as ion-carrier mechanism or carrier theory involves a metabolically produced substance (carriers) that combines with free ions. The ion-carrier complex can then cross membranes and other barriers not permeable to free ions and later dissociate to release ions into the inner space of the cell.
Active ion transport is selective process such that specific ions are transported across the plasmalemma by specific carrier mechanism.
Volumes of Nutrient Uptake
Higher production through higher cropping intensity also results in substantially higher nutrient uptake, which can range from 400 to 1 000 kg N + P2O5 + K2O/ha/year. The share of N, P2O5 and K2O in nutrient uptake is generally 35 percent N, 17 percent P2O5 and 48 percent K2O, in the ratio 1.0:0.5:1.4.
Thus, every tonne of N removed is accompanied by the removal of 0.5 tonnes P2O5 and 1.4 tonnes K2O on average. In addition to major nutrients, a grain production level of 10 tonnes/ha through a rice–wheat rotation (6 tonnes paddy + 4 tonnes wheat) can absorb about 3–4 kg of Fe or Mn, 0.5 kg Zn, 200–300 g of Cu or B but only 20 g Mo.
Thus, at the same production level, the uptake among nutrients by a crop can vary by more than 10 000 times (260 kg K vs 20 g Mo). Within the group of micronutrients itself, the uptake of Fe and Mn can be 200 times that of Mo.
For successful crop production, the crop must be able to access and absorb the indicated nutrients whether these are 150–200 kg of N or K2O or 15–20 g of Mo.
Nutrient uptake by a crop depends on a large number of factors, both controllable and otherwise. This is why large variations are encountered for a given nutrient or for a given crop even under similar conditions.
Nutrient uptake can differ owing to the differences among crops, genetic character of a variety, environment where they grow, fertility level of the field, yield level, luxury consumption, nutrient imbalances and post- absorption events such as lodging and leaf fall.
Thus, in order to produce 1 tonne of grain, the uptake by a given crop can vary 1.7-fold in the case of N, 2.3-fold in the case of P and 3.6-fold in the case of K among locations.
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Fate of Nutrients Applied to the Soil
The amounts of nutrients added through fertilizers and other sources are only partly utilized by the crop. There are four possibilities for what may happen to the added nutrients:
They enter the pool of available forms and are absorbed by the fertilized plants (recovered portion).
They are not absorbed but remain available and are partly utilised by the next crop (residual).
They are “fixed” and thus removed from nutrient cycling for longer periods.
They are lost from the soil (through ammonia volatilisation, leaching, and denitrification in the case of N).
Fate of Nutrients Absorbed by Crops
The nutrients taken up by a crop are distributed in different parts of the plant during its life span. In the case of grain crops, 70–75 percent of N and P, 25– 30 percent of K and 40–60 percent of S absorbed ends up in the grain, the rest stays in straw/stover.
In rice, more than 70 percent of the N absorbed is transferred to the grain while a greater proportion of K, Ca, Mg, Fe, Mn and B remains in the straw.
The absorbed S, Zn and Cu are distributed about equally in grain and straw. In groundnut, out of the nutrients absorbed, the kernels contain 41 percent of N, 52 percent of P, 28 percent of K, 11 percent of Mg and 1 percent of Ca.
The leaves and stalks contain 45–50 percent of total NPK absorbed and also the bulk of Ca and Mg. In potato, harvested tubers account for 80, 83–88 and 70–78 percent of total N, P and K absorbed, respectively.
In cassava, the proportion of absorbed nutrients present in tubers is 23 percent of N, 32 percent of P, 38 percent of K, 12 percent of S, 11 percent of Ca and 29 percent of Mg. In jute, the proportion of absorbed nutrients that is returned to the soil before harvest through leaf fall is particularly high.
Crop Recovery of Added Nutrients and their Implications
The recovery or utilization rate of an applied nutrient is the portion of the added nutrient that is taken up by the plants. It is expressed as a percentage of the nutrient amount supplied.
A recovery of 50 percent means that half of the fertilizer nutrients applied has been utilized by the fertilized.
The recovery rate for applied nutrient is often high for K (up to 70 percent), medium for N (35–70 percent), comparatively low for P and S (15–30 percent), and very low (less than 10 percent) for micronutrients.
In summary, 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. But 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.
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 the three mechanisms – diffusion, ion exchange and carrier hypothesis are employed in nutrient uptake but carrier hypothesis is used to explain absorption against nutrient concentration gradient.
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