Vegetable plant roots absorb nutrients through two distinctly different sequential processes. First, the nutrients must move from the soil to the surface of the plant roots. Second, the nutrients must be able to cross from the outside to the inside of the plant roots. Once the nutrient gets inside the plant, the nutrients can move upward to the leaves and developing vegetable.
Process 1: Movement of nutrient ions from the soil to the root surface
It surprises people unfamiliar with botany that plant roots don’t grow in search of nutrients. Instead for plant roots to grow into the soil, they must first encounter and absorb nearby nutrients to develop new root tissue. Since roots have no special radar, sonar or other device to detect nutrients around the corner or in deeper soil layers, they instead depend on, essentially, the nutrients coming to them.
Root interception. Soil particles such as sand, silt and clay form into soil aggregates or chunks bound with humus (organic matter), where most soil nutrients live. But since plant roots contact only a very limited amount of the total soil surface — about 1 to 2 percent — the process of roots making direct contact with soil particles (root interception) can’t be a very major explanation for the way plant nutrients move to the roots. Instead, the roots grow around these aggregates and usually don’t penetrate into them. Thus, most plant roots don’t contact most of the very extensive surfaces of the minute clay mineral particles and humus particles in the soil.
Mass flow through leaf transpiration. Plant leaves, however, induce more contact. Vegetable plants transpire water from their leaves, creating suction on the water at the root surface that draws the nutritious surface soil solution toward the plant roots. This process of mass flow caused by leaf transpiration explains how most plant nutrients (98 percent) move from the soil to the root surface. Nutrient dissolution in the soil solution is an important precondition allowing it to move with this mass flow process.
Diffusion. For some specific plant nutrient ions, the major movement mechanism is the very slow process of diffusion. One can liken diffusion to a concentrated crowd of people leaving a stadium to go home after a game, each taking a different path home. In soil, equate the pellet of fertilizer or band of nutrients to the slowly dispersing stadium crowd. The nutrient concentration begins very high around the pellet or fertilizer band, but over time, the nutrient ions very slowly move away from the concentrated zone (out of the stadium). This movement is in response to a high concentration gradient at the surface of the pellet or zone and a low concentration gradient existing further away from the pellet or zone. Diffusion operates by these highly concentrated ions moving as far away from one another as possible, and becoming exposed and available to the plant roots. Phosphate, potassium and zinc are the main nutrient ions moving as a result of diffusion.
Chelates. Plant micronutrient metal ions (iron, manganese, copper, zinc, cobalt and nickel) can move as a result of the action of chelates. Chelates are formed naturally by a wide variety of microorganisms, though some growers apply synthetic chelates. These chelates act as a nutrient’s taxi cab. Each chelate attaches to a metal micronutrient ion forming a neutral electrically charged particle, allowing the chelated nutrient to move to the root in response to mass flow of the soil solution.
Process 2: Movement of ions from the outside to the inside of the root
Once the plant nutrient moves from the soil via root interception, mass flow or diffusion, or chelate movement, the nutrient has only reached the surface of the plant root. The next process (absorption) is much more difficult to understand. First, consider the nature of plant roots. To provide protection from leaking their contents and from attack by microorganism pathogens, plant roots have developed an important barrier, the casparian strip.
The casparian strip. A corky type of deposit, envision the casparian strip as the mortar cementing bricks together. In this analogy, the bricks are the individual plant cells. As with a brick wall, the casparian strip consists of only one cell-wide layer completely surrounding the root. This band is similar to a sleeve and is located inside several cell layers from the outside cell surface. This could be envisioned as wearing an outside wool sweater with an inside jacket protecting one from the wind. The inside jacket would be similar to the casparian strip. The corky deposit surrounds the cell, as does the cement mortar around a brick, but it does not cover the front or the back of the cells.
The plant root cells embedded within the casparian strip force all nutrient ions to enter directly through these living cells. The nutrient ions must move from the outside to the inside of the root. This can’t occur by diffusion because the ion concentration inside the root is higher than the concentration outside it. Thus, if the roots were penetrated, a net flow of nutrients would leak from the plant roots. However, this does not occur. The corky deposit prevents leakage.
ATP. To push nutrients into the root in the face of higher nutrient concentration inside, the plant cells must exert energy. This energy comes from ATP (adenosine triphosphate), the energy molecule of all cells. The exact mechanism hasn’t been completely revealed to science. Some theories explain the facts we do know. Apparently, the plant cells contain in their cell walls a special molecule (called a carrier) with the ability to recognize each specific nutrient ion. Thus, the cell comprises a variety of these special carrier molecules, with one for each different nutrient. For example, separate carriers exist for calcium, magnesium, copper and zinc. This explains how plant roots can be selective in the nature of ions admitted into the plant roots.
Vegetable plant roots growing in cold soils often exhibit phosphorus (P) deficiency symptoms (purple coloration with slow growth). However, when the soil warms, the roots grow, resulting in the roots absorbing more P, thereby overcoming the deficiency symptoms. This observation has convinced scientists plant roots respond primarily by physiological, rather than by purely chemical processes in absorbing plant nutrient ions. This observation is supported by the fact plant roots require oxygen gas from the air for the roots to remain healthy. This oxygen gas is used by the plant to produce the ATP energy essential in pushing the nutrient ions into the plant.
Once the nutrient ions have been pushed through the living cells of the casparian strip, these ions move into the plant xylem tissue and are carried upward to the leaves and developing vegetable parts. These ions (mainly the metal micronutrients) may serve as components of various enzymes. Other nutrients (mainly nitrogen, phosphorus, magnesium and sulfur, along with calcium) become the major components of the plant as proteins, sugars, DNA, chlorophyll and a host of other compounds.
Source: Thomas A. Ruehr, Professor, Earth and Soil Sciences Department, Cal Poly State University, San Luis Obispo, CA 93407-0261. Adapted from Vegetables West magazine.