Roots do not intentionally grow towards a nutrient source. For nutrient uptake to occur, the individual nutrient ion must be in position adjacent to the root. Positioning of the nutrient ion can occur by one or more of three processes.
The root can “bump into” the ion as it grows through the soil. This mechanism is called root interception. Work by Barber estimates that perhaps one percent of the nutrients in a corn plant come from the root interception process.
The soluble fraction of nutrients present in soil solution (water) and not held on the soil fractions flow to the root as water is taken up. This process is called mass flow. Nutrients such as nitrate-N, calcium and sulfur are normally supplied by mass flow.
Nutrients such as phosphorus and potassium are absorbed strongly by soils and are only present in small quantities in the soil solution. These nutrients move to the root by diffusion. As uptake of these nutrients occurs at the root, the concentration in the soil solution in close proximity to the root decreases. This creates a gradient for the nutrient to diffuse through the soil solution from a zone of high concentration to the depleted solution adjacent to the root. Diffusion is responsible for the majority of the P, K and Zn moving to the root for uptake. Table 3 gives the relative importance of each mechanism in positioning nutrients adjacent to plant roots for uptake.
|Table 3. Percent of nutrients taken up by a corn crop
normally supplied by root interception, mass flow and diffusion
|Nutrient||Root interception||Mass flow||Diffusion|
|% of uptake possible|