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 affected mainly by three mechanisms: mass flow, diffusion, and root interception.
The three mechanisms may occur simultaneously, but one mechanism or another is usually favored by a particular nutrient element. For example, in Table 4.2, 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.
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 past a root from a high water concentration gradient to a low water concentration, nutrients are carried along for the plant to utilize.
Diffusion is a continual process in the soil whereby plant nutrients move from areas of higher concentration towards 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, soil acidity, the amount of organic matter, and the nature of the element itself.
Other factors that have been found to affect the 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 the 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.
Root Interception is the contact made between the growing roots and nutrient ions in soil solution. The roots grow into a new soil zone where there are pools of nutrients in solution. There is direct contact of the root with the nutrient which this results in direct nutrient exchange between the root and the soil thus the term contact feeding is used to describe this method of nutrient absorption.