SOIL NUTRIENT SUPPLY TO PLANT ROOTS

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There are three ways in which plant nutrients are supplied to roots of growing plants: 

1. Root Interception
2. Mass Flow
3. Diffusion

Understanding how nutrients are supplied to roots can be helpful in managing the “4 Rs” of nutrient stewardship (Right – Source, Rate, Time, & Place).  Following is a simplistic overview of each of the three ways that roots access nutrients and which nutrients are involved in these methods of root access.

Root Interception – Plant roots come into physical contact with nutrients as the roots grow and expand in the available soil volume.  Because of this, several root and soil factors will impact how effective the roots actually are in intercepting essential nutrients.  According to Dr. Stan Barber (1984) corn roots only occupy about 0.4 percent of the volume of the top 8 inches of soil, while soybean roots occupy only between 0.4 and 0.9 percent of this soil volume.

The basic crop root structure is important, including whether the roots are fibrous with multiple branches, taproot type of root with heavy central structure, long roots that explore large areas of soil, etc.  Root hairs and beneficial soil mycorrhizae greatly increase the amount of root surface effectively exposed to the soil.  (Soil mycorrhizae may extend 3 inches into the soil, increasing the effective root absorptive area ten times or more.) 

Soil structure and “tilth” is of importance in helping determine how effectively roots can explore their environment.  Soil type and organic matter are additional important factors.  Soil temperature, compaction, or soil saturation (amount of aeration) are known to inhibit root expansion and growth.  Finally, soil chemistry, including pH, is critically important in its influence both on the form that the nutrients are available in the soil solution (or adsorbed on soil particles or present in clay lattice), and on how effectively roots can absorb nutrients at the root-soil interface.

The amount of essential nutrients available to the growing plant depends on the nutrient concentration in the soil, soil cation exchange capacity, as well as how strongly the nutrients are adsorbed to the soil particles or absorbed in clay lattice.  Soil pH has already been mentioned but soil moisture has a huge impact on how open clay lattices are, as well as how re

Mass Flow – Mass flow refers to movement of nutrients dissolved in the soil solution.  Since mass flow depends on nutrients dissolved in soil water, effectiveness of this process is dependent on soil moisture (including percolation) and on plant growth and evapotranspiration.  The quantity of nutrients supplied to growing plants by mass flow is largely linked to solution concentration of the nutrients, plus amount of water transpired by the plant.  Temperature, humidity, and other climate factors help regulate this process.  The amount of root growth is important with mass flow, but mass flow nutrients are capable of moving relatively large distances (inches) to roots in the water supply.

Nutrients commonly supplied to the plant by mass flow include nitrates and sulfates, plus borate, chloride, calcium, magnesium and others.

Diffusion – With diffusion, nutrient ions move from areas of high concentration to areas of low concentration in the soil.  This process is generally slower than mass flow, effective distances of nutrient movement in the soil are not as great as with mass flow, and volumes of nutrients moving through this process are lower than with mass flow.  Seasonal movement of phosphorous in the soil by diffusion is typically less than 1/16 of an inch, and for potassium is only about ½ inch.  Since nutrients that move by diffusion tend to not move freely or over relatively large distances in the soil, root growth and volume is important in this process.  Part of what drives nutrient diffusion is that plant roots create zones of lower nutrient concentration as nutrients are taken up, and additional nutrients then diffuse into these root zones.

Diffusion is the primary mechanism of nutrient movement in the soil for phosphorus, potassium, iron, and zinc.  As these nutrients diffuse through the soil, they can be rendered immobile by soil adsorption, another reason for maintaining soil sufficiency levels.  Cool soils will slow diffusion, as will dry soils.  Dry soils limit the available water film around soil particles, effectively slowing or stopping the process.