Hydroponics mimics what occurs in a soil that is being cropped. In both the hydroponic and soil systems, the only elements that will be absorbed by plant roots are in an ionic form existing in either the nutrient solution or soil solution, respectively. In fact, the nutrient solution and its elemental constituents are similar to that of a soil solution although in much higher elemental concentrations. This was one of several factors that enabled the early 1850s plant nutrition researchers to determine what elements and their relative concentration levels needed to be in a nutrient element-enriched water solution in order to sustain normal plant growth (Jones, 2012).
In soil, there exists an equilibrium between the ions in the soil solution and their elemental forms found in what is called the “solid” phase as either an exchangeable cation or anion adsorbed on the surface of a soil colloid, such as clay and humus, or as an elemental component of a soil mineral. A discussion as to how ions move into and within the soil solution can be found in the book by Jones (2012). The solid colloidal phase in the soil acts as a “buffer” that is a significant factor in keeping the solution phase consistent in composition as well as
pH. Ionic root absorption occurs from within what is known as the rhizosphere, a thin cylindrical column that surrounds the roots that has unique biophysical-chemical properties. As elements are root absorbed, hydrogen ions are released, making the rhizosphere of lower pH than the surrounding soil. Also with elemental ion absorption, there generates a concentration gradient that results in the movement by diffusion of elemental ions from the surrounding soil solution, followed by a re-balancing of elements into the liquid phase from the solid phase driven by equilibrium forces.
What occurs in the soil in terms of maintaining a constancy of elemental content and concentration in the soil solution is due in part to the equilibrium forces that exists between the solid and liquid phases. Therefore, it would be desirable to maintain such constancy in elemental content concentration in a nutrient solution when brought into contact with plant roots in order to optimize nutrient element utilization by the plant. In the commonly used hydroponic growing techniques, flood-and-drain, nutrient film technique (NFT) and drip irrigation, there occurs an accumulation of elements in the rooting medium, or the root mass, as either an increasing concentration of elements in both solution and precipitate forms. Assay data and a discussion of this phenomenon can be found in the book by Jones (2014). Over time, this accumulation becomes a factor that affects both plant growth and the nutritional status of the plant during the growing period, created as a result of repeated applications of a nutrient solution to the rooting medium using the flood-and-drain and drip irrigation hydroponic growing methods and in the root mass when using the NFT hydroponic growing method.
Dr. J.B. Jones, Jr. is the author of several books including. Instructions for Growing Tomatoes in the Home Garden and Greenhouse. GroSystems, Inc., Anderson, SC (available in soft cover and e-book format for Kindle at Amazon.com).
Jones, Jr., J. Benton. 2012. Soil Fertility and Plant Nutrition Manual, 2nd Edition. CRC Press, Inc., Boca Raton, FL.
Jones, Jr., J. Benton. 2014. Complete Guide for Growing Plants Hydroponically. CRC Press, Inc., Boca Raton, FL.