What follows is a list of some of the testing variables we use to test our Oxyfertigation Growing Systems, growing processes, and Nutrient Solutions, and how some of those variables relate to our Growing Systems.
pH stands for potential hydrogen or power of hydrogen. It is the measure of the activity of the solvated hydrogen ion in a solution and indicates the degree the Nutrient Solution is acidic, neutral or alkaline. The pH scale goes from 0 to 14, 7 is neutral, any values below 7 are acidic, and any values above 7 are alkaline. The pH scale is logarithmic and each whole pH value below 7 is ten times more acidic than the next higher value. Thus pH 4 is ten times more acidic than pH 5 and 100 times (10 times 10) more acidic than pH 6. Plants growing in soil typically grow best in a pH value between 6.0 – 7.0. When outside (which ranges between 5.5 – 7.0), nutrients become less accessible for plants. Each Variety absorbs nutrients differently and the ability of the Variety to absorb a specific nutrient is affected by the pH level of the Nutrient Solution. The current versions of our Growing Systems enable the pH value of the solution to be adjusted. The raising and lowering of the pH is caused by adding a solution that can also possibly contain nutrients used by a plant.
EC and TDS
Electrical Conductivity (EC) is the measure of a material's ability to allow the transport of an electric charge, measured in Conductivity Factor (CF), Micro Siemens per square centimeter (μS/cm2) conductivity, or Parts per Million (PPM). It represents all the ions in the Nutrient Solution and provides a general number of how much of a nutrient is in a mix. However, it does not distinguish between any specific nutrient and thus is only useful as an approximation of relative Nutrient Solution density. TDS stand for Total Dissolved Solids and refers to the measure of all nutrient ions contained in a liquid. It is a measurement used to determine nutrient strength. TDS is not as accurate a representation of EC but much more commonly used. TDS is directly calculated from Conductivity using one of two different scales: NaCl (1EC = 500 ppm) and the 442 scale (1EC = 700 ppm). Different meters offer different conversion factors and some offer a sliding scale or the option to change the scale. An EC reading must be adjusted for temperature. EC/TDS is used as a general measure by our Growing Systems to monitor the amount of nutrient in the nutrient solution at a particular time.
Ion Selective Measurement
Ion-Selective Electrodes (ISE) consist of an ion-specific half-cell and a reference half-cell. The ion-specific cell gives a potential against the reference cell depending on the specific ion concentration. When the specific ion concentration (the sample or an ion standard) changes, the potential changes as well. Current ion-selective measurement becomes distinctly inaccurate in the presence of many different ions, such as can be found in the Nutrient Solution, thus rendering it a potentially inaccurate method for knowing the specific PPM of a particular nutrient. In the current versions of our Growing Systems, an ISE probe, or something performing a similar action, will report a PPM of an ion. Used stored reference values or computed adjustments, the value returned by the probe, can be used monitor the amount of a specific ion in the Nutrient Solution.
Vapor Pressure Deficit (VPD)
Vapor Pressure Deficit, or VPD, is a measure of the difference (or deficit) between the pressure exerted by the moisture currently in the air and the pressure at saturation. VPD units are most often expressed in standard pressure units such as millibars, kilopascals, or pounds per square inch. VPD is sometimes expressed in mass deficit concentration units such as grams of water per cubic meter of dry air, or grams of water per kilogram of dry air. VPD measurements can provide an indication of the current evaporation potential of the air, and is useful in creating the correct atmosphere for growing the plants. VPD value is useful when air temperature or humidity changes as it reflects the evaporation potential. VPD can be affected by airflow as well. In the current versions of our Growing Systems, VPD values can change at different heights in the Growing Planks due to the effect of the evaporation by the plants, Growing Room atmosphere (outside air or internal recirculated air) controlled by the Environmental Control Unit, heat and vapors from the Lighting Subsystem, atmosphere (air and/or CO2) made available to the Growing Planks under control of the Atmosphere Subsystem, or atmosphere affected by the various chambers storing the Nutrient Solutions. VPD is part of the overall Nutritional Formula to manage, predict, and control plant evaporation. Plants control decisions to shade, or to increase and decrease liquid take-up using its understanding of the VPD value. This affects Nutrient Solution take-up, resulting in the amounts of nutrients absorbed by a plant, which will increase the vitamins and the weight of the plant. In the current versions of our Growing Systems VPD is controlled in a much greater manner than in-field or greenhouse conditions. The VPD value can be used to measure, and to create or reduce crop stress. Management of the VPD value is part of a complete, comprehensive and specific Nutrition Formula for a particular Cultivar and a particular Variety.