Hydroponics water chemistry

Here is a simple starting point for students and growers interested in aquaponics and hydroponics water chemistry.

ACID-BASE: pH
is a term used to rather universally to express the intensity of the acid or alkaline conditions of water or soil. In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at 77 F. Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline. pH measurements are important in all fields of science and engineering and in plant science are significant by influencing the availability of the Primary and Secondary Nutrients noted above. Included in the Appendices are several charts that illustrate the effects of pH on these nutrients. The width of the bar determines the relative availability of each element with a change in pH.

AMMONIA
is a compound of nitrogen (N) and hydrogen (H) with the formula NH3. It is a colorless gas with a characteristic pungent odor. Nitrogen is the mineral element most in demand by plants and the fourth most common element in their composition, being outranked only by carbon (C), hydrogen and oxygen (O). Ammonia is typically formed in soils and water when organic matter such as plant and animal residues containing organic nitrogen compounds are decomposed by bacteria. Ammonia is steadily released into the tank water through the gills and the excrete of fish as product of their metabolism. Higher concentrations of ammonia in the range of 0.5 and 1 ppm can kill fish and plants do not absorb it as well as nitrates.

NITRITES
NO2 is an intermediate product of aerobic nitrification process and is formed when specialized soil and water Nitrosomonas bacteria oxidize and convert ammonia to nitrite. Nitrite levels typically are in the range of .25 to 1.0 ppm in a properly operating system.

NITRATES
NO3 is the final decomposition product of the aerobic nitrification process and is formed when specialized Nitrobacter bacteria oxidize the nitrites to nitrate. Nitrate levels typically are in the range of 2-150 ppm in a properly operating system. During system startup and prior to the nitrififying bacteria being fully established on the BioGarden trough aggregate and plant roots, spikes may occur in the levels of ammonia ( up to 10 ppm) and nitrite (up to 15 ppm ) and nitrate (up to 200 ppm). The nitrification process is affected by pH, temperature, and oxygen level of the water or soil. Optimum pH range for nitrification is 7-8.3; Optimum temperature range is 75-85 F; Optimum dissolved oxygen range of 1.5-3 ppm. When starting a bed for the season, it takes a few weeks to get nitrifying bacteria colonized in the beds. Without the bacteria, ammonia does not convert and so the ammonia builds up and fish can die. A practical way to start a system before introducing fish is to add a cup of pure ammonia (no cleaning ammonia -without detergent or surfactants) to the tanks and let it cycle through the beds for a couple weeks, encouraging nitrifying bacteria to colonize and thereby prepare the rock beds for fish ammonia.

PHOSPHATES
PO4 is released into water and soil by bacteria and fungi during the decomposition of plant and animal organic matter and the solubilization of phosphate containing minerals such as Apatite. Typical hydroponic nutrient solutions require phosphate concentrations of 30-100 ppm and they are adjusted for the particular plant to be grown.

OXYGEN
is required for the growth of plants, fish, earth worms and aerobic bacteria such as nitrifiers and decomposers that break down organic matter. For fish and deep water growing conditions, oxygen must be continuously supplied to tanks and troughs. In ebb and flow systems it is needed for the tanks but not the beds, as the beds are continuously replenished with fresh oxygen with each flood and drain, forcing out expired air and sucking in fresh air.

DISSOLVED SOLIDS
TDS is a measurement of the amount of ions in the water. These are a balance of both positive (cations) and negative (anions). Total Dissolved Solids measures all solids that pass through a 2 micron filter. This includes inorganic ions and organic compounds. Most test meter measures conductivity in millisiemens/cm. Laboratory measurement of TDS is typically more accurate that using meter and conversion as TDS depends upon the ions and organic dissolved in the water but for our operational purposes is an good estimate of actual TDS. I have attached a calculation tool that can be used to more accurately determine TDS based on a fairly complete list of anions and cations that are typically measured by a lab or utilized to formulate hydroponic nutrient solutions. Normal reading range is 800-1500 with 2000 as the upper limit.