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Water temperature is more important in aquaponics than air temperature. Many water chemistry factors are affected by temperature, such as the amount of toxic ammonia (un-ionized) present and the solubility of oxygen. It also directly impacts the health and survival of both fish and plants. Fish are poikilothermic, or cold-blooded. This means that their body temperature is dependent on water temperature. At extreme temperature, fish will stop eating, becoming lethargic and susceptible to disease. In plants, high temperature can reduce the uptake of essential plant nutrients, such as calcium, force early flowering in cool weather crops, and increase potential for plant roots pathogens like Pythium spp. For this reason, it is important to prevent wide swings in daily temperature. Shading or covering water surfaces, insulating fish tanks and plant beds, and utilizing passive or solar heating in greenhouses are strategies many producers employ. In temperate areas where temperature changes drastically from season to season, producers can alternate fish and plant crops seasonally to reduce heating or cooling costs.

The pH is a measure of the acidity or basicity of a solution. It is determined by the presence or absence of free hydrogen ions (H+), where the more H+ present, the more acidic a solution is. An acidic solution has a low pH. The pH is measured on a scale from 1-14, with 7 being neutral. A pH value below 7 indicates a solution is acidic and above 7 indicates a solution is basic. The pH is recorded on a logarithmic scale and thus is not intuitive for many practitioners. For example, if the pH of an aquaponic system measures 7, then after two weeks measures 5, the pH has not dropped by a degree of 2, but rather 100 times. Understanding the pH scale is critical for water management and correction.

Fish, plants, and bacteria have specific tolerance ranges for pH. While they can tolerate parameters outside their optimal range, sub-par conditions can greatly affect growth and survival. Fish can tolerate a wide range of pH, from 6.0-8.5, but they need to be acclimated slowly to changes. The pH is particularly important for plants and bacteria. All micro- and macro-nutrients are available to plants at a pH between 6.0-6.5 (Figure 16). Above or below this range, certain nutrients are not available to the plants. When pH exceeds 7.5, plants quickly become deficient in essential nutrients like iron, phosphorous, and manganese (Somerville et al. 2014). Conversely, low pH can have negative impacts on nitrifying bacteria. Below 6.0, the ability to convert ammonia to nitrate is greatly reduced.

exceeds 7.5, plants quickly become deficient in essential nutrients like iron, phosphorous, and manganese (Somerville et al. 2014). Conversely, low pH can have negative impacts on nitrifying bacteria. Below 6.0, the ability to convert ammonia to nitrate is greatly reduced.

There are many factors that influence pH. Nitrification (discussed in the following section) and fish stocking density drive pH down by producing H+ and CO2, respectively. Amendments are needed to bring pH up to suitable culture levels. Managing pH begins with consistent monitoring and recording. If pH is low, chemicals that increase total alkalinity, like calcium hydroxide (hydrated lime; Ca(OH)₂), agricultural lime (calcium carbonate (CaCO₃)), calcium potassium hydroxide (KOH), or potassium carbonate (K₂CO₃), can be used. The addition of

FIGURE 16 Soil pH and nutrients

4.0 pH 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.5 9.0 9.5 10.0

Acidic Alkaline

STRONGLY MEDIUM SLIGHTLY VERY SLIGHTLY VERY SLIGHTLY VERY SLIGHTLY MEDIUM STRONGLY

NITROGEN

PHOSPHORUS

POTASSIUM

SULFUR

CALCIUM

MAGNESIUM

IRON

MANGANESE

BORON

COPPER AND ZINC

MOLYBDENUM

CORPORATION