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Fertigation is the use of fertilizers in the appropriate combination, concentration and pH. Mineral nutrition is critical for optimal plant growth. Optimal nutritional conditions can vary between different plant species, for the same plant species at different times of its life cycle, for the same plant species at different times of the year, and for the same plant species under different environmental conditions. Even balanced aquaponic systems can experience nutrient deficiencies. Fish feeds do not necessarily have the right quantities of nutrients for plants, and generally have low iron, calcium and potassium values (see Chapter 5). Thus supplementary plant fertilizers may be necessary, particularly when growing fruiting vegetables or those with high nutrient demands. Synthetic fertilizers are often too harsh for aquaponics and can upset the balanced ecosystem. In general, iron is added as chelated iron to reach concentrations of about 2 mg/litre. Calcium and potassium are added when buffering the water to the correct pH. These are added as calcium hydroxide or potassium hydroxide, or as calcium carbonate and potassium carbonate. The choice of the buffer depends on the plant type being cultivated: leafy vegetables may need more calcium, while fruiting plants may need more potassium (Somerville et al. 2014c).
Any hydroponic nutrient solution begins with the water, and therefore it is essential to begin with laboratory analysis of a sample. The three main things to note are the alkalinity, the electrical conductivity (EC), and the concentration of specific elements. Alkalinity, which is a measure of water’s ability to neutralize acid, is usually reported in terms of mg/L of calcium carbonate equivalents (CaCO3). Alkalinity values may range from near 0 (in very pure or reverse osmosis- treated water) to more than 300 mg/L CaCO3. The greater the alkalinity of the water, the more the pH will tend to rise in the nutrient solution. Water source alkalinity is a much more important number to look at than its pH: the pH is simply a one-time snapshot of how acidic or basic the water is, while alkalinity is a measure of its long-lasting pH effect. Only once the water alkalinity is known will it be possible to select an appropriate fertilizer strategy. Depending on the alkalinity, it may be necessary to choose a formulation with a greater proportion of acidic nitrogen forms (ammonium or urea) or to add acid to neutralize the alkalinity and counter the pH rise (Mattson & Peters 2014).
EC is a measure of the total dissolved salts, including both essential elements and unwanted contaminants (such as sodium). EC is therefore a rough measure of water source purity. EC should ideally be less than 0.25 mS/cm for closed systems. The laboratory water analysis will also indicate which specific essential elements and contaminants are in the water. The concentration of essential elements should be taken into account when preparing a nutrient solution recipe (see below). Tap water can often contain significant levels of Ca, Mg, S and P. Sodium and chloride (table salt) are common contaminants in some waters; ideally these should be less than 50 and 70 mg/L, respectively (Mattson & Peters 2014).
Mineral nutrients are available in the form of liquids or as powder concentrates that are then diluted with water. Nutrients are available in different formulas that, when mixed together, provide all the essential elements. Usually, the calcium containing compounds are kept separate from the phosphate and sulphate compounds, because in high concentrations the calcium will combine with the phosphates and sulphates to form insoluble precipitates. A typical nutrient solution will be divided into 3 tanks: a calcium/iron tank, the macro/micro tank containing all the other nutrients, and an acid tank which is kept separate so that pH can be adjusted individually (Rorabaugh 2015).
A grower will start with a nutrient solution recipe – a list of inorganic compounds and their final concentrations in mg/L (milligram per litre) or mMol (millimole). The recipe needs to take into account the plant you want to grow, the regional location and environmental conditions, and the time of year. Table 3 shows a nutrient solution recipe for growing tomatoes in Las Vegas during winter. In weeks 0-6 the recipe is higher in nitrogen, calcium and magnesium to ensure good structure and vegetative growth. In weeks 6-12 the nitrogen is reduced and the potassium increased to enhance flowering (reproduction). From week 12 onwards the recipe is designed to maintain a balance between vegetative and reproductive growth (Rorabaugh 2015).
Table 3: Example of a nutrient solution recipe used by Sunco Ltd., Las Vegas NV, for tomatoes during winter (from Rorabaugh 2015)
Nutrient (mg/L)Week 0-6Week 6-12Week 12+N224189189P474739K281351341Ca212190170Mg656048Fe2.02.02.0Mn0.550.550.55Zn0.330.330.33Cu0.050.050.05B0.280.280.28Mo0.050.050.05
HydroBuddy is an open source program for the calculation of nutrient solutions for hydroponics. The programme enables one to find the amount of salt weights necessary for the preparation of nutrient solution with a given composition or, conversely, to determine nutrient concentrations within a solution based on a given fixed weight of salts. While the database contains pre-defined formulations, the programme can be customised to allow the addition of other preparations.
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