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Laboratory analysis of water and nutrient solutions, and understanding laboratory results.

There are many test kits for do- it yourself checking of water and nutrient solution quality with a range of prices and sophistication, but the quality of results and the time and cost of using these kits is such that growers are much better served by using top quality commercial laboratories for accurate, reliable and timely results at reasonable cost.

Water analysis

Growers must know the quality of the water being used to grow hydroponic crops, and must get the water analysed by a reputable laboratory. The attributes which must be measured by laboratory analysis are:

The pH and alkalinity

The conductivity

Calcium, magnesium, sodium and potassium concentration

Nitrate, sulphate and chloride concentration

Iron, manganese, zinc copper and boron concentration.

Silica is an optional extra analysis , but worth getting for crops which respond to silica (cucurbits, roses, strawberries and few others )

Water may contain other elements than these, but not usually in concentrations high enough to affect plant nutrition. Growers who draw their water from public drinking water mains may not need to get the water analysed, as most public authorities obtain regular analyses from independent or certified laboratories, and can provide analysis reports to their customers.

The pH and alkalinity gives some indication of whether or not the water will be corrosive and also how much acid will be needed to neutralise the alkalinity of the water. The amount of calcium , magnesium and potassium needed in nutrient solutions can be reduced by the amount already present in the water. Sodium is not a useful nutrient, but some sodium will be taken up by most crops, however many water supplies contain more sodium than the crop being grown can take up, and in these cases sodium will accumulate in recirculating nutrient solutions. There are many references that state that water containing more than so much sodium should not be used for hydroponics but each water supply needs to be considered in relation to the crop being grown and the frequency of dumping nutrient solutions that is acceptable to the grower.

Iron in water supplies is usually not in a form available to crops and it is not usual to reduce the amount of chelated iron in nutrient solution to allow for iron in the water. Iron can be a nuisance if it precipitates within the water supply system or in recirculating nutrient solution, and can cause severe problems if it blocks trickle irrigation systems or their emitters. In such situations specialist advice on removing iron from the water supply will be needed as different situations require different techniques for eliminating iron. Manganese in water supplies behaves in very similar way to iron but is rarely present in such high concentrations as iron. Zinc and copper are not usually significant components of natural water, but the result of corrosion of metals in contact with the water. The amounts of zinc and copper included in nutrient solutions can be reduced by the amount in the water supply. A few water supplies may contain toxic amounts of zinc. Many water supplies contain traces of boron but water in geothermal areas may contains several parts per million boron. If the boron concentration in a water supply is less than the required boron concentration in a nutrient solution then the amount of boron used should be reduced to allow for the boron in the water supply. If there is more boron in the water supply than needed by the crop, then it will accumulate in recirculating nutrient solution and become toxic to the crop. Tomatoes may show reduced growth when nutrient solutions contain more than 3 ppm B and may develop visual toxicity symptoms (marginal scorch of older leaves) with more than 5 ppm, but tolerance and susceptibility to B toxicity can vary between crops.

Microbiological Quality

The microbiological quality of water supplies is also important, especially if the water comes into contact with edible produce, in which case a minimum standard is potable quality.

Potable quality implies chemical quality suitable for drinking and freedom from human pathogens, but does not imply freedom from plant pathogens. Plant pathogens require higher doses of most sterilising treatments than human pathogens and hence the assumption that town water supplies are plant pathogen free is sometimes incorrect. The risk of plant pathogens in water from rivers, creeks, ponds, spring and greenhouse roofs is appreciable and water from all these sources is best treated to eliminate plant pathogens. Water from deep bores is usually pathogen free, but shallow bores may be contaminated.

Water disinfection treatments are discussed elsewhere in GHVI. Herbicide and growth regulator contamination of water supplies is a major risk for hydroponic growers and must be carefully in considered in relation to the nature and use of the water catchment area.

Nutrient solutions

Good crop management requires regular monitoring of the chemical composition of the nutrient solution. Each grower should keep daily records of nutrient solution pH, conductivity, and amounts of water, nutrient stock solutions and acid or alkali added. There should also be regular solution analyses, some authorities recommend fortnightly analysis, many growers find monthly analyses adequate, but the ideally the timing should vary and coincide with critical periods in crop development.

Routine laboratory analysis of nutrient solution should include:

Conductivity and pH

Nitrate- nitrogen, phosphorus, potassium, sulphate  S, calcium, magnesium, sodium and chloride, and the micro-nutrients , iron, manganese, zinc, copper and boron, with molybdenum and silica as optional extras.
Nutrient solutions have to contain a balance of cations and anions, and if the laboratory does not calculate and report the sums of cations and anions then growers should do so (this calculation is detailed in the conductivity page). The sum of cations (equivalents) should more or less equal the sum of anions (equivalents) and the sum of cations (equivalents) should be within a few units of the measured CF. A lack of agreement between the cation and anion sum suggest an error or inaccuracy in the analysis. The cation sum and CF will not agree at high CF values, or if sulphate concentrations are very high in the solution.

Some laboratories provide detailed information on their services on the internet, one such laboratory than can be thoroughly recommended is Hill Laboratories Ltd of Hamilton, New Zealand (Hill-labs.co.nz). This laboratory accepts samples from overseas by prior arrangement.

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