While we know hydroponics can provide advanced and balanced plant nutrition, sometimes a little reassurance or detective work is required when it comes to the analytical side of growing. Using laboratory testing of water, nutrient solutions, foliage, and growing mediums are all useful tools for both small and large growers alike and in these modern times are becoming standard practice in many indoor systems.

Laboratory testing is not as technical as it seems; a small sample is collected and sent away with the results usually returned within a few days. What can be confusing, however, is the interpretation and use of this valuable data to better optimize growth and plant performance or to simply identify a nutritional issue.

Lab testing can not only help with diagnosis of plant problems, but assist with basic adjustments to nutritional programs, determine the suitability of a water supply for hydroponic production, or ensure food safety guidelines are being met with microbiological testing.

Types of Lab Tests

The basic lab tests used by hydroponic growers are water, nutrient solutions, and foliar mineral levels. There are less common tests such as sap testing, analysis of growing substrates (widely used in the nursery industry for container mixes), and microbiological testing for the presence of food safety pathogens such as E.coli, which applies more to commercial growers. These tests are carried out by agricultural testing laboratories that must meet certain standards for accuracy to ensure reliable results.

The first step in obtaining any hydroponic analysis is to contact an agricultural lab and check their requirements for sending samples. Some labs will provide sampling kits including bottles for water/nutrients and bags for foliar samples to help prevent contamination, however, many just provide instructions on sample size, containers, packaging, and delivery methods to ensure the sample reaches the lab in good condition.

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Once analyzed, the lab will provide a report for each sample — basic reports may only contain the levels of variables tested, however, some labs may provide an interpretation or guidelines to optimal levels for the crop being grown.

Water Testing

Unless reverse osmosis (RO) or distilled water is being used for a hydroponic system, a water supply analysis is always a good idea. Even growers who consider their water supply to be of high quality often find surprises in their water analysis, many of which can usually be corrected or adjusted for. Most water supplies, particularly those based on well water or municipal supplies, do have some minerals present and it is always an advantage to know other parameters such as the pH, EC, and alkalinity.

A basic water supply analysis should test for: TDS (total dissolved solids), EC, pH, alkalinity, nitrogen(N), phosphorus(P), potassium(K), calcium(Ca), sulphur(S), magnesium(Mg), sodium, chloride, iron, manganese, zinc, boron, and copper, which are of importance to hydroponic plant nutrition. While the presence of low levels of macro elements such as N, P, K, Ca, Mg, and S in a water supply is not a concern, high levels of these will require adjustment of the nutrient formulation to keep the solution in balance. In most water supplies Ca and Mg are the most common elements found, while N, P, and K are much less likely to be at plant-usable levels.

Water analysis reports that show a high pH and moderate to high alkalinity (which is the strength of a high pH) give growers valuable information on how pH may need to be adjusted. Highly alkaline waters are best treated with acid before they are introduced to the hydroponic system, which provides a greater buffering capacity and less acid will be required overall to keep pH levels down over time.

Foliar Analysis

Foliar analysis is another useful tool for growers that can provide insightful and valuable information on the nutritional status of a hydroponic crop. Foliar analysis samples need to be collected correctly to be accurate; this involves taking leaves at the correct stage of growth, often the youngest mature leaf, combined from several different plants to ensure a representative sample is being tested. These are then sent directly to the lab before the leaves can deteriorate.

Foliar analysis is used for a couple of different reasons. First, it can routinely monitor nutrients in the plant tissue to help sustain growth and avoid nutritional disorders in the crop that impact yield. Second, it can be used as a diagnostic tool to identify nutrient deficiencies, toxicities, or imbalances when plants appear to be underperforming or showing strange symptoms.

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While foliar analysis gives an insight into the current nutritional status of a plant or crop, it doesn’t reflect what the plant is taking up at the time of testing. Foliar mineral levels are representative of the nutrients being taken up at the time tissue was forming, and for some slow-growing species that may have been several weeks previously. Regular foliar analysis does provide long-term information on how minerals levels change through the crop life cycle and when nutrient formulation adjustments should be made, particularly when plants move into the flowering and fruiting phases.

Foliar analysis reports returned from the lab usually state macro elements (N, P, K, Ca, S, Mg) as a percentage and micro elements as either ppm or mg/Kg which are equivalent. Some lab reports will use a comparison chart to indicate how the results stack up against optimal levels for the plant species tested; otherwise growers need to carry out their own comparison. There are many references for the ideal foliar minerals for commonly grown hydroponic crops and these may differ slightly from source to source, often the ideal level is stated as a range within which normal healthy tissue should fall (Table 1).

Table 1

Nutrient Solution Analysis

Sending samples of nutrient solution to an agricultural lab for analysis is a common practice with commercial growers, however, smaller systems also benefit from this particularly where recirculation of the nutrient solution is in use. Much like a water analysis, nutrient solution samples are collected in clean, rinsed bottles and sent off to the lab as quickly as possible. Samples can be kept under refrigeration, if necessary, before shipment.

The most commonly analyzed samples are of nutrient ‘leachate’ that is the nutrient solution that drains from the base of growing containers/slabs or beds. These samples give valuable information on how levels of each of the specific nutrient ions change as the solution flows through the root system, thus identifying any deficiencies, accumulation of elements, or general changes in balance of the nutrient formulation due to plant uptake.

Samples of nutrient taken from recirculating systems such as NFT, DFT, raft/pond type systems should be carried out after the solution has been in use for at least a week to assess how the balance of nutrient ions is changing over time. Data from these analysis reports then can be used to make adjustments to the nutrient solution — elements that are declining over time can be boosted and any accumulation, particularly of any unwanted elements such as sodium, can be identified.

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Use of solution analysis allows a grower to determine when a recirculating nutrient solution requires replacement to bring the balance of elements back in line with optimal levels. Solution analysis also allows a check to be made of system equipment such as EC and pH meters and probes. Samples should be tested for EC and pH with the grower’s meters before being sent off to the lab; these can then be compared to the lab reported EC and pH levels which are taken with fully calibrated equipment to ensure accuracy levels.

Nutrient solution samples are typically sampled for: EC, pH, nitrate, ammonium, total nitrogen, phosphorus, potassium, calcium, sulphur, magnesium, iron, manganese, zinc, boron, copper, and sodium. Nutrient solution analysis returned from the lab usually state the level of each element in either parts per million (ppm) or mg/l (milligrams per liter) which are equivalent. These, as well as pH and EC, should then be compared back to the ideal levels of each element that the original formulation or nutrient product contained. So, if a nutrient solution was used which contained 150ppm N and the analysis of the solution can back with 145ppm N, then this indicates the initial level was approximately what the crop was taking up. If the analysis report returned a much lower level, then this indicates N needs to be increased in the nutrient solution to account for the higher uptake rate.

Along with nutrient levels, unwanted elements such as the accumulation of sodium or any trace elements in the water supply can be checked with analysis reports, allowing replacement of recirculating solutions when levels become too high, thus reducing the risk of toxicity and growth issues.

Keeping good records of regular laboratory analysis reports is another important aspect of crop testing and allows trends to be seen as plants move through their life cycle or respond to different growing conditions. If cost is an issue, the most important lab test for any grower is the water supply; this can determine if water treatment is necessary, if any adjustment is required for nutrient solutions, and can help identify potential growth issues before they occur.