Plant analysis has long been an integral part of commercial greenhouse nutritional programs, but it’s also a potential tool for smaller growers.
Whether it’s to help diagnose mysterious foliar symptoms or simply reassure that all is well with nutrition, mineral analysis of the foliage can provide useful and often insightful information on the nutritional status of a plant or crop.
However, it does have limitations and some drawbacks that need to be understood before the data is put to good use.
The Process of Plant Analysis
Plant or foliar analysis involves taking a small sample of leaves—usually the youngest recently expanded leaf—from a number of plants and sending them off to an agricultural lab for mineral analysis. Plant analysis labs usually provide information or a kit containing sample bags and details of how to complete the sampling process.
This includes the number of leaves required and whether these should be washed, refrigerated, or otherwise treated before sending them in. If plants are particularly dusty or have been sprayed with any chemicals such as leaf shine or other products that may interfere with the results, then washing the sample with reverse osmosis or distilled water, followed by complete drying, is advisable. Leaf samples can deteriorate quickly and so should be sent off to the lab immediately.
The plant analysis lab report generated usually states each of the macroelements (N, P, K, Ca, Mg and S) as a percentage of dry matter, while microelements (Fe, Mn, B, Zn, Cu), present in much lower concentrations, are expressed as either parts per million (ppm), milligrams per kilogram (mg/k), or micrograms per gram (μg/g). Some lab reports will also include a guide to the ideal foliar level range of each nutrient for specific crops to compare the results against, but not all do.
Instead guidelines for most common grown hydroponic plants can be found in plant nutrition books and websites that indicate ideal levels for different species at different stages of growth. A list of foliar mineral levels at which deficiency or toxicity symptoms would be expected to occur are also useful to have on-hand when comparing plant analysis reports.
When to Use Foliar Analysis
Plant analysis can be used for different purposes. It has a vital role in crop research, where nutritional programs for new crops may be determined by analyzing the mineral content of well-grown, healthy plants. This data can then be used to formulate nutrient solutions or soil programs specifically aimed at maximizing plant nutrition for that crop.
For those running small trials, foliar mineral analysis may be used to determine how different treatments, supplements, products, growing environments, and formulations affect the nutritional content of the plant.
For growers, the most common use of foliar analysis is for when suspected deficiency or toxicity symptoms are starting to show on plants. Since many physiological, disease, and nutritional disorders can look quite similar, plant analysis is often used to rule out or confirm specific nutrient issues so that these can be rectified quickly and correctly.
Foliar analysis is often used alongside nutrient solution analysis to determine necessary changes or supplements to prevent deficiencies from continuing to occur. It is also commonly used to confirm that plant nutrition has been optimal and that the nutrient formulation is suitable for the current stage of growth.
Growers experiencing unexplained symptoms such as an overall lack of plant vigor, plant stunting, or low yields may use plant analysis to rule out nutrition as the specific cause of any issues, or to diagnose nutritional problems before they become more severe.
Interpretation of Plant Analysis Lab Reports
Recommendations for the ideal level of each nutrient in foliar samples vary slightly, depending on the reference source. For this reason, ideal levels are simply given as a range within which most healthy plants should fall. Typically, this is somewhat species dependant.
For example, normal foliar levels of nitrogen (N) in crisp head lettuce are often stated as being three to 4.5 per cent, while in tomato, a crop requiring a higher level of nutrition, are 4.5-5.5 per cent; deficient levels would be less than two per cent N. For potassium, ideal foliar levels are 4.5 to eight per cent in crisp head lettuce and four to six per cent in tomato, with deficient levels being below two per cent.
Micro or trace elements often have a broader range of normal levels in healthy plants. In tomato, iron levels should be within the range of 80-200 ppm and manganese should be between 50-300 ppm, with iron levels becoming deficient at below 60 ppm and manganese below 20 ppm.
By comparing the foliar mineral levels returned on a recent lab analysis to the healthy range for the plant species being grown, it’s possible to see which nutrients fall outside the recommended range. This can help pick up any potential issues before a stage of deficiency has been reached and plant growth becomes affected.
Limitations or Issues with Foliar Testing
While plant foliar analysis is an extremely useful tool, it does have drawbacks and limitations that all growers using this process need to be aware of. First, the foliar nutrient levels returned on the lab report are more representative of plant nutrition at the time the leaf was forming. For slow-growing crops, this may have been several weeks ago.
By using both foliar analysis and nutrient solution analysis, a better picture can be drawn up as to when any particular element was deficient or needed boosting in the plants’ feed schedule.
Secondly, if odd results do appear on a foliar mineral level analysis report, it’s important to remember that contamination can be an issue. Copper fungicides, for example, can give incorrect copper readings of foliar samples and although leaf samples can be washed before sending off to the lab, some compounds may become incorporated into the waxy leaf cuticle and not be removed.
Another factor to consider is that foliar analysis does not take into account the elements (such as potassium) that are present or required by fruit. Heavy fruiting crops, such as tomatoes, partition considerable amounts of potassium into fruit tissue, so this needs to be considered if using foliar tissue analysis to formulate a new nutrient solution or adjust a current one.
Perhaps one of the most common mistakes is that while a mineral deficiency problem may show up in the report, this may not necessarily be directly caused by a lack of that element in the nutrient solution. Other factors also affect plant uptake and nutrient transportation, and subsequent foliar mineral levels.
In this case, boosting levels of the element shown to be low or deficient in the nutrient solution will not improve the problem. Other factors need to be addressed. Calcium is a common example of this, and iron is another. Calcium-related disorders such as blossom end rot and tip burn in lettuce and many other plants are a result of a lack of calcium incorporated into new developing tissue. (For help with troubleshooting problems, see What's the Problem? Your Guide to Hydroponic Troubleshooting)
However, in hydroponics, where calcium is typically supplied in most nutrient products at fairly high levels, a lack of calcium in the root zone is hardly ever the cause of the deficiency symptoms. In this case, low foliar calcium levels on a plant analysis report are often a direct result of growing conditions, which have limited plant uptake of calcium from the hydroponics solution.
These conditions include high humidity, warm temperatures, and lack of airflow. All of these restrict transpiration from the foliage, in turn restricting calcium uptake and transportation within the transpirational flow inside the plant. Increasing calcium levels in the nutrient solution to address these low foliar calcium levels (when levels are not actually deficient) will not assist with induced mineral deficiencies.
Nutrient interactions can also induce nutrient deficiencies in the foliar analysis tests despite there being no actual lack of a particular element in the nutrient solution. An example of this is in the use of high levels of ammonium (NH4), which also restricts calcium uptake in the root zone. This can induce calcium deficiency in the foliage and fruit of the plant that subsequently shows up on the foliar analysis reports.
Under certain growing conditions, such as sub-optimal temperatures, certain plants are unable to take up iron or phosphorus efficiently and may show low foliar levels, despite these elements being present in the nutrient solution at good levels.
If the plant roots have been suffering from a root rot pathogen or root die-back, foliar mineral analysis may show multiple deficiencies in many nutrients; however, this is simply due to the root system being unable to take up sufficient minerals from the nutrient solution due to tissue damage. A lack of oxygen in the root zone can also induce mineral deficiency. This is also due to damaged roots being unable to take up the nutrients required for healthy foliar levels.
To avoid these types of issues it is usually beneficial to carry out both nutrient solution and foliar mineral level testing, particularly in recirculating systems where certain nutrients may become depleted rapidly. This data can then be used to determine if there is an ongoing problem with the formulation, or lack of any one essential element in the nutrient solution causing low foliar levels, or if there is another issue effecting uptake and distribution within the plant.
Where the nutrient product or formulation is found to be the cause of low, deficient, and even toxic levels of any element as shown up in a plant analysis report, this data can then be used to adjust levels in the nutrient solution. Increasing deficient levels of most elements in the nutrient solution will rapidly lead to higher levels of uptake and incorporation into new leaf tissue.
For some elements, particularly the micro or trace elements, foliar sprays can also assist to help quickly over come a deficiency issue. A follow up leaf analysis can then be used to confirm the deficiency or toxicity problem has been solved.
Plant analysis is likely to become an increasingly important aspect of indoor hydroponic production as growers come to understand more of the complexities of crop nutrition, boosting growth and maximizing yields.
While hydroponics may seem like the ideal tool to provide the optimal level of each element in a well-balanced nutrient solution, in reality, nutrition is much more complex, and a little monitoring and plant analysis can go a long way to reaching the full potential of the system.