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Cannabis Nutrient Tissue Analysis: Ins and Outs for Sample Requirements and Other

 

First placed online Nov 2020.

 

Excerpt from larger body of work surrounding working with cannabis tissue analysis to create optimised nutrition for your cultivars and growing methodology. This excerpt is put online for my clients to guide them through some important ins and outs (lab tests required, basics of sufficiency ranges, factors that affect nutrient uptake, complexities of working with tissue analysis etc) for the agronomy (crop nutrition) work we conduct. Bookmark this page because there is currently some very important research being conducted on cannabis sufficiency ranges and I will update this material with the findings when they are released.

 

Factors that Affect Nutrient Uptake and Translocation  

 

I often talk to cannabis growers who believe that when their plants display a nutrient deficiency symptom the problem can be fixed by providing more of the suspect nutrient to their plants. The logic here is straight forward; the plants are displaying symptoms of a deficiency. As a result, the grower reasons that if he or she supplies more of the nutrient that seems to be lacking the problem will be fixed. However, often things are not this straight forward.

 

First off, analysing deficiency symptoms can be very complex. For example, many symptoms appear similar; e.g. N and S deficiency symptoms can be very alike, depending upon plant growth stage and severity of deficiencies.

 

Secondly, multiple deficiencies can occur at the same time. If more than one nutrient is deficient, visual symptoms may be misleading.

 

Third, excess of one nutrient can antagonize another nutrient, resulting in that nutrient becoming deficient.

 

Lastly, there are pseudo (false) deficiency symptoms (visual symptoms appearing similar to nutrient deficiency symptoms) to consider. Potential factors causing pseudo deficiency include, disease, suboptimal temperatures, sodium chloride contaminants in the water supply and/or substrate, genetic abnormalities, herbicide and pesticide phytotoxicity, insects, and suboptimal root zone conditions (e.g. oxygen deprivation).

 

Let’s firstly consider what it means to have a deficiency. This means that a nutrient or nutrients in the plant’s tissue falls below what is termed the nutrient sufficiency range. A plant’s nutrient sufficiency range is the range of nutrient amount necessary to meet the plant’s nutritional needs and maximize growth. Nutrient levels outside of a plant’s sufficiency range cause overall crop growth and health to decline due to either a deficiency or toxicity. Where symptoms of a deficiency are concerned, there are several reasons why a nutrient might fall below the sufficiency range in the plant tissue. The nutrient could be completely absent, or there could be not enough of the nutrient being provided to the plant, or there could be enough of the nutrient being provided, but the nutrient is unable to get to the plant tissue. To establish which of these factors it is growers need to analyse what is usually required re the concentration of a nutrient that should be provided to the plant via the hydroponic nutrient solution and then evaluate whether that element is in an adequate concentration in solution.

 

Just as importantly you also need to consider the ratios of that nutrient inline to other nutrients because nutrient uptake occurs via proteins embedded in root membranes that catalyse the transport of nutrients across the membrane. Similar cations and similar anions compete for binding to specific carrier proteins, whereas the uptake of cations versus anions occurs through different transport proteins. [1] As a result, an excess of one nutrient can result in what is termed nutrient antagonism, whereby the excess of one nutrient impedes the uptake of another nutrient. For example, it is well known that a high K and Ca can induce Mg deficiency in crop plants.  Many growers may identify a Mg deficiency and conclude more Mg is required. However, adding more Mg to solution is unlikely to correct the problem and the appropriate course of action would be to reduce Ca and/or K to ensure more Mg is available to the plant.

 

Another key problem I regularly encounter when working with cannabis tissue analysis is that high Ca reduces K uptake. For example, I often find many of my clients are using far more K than is optimal for cannabis; however, even at extremely high ppm of K (e.g. 350 ppm K) the tissue is low on K. This is typically caused by high/excess Ca in solution and by simply reducing Ca we quickly ensure that adequate K is getting to the leaf and flower at lower ppm than they were previously running in solution (e.g. we may drop K from 350 ppm to 260 ppm with lower Ca and get higher K in the plant tissue).

 

A report by the Virtual Fertilizer Research Center in 2015[2] looked at 96 scientific research papers in which single nutrient effects and their interaction effects were studied. Within these papers, 116 nutrient interactions were identified: 17 cases of these were antagonism.

 

Most antagonistic reactions were between nutrients such as zinc (Zn), copper (Cu), iron (Fe), calcium (Ca) and magnesium (Mg). These are all cations, or positively charged molecules or atoms.

 

While the science that underpins nutrient antagonism is complex, what it comes down to is the excess of any one nutrient can potentially antagonize the uptake and movement into the plant tissue of another nutrient (or nutrients’ plural) resulting in a deficiency or deficiencies. Depending on the extent of the deficiency this can present with visual deficiency symptoms.

 

Another issue that can occur is a deficiency of a nutrient can lead to deficiency of another nutrient. For example, low Mg can lead to low P in the tissue. This is because interactions between P and Mg occur in plants, as Mg is an activator of several kinase enzymes and activates most reactions involving phosphate transfer.[3] Therefore, adequate supply of Mg is necessary to prevent P deficiency. This situation (low Mg inducing P deficiency) may result in visual deficiency symptoms of Mg or P or both. This makes what is actually occurring hard to diagnose because while both nutrients are deficient an array of visual symptoms can appear, leading growers to the wrong conclusion.

 

Lastly, on the subject on nutrient ion levels in solutions and substrates, another issue that can occur – albeit rare in most hydroponic growing situations – is NaCl (sodium chloride) toxicity which presents with visual symptoms – generally as chlorosis and/or necrosis. High levels of the cation sodium (Na+) in the rootzone interferes with the cations Ca++, Mg++ and K+ uptake while high concentrations of chloride can cause phytotoxicity problems. This phytotoxicity results from accumulation of chloride in the leaves. In one cannabis study, it was found that NaCl in excess of 5 mM in the rootzone may decrease yield and potency of cannabis.[4] NaCl toxicity typically occurs because of a poor source water supply or a contaminated organic substrate or fertilizer. For example, some poorly treated coir substrate products can possess high levels of sodium and chloride. The thing is that unless growers test their substrate and water supply they often see NaCl issues as a deficiency or toxicity and add or subtract nutrients in an attempt to correct things. This in all instances fails. The only way to correct this issue is to remove the source/s of the high NaCl from the growing equation.

 

What this all boils down to is the uptake and utilization of nutrients depends not only on the quantities but also on the ratios among nutrient types. If a particular nutrient is deficient, yields can be negatively impacted. A similar reduction in plant growth can arise when a particular nutrient is present at a concentration that is too high. All the nutrient ion types need to be within their respective ranges if plant productivity is to be optimized. Departure from these optimal levels in any of the nutrient ion types will have an influence on others as well.

 

Environment

 

Environmental factors can have a profound effect on the nutritional status of the crop. This is because fertilizers are not plant food. They are no more food for plants than iron, calcium or zinc supplements are food for people. Plants produce their own food from water, carbon dioxide, and light energy through photosynthesis. This food (sugars and carbohydrates) is combined with plant nutrients to produce proteins, enzymes, vitamins, and other elements essential to growth.

 

Anything that reduces or stops sugar production in leaves can lower nutrient absorption. Thus, if a plant is under stress because of low light or extreme temperatures etc, nutrient deficiency may develop.

 

For example, heat stress can reduce plant photosynthetic and transpiration efficiencies and negatively impact plant root development. Based on a few past studies, it is known that heat stress often decreases the concentration of nutrients in plant tissues or decreases the total content of nutrients in the plants, though effects can vary among nutrients and species. Heat stress can also disrupt enzymes involved in nutrient metabolism. Decreases in nutrient acquisition with heat stress could potentially be caused by several factors, including a decrease in root mass or surface area and/or a decrease in nutrient uptake per unit root. [5]

 

Vapor Pressure Deficit (VPD), the combination of RH and temperature, also affects nutrient uptake. For example, a deficiency of K, even where there is adequate K in solution, can come from a high temperature with low humidity which increases the VPD, so much so that Ca transport is favoured over K uptake. This results in the plant becoming K deficient despite there being enough K in solution because the transport of another nutrient is able to out compete it due to high VPD. The solution is not to increase K, nor is it to decrease Ca. The solution in this case is to bring the VPD (temperature and humidity) to a more ideal level, so that the absorption of Ca and K can be optimized.

 

In general, high VPD (low humidity) can cause several issues in nutrient uptake – namely that the rate of transpiration increases, increasing water and nutrient movement through the plant. As a result, when working with tissue analysis I often see excess of the micronutrients, particularly Fe, Cu and Zn, where under the same levels of micronutrients in solution at lower VPD (higher humidity) this wouldn’t be occurring. Conversely, under high humidity (low VPD) water and nutrient transport into the plant can be reduced (due to reduced transpiration) and this can result in lower levels of certain nutrients in the tissue.

 

Other environmental factors can also play a key role in determining transport. For example, low nutrient solution and/or substrate temperature often causes a deficiency of nutrients in plants because nutrient absorption at the root level is hindered by low temperature. Studies have shown that low rootzone temperature causes a significant reduction in total N, P, and K uptake under hydroponic growing conditions. Other nutrients such as iron are also affected.

 

Low root zone oxygen also significantly affects nutrient uptake. In one study, the consequences of oxygen deficiency on the root system of tomato plants in soilless culture at the beginning of the flowering stage were assessed over a 72‐hour period. Oxygen deficiency of the nutrient solution had immediate effects on the water and nutrient uptake of the whole plant. The root asphyxia of a tomato plant caused a 20 to 30% decrease of water uptake after 48 hours. After 10 hours it also leads to the end of the uptake process of the nutrients except nitrates. Potassium (K) was the nutrient most sensitive to oxygen deprivation. Nitrate uptake was the least affected by oxygen deficiency.[6]

 

In general, the water – oxygen relationship in the substrate plays a crucial role in nutrient uptake. For example, overdrying of the substrate through insufficient fertigation also has a profound impact on nutrient uptake.  Most of the nutrients a plant needs are dissolved in water and then absorbed by its roots. In fact, in soil growing, 98 percent of the nutrients are absorbed from the soil-water solution, and only about 2 percent are actually extracted from soil particles. Hydroponic substrates follow a similar trend to this. If the substrate becomes too dry nutrient availability is greatly reduced and this will reflect in the plant tissue.  Conversely, if the substrate becomes too wet and, as a result, oxygen deficient this too severely impacts on nutrient uptake.

 

Fertigation frequency also affects the nutrient status in the tissue. High fertigation frequency (e.g. 5 – 6 feeds daily) versus low fertigation frequency (e.g. a single feed per day) under the same ppm of each nutrient ion in solution will have different outcomes on the nutrient status of the tissue, particularly pertaining to the micronutrients and P.

 

pH also affects nutrient availability and uptake, as does EC. Too high an EC increases osmotic pressure limiting water and nutrient uptake while a low EC increases the mass flow of water and nutrients into the plant. A high EC doesn’t necessarily relate to the nutrient solution; nutrient buildup in the substrate can result in excessive EC in the rootzone. On the other hand, the pH of a substrate where charged colloids are present can influence the availability of the individual ions within that substrate. As pH changes in the substrate one particular nutrient ion may gradually become more insoluble, leaving less of that ion available to act as a nutrient. For example, If the pH of the growing medium rises above 6.5, iron, manganese, zinc, copper and boron start to become less available for plant uptake. As the pH drops from e.g. 6.5 to 5.5, these elements become more soluble and more readily available to the plant.  Molybdenum, on the other hand, is the opposite. It becomes soluble at high pH and insoluble at low pH. Conversely, if the pH of the substrate drops too low this may increase micronutrient uptake to a point where some micronutrients creep towards excessive levels in the tissue. In general, there are all sorts of chemical reactions that can occur between different elements in hydroponic solutions and substrates and pH is an important determinant in many of these processes.  It has been shown by scientists at NCSU (North Carolina State University) that cannabis has a wide pH range when compared to many other crops but this is determined by there being appropriate amounts of nutrients available across a wide pH range, and therefore growers should still focus on maintaining pH within optimal range.

 

Deficiency or Excess Symptoms Can be Cultivar/Genotype Specific

 

Often when I do consults with grows that are producing multiple cultivars under the exact same nutrient and environmental conditions (not ideal, but this is often a commercial reality of scale) we may find that 99% of the cultivars being produced under a given nutrient regime grow well while 1 or 2 cultivars start expressing nutrient deficiency or excess symptoms under the same nutrient regime. This is generally fixable by bringing back (toxicity) or increasing (deficiency) a nutrient or nutrients being provided to the plants. However, in some cases by correcting the nutrition for one or two cultivars, nutrient optimums for other cultivars is compromised. This really comes down to the fact that optimum crop nutrition for cannabis is cultivar specific and some cultivars thrive under higher or lower levels of nutrients than others. I’ll talk more about cultivar specificity of cannabis nutrition in a moment.

 

For now, you can perhaps tell, there are numerous factors that can affect nutrient uptake and translocation. This is reflected in the nutrient status of the plant tissue. It is often not simply a case of identifying a deficiency of a nutrient and adding more of that nutrient to correct the problem.

 

Nutrient Sufficiency Ranges

 

Nutrient sufficiency or critical nutrient concentration is a relative term because an absolute sufficiency cannot necessarily be determined. Nutrient sufficiency is a measure of nutrient concentration in the plant and is determined by plant tissue analysis. It is preferably expressed as a range of concentrations rather than a single concentration. Sufficiency of a given nutrient lies between critical deficiency value and an excess or toxic concentration. See following nutrient sufficiency ranges for cannabis.

 

Cannabis Sufficiency Ranges 

 

 

 

 

 

 

 

 

 

 

 

 

 

Sufficiency Ranges for Cannabis from a Colorado Lab

 

  Low Sufficient High
N% < 3.3 3.3 to 4.76 > 4.76
P% < 0.24 0.24 to 0.49 > 0.49
K% < 1.83 1.83 to 2.35 > 2.35
Ca% < 1.47 1.47 to 4.42 > 4.42
S% < 0.17 0.17 to 0.26 > 0.26
Mg% < 0.40 0.40 to 0.81 > 0.81
Zn ppm < 24 24 to 52 > 52 8
Cu ppm < 5 5 to 7.1 > 7.1
Fe ppm < 100 100 to 150 > 150
Mn ppm < 41 41 to 93 > 93

 

Cannabis Averages from a California Lab

 

Units are mg/kg unless otherwise stated

 

  Average
N by percentage 5.6%
P 6,604
K 29,030
Ca 41,882
S 4,596
Mg 7,775
Zn 74.0
Cu 10.8
Fe 149.8
Mn 89.7
Mo 1.5
Si 884.70
Cl 2,759
Na 253.13

 

The nutrient status of a crop can be measured through lab tissue analysis and the results can be measured against sufficiency ranges.

 

The sufficiency range for any crop is simply the range of concentrations normally found in healthy, productive plants during growth. Ideally, sufficiency ranges are developed through research that includes side-by-side crop trials where differing levels of nutrients are applied and yield is measured. Additionally, as cannabis is a medicinal oil producing crop, any such side-by-side trials would need to measure cannabinoid production.  To date, this hasn’t occurred with high THC cannabis (or at least it hasn’t occurred at an academic level). Thus, cannabis sufficiency ranges are a work in progress and these ranges only really tell us that the nutrients found in the tissue should be adequate to produce healthy growth (i.e. visual nutrient disorders shouldn’t present within these ranges), but not necessarily optimal yields and cannabinoid percentages.  In other words, cannabis sufficiency ranges tell us only that the nutrients are sufficient and not necessarily optimal. Other than this, it is well established that optimum crop nutrition for cannabis is genotype specific. This complexifies matters further because optimal crop nutrition may vary cultivar to cultivar. What this means is that when looking at these quite wide sufficiency ranges that somewhere within each nutrient sufficiency range is the ideal for producing crop optimums.

 

Genetics/Cultivar Influences Optimums in Cannabis Nutrition

 

Cannabis growers cultivate a wide variety of cultivars/hybrids. Each cultivar/hybrid has different nutritional requirements than the other cultivars/hybrids. Nutritional programs should be adjusted according to the specific cultivar’s/hybrid’s requirements.

 

For example, one study found when looking at the potassium requirements of cannabis in two separate cultivars that:

 

[Quote]

 

The plants were exposed to five levels of K (15, 60, 100, 175, and 240 ppm K). Growth response to K inputs varied between genotypes, revealing genetic differences within the Cannabis sativa species to mineral nutrition. Fifteen ppm of K was insufficient for optimal growth and function in both genotypes and elicited visual deficiency symptoms. Two hundred and forty ppm K proved excessive and damaging to development of the genotype Royal Medic, while in Desert Queen it stimulated rather than restricted shoot and root development. The differences between the genotypes in the response to K nutrition were accompanied by some variability in uptake, transport, and accumulation of nutrients. For example, higher levels of K transport from root to the shoot were apparent in Desert Queen. “ [7]

 

[End Quote]

 

Landis et al (2019) found that significant differences in nutrient concentrations occurred among CBD (hemp) cultivars, suggesting that broader target nutrient ranges may be appropriate for cannabis. This study concluding:

 

[Quote]

 

“This study identified significant differences in leaf tissue nutrient concentrations among greenhouse grown CBD hemp Cannabis cultivars, which suggests nutrient uptake, partitioning, and/or utilization may differ among cultivars.”[8]

 

[End Quote]

 

As an example of significant differences in leaf tissue nutrients, here are some results from one cannabis tissue analysis I ran for a US based client in early 2020. From this analysis you can see quite significant differences between calcium (Ca) and phosphorous (P) when looking at two separate cultivars grown under the exact same nutrient program in the exact same environmental conditions.  Both cultivars had tissue samples taken for analysis at week 5 of flower. Units are mg/kg.

 

 

Element

GMO-Wk5-RW 1/2 PK Week 5 Rockwood Flower SS-Wk5 – RW 1/2 PK Sunset Sherbet Rockwool

Cannabis Average Values

 

Phosphorous 5,666.22 7,456.89 6,604.4
Calcium 38,782.2 52,375.00 41,882

 

From this analysis you can see quite significant differences between Ca and P when looking at two separate cultivars grown in the exact same environment under the same nutrient program where tissue samples were taken at the same week of flowering. These differences are common (the norm rather than the exception) when tissue analysis is conducted on different cannabis cultivars.

 

Author’s note: On the point of optimised cannabis nutrition being cultivar specific. Clients sometimes ask me if they can produce all of their chosen cultivars under the same nutrient regime without issue.

 

The answer to this is, in general, yes. Most cultivars will do well under the same nutrient regime when this regime is tailored to the environment (VPD etc) and growing methodology.

 

The reality of commercial cannabis production is that it requires homogenous substrates, crop nutrition and fertigation. The reality is also that multiple cultivars are typically produced under the same environmental conditions (light, temp, RH etc).

 

Studies have shown that different cannabis genotypes have different optimums with regards to air temperatures, CO2 and light levels; studies have now also shown that different cannabis cultivars have somewhat differing nutrient optimums (e.g. some cultivars may thrive under higher K than others), so the chances are that producing multiple cultivars in the exact same environmental conditions, under the exact same nutrition is always going to be somewhat of a compromise. However, producing multiple cultivars under homogenous conditions (crop nutrition, environment, fertigation strategy etc) is also a commercial reality and the aim is to get the best production possible out of all of the cultivars being grown in any given setting. 

 

Anyway, back to tissue analysis and sufficiency ranges….

 

There are also other issues when dealing with tissue analysis. For example, nutrient concentrations in the plant vary during the crop cycle and also vary among plant parts. Interpretations of nutrient sufficiency ranges were developed for specific growth stages and specific plant parts. Erroneous interpretations can be made by sampling the wrong part of the plant and/or sampling at the wrong growth stage.

 

For example, tissue age influences the nutrient concentration; several examples are reported in the literature, including studies in alfalfa, potato, corn, peach, and many other agricultural and horticultural crop species. Although some exceptions may occur, concentrations of nitrogen, phosphorus, potassium and sulfur tend to decrease with tissue aging. On the other hand, calcium and magnesium concentrations tend to increase in older tissues (low mobility). The dynamic nature of the plant tissue mineral composition tends to restrict the use of leaf analysis for nutritional diagnosis.

 

Additionally, to supply adequate nutrition for optimum plant growth, the physiological stage of development must be considered when adjustments to nutritional regimes are required. This requirement is attributed to the fact that nutrient concentrations in plant tissues and the demand for those nutrients fluctuate with the stage of plant development.[9] For example, plants may absorb different ratios of nutrients at lower or higher rates as flower buds begin to develop than they do during periods of rapid vegetative growth.

 

While research on cannabis is still lacking, other flowering/fruiting crops such as tomato provide some insight into this. For example, in one study with tomato, where plant and media nutrient analysis were conducted at three stages of plant development, it was found;

 

[Quote]

 

“Plant biomass increased linearly as fertilizer level increased or as time progressed. Generally, concentration of extractable nutrients in the medium increased linearly with increases in nutrients in the solutions. However, as time progressed, N concentrations in medium rose, but P, K, Ca, and Mg in the medium fell. Concentrations of N, P, or K in leaves increased as nutrition increased, but Mg or Ca in leaves had no significant changes with increased nutrient supply. With time the N, P, Ca, and Mg concentration in tissues fell, but K rose. The concentrations for K in the medium decreased as plant growth progressed. Potassium in the medium was lowest at flower initiation and at fruit formation indicating that the K supply was depleted and that possibly not enough K was supplied at the advanced stages.”

 

[End Quote]

 

Of course, cannabis follows a very similar trend to this. That is, the N requirement drops and the P and K requirement increases during flower. This can be seen through running tissue analysis in unison with substrate analysis where N begins accumulating in the substrate while much higher levels of P and K are removed from the substrate during flowering when compared to vegetative growth. In short, the balance between crop yield, quality and nutrient status can be manipulated by exploiting nutrient interactions.

 

The problem is that, to date, very little serious research has been conducted with high THC cannabis to establish sufficiency range optimums at the various stages of growth. And arguably, the most reliable sufficiency ranges we have to date were developed by Landis et al (2019);[10] however, these ranges were established with greenhouse grown, low THC hemp varieties using vegetative plants that were 12 weeks old. Therefore, given they were not flowering plants, during which period high levels of biomass production occurs, these sufficiency ranges are unlikely to be optimal for high THC flowering cannabis varieties.

 

Other issues also present when interpreting tissue analysis. Martin and Matocha (1973) state that “the basic principle of the use of plant analysis is that the chemical composition of the plant reflects its nutrient supply in relation to growth.” They caution, however, that “the chemical composition of any plant is a `result’ of the interaction of nutrient supply and plant growth. Any factor that limits growth may cause other nutrients to accumulate in the plant.” They point out that in using plant analysis as a diagnostic tool, “we are in effect attempting to infer a cause and effect relationship from two results (yield and nutrient concentration), either of which may have been brought out by some other factor.” For example, water stress caused by cold temperatures can result in reduced tissue nutrient even though the nutrient supply in solution would be considered adequate under normal conditions.[11]

 

Another limitation of plant analysis is that it usually detects only the one element that inhibits plant growth the most. Rarely are two or more elements acutely deficient at the same time. A plant, for example, may be deficient in K, but because K is limiting growth there may be sufficient P for the reduced amount of dry-matter production, even if P is low in the nutrient solution. When K is added to correct the problem dry-matter production increases sharply, then P becomes deficient.

 

Basically, working with sufficiency ranges and tissue analysis is far more complex than some would paint it.  Further, achieving optimum yields comes down to providing the plant with the entire package, not just aspects of it. Too often, I deal with clients who seem to be looking for a magic bullet in their nutrient regime that acts to correct less than optimal environmental conditions. It’s, however, imperative that growers understand that what drives yields is a combination of environment (air temperatures, RH, day-night DIF, CO2 levels, light PPFD, rootzone conditions, pest and fungi prevention and management etc) along with crop nutrition. The two (environment and nutrition) are indivisible in driving plant growth and all limiting factors must be excluded from the growing equation in order to realise optimum yields.

 

Understanding Sufficiency Ranges

 

 

 

 

 

 

 

 

 

 

The graph shows a general crop yield-response to fertilizer application. Generally speaking, higher fertilization level gives higher yields, but only up to a certain point. Beyond this point, addition of fertilizers will not increase yields and may even reduce them as a result of nutrient toxicity and/or too high levels of nutrient salts in the root zone which leads to osmotic stress.[12]

 

First off, we have the deficiency range or seen here as “deficiency symptoms”. In the deficiency range, visible nutrient deficiency symptoms are evident and crop yield is less than 75% of maximum.

 

However, for many nutrients, yield decreases occur before visible deficiency symptoms become apparent. This is defined as falling within the ‘hidden hunger’ and/or ‘critical range’ where the plants are being underfed but show no visible deficiency symptoms. Where “hidden hunger” is concerned, as the name implies, the plant is hungry but we cannot see it (i.e. symptoms of hunger are “hidden”). Because the exact concentration of a nutrient below which yields decline is difficult to determine precisely, some experts define the critical level as the nutrient concentration at 90 or 95% of maximum yield. However, hidden hunger can be present well before this without visible signs of a deficiency.  For example, a grower may only be achieving 85% of the maximum possible growth/yield before visual symptoms of a deficiency present.

 

In fact, scientifically speaking, the expression of nutrient deficiency symptoms varies for acute or chronic deficiency conditions. Acute deficiency occurs when a nutrient is suddenly no longer available to a rapidly growing plant. Chronic deficiency occurs when there is a limited but continuous supply of a nutrient, at a rate that is insufficient to meet the growth demands of the plant.

 

Most of the classic deficiency symptoms described in textbooks and online are characteristic of acute deficiencies. However, the most common symptoms of low-grade, chronic deficiencies are a tendency towards darker green leaves and stunted or slow growth. So basically, where a chronic deficiency is present, the plants leaves are dark green. As such, the plant looks healthy to novice growers (if it’s dark green its healthy right?). The problem is that dependent on the degree of a chronic deficiency many novice growers are unlikely to be able to spot that growth rates are less than optimal. The only way of knowing that hidden hunger or a chronic deficiency is present is through lab analysing the plant tissue or through having another plant that is being fed with more ideal nutrition to measure growth rates against.

 

This is something that hydroponic nutrient manufacturers and others typically forget to mention. That is, many ‘hydro’ growers have been led to believe that if a nutrient deficiency or excess is present then the plant will tell them this through displaying symptoms of excess or deficiency.

 

Nothing could be further from the truth! The fact is that yield losses can occur long before signs of excess or deficiency become apparent.

 

Back to our graph; when we increase feed levels we reach a relatively wide ‘sufficiency range’ where there is enough nutrient to ensure healthy growth. As you can see, there is some degree of tolerance within the sufficiency range where, if say we were in the middle of this range, we could marginally increase or decrease nutrient levels without compromising plant health.  When working with tissue analysis, in fact, the aim is to keep nutrients at about mid-range to accommodate for subtle shifts. Going too high in the sufficiency range is not ideal, nor is having nutrients too low in the range. We’ll talk more about this later.

 

Moving into the ‘luxury consumption range’, which describes the accumulation of nutrients above what is required for healthy plant growth and function i.e., “luxury consumption” is a process described for numerous plant species, mostly related to K uptake. ‘Luxury consumption’ of K usually does not affect growth and development of plants, but it was previously reported for numerous species including cotton, that excessive K fertilizer reduces plant biomass.[13] Thus, while luxury consumption provides no benefits to growth this level of fertilization may stand to impair growth – although this hasn’t been fully investigated in cannabis at this point in time.[14]

 

As we apply more nutrients we move into the toxicity range. Toxicity is broken into ‘incipient toxicity’ (where nutrients accumulate in the plant tissue to such a degree that they start to become toxic and growth rates begin to decline) and toxicity (where nutrients are supplied at such high levels they are immediately toxic, greatly impacting on plant health). As the name implies, incipient toxicity describes being in an initial stage; “beginning to happen or develop”. Therefore, incipient toxicity is where excess nutrients slowly begin to accumulate in the plant tissue to such a degree that they start to become toxic.  Signs of excess won’t necessarily become apparent for some time. However, growth will be impaired long before nutrient excess signs become apparent. Therefore, while growers are providing too much nutrient (enough nutrients to impair growth) they can be completely unaware of this because the visual symptoms that growers attribute to excess aren’t apparent. Nevertheless, they are losing yield because this excess is hidden and is impacting on growth.

 

For practical purposes, the point of importance is the critical level at which yield declines from an increase in nutrient concentration – the so-called toxicity range at which point excessive levels of a nutrient reduce growth. If the nutrient supply is increased sufficiently, yields decline either because of an imbalance with other plant nutrients or direct toxic effects of the nutrient excess. Phosphorus, for example, at excessive levels can antagonize the uptake of copper, iron and zinc and be out of balance with respect to nitrogen or potassium, but it is rarely toxic per se (i.e. too much phosphorus will impair growth due to antagonizing copper, iron and zinc but phosphorus toxicity symptoms are unlikely to be apparent because P toxicity is extremely rare in most plant species). Thus, while too high phosphorus application will reduce yields through antagonizing other key nutrients such as copper, iron and zinc, visual toxicity symptoms are unlikely.

 

Last of all we have the lethal range, where nutrients are applied at such a high degree they kill the plant. This one pretty much speaks for itself, albeit toxicity symptoms would typically present well before we hit the lethal range and steps could be taken to prevent crop death.

 

Cannabis Nutrient Optimums and Sufficiency Ranges – Overview

 

The sufficiency range for a crop of each nutrient has been determined primarily by university research.

 

In theory, the determined sufficiency range is the area where an increase in tissue nutrient concentration is not accompanied by an increase in growth. However, the sufficiency range for each nutrient is based on a particular stage of growth and a specific plant part of each crop.

 

To supply adequate nutrition for optimum plant growth, the physiological stage of development must be considered when adjustments to nutritional regimes are required. This requirement is attributed to the fact that nutrient concentrations in plant tissues and the demand for those nutrients fluctuate with the stage of plant development.[15] In general, the concentrations of nutrients within the dry matter of plants declines as the age of the plant progresses.[16]  The concentrations of most nutrients reduce as plants grow because structural components make up a larger proportion of the plant changing the nutrient content to dry matter ratio. Rapidly growing plants may actually have taken up more nutrient than a stunted plant, but proportionately more aboveground growth dilutes it down.

 

In general, the tissue contents of N, P, and K decrease as the stage of plant development progresses and the contents of Ca, Mg, Mn, and B often increase.[17]  Therefore, the optimum concentrations of mineral nutrients are generally less in older plants than in younger plants. [18]

 

Further, plants also may absorb different nutrients at higher levels and differing ratios as flower buds begin to develop than they do during periods of rapid vegetative growth. For example, cannabis preferentially requires higher P and K and lower N during flower, while higher N and lower P and K during vegetative growth elicits better growth outcomes.[19]

 

One factor to consider is that leaf tissue analysis does not take into account the elements such as potassium that are present or required by flowers. Heavy flowering crops, such as cannabis, partition considerable amounts of potassium into bud tissue, so this needs to be considered if using tissue analysis to formulate a new nutrient program or adjust a current one.

 

As previously noted, to date, arguably, the most reliable sufficiency ranges for cannabis were developed by Landis et al (2019);[20] however, these ranges were established with greenhouse grown, low THC hemp varieties using vegetative plants that were 12 weeks old. Therefore, given the tissue that was sampled came from plants that were not flowering, during which period high levels of biomass production occurs, and at which point the P and K requirement of cannabis increases while N requirement reduces, these ranges may not reflect ideals for cannabis during the reproductive phase of growth. As such, the database for cannabis nutrient sufficiency ranges is still a work in progress and will be further refined over time as scientific studies determine optimums at different stages of growth. For now, having used the Landis et al (2019) sufficiency ranges, during both vegetative and flower, they provide reasonably good outcomes when developing a crop nutrition program. However, the theoretical statement that the “sufficiency range is the area where an increase in tissue nutrient concentration is not accompanied by an increase in growth” is too simplistic where it applies to cannabis given 1) cannabis sufficiency ranges are still a work in progress; 2) inadequate data exists for flowering cannabis plants; 3) given my own experiences working with these ranges, growth benefits are achieved outside of these ranges pertaining to several nutrients and 4) these ranges don’t provide much in the way of insight into the ideal NPK ratio changes that occur between vegetative and reproductive growth.

 

The other thing, of course, is cannabis sufficiency ranges don’t; 1) factor in cultivar; i.e. each cultivar may have slightly different nutrient optimums and 2) cannabis is produced for, among other things, cannabinoids, while sufficiency ranges only factor in healthy growth. For example, it has now been shown that reducing K below sufficiency at a given point of the flowering cycle can result in improved cannabinoid production. That is to say, a visually healthy looking plant doesn’t necessarily produce the highest cannabinoid yield. Other than this, based on my own findings, yield improvements can be realised by making subtle tweaks to the crop nutrition during the flowering cycle. That is, achieving optimum yields with crop nutrition is not a simple case of ensuring all the nutrients fall somewhere within their deemed to be sufficient ranges.

 

Working with Tissue Analysis

 

Plant tissue analysis is a very effective way to monitor and optimise your nutrient regime for each stage of growth. Fortunately, tissue testing for high THC cannabis is now possible and there are several laboratories who conduct this testing in North America.

 

However, the interpretation of the results requires some experience and skill. Since results are dependent on many factors, it is essential to understand these factors and to conduct other tests, such as substrate analysis and side-by-side growth trials under slightly different nutrient regimes in order to establish optimum crop nutrition. Other than this, as I have previously highlighted, numerous factors can affect the nutrient status of the crop. The complexities that these factors create cannot be understated and some expertise is required to analyse and correct crop nutrition to optimum.

 

Routine v. Diagnostic Analysis

 

Plant tissue analysis falls into two major categories: routine analysis and diagnostic analysis. Routine analysis is for problem prevention, whereas diagnostic analysis is for problem solving.

Routine analysis allows growers to monitor nutrient uptake in the plant during production. This requires a grower to take multiple samples (e.g., vegetative growth, week 3 of flower, week 5 of flower etc) over the course of the crop cycle to refine a nutrient program and/or to identify problems or issues before they become serious concerns.

On the other hand, growers can use diagnostic analysis to diagnose deficient or toxic nutrients when the plants are displaying symptoms of deficiency or excess. This process helps identify the specific problem and is primarily a corrective tool. Plant tissue analysis is especially useful in determining micronutrient levels in the plant and has a greater level of accuracy than a substrate test using a water-based extraction.

 

Diagnostic Analysis

 

By its very nature, diagnostic tissue testing is only undertaken after a problem has been recognized. Often a grower will see some visual clue that the crop is not as it should be. At this point, information to help make a diagnosis is needed, one component of which is tissue analysis. Other information, such as solution and substrate testing, climatic data and pesticide and fertilizer records will often be needed before the problem is correctly identified.

 

To accurately diagnose a nutritional disorder, growers can take samples from the symptomatic portion of the plant. For example, if the lower leaves display toxicity symptoms then cultivators should sample these leaves. Because the critical lower deficiency ranges and toxic upper ranges for cannabis are not known at this time, cultivators should collect a good control sample to compare to their problem sample and submit the two samples separately for analysis.

 

Routine Analysis

 

The most important use of plant analysis is as a monitoring tool for determining the adequacy of current fertilization practices. Sampling a crop periodically during the crop cycle provides a record of its nutrient content that can be used in future. With nutrient solution test information and a plant analysis report, a grow operator can closely tailor fertilization practices to specific substrate-fertigation-cultivar needs.

 

Taking and Sending Tissue Samples

 

Careful sampling ensures the effectiveness of plant analysis as a diagnostic tool. Accuracy starts with the right sample. Use care when collecting, preparing and sending plant samples to the lab because the analysis is only as good as the sample. If a poor sample is taken, inaccurate results are likely. Follow these steps to ensure accuracy.

 

  • If you are wanting to test several cultivars, each cultivar needs to be tested separately of the other cultivar/s. This means taking leaf samples from each cultivar and bagging and submitting them to the lab as separate samples. I.e., if you are testing three cultivars you will be submitting three separate samples to the lab for tissue analysis.
  • Leaf tissue analysis submissions require about 15-20 fully formed leaves per cultivar taken near the top of the plant. If you have several of the same cultivar growing in the same area spread the sample taking across several plants.
  • The uppermost, recently mature leaves from a plant will provide the most ideal plant sample. Typically, young developing leaves and older mature leaves will not accurately reflect the nutrient status of the whole plant. Note re tissue samples: Chronic deficiencies or excesses of certain nutrients may indicate a sampling problem. Since calcium accumulates in lower leaves as cell walls develop, consistently low levels of this element when there are no visual deficiency symptoms may indicate the sample is being taken too near the growing point. Likewise, consistently high calcium and low potassium may indicate the sample is being taken too far down from the growing point.
  • Use healthy tissue. Samples should not come from plants that have experienced long periods of stress. This stress includes, but is not limited to: drought, heat, water or nutritional stress, mechanical damage, disease damage and insect damage.
  • Avoid contamination. If there is a chance that the plant sample could have fertilizer residue, soil, or other forms of contamination (e.g. fungicides and/or pesticides) on the leaves or petioles, rinse the sample with deionized water (e.g. RO). Tap water may contain ions such as iron, calcium and magnesium, which can affect analysis. Dry the leaves with paper towels after washing, prior to packaging them. After rinsing the plant tissue it is critical to get samples to the lab quickly.
  • Tissue testing is not recommended if the crop has received foliar sprays containing nutrients, especially micronutrients. There is no way to completely remove residues from leaf surfaces and these residues result in higher test results than is actually in the plant tissue.

 

Procedure for packing samples

 

  • Place leaves between folded up dry paper towel to absorb moisture during shipping in a ziplock bag.

 

  • Label the bag/s with your sample name which should include an abbreviated strain/cultivar name and stage of growth, pH and EC at the time of taking tissue samples.

 

              Example SC F20 6.0/2.2

 

This stands for Slime Cookies, Flower Day 20, pH 6.0, 2.2 EC.

 

It is also worth keeping (and putting on file) concise notes on the environmental conditions the plants were grown under at the time tissue samples were taken. For example, the day and night temperature + day and night relative humidity (RH), CO2 levels, light intensity/PPFD/DLI and fertigation frequency (i.e. number of feeds per day). This information can be filed with the tissue and water/nutrient lab results when you receive them. By filing this data along with lab results it enables you to cross compare tissue analysis data from different dates and e.g. see differences where the same strains are grown under differing environmental conditions. This will enable you to make informed decisions later about the best nutrient regime to run under given environmental conditions.

 

Delivering the Samples

 

Decomposition of a plant sample before it reaches the laboratory will result in a loss of carbon (as CO2 through respiration and microbial activity) and the concomitant concentration of most other elements, thereby giving erroneously high readings.

 

Immediately send the samples to a lab certified in your state to conduct leaf-tissue analysis. Try to collect the leaf sample at the beginning of the week so delivery will not be delayed over the weekend.

 

Always Send a Nutrient Solution Sample to the Lab

 

Often what you think you have in the feed solution differs from what is actually there. This really comes down to, in most cases, inaccurate percentage fertilizer listings or, in some cases, mixing errors.

 

However, because what is in solution is often different from what should be there, at the same time as running tissue analysis it is necessary to analyze the nutrient solution that you are feeding to the plants in order to have accurate nutrient solution data to cross reference against the tissue analysis data.

 

The amount of nutrient solution needed for testing is generally about 200ml submitted in a sturdy, leak proof container. Check with your lab re their sample requirements.

 

What Tests to Ask the Lab For

 

Tissue Tests

 

Typically, a leaf tissue analysis will include the following macronutrients: nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), and magnesium (Mg). Labs might not include sulphur (S) or may do so at an additional charge. Tissue analysis should also determine the following micronutrient concentrations: boron (B), copper (Cu), iron (Fe), manganese (Mn) and zinc (Zn). Some labs charge extra and require a larger sample amount for molybdenum (Mo) determination, and some will also test for sodium (Na) and chloride (Cl). Some fertilizers and source water may contain these elements so it is recommended to test for Na and Cl. A recent study showed that 5mM of NaCl in the rootzone of cannabis impacted on inflorescent and cannabinoid yield. Further, organic substrates such as coir can contain quite high levels of Cl which reflects in the tissue of the plant causing issues potentially with toxicity, but also altering N uptake. For this reason, it is advisable that coir growers always test the plant tissue for Cl.

 

Some labs will also test for silica. However, generally this is an extra test that needs requesting and will incur additional cost. In some cases, some labs do not offer tests for silica at all.

 

It is important to note that various labs offer standard tissue testing packages. Some of these packages are extremely comprehensive, reporting on an array of macro and micro nutrients, along with silica and heavy metals found in the tissue while other labs may have a standard package that only reports on the basic macro and micronutrients. In some cases, these ‘standard’ packages may not test for all the macro and micronutrients you would like reports on and, as a result, you need to request and pay for further tests beyond the standard tissue testing package.

 

Because labs offer different standard packages it is important to speak to the lab and find out what exactly they test for in their tissue test package.

 

Be sure, at a minimum, to request tests for N, P, K, Ca, Mg, S, Fe, Zn, Cu, Mn, B and Mo. For growers who produce in peat and coir it is also worth testing for Na and Cl because these elements can be problematic in organic substrates and testing for them will alert you to any issues. Further, for those who use municipal water supplies it is worth testing for Na and Cl because there can be excess of one or both of these nutrients in the water supply.

 

It is important to note that because high THC cannabis is still federally illegal in the US, any tissue tests should be run by a lab that offers this testing in your state, so as samples are not sent across state lines (in breach of federal law). Other than this, in some cases, labs will not test cannabis flower while they will test cannabis leaves. This is really due to the federal legality situation where hemp is legal while high THC cannabis is federally illegal. Thus, through testing leaf material only, the lab really doesn’t know whether the sample material is from a hemp plant or a high THC cannabis plant.

 

What the Lab Reports

 

Lab reports vary from simple listings on each nutrient by percentage or mg/kg (ppm) with no indication of sufficiency ranges while other labs will report with sufficiency ranges, while yet other labs may show results next to cannabis averages. For example, Wallace Labs in California reports results against cannabis averages which fall, for the most part, within the current established cannabis sufficiency ranges.

 

With most labs, the macronutrients N, P, K, Ca, Mg and S are usually expressed as percentage or as g/kg and the micronutrients as either mg/kg (=ppm) or microgram/ gram.

 

However, some labs may report some of the macronutrients as mg/kg (=ppm) along with reporting e.g. N as %.

 

What does the lab do and how long does it take to get results?

 

Plant tissue analysis is a laboratory procedure that involves certain steps:

 

Step 1) Preliminary preparation – drying, grinding, and mixing plant tissue

Step 2) Destruction of organic matrix – strong acids or very high temperatures are used during this step.

Step 3) Analysis of mineral residue for nutrient content – many different techniques can be used for this step

 

The method used by the laboratory in testing may greatly affect the meaning of the reported results. Many laboratory procedures, all radically different in approach, have been developed for plant analysis. For example, tests for P, K, etc., range from exotic neutron magnetic resonance (NMR) techniques to field quick-test kits. However, growers should patronize laboratories offering agricultural tests.

 

Plant tissue analysis is a more time-consuming process than substrate or nutrient solution testing, therefore leading to longer turn-around times. Plant tissue analysis is also more expensive than soil or nutrient solution testing.

 

The time for testing to be completed must be held to a minimum. A reasonable time frame for this process is 3 to 5 working days, for diagnostic samples. Due to short crop cycle times of indoor grown cannabis, the quicker the lab turnaround time the better.

 

Water (Nutrient Solution) Tests

 

Again, I stress, at the same time as running tissue analysis it is important to analyse the nutrient solution being fed to the plants at the time tissue samples are submitted to the lab.

 

When requesting water/nutrient solution tests, these tests at minimum should mirror the macro and micronutrients that are being tested for in the plant tissue. Therefore, at a minimum, the water/nutrient solution tests need to report on N, P, K, Ca, Mg, S, Fe, Zn, Cu, Mn, B and Mo. Ideally the tests should also include Na and Cl.

 

Substrate Testing

 

Routine Analysis

 

Growers should monitor the pH and EC, in the root environment, as a matter of course throughout the crop cycle.

 

However, for routine analysis of crop nutrition, lab testing the substrate for pH, EC and nutrients can prove helpful for establishing an optimised nutrient regime.

 

That is, there are two key things going on where it comes to the nutrient availability to the plant. Firstly, there is what is provided to the plants via the nutrient solution. The nutrient levels and ratios, pH and EC of the nutrient solution are reasonably easy to control.

 

Secondly, there is what is occurring in the substrate root environment of the plant re nutrient levels and ratios, pH and EC. This is far more unpredictable and less easily controlled than controlling what is fed to the plants via the feed solution. For example, substrates retain a percentage of the nutrients that are irrigated into the media, even where high rates (>30%) of runoff occur. Substrates such as perlite are shown to retain significant amounts of nitrogen, phosphorus, and calcium. One study showed that approximately 5–7% of all nutrients that were supplied run-to-waste to perlite grown tomato were retained by the substrate.[21] This level of retention is notable in that it causes an imbalance of nutrients in the plants rootzone.

 

Additionally, the surfaces of particles and other solids in hydroponic substrates bear permanent and/or variable electrical charges. The surface charge properties and the dissolution characteristics of the substrate solids are important since they affect the ionic composition of the nutrients in the substrate. [22] For example, organic substrates such as coir have negatively charged exchange sites which bind the positively charged cations (e.g. Ca++, Mg++, K+, NH4 +). The affinity of cations for negatively charged surfaces under equal concentration in the soil solution of the substrate is affected by ion characteristics such as valence, size and water status. For example, the solubility of NO3 −, the major N source for plants grown in soilless culture, is very high. Its affinity to negative charged surfaces is very low compared with that of H2PO4 − or SO4 2−, and therefore, the concentrations of NO3 − in the substrate solution is nearly unaffected by adsorption reactions. NH4 + is also an N source for plants in soilless production. Unlike NO3 −, the high affinity of NH4 + to negatively charged surfaces affects its availability to the roots. Similarly, divalent cations such as Ca++ and Mg++ have a high affinity to being bound by the negatively charged substrate particles while K+ only has a single valence and therefore isn’t bound to the same extent.

 

Further, organic substrates such as coir and peat have unique chemical properties. For example, coir naturally contains potassium (K). This K is released from the substrate particles into the soil solution, thus making it available for plant uptake. The level of K released into the soil solution during the crop cycle is quite unpredictable and varies among coir products. As a result, substrate monitoring provides an insight into how much additional K, beyond that being supplied by the nutrient solution, is available in the soil solution.

 

All substrates have unique properties that influence nutrient holding and release. For example, one study that investigated coir, peat and rockwool nutrient retention in the root zone found the different substrates significantly influenced the accumulation of N, P, K, and S nutrient in crops. Where plant tissue analysis was concerned, in general, all nutrients showed the highest tissue accumulation in crops produced in coir and the lowest accumulation in crops grown in rockwool. Where nutrients in the substrate were concerned, different substrates showed significant differences in nutrient uptake by crops and nutrient residue/buildup in substrates, resulting in obvious differences in nutrient balance. The coir generally showed the highest nutrient uptake by crops, especially for P, K, and S. Moreover, the coir also showed the highest P residue in substrate. However, the highest residues in the three substrates of other nutrients (e.g., Ca, Mg, and S) were generally found in peat.[23]

 

In short, there are all sorts of things that can occur that impact on the nutrient levels and ratios in the substrate. The only way to establish what is occurring at this level is to periodically test the substrate for macro and micro nutrients.

 

Diagnostic Analysis

 

Where diagnostic tissue testing is required, due to plant health issues (i.e. symptoms of nutrient deficiency or excess), substrate along with tissue testing provides invaluable information pertaining to whether any nutrients are accumulating in the substrate to toxic levels. In fact, from a diagnostic level, substrate testing – beyond tissue and nutrient solution testing – is required to look more closely at what is occurring in the rootzone re nutrient levels and ratios, pH and EC.

 

Substrate Tests Required

 

It is important to note that the test required for hydroponic substrate analysis is called the Saturated Paste Extract test, which is also known as the Soil Paste Extract test.

 

Saturated Paste Extract (SPE) testing is a water-soluble test that helps identify what is happening in the substrate short term. It is a good tool for determining what nutrients are soluble in the soil solution, including high sodium or salt levels.

 

In the SPE procedure cores of substrate are taken. Water is mixed into the collected substrate to the point of a glistening paste, the mix is allowed to sit, and then the liquid phase is separated from the substrate for analysis. The substrate solution is altered because more water is added than it could have held at container capacity — possibly 10 to 15 percent excess water. The pH is usually a little higher and EC and nutrient levels a little lower than the root would experience due to dilution.

 

An advantage of the SPE is that you do not need to consider the starting moisture content of the growing medium as with the other procedures. For example, if the sample is wet due to a recent irrigation, then less deionized water is required to saturate the growing medium sample compared to a dry sample.

 

The disadvantage to this procedure is that it is slightly destructive to the roots because substrate samples need to be extracted from the root zone, meaning you are also extracting some root material.

 

From your perspective, submitting a sample for SPE testing comes down to collecting a sample (generally about 500 ml) of substrate and submitting this to the lab. Check with the lab re their requirements as to the amount of substrate you will need to submit for SPE testing.

 

Tests should include EC, pH, N, P, K, Ca, Mg, S, Fe, Zn, Cu, Mn, B, Cl and Na. It is important to note that, in general testing substrates for the basic micronutrients has its limitations. I’ll talk more about this shortly. Additionally, few laboratories conduct saturated paste extract Mo analysis. Molybdenum is present at very low levels in substrates, much lower than most of the other nutrients, making an accurate determination difficult. Most plants have a low requirement for Mo, and slight differences in soil Mo levels can impact plant performance. Therefore, substrate tests for Mo are of limited use and are rarely conducted.

 

Note: It is important to specify to the lab that you require a saturated paste extract test. Without specifying this, often labs will run a total nutrient test or exchangeable nutrient test which is done to determine fertilizer recommendations in soil agriculture. This test is pretty much worthless where it comes to looking at what nutrients are available, at what levels, in the soil solution of a hydroponic substrate.

 

The Complexities of Substrate Testing

 

A major problem in substrate growing systems (and soils) is to define and measure the nutrient concentration in the root environment accurately.

 

It is likely that EC, water content and ion concentrations are not uniform in the root environment of the substrate[24]. For example, De Rijck and Schrevens measured ECs in the rockwool slab of a tomato crop of between 4.5 and 10 dS⋅m–1 for a supply EC of 2 dS⋅m–1[25].

 

The plants themselves react to the different conditions in the root environment. Roots in good condition may compensate water and nutrient uptake for roots in bad condition.[26] A healthy plant reacts to the most favourable conditions in the root environment. Sonneveld and Voogt grew tomato plants with split root systems and supplied nutrient solution of different concentration to the two halves of the root system. They found that yield was maximum when at least one half of the root system was exposed to optimal nutrient concentrations (EC at 2.5–3.0 dS⋅m–1).[27] In addition, most of the nutrients were absorbed by the root half exposed to high nutrient solution concentration, while most of the water was taken up by the other half. Similar results were obtained by Sonneveld and De Kreij with cucumber.[28]

 

Basically, water and nutrients are unevenly distributed in the root environment and, therefore, the concentration of the nutrients in the substrate sample you collect may not adequately characterize the conditions in the root environment as a whole.

 

Micronutrient Testing in Organic Substrates 

 

Something to be aware of is that substrate testing often does not provide accurate information on the micronutrient levels in the rootzone. Because of this, often tissue analysis offers better data in which to refine the micronutrients being provided to the crop via the feed solution.

 

While available micronutrient levels in the substrate are important for the growth, in organic substrates such as peat and coir the micronutrients are often complexed by organic compounds. Hence, the concentrations of these micronutrients in a saturation extract are quite low. Therefore, it is difficult to distinguish between deficient and adequate levels. In evaluating 15 extractants, Berghage et al. found that extractable levels of iron, manganese and zinc could be increased greatly by using weak solutions of various salts, acids or chelates in the saturating solution with the saturation extract procedure.[29]

 

Saturation with a 0.005 M DTPA solution was found to most consistently increase extractable micronutrient levels while having only a minor effect on the other key test parameters: total soluble salts and extractable levels of nitrate, phosphorus, potassium, calcium, magnesium, sodium and chloride.

 

When using a lab for substrate testing it is important to understand that testing procedures they use to determine the accuracy of lab results. Because of this it is always worth talking to your lab about their testing standards (method, accuracy etc).

 

Taking a Substrate Sample for Saturated Paste Extract Testing *Organic Media (Peat and Coir)

 

It is best to take substrate samples just before the next fertigation.

 

To obtain a representative substrate sample it is necessary to combine several sub samples in order to obtain an average value. Depending on the size of the crop, samples from about 3 – 5 different pots are usually required. The samples should be obtained from uniform plants that are under the same nutrient regime and fertigation strategy, the same cultivar, age and in the same size container. Collect your samples at the same time between fertigations, i.e., just before the next fertigation. Avoid sampling the top 4-5 cm of media since there are usually very few roots in this zone, and the salts tend to be higher due to evaporation of water from the substrate surface. Collect samples from the mid-range to lower range of the root system in the pot. You can safely remove about 5% of the media without harming the plant.

 

Use fresh, moistened growing media to replace the media removed during the extraction of your sample.

 

Blend your samples together thoroughly and submit the required amount of sample (from this blend) needed for lab testing.

 

Testing the Soil Solution in Rockwool

 

Testing the soil solution in rockwool doesn’t require taking a media sample. Instead, it is relatively easy to use a syringe and extract some solution from the rockwool and submit this sample to the lab for a water/nutrient solution test. It is best to take the liquid sample from several points during extraction and combine these for testing to get a reasonably decent representative sample.

 

More to this article will be available in an upcoming book I am currently working on. This material will feature examples of tissue analysis and what steps were made to correct the nutrient to bring it into line to optimum.

 

References

 

[1] Marschner, P. (2012). Marschner’s mineral nutrition of higher plants. 3rd Edn. London: Academic Press.

[2] Rietra, R. P. J. J., Heinen, M., Dimpla, C., & Bindraban, P. S. (2015). Effects of nutrients antagonism and synergism on fertilizer use efficiency. (VFRC report; No. 2015/5). Virtual Fertilizer Research Centre.

[3] Niu Y, Jin G, Li X, et al. Phosphorus and magnesium interactively modulate the elongation and directional growth of primary roots in Arabidopsis thaliana (L.) Heynh. J Exp Bot. 2015;66(13):3841-3854. doi:10.1093/jxb/erv181

[4] Yep, Brandon & Gale, Nigel & Zheng, Youbin. (2020). Aquaponic and Hydroponic Solutions Modulate NaCl-Induced Stress in Drug-Type Cannabis sativa L. Frontiers in Plant Science. 11. 10.3389/fpls.2020.01169.

[5] Giri, Anju et al. “Heat Stress Decreases Levels of Nutrient-Uptake and -Assimilation Proteins in Tomato Roots.” Plants (Basel, Switzerland) vol. 6,1 6. 19 Jan. 2017, doi:10.3390/plants6010006

[6] Philippe Morard, Ludovic Lacoste & Jérôme Silvestre (2000) Effect of oxygen deficiency on uptake of water and mineral nutrients by tomato plants in soilless culture, Journal of Plant Nutrition, 23:8, 1063-1078, DOI: 10.1080/01904160009382082

[7] Saloner A, Sacks MM and Bernstein N (2019) Response of Medical Cannabis (Cannabis sativa L.) Genotypes to K Supply Under Long Photoperiod. Front. Plant Sci. 10:1369. doi: 10.3389/fpls.2019.01369

[8] Landis, H.; Hicks, K.; Cockson, P.; Henry, J.B.; Smith, J.T.; Whipker, B.E. Expanding leaf tissue nutrient survey ranges for greenhouse cannabidiol-hemp. Crop Forage Turfgrass Manag. 2019, 5, 1–3.

[9] Mills, H.A. and J.B. Jones Jr. Plant Analysis Handbook II. Micro Macro International, Athens, Ga., 1996.

[10] Landis, H., Hicks, K., Cockson, P., Henry, J. B., Smith, J. T., & Whipker, B. E. (2019). Expanding Leaf Tissue Nutrient Survey Ranges for Greenhouse Cannabidiol-Hemp. CROP FORAGE & TURFGRASS MANAGEMENT, 5(1). https://doi.org/10.2134/cftm2018.09.0081

[11] Martin, W. E., and J. E. Matocha. 1973. Plant analysis as an aid in the fertilization of forage crops. In Soil Testing and Plant Analysis. Revised. L. M. Walsh and J. D. Beaton (eds.), pp. 393-426. Soil Sci. Soc. Amer., Madison, WI.

[12] Mohamad Zamri Sabli, PhD thesis (2012) Fertigation of Bell Pepper (Capsicum annuum L.) in a Soil-less Greenhouse System: Effects of Fertiliser Formulation and Irrigation Frequency

[13] Chen, B., Chai, Z., Sheng, J., Zhang, W. and Jiang, P. (2017). Effects of potassium fertilizer 53 on the physiological mechanisms of cotton fiber quality. Pak. J. Bot., 49, 935-943

[14] Saloner A, Sacks MM, Bernstein N. Response of Medical Cannabis (Cannabis sativa L.) Genotypes to K Supply Under Long Photoperiod. Front Plant Sci. 2019;10:1369. Published 2019 Nov 18. doi:10.3389/fpls.2019.01369

[15] Mills, H.A. and J.B. Jones Jr. Plant Analysis Handbook II. Micro Macro International, Athens, Ga., 1996.

[16] Marschner, H. Mineral Nutrition of Higher Plants, Second Edition. Academic Press, New York, 1995.

[17] Mengel, K. and E.A. Kirkby. Principles of Plant Nutrition, Second Edition. International Potash Institute, Bern, Switzerland, 1979

[18] Marschner, H. Mineral Nutrition of Higher Plants, Second Edition. Academic Press, New York, 1995.

[19] Yep, Brandon & Gale, Nigel & Zheng, Youbin. (2020). Comparing hydroponic and aquaponic rootzones on the growth of two drug- type Cannabis sativa L. cultivars during the flowering stage. Industrial Crops and Products. 157. 1122881. 10.1016/j.indcrop.2020.112881.

[20] Landis, H., Hicks, K., Cockson, P., Henry, J. B., Smith, J. T., & Whipker, B. E. (2019). Expanding Leaf Tissue Nutrient Survey Ranges for Greenhouse Cannabidiol-Hemp. CROP FORAGE & TURFGRASS MANAGEMENT, 5(1). https://doi.org/10.2134/cftm2018.09.0081

 

[21] Sanjuan Delmás et al. Applying nutrient dynamics to adjust the nutrient-water balance in hydroponic crops. A case study with open hydroponic tomato crops from Barcelona. Scientia Horticulturae, Volume 261, 5 February 2020.

[22] Silber, A. Chemical Characteristics of Soilless Media. IN Soilless Culture Theory and Practice Second Edition 2019. Chap 4 Pages 113-148

[23] Xiong, Jing et al. “Comparison of Coconut Coir, Rockwool, and Peat Cultivations for Tomato Production: Nutrient Balance, Plant Growth and Fruit Quality.” Frontiers in plant science vol. 8 1327. 2 Aug. 2017, doi:10.3389/fpls.2017.01327

[24] Hans-Peter Kläring. Strategies to control water and nutrient supplies to greenhouse crops. A review. Agronomie, EDP Sciences, 2001, 21 (4), pp.311-321. ff10.1051/agro:2001126ff. ffhal-00886120f

[25] De Rijck G., Schrevens E., Distribution of water and nutrients in rockwool slabs, Sci. Hortic. 72 (1998) 277–285.

[26] De Jager A., Effects of localized supply of H2PO4, NO3, Ca and K on the concentration of that nutrient in the plant and the rate of uptake by roots in young maize plant

[27] Sonneveld C., Voogt W., Response of tomatoes (Lycopersicon esculentum) to an unequal distribution of nutrients in the root environment, Plant and Soil 124 (1990) 251–256.

[28] Sonneveld C., De Kreij C., Response of cucumber (Cucumis sativus L.) to an unequal distribution of salts in the root environment, Plant and Soil 209 (1999) 47–56.

[29] Berhage, R.D., D.M. Krauskopf, D.D. Warncke, and I. Widders. 1987. Micronutrient testing of plant growth media: Extractant identification and evaluation. Comm. Soil Sci. Plant Anal. 18:1089-1110.

 



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