Plants require 18 essential elements. Three of the basic elements are obtained by the plant from either water or carbon dioxide in the air [Carbon (C), Oxygen (O), and Hydrogen (H)]. Additionally, the plant derives from the roots three primary/macronutrients – meaning they are needed in large quantities – [Nitrogen (N), Phosphorus (P), and Potassium (K)] as well as three secondary nutrients [Magnesium (Mg), Calcium (Ca) and Sulfur (S). The final 9 nutrients are considered essential micronutrients, and these too are derived from the plant’s roots. “Essential micronutrient” means the plant requires micro-doses of these 9 nutrients to complete some aspect of its lifecycle. Deficiencies in essential micronutrients can result in problems with respect to plant growth, development, and reproduction. The final 9 essential micronutrients are: Boron (B), Chlorine (Cl), Copper (Cu), Iron (Fe), Molybdenum (Mo), Manganese (Mn), Nickel (Ni), Zinc (Zn), and Silicon (Si). Each micronutrient plays an important role in the plant lifecycle, even though the required dose can often fall below 10 ppm concentration in a deep-water culture system.
Of particular interest in this study by Hydra Unlimited (led by Chief Scientist Hank Bonnah) is Silicon, often described as elemental Silica or Silicon-Dioxide (SiO2). Silica is an intermediate uptake nutrient, meaning it is taken up by the plant through the roots at about the same rate as water uptake through evapotranspiration. In nutrient solutions, silica is typically derived from Potassium Silicate (K₂O₃Si) or Metasilicic acid (H2O3Si), which disassociates in water, thus liberating silicon to be taken up by plants at reasonably high levels (depending upon plant needs). Silicon performs several important functions within the Cannabis sativa lifecycle:
- Mechanical strength and resistance to lodging (the bending of plant stems due to lack of turgor and/or fiber weakness).
- Abiotic Stress Resistance.
- Improvement of leaf exposure to light.
- Decreased susceptibility to pathogens and root parasites.
- Silicon can also alleviate imbalances between zinc and phosphorus supply as well as promote the use of Cannabis sativa in the bioremediation of heavy metals contaminated soil.
- And finally, Silicon can improve water use efficiency in relation to stomatal closure and decrease in transpiration rates.
So, now we have a sense of the importance of silica as an essential micronutrient. Whether it is a macronutrient, secondary nutrient, or essential micronutrient, it is important to track plant consumption of the nutrient mix being offered to plants growing in recirculating deep-water culture systems. At a minimum, keeping track of EC or electrical conductivity is critical in understanding plant health and harvest success. Unlike substrate methods wherein fertigation runoff (a requirement to assure the nutrient-depleted root mass is overwatered to replenish nutrients with a subsequent dry-back), recirculating deep-water culture always presents the root mass exactly the necessary nutrient concentration without the 10-20% leachate runoff/waste of substrate fertigation. In fact, one can only surmise the nutrient concentration in substrate/fertigation farming techniques. In recirculating deep-water culture, the cultivator has the opportunity to evaluate nutrient concentrations and consumptions across the 15 root-delivered nutrients. All that is necessary, beyond utilizing a recirculating deep-water culture system, is access to an A2LA-accredited lab capable of evaluating nutrient water [internet keyword search: water quality lab].
Hydra Unlimited has investigated silica uptake during the growth phase commonly known as 'stretch.' "If we break down the typical growth phases of Cannabis sativa into three parts over a 12-week period, we might see something like this: Vegetation, Stretch, and Flower."
In more scientific terms, the “stretch” phase of a developing stem of Cannabis sativa contains a specific growth region called the ‘snap point’ where the fiber-enriched bast tissues considerably change their mechanical properties. Bast tissues are tissues along the stem axis of the Cannabis sativa, which make up part of the phloem fibers. The “snap point” is present during a specific period of plant development—the fast growth phase, where elongation and secondary wall thickening of the stem occurs. The snap point disappears when stem growth is completed, and the cultivars transition into the flowering phase.
Hydra Unlimited's hypothesis: "We hope to see biomineralization in the form of silica uptake during this stretch/snap point growth phase. We will investigate silica uptake spanning from week 4 through week 8 and expect an acceleration of silica uptake at the start of the snap point with a slowing of silica uptake at the close of the snap point. We will perform weekly nutrient water sampling taken from 4 parallel systems occupying the same grow room. We will attempt to overlay the plant growth index (canopy volume) with measured silica concentration in the nutrient water."
The boundary conditions – How the experiment is set up
- The assessment starts at week 4 with a complete reset of 3 systems (water change out with fresh nutrients / pH adjustments) at week 5.
- The systems that receive a complete reset will be called systems B, C & D.
- System A will remain unchanged, charged with the present “pre-snap point” nutrient water from week 4. This will be the experimental control system.
- Only nominal silica adds will occur based upon needed adjustments of pH in that the silica-containing adjunct [Potassium Silicate (K₂O₃Si)] is part of a pH balancing solution.
- The cultivars that makeup systems A & B were of lesser clones (meaning less vibrant than average). Cultivars that makeup systems C & D are, generally speaking, average or better than average clones. This is important in that resulting cultivar vitality is directly related to the vitality of the clones used (other factors aside).
The evaluation and assessment – By week
Week 4 - Jump off point.
Here are all 4 systems with a spider chart (left) showing not only silica (SiO2) concentration in mg/l (ppm) but also sodium (Na), chloride (Cl), and Sulfate (SO4). Silica is sitting at 25 ppm for Systems B, C, and D and at about 40 ppm for System A.
The graph on the right shows plant progress in canopy volume as calculated by the growth index. This is the average canopy volume per system.
Week 5 – Water change-out for systems B, C, & D. Carry-over for System A (control)
The spider chart shows the replenishment of systems B, C, and D. We see the results of system A, being left as control, entering the snap point wherein A is beginning to consume residual Silica. We can also see from the graph on the left an acceleration of the growth index aligning with snap point “plant-stretch” progress.
Week 6 – The “stretch” continues.
The spider graph reveals the plants are eating every bit of silica they can. The system is now depleted of this important micronutrient. On the right, we can see the snap-point is fully accelerating in terms of the calculated canopy volume. We also see an accumulation of Na and Cl in the system. It is unclear as to the increasing source of these elements and begs further study.
Here we also get a sense of the difference between below-average and above-average clones. System A and B cultivars begin to show their lack of vitality as compared to the cultivars in systems C and D. Independent of vitality, all are still consuming and depleting the available silica.
Week 7 – Stretch begins to slow
We begin to exit the snap point for the cultivars as we see a corresponding change in slope (rate of change of plant progress). And yet, the plants continue their voracious appetite for Silica. [Note: control system A; growth index measurement anomaly week 7 has under-represented actual growth index. Corrected for the following week].
Week 8 – Exit of the snap point
The entire story is told by the spider graph on the left. The plants in all four systems end their massive uptake of silica and exit the snap point. As stated earlier: The “snap point” is present during a specific period of plant development—the fast growth phase, where elongation and secondary wall thickening of the stem occurs. The snap point disappears when stem growth is completed.
"The hypothesis stated earlier cannot be fully validated without tissue samples to verify silica translocation to the bast fibers for each week of the snap point. And while we will leave the tissue assessment for further study, what can be learned is the incredible power of Hydra Unlimited’s recirculating deep-water culture system for nutrient assessment. No other cultivation technique affords this level of insight into what the plants are actually eating as they progress through their life cycle," the team says.
"We can take the recognition of silica depletion to the next step. The graph below depicts the resulting growth success as a function of nutrient concentration. Too few or too much of a particular nutrient can adversely affect plant success. In our silica example, a case could be made where silica has fallen into the deficiency zone. Remediation steps of offering replacement silica to the water in the form of potassium silicate or metasilicic acid could be considered in order to replenish and return silica concentrations back into the cultivator’s planned “adequate zone” for silica concentration."
"The methods and techniques shown here can also be applied to the next level of crop steering – beyond manipulating the environment. We have the potential to tailor our nutrient choices to modulate the expression of phenotype and potentially influence genotype as well as chemotype expression. Through the use of Hydra Unlimited’s recirculating deep-water culture system paired with water chemistry assessment and data visualization, it’s possible to bring crop steering to the next level," the team says.
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