The Rhizosphere

The rhizosphere is an ecological niche composed of a plant’s root system, soil interface, and all associated biology.

A gram of quality topsoil contains more life forms than there are humans on earth. This universe contains bacteria, fungi, nematodes, and other microbes.

The rhizosphere can also be inhabited by earthworms, soil dwelling mites, and insects at various life stages.

These organisms compete, cooperate, and communicate by exchanging biochemicals including nutrients, hormones, terpenes, and volatile organic compounds (VOCs).

Beneficial Microbes

Plants have co-evolved with microbial life and have formed mutually beneficial relationships with a diversity of organisms.

This mutualism began 420 million years ago when plants first colonized land, and before they had developed roots.

The complex architecture of plants is built on these ancient relationships.

Microbes serve many functions for plant health including immune signaling, nutrient provision, and pathogen suppression.

These functions are so important that a large percentage of the net energy produced by plants is transferred to the soil to support these communities.

Nutrient Acquisition

Roots are the primary pathway for nutrient acquisition from the soil-water solution. The efficiency of this function is related to root surface area, environmental conditions, and microbial interactions.

Microbes enable plant nutrient uptake in several important ways:

• Bacteria fix Nitrogen in the soil and convert it to plant available nitrates.

• Phosphate solubilizing microbes convert Phosphorus compounds for plant assimilation.

• Potassium mobilizing bacteria have been linked to higher crop yields.

• Mycorrhizal fungi can break down rocks and mobilize trace elements like Iron.

Mycorrhizae

In diverse soil ecosystems, networks of hyphae composed of multiple fungal species often interconnect plants, enabling resource exchange and communication between neighbors.

The small size of these hyphae enables penetration of soil pores that the plant root would not be able to access otherwise.

Some mycorrhizae excrete enzymes that are toxic to nematodes and other soil dwelling pests.

Pathogen Suppression

Microbial communities can suppress pests and disease in several ways.

• Competition – Beneficial microorganisms can outcompete pathogens for food sources and physical habitat when environmental conditions promote their proliferation.

• Parasitism – Various bacterial and fungal microorganisms can parasitize plant pests including nematodes, soil dwelling insects, and other microbes.

• Chemical Warfare – In addition to metabolites that inhibit pathogen growth, microorganisms produce more than 1300 volatile organic compounds (VOCs) of diverse chemical classifications.

• Immune Response – Microorganisms can solicit plant immune responses through hormone signaling pathways.

Bioprotectants

Organic cultivators can build living soils to benefit from the incredible diversity of beneficial microorganisms contained in quality compost.

Compost

Quality composts are nutrient dense and biologically diverse.

Compost is rich in organic materials which will increase the buffering capacity of the soil. This relates to the soil’s ability to store and exchange nutrients and is calculated as the Cation Exchange Capacity.

An important compost quality metric to consider is the Carbon to Nitrogen or C:N ratio.

Microorganisms tend to consume about 20 parts of Carbon to 1 part Nitrogen. A C:N ratio below 20 indicates that ample Nitrogen was provided to fuel the compost process.  Compost with a C:N ratio above 30 should probably be avoided.

Compost should smell rich and inoffensive.  Foul smells indicate that conditions favor bad actors that off gas nutrients rather than fixing them in the soil. 

Living Soil

Living soils are diverse ecosystems that promote plant health and productivity.

Compost provides beneficial microbes, plant available nutrients, humates, and other goodies to organic soil mixes.

These soils are based on fibrous grow media to create a soil structure with an ideal air to water ratio and enable microbes to thrive in aerobic conditions.

Plants can optimize soil conditions for specific members of the microbial community to produce inputs that the plant requires based on its changing needs.

This is achieved by the selective sharing of root exudates which provide nutrients and can change the PH of the soil to benefit uptake.

Lab analysis can provide insight into the chemistry of organic soils. The ratios of macro and micronutrients affect plant uptake and are more important than the absolute values present.

Organic liquid feeds provide the fastest crop response at the highest expense. These products should never be fertigated through irrigation systems for fear of biofilm.

Conventional Horticulture

In conventional horticulture rooting media are inert substrates that provide structure for root development. Common substrates include rockwool, coco coir, and soilless mixes.

Nutrient salts are applied in solution with irrigation water to feed the plant.

Potential Hydrogen (PH) of nutrient solution is important to monitor when using these substrates. PH is a measure of acidity, and many nutrients become unavailable when acidity is out of range.   

While very efficient at producing biomass, these systems can represent an ecological void that pathogens are eager to populate. The use of microbial inputs can help to fill this void.

Hydroponics

Cannabis Plants grown in hydroponic systems can be susceptible to root rot, characterized by slimy and discolored roots.

This disease can impair or kill infected plants. Systems where water is shared can enable the spread of Fusarium and Pythium species that contribute to this disease.

These fungal and oomycete pathogens can reproduce by sporulation and are difficult to eradicate once they have established in cultivation facilities.

In some cases, addition of microbial inoculants has been shown to inhibit algal growth in nutrient solutions. Light must be excluded, and water temperature and oxygenation monitored to manage disease pressure.

Cool water contains high levels of dissolved oxygen. As water warms, its oxygen holding capacity diminishes. These conditions limit oxygen uptake by the plant and promote disease development.

Root Oxygen

Oxygen availability for uptake by roots is required for plant health and productivity. Various inputs are incorporated into soil and soilless mixes for aeration.

Peat Moss has excellent water holding and drainage capacity. It is also acidic and can become hydrophobic or hard to re-wet when it dries out. 

Coco Coir is the pith surrounding coconuts.  This farming byproduct is processed to create a soil input with a neutral PH, and high water holding capacity.

Perlite is a volcanic glass which expands when heated to form an inert medium that is used as an aeration input in soils.

Other input which can serve a similar function include vermiculite, pumice, and rice hulls.

Dissolved oxygen in irrigation water relates directly to plant performance. In addition to water temperature management, dissolved oxygen levels can be dramatically improved through use of nanobubble water treatment technology.

Germination

When a seed germinates, it’s root radicle emerges from the base and begins growing down into the soil

The root’s growing tip is protected by a root cap which continually sloughs off and is replaced as the root penetrates abrasive soils.

The root tip includes zones of cell division, elongation, and differentiation. The root meristem is the area of rapidly dividing cells closest to the root tip, these cells elongate to drive root growth, then specialize into various root cells within one centimeter of the growth tip.

The root has an outer layer of cells called the epidermis which surrounds the vascular tissues that enable water and nutrient transport. The root cap, and epidermal cells secrete mucilage, which lubricates the root path, aids nutrient acquisition, and contributes to soil structure.

Cloning

Cannabis plants are commonly propagated vegetatively or cloned. This process involves humidity, temperature, and stagnant air conditions that favor pathogen establishment.

Beneficial Bios

The soil matrix serves as a habitat for microarthropods, nematodes, and insects at various life stages.

Nematodes are a diverse class of microscopic worms some of which parasitize plant roots.

Predatory nematodes (Steinernema feltiae) can serve as an effective biocontrol for thrips and fungus gnat larvae. These nematodes penetrate their target and release symbiotic bacteria which kills the host. Nematodes then multiply within the cadaver to complete their life cycle.

Nematodes are usually delivered through irrigation systems where they are injected downstream of sand filters.

Hypoaspis mites (Strateolaelaps scimitus) are broadcast on the soil surface in a bulk format to target the same pest species.

Root Aphids are a significant pest in cannabis and are extremely difficult to manage. Ants will introduce them to your crops to farm them for honeydew and Nitrogen.

Metarhizium species of entomopathogenic (insect killing) fungi are one of the few efficacious controls for this scourge.

Root Development

As the tap root develops, radial roots begin to initiate along its length in response to hormone signaling from the root tip. As these radial roots develop, they in turn will initiate radial roots, until the soil is colonized with a fibrous root mass.

Root hairs, which are extensions of epidermal cells, develop along roots and increase the surface area to enhance the absorption of water and minerals.

Air pruning pots are recommended for all stages of plant growth.  These pots allow for faster drying of soil media between irrigation cycles.  These pots also benefit root structure by air pruning root tips and encouraging development of radial roots.

Once plugs have been transplanted, and thoroughly watered in, it is best to let the soil dry out as much as possible before the first irrigation.  Several days can be required for this initial dry-back in organic soils. 

This will motivate the roots to expand throughout the soil and help maintain a healthy air to water ratio in the soil media. 

A well-crafted root system enables small plants to take off when transplanted.

Water Balance

These responses can be activated for generative crop steering by increasing dry-back of the media.

Irrigation Efficiency

In conventional cultivation, irrigation is provided multiple times per day. Each irrigation cycle is designed to provide 20% drainage to flush excess salts from the grow media.

In organic soil systems, excess irrigation water will leach mobile nutrients from the soil and should be minimized.

Irrigation shots are designed to provide even hydration without channel pouring through the soil. The first shot in the morning is very brief (less than a minute) and longer shots can be scheduled when soil has absorbed water and expanded.

Whenever possible, watering should take place in the morning to enable nightly dry-back and minimize the contribution of irrigation to nighttime humidity.

Installation of mulch will improve irrigation dynamics in most systems. When dry back is desired, the driest plant in the crop should be on the verge of wilting before irrigation repeats. 

The best way to determine the extent to which a plant in your system can dry back, is to withhold water from a small number of plants until they start to droop, then weigh the pot. 

Results

Soils can be sampled to quantify microbe biomass, community composition, and to identify specific players within the microbiome. This is achieved through analysis of DNA, lipid biomarkers and other laboratory processes.

Scientists have only studied a small fraction of soil microorganisms. While providing interesting insights, it can be difficult to extract actionable information about soil performance from microbial assays.

The answer to complexity is often simplicity. The most effective way to understand how different rhizosphere inputs will affect plant performance is through plant growth trials.