Osmosis: How Cannabis Plants Absorb Water and Nutrients

green leaf
October 01, 2020

Absorbing water and nutrients is the primary function or responsibility of the root system. Leaves, too, play a role. Through their stomata, the plants can also assimilate moisture from the air or rain. And some growers take advantage of this by foliar feeding to remedy pH or avoid a nutrient deficiency or to boost growth. This intricate and critical process is only possible because of the “osmosis” phenomenon.

The term “reverse osmosis” might come to mind. You may have heard of it, or actually use it as a filtration system to soften hard water. That is, of course, entirely unrelated.

Concerning cannabis, osmosis is the process or mechanism that makes moisture absorption and nutrient uptake possible. An understanding of this phenomenon has no bearing on your ability to grow plants successfully. It does give you a better appreciation of the inner workings of their growth.

What is Osmosis?

By definition, osmosis refers to a fluid’s movement through a semipermeable membrane to a region with higher solvent concentration until both sides reach equilibrium.

Sounds complicated?

For you to understand this concept, let’s use a little imagination.

Imagine a container filled with water. The molecules that constitute water are in constant movement. Besides bumping into each other, they will also hit and bounce off the container’s interior surface. Although we do not see it with the naked eye, there are millions of collisions occurring.

“Pressure” is the term used to measure the number of collisions per square meter per second. Put another way, it is the “net force” exerted by all the collisions within a given time frame.

Now, imagine there is a barrier in the container dividing the water into two equal parts. Let’s assume that this barrier has exceptionally tiny holes, enough for water molecules to pass through but not other large particles.

Due to their constant movement, most of the water molecules would hit and bounce off the wall of the barrier like it would on the container’s interior surface. Some would chance upon the tiny holes and pass through. This happens on both sides, in which water molecules move from one side to the other side and vice versa.

For clarity, let’s label the two regions of water A and B.

Let’s dissolve salt into side A. Salt molecules are comparatively larger than water molecules and, therefore, could not pass through the tiny holes. In this scenario, the total number of collisions on each side would be the same, which means that the pressure in A and B are equal.

A fundamental difference is this. In A, the collisions not only involve water molecules but also that of salt. In B, it is purely water molecules.

Fewer water molecules pass through the barrier than B to A due to salt. Hence, there is a net movement of water from B (no salt) to A (high concentration of salt).

The barrier refers to the semipermeable membrane, while the movement of water is the phenomenon called osmosis. Salt, on the other hand, could be any substance dissolved into the water that comprises molecules larger than that of water.

What Is Osmotic Pressure?

Osmotic pressure is the minimum pressure that needs to be applied to a solution to prevent the inward flow of water across a semipermeable membrane, nullifying osmosis.

Something to Consider

The flow of water from one side to the other is because of having more collisions. This movement will continue until the number of molecule collisions on both sides becomes the same, which can occur in two ways.

1. One scenario would have a substance dissolved in the water on both sides at different concentrations. The movement of water, in this case, would be until the level of dissolved solids in both sides equalize.

2. Another way is when the water movement raises the pressure on the side that contains more dissolved substances. If the pressure is high enough, the number of collisions on both sides would eventually equalize.

You could consider osmotic pressure, in this regard, as a measure of osmotic strength.

How Does Osmosis Affect Marijuana Plants?

The way plants absorb nutrients and water is based on the principle of osmosis. It is why, besides sustenance, they can stand upright, leaves, and stems stay healthy – not droopy, and definitely not wilted.

Imagine a bag made of a semipermeable membrane, filled with water and salt substances. If you dip this bag into a solution that contains more dissolved solids (higher osmotic pressure), guess what happens? The bag collapses because water is drawn out.

On the other hand, if you were to put the bag in a solution that contains fewer dissolved solids (lower osmotic pressure), it is the water from outside that flows in.

Why is this Relevant?

As you can see, semipermeable membranes are a part of nature – plants and even humans. Cell walls are an example. Osmosis is an integral part of life.

Think of root hairs as the barrier in a container. Their cells are semipermeable, and water could pass through without trouble but not the dissolved nutrients. For the plants to absorb the nutrients, they rely on the pressure difference – outside the roots and inside.

Once the concentration of dissolved solids in the root zone rises, so too does the osmotic pressure. It results in water molecules flowing into the plant through xylems – vascular tissues that conduct water and dissolved nutrients.

Have you ever wondered why on chilly mornings when the soil is damp, you sometimes see droplets of water on the edges of leaves? It happens because the high osmotic pressure gradient in the root zone has forced too much water to flow into the plants, which then flowed out through the veins at the leaves’ edges. “Guttation” is the term used by botanists to refer to this phenomenon.

How do plants control the osmotic pressure in the roots? Sugar is the key. As you know, it is the leaves that manufacture sugar during photosynthesis and subsequently transported to the roots via phloem tissues. Plants can convert sugar to starch, which is mildly soluble and does not add much to osmotic pressure. They can do this to reduce osmotic pressure in the roots. On the other hand, they can also convert starch to sugar to increase osmotic pressure.

What about the nutrients?

Roots use a process called “active transport” to absorb nutrients. But they can only absorb nutrients in a soluble form – dissolved in water. If they are in an insoluble form, the plants cannot assimilate them.

For example, you add some iron filings into the soil to address iron deficiency. That is not going to help because the iron filings are not in a soluble form. Hence, the nutrients we provide to the plants are soluble and ready-to-use.

Adding nutrients does present a conundrum. Because a higher concentration of solids increases the osmotic pressure in the root zone. Remember, as the pressure increases, the plants’ capacity to absorb water diminishes. Once the outside pressure is higher than inside the plants, how does osmosis work again? Water molecules from a region of low concentration of solids flow into the zone where the level is higher.

Quite simply, add excessive nutrients, and the roots could no longer absorb water and nutrients. Worse, water flowing out of the plants causes dehydration. Leaves turn brown around the edges as they dry up. In this scenario, we have to deal with a common problem that many beginners encounter – a nutrient burn. Read everything there is to know about cannabis leaves for a better understanding of this.

Controlling the Osmotic Pressure in the Root Zone

At this point, you understand the concept of osmosis and how osmotic pressure works. The next question is, “How do we control osmotic pressure to the plants’ advantage?”

On the surface, osmotic pressure in the root zone rises when you add nutrients.

But there’s more at play here

For starters, think about what happens when you add nutrients at a time when the soil is wet. Of course, the nutrients are well diluted. Let’s assume that it is at the right concentration, so all is well and good. In time, the medium loses water due to plant intake and evaporation. Nutrients, on the other hand, remain. During subsequent feedings, wet and dry cycles, its concentration can keep on increasing until such time it exerts excessive osmotic pressure, causing problems for the plants.

The above scenario is more of a problem when growing in pots. Unlike in the ground outdoors, most of the unutilized nutrients stay in the container. Perhaps some will drain out, but most will remain and accumulate.

Before you start worrying too much about nutrient accumulation in pots, you are unlikely to encounter this problem. Use high-quality nutrients, provide the correct mixture at reasonable doses – that should be enough to keep you and the plants out of trouble. If the need arises, you can always flush the medium to get rid of the excess.

Another scenario that you need to consider is the osmotic pressure inside the plants. Is it high enough that instead of losing water, water and nutrients flow in from soil? The plants do have a say in this matter, producing sugar, which increases osmotic pressure in their system. It means that when the plants are most active and growing at a vigorous rate, they need generous levels of nutrients. During phases of their life cycle, when growth is slow, keep the nutrients to a minimum to avoid accumulation.

An excellent example to understand this concept are cuttings. Once you obtain them, they lose water and focus their energy on developing roots. Sugar levels during this period are low, which means osmotic pressure is also low. Therefore, it is in the best interest of the cuts to keep the external osmotic pressure even lower. Adding nutrients into the medium at this time is a huge mistake because they increase the osmotic pressure. Already with a low water level, the cuts lose even more water and consequently die.

Controlling the Nutrient Level

A challenge with nutrients, especially as a beginner, is determining how much nutrients to provide. Granted, the plants can tolerate a degree of variance – a little high or low – they do much better when the nutrient level is stable. It is for this reason that some growers practice providing small doses frequently that a large dose occasionally.

Stabilized nutrient levels increase osmotic pressure only if they are in a solution, in which case they are available to the plants. Without water, they are practically useless to the plants. An ideal scenario would be storing the nutrients in an insoluble form in the medium. And then there should be a means to slowly convert the nutrients to a soluble form – enough to feed but not overwhelm the plants.

Let’s disregard the impact of chemical fertilizers on the soil ecosystem or the possibility of leaving toxic residues. An advantage these products have is that they are already in a form usable by the plants. Once applied or mixed into water, they dissolve and thus increase osmotic pressure. Provide the correct quantity and plants can absorb them without trouble. Too much, of course, leads to a lockout.

Organic fertilizers are not the same. Often, they are constituents of organic compounds that do not dissolve rapidly. They need microbes that consume the organic matter to decompose, thereby releasing plant-usable nutrients to mix into water. In this regard, these natural fertilizers are far superior to chemical-based ones. Although some manufacturers have produced slow-action chemical fertilizers by coating them with polymers.

Using organic fertilizers offers one more unique advantage. As the microbes break them down, they leave “humus” or decayed organic matter. Humus enhances the soil’s ability to store water and nutrients.

Nutrients attach and detach from humus in a process called “equilibrium reaction.” If the nutrient concentration is high, the rate of attachment is also higher than the separation. Effectively, those that are bound are removed from the water. Suppose there are less dissolved nutrients in the water. In that case, humus releases nutrients attached to it to fill the gap and maintain an equilibrium.

The action of equilibrium reaction is better known as “buffering.” You keep hearing about this – about how organic fertilizers provide better buffering. Now, you understand how this process works.

Now, the term buffering is not limited to humus. It is also an inherent property of clay mediums. Materials such as feldspars and silicates are chemically active. Hence, nutrients can attach or detach from them.

On the other hand, sand is chemically inert. Nutrients cannot attach to them. For that reason, nutrient level in sandy soils tends to fluctuate more than in clay soil and easily washed away.

Osmosis Sustains Plant Life

As you have read, the osmosis process is the mechanism by which the plants absorb water and nutrients. The truth is, everyone who starts growing marijuana already understands the importance of providing sustenance. Well, there you have it. Now, you also understand how osmotic pressure makes it possible for the uptake. At the same time, it could also be the reason for nutrient burn, lockout, or dehydration.