Roots do not work like sponges. They are picky about what they bring into the plant. Here is Chapter 8: “Roots and Plant Hydraulics” from my new book, Marijuana Cultivation Reconsidered.
Why You Need to Know: All manner of foolishness flourishes when growers fail to understand how roots take up water and nutrients, or how water and nutrients move throughout the plant. Don’t be one of the fools.
It is worthwhile to know a few things about ground tissues. Understanding how they work will save you money when you shop for products that claim to sweeten your buds or when you sort through the advice offered indiscriminately on the Internet.
The root is comprised of four nested layers of tissue: the outermost layer is the epidermis; just inside that layer is the cortex; this is followed by the endodermis; and finally, at the core, is the stele. There are also root hairs, which are extensions of the epidermis which serve to greatly increase the root’s surface area. And, finally, at the growing tip of the root (the meristematic region) are specialized cells known as the root cap. Some truly amazing things go on in the root cap. None other than Charles Darwin understood the root cap of the plant to function “like the brain of one of the lower animals.”32 33 34 The root cap can detect:
other members of its own species
its own roots
In short, this is an amazing part of the plant.
Roots bring water and nutrients into the plant using osmosis to absorb water, and active transport to absorb nutrients. (Note the use of the word “absorb”–with a B–which connotes transport across a barrier and into another space. When we consider soil properties in a later section of this book, we will discuss adsorption–with a D–which means to stick onto, rather than transport into.) This fact is worth emphasizing: Roots use osmosis to absorb water, and active transport to absorb nutrients.
Counterintuitively, water and nutrients do not enter together with the ions that are dissolved in water. That’s an important point to remember. They enter separately. The second point to remember is that plants do not use nutrients in their organic form. They cannot use nutrients until they have been broken down into their inorganic form. We will get into that later in the book, just note it for now.
An additional mechanism is used within the plant to move water and other solutions around. That mechanism is called convection. So, that’s three ways of moving water into and through the plant: osmosis, active transport and convection (where the first two bring water into the plant and the third moves it around). Let’s have a look at each.
Understanding osmosis is critical to your understanding of roots. Please take the time to comprehend osmosis because doing so will serve you in many ways as a gardener. To begin, it will help to become familiar with a few terms:
Solute: This is the stuff that gets dissolved. Here, we mean mineral solids (nutrients)
Solvent: This is the stuff that dissolves the solute. In this case, we mean water
Solution: A combination of solute and solvent
Dilute: Lacking in, or having a low level of, solute
Concentrate: High in solute
Osmosis is the transportation of solvent through a semi-permeable membrane and into a concentration of higher solute in whichever direction will enable it to equalize concentration on either side of the membrane.
We always want osmosis to create pressure (which results in turgor, or the stiffening of pressurized tissue) in the plant cell. This causes the plant to stand upright.
Osmosis becomes a factor when the dissolved chemicals on one side of a semi-permeable membrane cannot pass through the membrane in order to equalize concentrations. Instead, water moves through to achieve the same result. Hence:
If the plant cell has a high concentration of solute (think dissolved nutrient) and the water outside the cell has a low concentration of solvent (think pure water), we describe that cell as being in a hypotonic solution and the plant cell will absorb water, resulting in turgid plant tissue.
If, on the other hand, the plant cell has a high level of solvent and the material surrounding the cell has a high concentration of solute, we describe that cell as being in a hypertonic solution and the plant cell will lose water, resulting in flaccid plant tissue.
When a plant cell is in a solution equal in concentration to the fluid surrounding it, it is said to be in an isotonic solution. This does not mean that no transport is occurring; it means that “water in” is in balance with “water out.”
Active transport costs the plant energy; that’s why it’s called “active.” Active transport is the use of chemical energy (ATP in this case) in order to move solution against the concentration gradient. That means moving mineral nutrient ions from an area of low concentration into an area of high concentration, or, in other words, not equalizing the concentration but instead creating a greater concentration disparity–just in the opposite direction of osmosis.
Plants need to engage in active transport after they have exhausted their ability to absorb nutrient through diffusion. (In contradistinction to osmosis, diffusion is simply the tendency of molecules to evenly distribute themselves; for example, in the way an odor can become detectable at a distance from where it originated.) Since the plant’s need for ionic nutrient is greater than the supply available outside the plant in the soil, the plant must have a way to move that nutrient beyond equilibrium and against a concentration gradient.
Plants are able to do this by using ATP to change the shape of proteins in the cell membranes. When the proteins change shape they are able to bind to ionic (charged) nutrients and deposit them on the opposite side of the membrane, which, in turn, causes another change in protein shape and results in another transport of ions, this time in the other direction. Simply put, there’s a trade. After the trade has occurred, an additional ATP molecule is needed to repeat the process.
Plants use H⁺ (hydrogen) and OH⁻ (hydroxyl) ions, which are products of plant metabolism, to trade for mineral nutrient ions that adhere to soil particles, such as NO3⁻ (nitrate) or K⁺ (potassium). This mechanism is so effective that plants are able to move some nutrients against a gradient with a difference in concentration of 10,000 times!35 Plants do not secrete H⁺ (hydrogen) and OH⁻ (hydroxyl) ions in equal measure; instead they tend to favor H⁺, which means that the plant is actively acidifying its soil (this occurs in addition to decomposition and consequent carbonic acid accumulation.) We will get into this a little more later on because it has implications when you mix or buy potting soil.
Convection is the loss of water through the leaves of the plant. This loss creates a lower pressure gradient at the top of the plant which the water moves toward in order to fill. This phenomenon of the plant pulling water upward is called the Cohesion-Tension (or C-T) mechanism and it is the primary mechanism for moving water through a plant.
Water sticks to itself because of its hydrogen bonds (cohesion). In the small tubes of the xylem, these bonds provide enough tension to enable the low pressure at the top of the plant to literally pull water great distances upward without ever breaking this tension.
In Summary: Roots are amazing, almost “smart” things. Osmosis, active transport and convection carry water and nutrients into and throughout the plant. Plants do not absorb nutrient broth the way a sponge soaks up water from a countertop; mineral nutrient ions and water enter separately. Excessive concentrations of nutrient solution in the soil can reverse the direction of osmosis, killing your plant. Understanding how uptake and transport mechanisms work also resolves the question of whether or not you need to “flush” at the end of each growing cycle. Flushes are pure nonsense, as you now know. Only someone without an understanding of these transport mechanisms would claim differently.
32. Darwin, C., & Darwin, F. (1897). The power of movement in plants. Appleton.↩
33. Barlow, P. W. (2006). Charles Darwin and the plant root apex: closing a gap in living systems theory as applied to plants. In Communication in Plants (pp. 37-51). Springer.↩
34. Baluska, F., Mancuso, S., Volkmann, D., & Barlow, P. W. (2009). The ‘root-brain’ hypothesis of Charles and Francis Darwin: Revival after more than 125 years. Plant Signal Behav, 4(12), 1121-1127.↩
35. Taiz, L., & Zeiger, E. (2010). Plant Physiology, Fifth Edition (Fifth ed.). Sinauer Associates, Inc.↩
If you are finding this information useful, why not pick up a copy of Marijuana Cultivation Reconsidered? It contains this information, plus so much more. Marijuana Cultivation Reconsidered is over 300 pages, with over 90 images and illustrations. Danny Danko describes it as “meticulously researched and enormously useful. . . a must-read for any grower striving to learn more than the basics and think outside the parameters of ‘conventional wisdom’ and horticulture folklore.”