Life is only possible because of a complex exchange of chemicals enabling cell growth. Healthy cells rely on a constant supply of essential nutrients. We consume food, which is magically broken down into nutrients to fuel our cells. (Okay, it’s not magic but we are here to talk about trees and not the human digestive system). So like any other living thing, trees need food to survive. However, the food a tree requires does not come from the 5 main food groups.
Trees obtain their food from the soil in which they are rooted.
What Nutrients a Tree Needs to Survive
The next bit contains a whole bunch of chemistry but who among us didn’t love science class (Okay, maybe just me but honestly this is cool). 95% of a tree is made up of carbon, and water [1]. The other 5% is made up of 13 essential nutrients that have specific roles in the healthy function of a tree (fig. 1). Of the 13 essential nutrients, there are 3 categories:
Primary Macronutrients: These nutrients are highly abundant in the soil, and are the main ingredients used in fertilizers.
Nitrogen (N): Responsible for green, leafy growth. Necessary for building proteins and precursors to chlorophyll, which performs metabolism and respiration functions (AKA photosynthesis).
Phosphorus (P): Required for energy transformation used for growth of roots, flowers, fruit and bulbs.
Potassium (K): Required for control of stomata cells in leaves, which are responsible for respiration and movement of water. This is important for appropriate stress responses, such as drought resistance.
2. Secondary Macronutrients: These nutrients are also abundant in the soil, but are only required in moderate amounts.
Calcium (Ca):
Part of the cell wall, therefore required for proper cell formation. Lack of calcium will result in gelatinous leaves due to a lack of fully formed cell walls [1].
Magnesium (Mg):
Required by the chlorophyll for photosynthesis.
Sulphur (S):
Required for protein formation. Fun fact- Sulphur is a natural scent molecule and plants like garlic contain LOTS of Sulphur [1].
3. Micronutrients: These minerals are ions required in minimal amounts, and are typically present in sufficient amounts if the pH of the soil is maintained. What is an ion and what is pH you ask? Yay, more chemistry, stay tuned.
Boron (B), Copper (Cu), Chloride (Cl), Manganese (Mn), Molybdenum (Mo), and Zinc (Zn).
As of now there is no UberEats for trees, so where do these nutrients come from?
Well, soil is a mixture of rock, clay or sand particles, molecules such as water, and elemental minerals (fig. 2). The elemental minerals are the nutrients required for plants and trees. Soil is ‘food’ delivery for trees- "TuberEats" if you will. The concentration and availability of these minerals (nutrients) depends on the following 3 factors:
Organic matter in the soil
Also known as dead or decaying stuff, this is the brown and green matter in the soil. Brown matter, such as yard or food waste, is rich in carbon, while green matter, such as leaves or grass clippings, are high in nitrogen.
pH of the soil
You were warned. Here is your chemistry lesson for the day! The acidity of something is its pH. The pH of soil is based on the concentration of hydrogen ions (which are charged molecules) within the soil mixture (fig.2). pH is represented by a scale (fig. 3), with 0-7 being acidic (i.e. lemon juice is between 2 and 3), 7-14 being alkaline (i.e. bleach is between 11-13), and 7 being neutral (i.e. water). Ideal conditions for growth and normal life are at a pH of 5.5-7. However, some nutrients thrive at different pH levels. Acidic soils are known to be high in micronutrients, but low in macronutrients, while alkaline soils are low in micronutrients and high in macronutrients [2].
Microbes colonizing the soil
In a previous blog, we discussed the microbial colonization of roots in terms of fungal microbes. But microbes can also be bacteria. Both fungi and bacteria in soil aid in the decay of organic matter, releasing additional macronutrients for plant consumption. It has also been proven that microbes naturally excrete certain ions via biochemical processes, which we won’t get into, but this aids in the availability of many micronutrients [3].
So, how does a tree eat?
Trees do not have a mouth or a stomach like you and I. So how do they consume their nutrients? Through the roots of course. The nutrients are present in the soil surrounding the roots of the tree. For a root to “consume” the nutrients, they must first be adjacent to the root. There are mechanisms utilized by roots to concentrate nutrients adjacently for uptake:
Sometimes, roots are just in the right place at the right time and will bump into an ion as they grow. This is called interception (fig. 4). This is a convenient method for trees with fast growing roots, such as poplar or willow trees.
Mass flow is essentially the constant flow of water on earth. Water is always flowing, carrying along particles and minerals it picks up along the way. As water flows past roots, certain nutrients the tree needs will become concentrated and available for uptake (fig. 5).
Diffusion is the passive movement of nutrients from a high concentration to a lower concentration. Root areas are considered low in nutrient concentration. Think of it like this: you throw a banana peel out the window while driving (it is biodegradable, it is not littering if you throw it hard enough into the forest). That banana peel becomes organic matter, releasing high concentrations of potassium into the soil. A neighboring tree is lacking potassium, and its roots will naturally diffuse the high concentration of nutrients from where your banana landed to the nutrient depleted roots (fig. 6). That's the diffusion of nutrients.
Once the nutrients are adjacent to the roots, active and selective uptake transfers nutrients from the soil to the inner workings of the tree. Selective uptake means that nutrient-specific protein carriers line the outer membrane of the roots [4], and carry nutrients into the tree only as needed for survival (fig. 7). This is why there are no obese trees! Nutrient uptake is proportional to the tree's needs, not the availability in the soil. If human nutrient consumption was this selective, no one would need to go back for seconds (unless maybe if it's grandma's cookies or mom’s meatloaf, which are too good not to have seconds).
Ok so long story short(ish), trees are alive, and thus require essential nutrients to survive. While they may not be snacking on cookies, trees need food too.
Resources
Alabama Cooperative Extension System. (Nov. 2018). Essential Plant Nutrients. https://www.aces.edu/blog/topics/farming/essential-plant-elements/
2. Direct Garden Supply. (Jul. 2020). The pH Nutrient Chart: Your Guide to Maximize Plant Nutrient Uptake. https://directgardensupply.com/blogs/knowledge/plant- ph-nutrient-chart
3. Merchant, S.S. and Helmann, J.D. (2012). Elemental Economy: microbial strategies for optimising growth in the face of nutrient limitation. Advances in Microbial Physiology, 60: 91-210. DOI: 10.1016/B978-0-12-398264-3.00002-4
4. Mengel, D. (1995) Roots, Growth and Nutrient Uptake. Department of Agronomy Publication, 95(8). Purdue University, IN. https://www.agry.purdue.edu/ext/pubs/agry-95-08.pdf
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