By Jeff Lowenfels
The following is an excerpt from the book, Teaming With Nutrients.
No matter where a plant grows, no matter how complex its flowers or fruits, no matter what its seeds look like or what kind of leaves it grows, all it takes for that plant to survive and reproduce are a mere seventeen of the ninety naturally occurring elements. (There are twenty-seven or so other elements, but these are man-made and thus could not be required for plants to survive.)
Just seventeen elements sustain life–and not just plant life, but yours and mine, as well. We eat plants and/or animals that eat plants to survive. Combining these seventeen elements results in all of a plant’s beauty, its physical structure, organs, and ability to sustain life on Earth. This is astounding.
Plant scientists have defined these seventeen elements as the essential nutrients. They are all needed for plants to survive (that is, to grow and reproduce). No other element can replace an essential nutrient because none can carry out its functions. Are there other essential nutrients besides the seventeen discussed here? Probably, but their exact functions have not yet been determined. Researchers are conducting hydroponics studies in which an element is left out of the nutrient solution to test its effects on plant health and growth. This research will likely identify new essential elements, but their necessary quantities are so small that the search is no doubt a difficult one.
A plant may contain other elements in addition to the essential elements, and some of these actually benefit the plant. There may be between thirty and sixty nonessential elements in a plant. Seaweed is famous for containing sixty natural elements. Some nonessential elements play positive roles and improve the plant in some way, but at the end of the day, the plant can survive without them. However, a plant cannot grow and reproduce without each of the seventeen essentials.
Here, again, is something to contemplate while gardening or when looking at a tree or following the progress of an amaryllis as it goes from bulb to flower in mere weeks. The combination of a mere seventeen substances makes all you are looking at. You can’t even play a decent card game with seventeen cards, but you can build plants by combining seventeen elements. Do I have your attention?
Essential Nutrients
Most gardeners can name many of the essential nutrients. The macronutrients are the ones required in the greatest quantities. Three of these are always represented on fertilizer packages as the N–P–K trilogy: Nitrogen (N), phosphorus (P), and potassium (K). Because it often comes up, the symbol K is used for potassium not because the letter P was already taken by phosphorus, but because it comes from the Latin name kalium. Beyond this trilogy, some gardeners are familiar with other macronutrients, such as sulfur, calcium, and magnesium. These are also used by plants in large amounts. Carbon, hydrogen, and oxygen are also macronutrients.
The second category is micronutrients, which are sometimes called trace minerals. The lack of iron, manganese, zinc, copper, molybdenum, boron, chlorine, or nickel can cause plants to do poorly. Although the name micronutrient might suggest they are less important than the macronutrients, they have the same degree of importance. They are essential, but only tiny amounts are required. The micronutrients are present in most soils and don’t have to be added very often unless there is something way off balance. But they have to be there or the plant will not survive or reproduce.
The list of essential plant nutrients is not a very long one, and most should be familiar to you because they are in your own daily diet (just check your vitamin and mineral supplements bottle). If you’re going to be a really good gardener, though, you need to really understand a lot more about them.
Macronutrients
Some of the macronutrients are familiar to gardeners in general terms, and some are even familiar in specific ways. Most gardeners, for example, associate a yellowing lawn with a lack of nitrogen. Understanding what each of these nutrients does in cellular terms, however, is the best way for a gardener to be able to assess and address problems in plants, should they arise.
Hydrogen, Oxygen, and Carbon
Hydrogen, oxygen, and carbon account for a whopping 96 percent of the mass of a plant. Carbon and oxygen each make up around 45 percent and hydrogen 6 percent. That leaves only 4 percent for the other fourteen essential elements.
Water (H2O) and carbon dioxide (CO2) are the sources of a plant’s hydrogen, oxygen, and carbon. Carbon dioxide and oxygen can be dissolved in water and enter plants via the roots. However, these gasses mostly enter through stomata on the leaves. Water is the source of hydrogen, and the breakdown of water results in the release of oxygen as a by-product. The carbon and oxygen used in photosynthesis comes from carbon dioxide. Individual plant cells have direct access to air and water at all times due to the unique structure and characteristic of cell walls, which surround every plant cell. You know it as the apoplastic pathway.
Despite their overwhelming presence in the makeup of plants, however, these elements are not considered fertilizer. This is not to say plants don’t need water to survive or that a gardener can’t supply it. In an enclosed situation, carbon dioxide can be pumped in to help plants grow, but this is not the norm. Hydrogen, oxygen, and carbon are clearly essential, but they are non-mineral nutrients, not fertilizers, and so they are only tangentially covered in this book.
Nitrogen
Nitrogen (N) is crucial to plant growth. One could argue why it is so important, but I settle on its presence as the backbone of amino acids, the structural building blocks of proteins, one of the four kinds of molecules that make up life. No nitrogen, no proteins.
Enzymes are proteins. These catalysts are required for all activities in a cell. Nothing happens on a cellular level without an enzyme being involved. And when something doesn’t happen, it’s usually because an enzyme is missing. Photosynthesis and respiration absolutely require nitrogen and the enzymes necessary to drive these processes. Proteins are also the molecules that make cellular membranes semi-permeable. They are the channels, carriers, and motors that are necessary for sufficient quantities of water and any quantity of the other essential nutrients to get into a plant cell.
Some would argue that a more important role for nitrogen is its role as the base element for nucleotide molecules. These are the building blocks of DNA and RNA, the blueprints and translators, respectively, of the genetic code. Much of a plant’s cellular activity, including the production and use of DNA and RNA, is to ensure there is an adequate supply of those nitrogen-based enzymes to carry out cellular activities.
If that isn’t enough, nitrogen is also an essential part of the chlorophyll molecule (C55H72MgN4O5). Without those four nitrogen atoms, there is no photosynthesis. Therein, incidentally, lies the answer tot he yellowing lawn: a lack of nitrogen means there is less of chlorophyll’s green pigment.
Nitrogen remains mobile once inside the plant, meaning it can be transported to where it is needed. This mobility is also why the first signs of yellowing from a lack of nitrogen occur in older leaves. Nitrogen is so critical to new growth that the plant will rob nitrogen from older cells in order to grow new ones.
Outside of the plant itself, nitrogen also has a great influence on the pH of the soil, which has a direct influence on the uptake of all nutrients. The pH in the rhizosphere goes up when No3 is added because hydroxyl ions (OH) are released. This increases the solubility of iron and aluminum phosphates.
Most plants in a garden are about 3 to 4 percent nitrogen by weight. By way of comparison, your body is around 3 percent nitrogen, which makes sense because we really are what we eat. Only carbon, hydrogen, and oxygen exist in higher concentrations in plants (and humans).
The Earth’s atmosphere consists of a whopping 78 percent nitrogen. Unfortunately, atmospheric nitrogen (N2) is off limits to plants because nitrogen atoms form extremely strong, triple covalent bonds with each other. However, it is precisely this ability of nitrogen to form triple bonds (due to empty pairs of electrons in its valence orbit, remember) that makes it so chemically important.
Those triple bonds are incredibly difficult to break apart. Until the early 1900s, when chemists solved this puzzle, atmospheric nitrogen bonds could only be broken by biological means via bacteria and Archaea. Making nitrogen usable by plants is nitrogen fixation, and the soil microbes responsible for this are known as diazotrophs. So important is nitrogen that plants will work with other organisms to get an adequate supply. Up to 50 percent of the nitrogen in your garden can come from nitrogen-fixing organisms.
The most familiar diazotrophs are Rhizobia, a group of soil bacteria that form symbiotic relationships with legumes. Rhizobia provide the enzymes necessary to break apart triple-bonded atmospheric nitrogen molecules, and the plant provides the housing for this activity, as well as carbon-based exudates that the bacteria consume. The amount of oxygen has to be limited for the enzymes to work, and the root nodules provide such an environment. There is enough nitrogen for the legume and the bacteria to share. Up to 20 percent excess nitrogen is produced as well, and it moves into the soil and soil food web, where much of it is brought to plants by another group of symbionts, mycorrhizal fungi. Many commercial mixtures of Rhizobia are available, and we no know that there has to be a specific match between the right species of Rhizobia with eh right kind of plant.
Less is known about Frankia, the so-called filamentous bacteria, or more properly actinomycetes. These nitrogen-fixing bacteria associate with actinorhizal plants that include alders, several trees and shrubs found in the South Pacific, and some more familiar plants used as landscape material, such as oleaster, bayberry, dryas, and the edible sea buckthorn. Most of these are considered pioneer plants, alder in particular, and their symbiotic relationships with Frankia bring needed plant-available nitrogen into poor soils.
Aside from biologically fixed nitrogen, there are relatively few natural sources of nitrogen. Those that exist are primarily deposits of nitrogen-based minerals and guano, both of which were severely exploited in the 1800s, actually resulting in wars: the rise of the modern German, U.S., and British navies; and the start of the modern nation building. Fears that the world would run out of useable nitrogen to supplement the biologically fixed supply had developed, and it appeared that the world would not be able to conduct enough agriculture to sustain a growing population.
Around the start of the twentieth century, the German chemist Fritz Haber figured out how to fix nitrogen. Carl Bosch, an industrial engineer, scaled up the process so it cuold be accomplished on an industrial scale, and the rest is history. Today the Haber-Bosch process provides over half a billion tons of artificial manures each year, requiring a staggering 5 percent fo the world’s natural gas production to do so. This artificially fixed nitrogen sustains more than a third of the world’s food production.
To keep learning about nutrients, find Teaming With Nutrients by Jeff Lowenfels at the AcresUSA Bookstore.
About the Author
Jeff Lowenfels is author of the acclaimed quadrilogy known as The Teaming Series” (specifically: “Teaming with Microbes, The Organic Gardener’s Guide to the Soil Food Web”, “Teaming with Nutrients, The Organic Gardener’s Guide to Optimizing Plant Nutrition”, “Teaming with Fungi, The Organic Growers Guide to Mycorrhizae” and “Teaming With Bacteria, The Organic Gardener’s Guide to Endophytic Bacteria and The Rhizophagy Cycle.” These books will change the way you grow plants…that is a guarantee unless you are already familiar with the soil food web.