Plant Nutrient or Food

    Is it a Plant “Nutrient” or “Food”?

    This is somewhat of a trick question, and falls often in the realm of semantics.  Many experts insist there is a distinction; I do not.  The dictionary defines a “food” as “any nutritious substance…that plants absorb, in order to maintain life and growth.”  In that broad sense then, any nutrient essential to the growth of a plant would be considered a “food.”  There are 16 such essential plant nutrients meaning that withholding any one of them eventually results in the death of the plant—faster for lack of some, slower for lack of others—the result however, is the same. 

    The essential nutrient list includes many so-called “minor” nutrients (note: the term “minor” means they are needed in smaller, but no less important quantities than other nutrients).  These “minor” nutrients are generally adequately supplied by the minerals that make up all soils, in quantities sufficient for normal growth.  Common exceptions to that rule in Utah and the arid/semi-arid western US are iron (Fe) and zinc (Zn), due to the fairly unalterable alkaline (high pH) character of the soils (a certain topic for a future post).

    Six of the essential nutrients are the primary “macro” nutrients needed in large (percentage level) quantities in the plant, namely carbon (C), oxygen (O), hydrogen (H), nitrogen (N), phosphorus (P) and potassium (K).  The first three (C, O and H) which make up the bulk of plant tissues (carbohydrate, cellulose, wax, lignin, etc.) are obtained readily from air and water absorbed by the plant.  The other three (N, P and K) are supplied by the soil, and are those nutrients which are the most often deficient naturally in plants.

    The distinction that some experts make between “food” and “nutrient” is as follows.  The elements C, O and H are the primary building blocks of energy storage compounds (fats, starches and sugars) and cell structures (cellulose, etc.).  Starches and sugars manufactured through photosynthesis are the bulk energy storage molecules used by the plant to supply roots (and aboveground tissues during the night) with the energy needed to function, and hence are deemed plant “foods.” The mineral elements (primarily N, P and K) are facilitator “nutrients” needed in the compounds and by the processes through which the “foods” are made. 

    To this expert, that is a weak distinction, indeed.  If one withholds the “nutrients”, the metabolic processes shut down; if one exhausts plant energy stores, the metabolic processes shut down.  Either way the plant dies, so whether defined as “food” or “nutrient” the plant relies equally upon them to “…maintain life and growth.”

    As a soil scientist, I am fond of the nutrients that come from soil, or supplemented to the soil from decomposing organic matter or mineral fertilizers, particularly N, P and K.  In the next couple of paragraphs, I would like to comment briefly on the importance of these elements in the plant and the reason I do not make any distinction between plant “food” and “nutrient.”

    Nitrogen (N)

    This element is the most dynamic and most often limiting soil-derived nutrient. Nitrogen is required in large quantities by all living things, and therefore, competition for N is very high.  Nitrogen is the root element of amino acids that are the building blocks of proteins, enzymes, DNA, RNA and many other critical compounds and structures in all living things.  Additionally in plants, N is a root element in the chlorophyll molecule that is critical to photosynthesis, the process that manufactures the sugars and other carbohydrates the plant needs for energy and cell structures.  Hence, without N, plants cannot “…maintain life and growth.”

    Phosphorus (P)

    This element is important in the manufacture of phospholipids that form the protective membranes that line each plant cell and regulate the transfer of water and nutrients and other compounds into and out of the cells.  This is most critical in the growth and function of roots where the active uptake of water and nutrients from the soil is foundational to plant function.  Moreover, P is the base element of ATP, ADP and NADP that are the energy transfer molecules that drive photosynthesis and other metabolic functions in plants, without which the plant cannot “…maintain life and growth.”

    (Incidentally, the reaction in the plant facilitated by NADP, a P-bearing compound, is that which releases gaseous oxygen, without which YOU cannot “…maintain life and growth” either!)

    Potassium (K)

    This nutrient is required in similar quantities comparable to N in the plant.  Curiously, K is not a building block of any plant tissues or metabolic compounds, but remains in dissolved ionic form in plant cell solutions.  Potassium ion, as such, is critical to maintaining electrolyte concentrations in plant cells, facilitates the transfer of other nutrient elements through the plant, and features preeminently in the control of the opening and closing of leaf pore openings (or stomates) through which carbon dioxide enters, and oxygen and water vapor exits the plant.  A lack of K in the plant means these pores cannot open, preventing carbon dioxide entry, and water flow out of the plant, thereby shutting down the whole supply train of raw materials (nutrients, water, and carbon) needed to “…maintain life and growth.”

    Last Word

    All the essential elements are both “substance” and “nutritious” in the sense that they provide the building blocks for all compounds, structures and tissues in the plant, and therefore enable the entire system to function properly and “…maintain life and growth.”  Remove even one from the whole and the whole ceases to function; supply (or “feed”) the whole with each in sufficient quantity, and the whole thrives.

    Grant Cardon, USU Extension Soils Specialist

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