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Functions of Major Plant Nutrients

Posted on: August 28th, 2014

In the spring when you add fertilizer–you know what you are adding, yet do you know why?  Every nutrient has a function and each function is important for healthy strong plant growth.  Providing the crop with the proper nutrients, at the proper time, in the proper proportions is the goal.

The elements found in order of greatest abundance in plants consist of:  carbon (C), oxygen (O), hydrogen (H), nitrogen (N), potassium (K), calcium (Ca), and phosphorus (P).  Carbon, oxygen, and hydrogen are all components of carbohydrates.  The major groups of carbohydrates are sugars, starches, and cellulose.  The sources of (C, O, & H) in plants are derived from carbon dioxide, oxygen and water–these elements are not normally included in the listings of plant foods or fertilizers.

NITROGEN:  The most ubiquitous of the plant foods.  Nitrogen is found in all amino acids, proteins, and enzymes.  Every life process involves one or more of these chemical groups and life as we know it–cannot exist without nitrogen!

POTASSIUM:  The second most abundant mineral nutrient in plants, with requirements being close to those of nitrogen.  In terms of physiology, potassium can be called the “Universal Helper.”  Potassium fills very few roles by itself, but most of life processes require potassium.  Potassium raises the osmotic pressure in cells, which is the gradient force that allows soil water to be absorbed by plant cells.  The higher the cell potassium level, the higher the osmotic pressure, then the greater attractive force for soil water.  The short version–potassium helps move nitrates across root cell membranes.  Potassium helps maintain leave and stem cell turgor by increasing the osmotic pressure–which increases the cell’s resistance to penetration by pests and aides in the plant’s general water movement.  Potassium also functions in the opening and closing of the stomata by controlling the movement of the guard cells.  This allows proper gas exchange in the leaves, provides raw materials for photosynthesis and respiration, along with respiration and discharge of waste materials and transpiration control.  So in short, Potassium:

  • Activates enzymes.  Potassium activates over 60 plant enzymes.  These enzymes (biological catalysts) are inactive or activity is reduced if potassium is absent.  Every life system in the plant is controlled or influenced by one or more of these enzymes.
  • Functions in nitrate movement across the root cell membranes and also nitrate transport up the plant.
  • Functions in moving photosynthate from point of production in the leaves to the fruit, stem, and root.
  • Increases root cell’s ability to absorb soil moisture
  • Increases transpiration and efficient use of water
  • Functions in stomatal open & closing thereby controlling gas exchange for photosynthesis & respiration
  • Increases cell turgor
  • Helps cells maintain activity in cold temperatures
  • Most people consider a deficiency of potassium reduces the test weight of the fruit as well as the full development.  Potassium is the important carrier for all nutrients into and up the plant.

CALCIUM:  Is found in large quantities in plants but it’s functional roles are few.  Calcium is used mainly as calcium pectate, which essentially is the glue that holds the plant together.

PHOSPHORUS:  is found in lesser amounts than nitrogen or potassium in plants and has fewer specific roles; however, these roles are critical.  Phosphate is the energy-rich reactive group in ATP (Adenosine triphosphate).  All energy requiring processes, in ALL living cells, are directly or indirectly coupled to ATP. 

No (P) = no ATP.  NO ATP = no energy transfer.  No energy transfer = no life.

Phosphorus is a component of genetic code materials DNA and RNA which control growth and reproduction.  Phosphorus is a component of phospholipids which are the primary structural chemicals of some cell membranes which allow the cell to hold its integrity and also serve as metabolic centers.  In short, phosphorus:

  • Provides energy storage and transfer
  • An integral part of DNA & RNA which control the plant’s genetic code
  • Is part of the cell membrane material phospholipid
  • Is a vital part of the ATP system.  All energy requiring processes, in living plants, are directly or indirectly coupled to ATP

It is an axiom that any life system in an organism is influenced by each of the other life systems in that organism.  A single life system or plant nutrient cannot by isolated from the whole.  Liebig’s “Law of Minimums” states:  ‘…that if a plant is deficient in one nutrient, that nutrient will be the limiting factor in the growth of that plant.’  Therefore, if a plant is deficient in potassium, adding excess nitrogen will not improve the growth.