Plant Nutrition: Essential Elements for Plant Growth

Plant nutrition is a diverse topic ranging from an understanding of the parts of a plant, essential elements plants need, the methods by which nutrients are absorbed and physiological symptoms of nutritional disorders.  This article, part of a series on each of these topics, focuses on the essential elements of plant nutrition.

Plant nutrition is at the core of food production techniques such as gardening, aquaponics and hydroponics.  In aquaponics the nutrients are provided as a by-product of the aquaculture, which is composted by natural bacteria.  Hydroponics, in contrast, requires the essential nutrients to be dissolved in water (or a concentrated solution of these nutrients to be diluted in water) and then the nutrient solution is made available to the plants.  Organic gardening can use recycled plant fibers that have been composted.  Regardless of the method you use, a thorough knowledge of plant nutrition is critical to managing your crops.

Essential elements necessary for plant growth are split into three categories: essential mineral elements, essential nonmineral elements and beneficial elements.  The criteria for elements to be considered essential was first set forth by Arnon and Stout in 1939:
  1. The element must be required for the completion of the life cycle of the plant
  2. The element must not be replaceable by another element in whole
  3. The element must be directly involved in the metabolism of the plant, i.e. required for a specific physiological function in the plant.
Water makes up the majority of the plant fresh weight, from 80 to 95 percent, with the exact percentage dependent on environmental and nutrient conditions at the time of analysis. Of the 90+ naturally-occurring elements found in plant tissues, only 16 are considered essential for plant growth. The first three are the essential nonmineral elements and they are, with their corresponding concentration in dry plant tissue:
  • Carbon, 45%
  • Oxygen, 45%
  • Hydrogen, 6%
Carbon is supplied from carbon dioxide, oxygen from both carbon dioxide and water, and hydrogen from water.   As you can see oxygen is one of the most abundant elements in plants and it is the access to oxygen that set growing methods like aquaponics apart from traditional dirt farming.  The growing medium can range from gravel, sand, light expanded clay aggregate (LECA), rockwool, sawdust, vermiculite and others.  The growing media only serves four purposes for plants: supply of water, oxygen, essential nutrients and supports the plant root structure.  By using a growing medium that cannot be compacted like soil, plants roots have increased access to oxygen and carbon dioxide, producing larger, more vibrant and productive plants.

The essential mineral elements comprise the remaining essential elements and are further divided into two categories, macronutrients and micronutrients.  The macronutrients and their relative percentages in dry plant tissues are:
  • Nitrogen, 1.5%
  • Potassium, 1.0%
  • Calcium, 0.5%
  • Magnesium, 0.2%
  • Phosphorus, 0.2%
  • Sulfur, 0.1%
The micronutrients are:
  • Chlorine, 0.01%
  • Iron, 0.01%
  • Boron, 0.002%
  • Zinc, 0.002%
  • Manganese, 0.005%
  • Copper, 0.0006%
  • Molybdenum, 0.00001%
Beneficial elements can promote plant growth, but fail to meet Arnon and Stout's criteria of essential.  They are silicon, nickel, sodium, aluminum, cobalt, selenium and vanadium to name a few.  Each of the essential elements has a role in the life of the plant, from regulating biochemical reactions to food storage to various metabolites for plant growth.  It's important to note plants have the ability to absorb a wide variety of elements present in soil.  The danger of pollution is plants are capable of absorbing toxic elements it may not actually use in development and growth.  Regardless, the toxicity of the element is not mitigated once absorbed by the plant and poses the same health risks should the plant be consumed.  A brilliant new scalable green technology has emerged to take advantage of this basic premise.  Called grey water reclamation this technology allows water polluted by anything from waste to heavy metals to be filtered using plants, which then absorb the elements.  The filtered water is then recycled back into the system.  The applications have ranged from shower and toilet water in homes, to grey water in office buildings.

The top three limiting nutrients, from a crop production standpoint, are nitrogen, phosphorus and potassium.  It was Justus von Liebig who developed the Law of the Minimum which states that crop yield is proportional to the amount of the most limiting nutrient, whichever nutrient it may be.  Once the deficient nutrient is provided, crop yield may increase until another nutrient becomes deficient and the Law of the Minimum would then apply to that nutrient.

Nitrogen is arguably the most important of the nutrients in an aquaponic system.  It is absorbed by plants as either ammonium ions or nitrate ions and then further reduced into organic compounds.  The fish by-product is primarily ammonia, a combination of ammonium and toxic free ammonia.  The bacteria cycle breaks down free ammonia to create nitrites and then nitrates.  In plants, nitrogen is an essential component of amino acids, the building blocks of proteins, which can be used in the structure of the plant, and nitrogen can be found in enzymes and coenzymes, which regulate the biochemical reactions in plants.

Phosphorus plays an important function in energy transfer in plants, specifically in the formation (and reduction) of phosphate bonds in ATP, a chemical energy compound.  The phosphate bonds needed to form ATP are produced via photosynthesis, while the energy processes required to grow the plant break down the bonds for the energy stored in them.  All cell membranes in plants comprise a lipid structure containing phosphorus which regulate the flow of compounds in and out of the cell.  Phosphorus is also part of the sugar phosphates which form structural components of nucleic acids DNA and RNA.

Potassium is absorbed as a cation and with only one oxidation state it balances the uptake of anions such as nitrate, sulfate and chloride.  It is also used as an activator for many enzymes and is required in high levels for protein synthesis, however it does not form a structural part of the plant.

The mere presence of the essential elements does not guarantee their availability to the plant.  The pH level of the aquaponic system dictates the rate at which the nutrients are absorbed.  pH is a measure of acidity and alkalinity.  As a logarithmic function, a one-unit change in pH is a ten fold change in hydrogen ion concentration.  Note in the chart below that Resh's optimum pH range is based on the hydroponic growing system, not aquaponics.

pH affect on nutrient availability.  Resh 2001.
This chart can be invaluable in diagnosing nutrient deficiencies in plants.  As an example we can see that as pH rises from 6.0 to 8.5 (a dramatic increase by any standard) we find a decrease in the amount of iron, manganese and boron available.  Plants requiring significant percentages of these elements would show some physiological symptoms of nutrient deficiency.  It is important to catch an imbalance early, since a deficiency of one element can impair the plant's ability to accumulate other elements.  In this case, the physiological symptoms characteristic to a single element deficiency are obscured with the symptoms of another, making it almost impossible to determine which elements are causing the deficiencies.

An excellent technique to detect nutrient deficiencies is to place different plant species with varying levels of susceptibility in the same system.  A common example is planting cucumbers in the same system as tomatoes.  Cucumbers will manifest symptoms of calcium deficiency long before tomato plants will.  While you may lose the cucumbers, you can remedy your system before losing your tomatoes.  The value of this technique is that you may still have a productive harvest instead of losing your entire crop.

In the chart above, Resh's pH range is selected to optimize nutrient uptake in a hydroponic system where plants are the only living component.  In aquaponics, however, we have three different living systems present: fish, plants and bacteria, each with a different optimum pH range.  We compromise and come up with an optimal range of 6.7 to 6.9.  This is very important to note.  Since the overall pH of the system is outside the ideal range of a particular component, smaller fluctuations in pH can have a bigger impact.  That is why pH is one of the key parameters to be monitored using programs like Aquaponics Tracker.  For example, if a pH fluctuation shifts outside the acceptable range for the bacteria, the bacteria may go dormant (or die), preventing the conversion of fish waste to plant nutrients.  The plants will begin to show symptoms of nutrient deficiency, but more importantly the lack of bio-filtration means accumulating toxic levels of ammonia in your fish tank.  When keeping track of system parameters it is important to assign notes to your system readings, such as tracking physiological changes in plants and fish, to link symptoms with readings and allowing you to check back should a similar incident arise again.

As I stated at the beginning of this article, understanding plant nutrition is required for managing your system.  By understanding the role nutrients play in plant development and health you have a powerful tool to diagnose physiological symptoms of plants in your systems.  While you are relieved from the careful measuring nutrient salts to dissolve in water required by hydroponics (or diluting concentrated solutions from bottles), you are responsible for the feed you provide your fish.  If your fish are denied proper nutrition, they in turn will create a poor by-product to feed your bacteria.  Always remember, "Garbage in, garbage out."