I just replied to a comment in my Fish Fertilizer Post which said, “It’s surprising the article makes no mention of the full spectrum of minerals present in sea food, and therefore the fertilizer. Sea water is known to have an astounding 82 elements (don’t have the link, please Google it) The only thing that prevents us from using sea water as fertilizer is the high sodium content. Fish do the wonderful job of filtering out that excess sodium and leaving you with extremely mineral rich organic matter ! “.
A couple of weeks ago at the Guelph Organic Conference, one of the salespeople selling an Australian sea salt extract, claimed that his product contained 99 nutrients that plants need.
I found the following claim on a company website; “Azomite – Organic Trace Mineral Powder – 67 Essential Minerals for You and Your Garden”. Azomite is a brand name product made from “special” rock dust.
Why does fertilizer only show three nutrient numbers, NPK, when plants need either 67, 82 or 99 nutrients? Inquisitive gardeners want to know.
Nutrients vs Elements
You might have noticed that the comments above used different terms. One said elements and another said nutrients. “Element” is a specific chemical term that describes a single type of atom. Carbon is an element, and aluminum is an element. All known elements are listed on the Periodic Table (see above) which shows around 118. Of these 94 exist naturally and the rest are man-made in the lab and are very unstable.
The term nutrient has various meanings, but in terms of plant growth it usually refers to the elements, minerals, or simple compounds that plants use. In most cases these are elements or ions of elements, but a few like CO2 and water are simple compounds. Of course, a molecule of water (H2O) consists of just two elements, hydrogen and oxygen. For the benefit of this post the term nutrient will only refer to elemental nutrients and does not include a wide range of organic molecules that could also be called nutrients.
Regarding the statement “Sea water is known to have an astounding 82 elements (don’t have the link, please Google it)” – I did Google it. According to Standford University (ref 2) there are 42 elements or 47 minerals and metals in sea water.
Essential Plant Nutrients
The essential plant nutrients include carbon, oxygen and hydrogen which are absorbed from the air. The other essential nutrients, which are obtained from the soil, (or water in the case of water plants) include:
- the macronutrients: nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), sulfur (S), magnesium (Mg)
- the micronutrients (or trace minerals): boron (B), chlorine (Cl), manganese (Mn), iron (Fe), zinc (Zn), copper (Cu), molybdenum (Mo), nickel (Ni). and cobalt (Co)
That is a total of 18. There is still debate as to whether silicon, nickel, chlorine and cobalt are essential.
What Nutrients Do Plants Use?
You might think we just answered this question, but plants can use additional nutrients which are non-essential. This means plants will use them if available, but they do not need them in their diet. Some of these nutrients are only found in certain types of plants.
Non-essential nutrients, which are also called beneficial nutrients, include aluminum (Al), silicon (Si), selenium (Se), sodium (Na), vanadium (V) and gallium (Ga). (ref 1)
Silicon is used to strengthen cell walls, which makes plants more drought resistant.
Sodium replaces potassium in certain reactions, is used by C4 plants and helps control osmotic pressure.
Vanadium is used by green algae.
This brings the total of useful nutrients to 24.
Plants also absorb other elements like cadmium and lead. These may provide some benefit to the plant but current evidence suggests they don’t.
Benefit of More Elements
There are 94 natural elements and plants use 24 of these. What is the benefit of the remaining 70 to plants? Nothing.
The myth that plants benefit from more nutrients is a consequence of marketing, based on peoples belief that “more is better”. If some nutrients are good for plants, other nutrients might also be important. This is not the case.
Most garden soil contains plenty of the micro-nutrients. Unless a soil test says differently, assume your soil has enough of these. Buying extra nutrients that plants don’t need is a waste of money and resources.
- Forms of nutrients in soil and their functions in plants; http://eagri.tnau.ac.in/eagri50/SSAC122/lec05.pdf
- Over 40 Minerals and Metals Contained in Seawater; http://www.miningweekly.com/article/over-40-minerals-and-metals-contained-in-seawater-their-extraction-likely-to-increase-in-the-future-2016-04-01/rep_id:3650