5.1: What is a Nutrient?

“All the great agricultural systems which have survived have made it their business never to deplete the earth of its fertility without at the same time beginning the process of restoration.” ― Albert Howard, The Soil and Health: A Study of Organic Agriculture

A nutrient is the link between soil elements and plant nutrients.

Farmers, gardeners, horticulturalists, crop consultants, soil scientists, and agronomists have differing ideas regarding nutrients. Perhaps that is because the term soil fertility is often conflated with plant nutrition since they are closely related concepts. In this section we define nutrients in terms of the origins and relationship of elements to each other, focusing on the nutrients that should be present in soil to grow healthy plants. This is a short summary of the role of the anion nutrients in the life of a plant. Later sections will cover how these nutrients cycle in the soil and how they are added to soils as fertilizers at various points in the cycle. The reader can also find out more about individual nutrients in plant physiology in a course that focuses on crops and plants.

Elements for Growing

Plants, animals, microbes, and other life forms need external sources of elements in order to grow and reproduce. Elements are “irreducible” constituents, that is, they are materials that cannot be further simplified. The earth is made up of about 90 naturally occurring elements. Elements are shown in Figure 1. It is a table arranged in repetitions or a “periodic” way to show how many protons or neutrons are in the nucleus of each element. (That number of protons coincides with each element’s “periodic” number). As most readers probably recall from high school chemistry, each letter(s) in the box is a shorthand name for an element. For example, Oxygen is element #8 and is abbreviated as “O”. Oxygen and silicon (Si) (element #14) are the most abundant elements on earth, next are aluminum (Al) (#13) and iron (Fe) (#26). Iron is abbreviated “Fe” because the Latin word for it is “Ferrous”.

Some elements are nutrients, while others are not. For example, all plants require iron, but none require aluminum, and very few require silicon. Sodium, which is very abundant in nature, is not required by plants, and though some plants can use it, it is toxic in high concentrations. Animals, on the other hand, do require sodium. To recap – not all elements are nutrients, but all nutrients are composed of elements.

Minerals

Minerals are generally defined as naturally occurring, homogenous, and inorganic. In this sense, inorganic simply means “does not contain carbon”. Each mineral has a well-defined color, hardness, crystalline structure, and chemical composition. Some minerals are composed of a single element, as is the case with both gold and silver.

It gets confusing when organic farmers also refer to some things they use, like kelp, as “a mineral” for animal feed. Kelp is a seaweed (actually now it is classified as an animal but that is another story) that is dried down to flakes–it contains trace minerals like iodine, but kelp is not itself a mineral.

The amounts of nutrients naturally occurring in a given soil depend on the parent rock material of the soil as well as the climate that has weathered the rock. Soil fertility is defined by Henry Foth as “the quality that enables a soil to provide the proper compounds, in proper amounts, and in the proper balance, for growth of specified plants when temperature and other factors are favorable.”

Figure \(\PageIndex{1}\): The sustainable agriculturalist’s period table of elements important in plant nutrition. The period table has more than 115 elements, many with higher period numbers are not found in nature. Important nutrient Anions are colored green, Cations are light blue.

Why are we talking about the periodic table of elements in a soil course? It is because we go back to this table to find the foundation of chemical attractions, soil fertility, and plant nutrition. We need to get re-acquainted with the elements just a bit. Many of us have seen the periodic element table, but not in the context of agriculture. It is rewarding to understand the outcome--fertilizers and how to use them in organic agriculture--when one first sees the whole picture of how the elements relate to each other in the table as well as the element’s role in the plant.

Let’s start by looking at a few “agricultural” elements and their period numbers and how this ends up giving them a slightly positive or negative charge. Remember that electrons have a negative charge, they are symbolized as e-. Protons have a slight positive charge, they are hungry to mate with an e-.

Table \(\PageIndex{1}\): Agricultural Elements

Element	Symbol	Periodic Number	Protons	Electrons	Charge	Outcome	Ionic Form of the Element
hydrogen	H	1	1	0	+1		H+
helium	He	2	2	2	0	stable
carbon	C	6	6	6	0	Stable
oxygen	O	8	8	8	0	stable
nitrogen	N	7	7	7	-1	anion	varies
sodium	Na	11	11	11	+1	cation	Na+
magnesium	Mg	12	12	12	+2	cation	Mg+2
phosphorus	P	15	15	15	-1	anion	varies
sulfur	S	16	16	16	-2	anion	varies
potassium	K	19	19	19	+1	cation	K+
calcium	Ca	20	20	20	+2	cation	Ca+2

Ionic forms

The following representations of the elements show the same information as in Table \(\PageIndex{1}\). Recall that the innermost ring contains just two electrons. The first circle below shows a basic atom and the number of electrons in each orbital ring that would make it stable When the electrons and protons are equal, the element is very stable and unreactive, like helium (He) and carbon (C).

Figure \(\PageIndex{2}\): Helium and Calcium's protons and electrons.

Anions

The nutrient elements are always in some ionic form or compound when they are in soil solution and are ready to be taken up by plant roots. You’ve previously read about cations (cat-eye-ons) and the diagram below really shows what an anion is. Anions can be thought of as elements with a slight negative charge. Anions carry one or more extra electrons. Elemental N and P both carry one more electron in their outermost atomic ring than they have protons to balance with in their nucleus. Sulfur has two extra electrons and thus an ionic charge of -2.

The anions Nitrogen, Phosphorus, and sulfur are more reactive than more stable elements because the negative charges from electrons in their outer orbit want to combine with something positive. You can see that Nitrogen (N) has 2 electrons in the inner orbit and only 5 in the second orbit. It 'looks for' three more electrons to fill its outer shell. Phosphorus and sulfur have 2 electrons in the inner ring, 8 in the next ring, but only 5 and 6 electrons respectively in the third ring. So again, they are not stable and want to combine with other elements and share ionic bonds. Most often the compounds formed by anions are variable and can include oxygen (sharing its electrons) or hydrogen (sharing its proton).

Figure \(\PageIndex{3}\): The anions Nitrogen, Phosphorus and Sulfur.

Cations

 

The debate over soil balancing and the ratios of Ca⁺² and Mg⁺² for example, can be better understood by noting the relationship between the two elements as shown in the period table. Calcium is a “bigger” element than magnesium, that’s how it is thought to “open” the soil when it inserts itself into a layer of clay. Organic farmers use knowledge like this as a substitute for synthetic, chemical shortcuts to soil, plant, and animal health. Adding calcium to soil helps soil structure while magnesium, a smaller ion, may 'tighten' soil.

Figure \(\PageIndex{4}\): Calcium takes up more space because it has more electrons than Magnesium. (Copyright; author via source)

Note the Magnesium molecule is smaller because it has one less ring of electrons. Both of these cation molecules have two electrons hanging out in the outer ring.