MICRONUTRIENTS - THE ROLE OF THE SOIL - PLANT MICRONUTRIENTS

 

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From the building block element hydrogen, creation produced all of the other elements of our universe – for example, potassium, iron and uranium. Each heavier element is a descendant of the one that preceeds it. It's like saying that son John looks and acts just like his grandad. With the elements, however, the relationships occur with predictable regularity.

Of the hundred-plus known elements, twenty are thought to be essential or functional in the growth of plants. All twenty are not required by all plants, but all are needed by some plants. For plant metabolism, carbon, hydrogen and oxygen are obtained from carbon dioxide and water. All the other elements are considered mineral nutrients and are ordinarily taken from the soil by the plant roots.

But now let's get the proper perspective. Nearly 95 percent of the green, wet weight of a plant may be made up from carbon, hydrogen and oxygen – while only some 5 percent of the plant may be constituted of the mineral elements. Still, without those mineral elements being taken into the plant in the proper amounts and balance, there can be no satisfactory growth.

NUTRIENTS
Organic	Primary	Secondary	Micronutrients	Functional
C Carbon H Hydrogen O Oxygen	N Nitrogen P Phosphorus K Potassium	Mg Magnesium Ca Calcium S Sulfur	Mn Manganese Zn Zinc Fe Iron Cu Copper B Boron Mo Molybdenum	Na Sodium V Vanadium Co Cobalt Si Silicon Cl Chlorine

Each of the twenty elements now known to be essential to plant nutrition plays a series of roles in the growth and development of plants, and when inadequately supplied by the soil, growth and yields are reduced. A few of the elements indicated are "functional" nutrients, which means that they function in plant metabolism in a non-specific manner. They are sodium, vanadium, cobalt, silicon and chlorine, and are defined here by color.

As chemists and plant nutritionists sharpen their skills, the roles of these functional elements may be more clearly defined and new elements may be added to the list.

Notice how boron and molybdenum are off by themselves while manganese, iron, copper and zinc are more closely related. Later we will see that these two sets of micronutrients act differently in the soil.

Mn	Manganese
Zn	Zinc
Fe	Iron
Cu	Copper
B	Boron
Mo	Molybdenum

Let's focus now on this abbreviated section of the periodic table that lists all chemical elements in an orderly way.

Our concern centers around the six micronutrient elements – their reaction in the soil, mode of entry into the plants, and the role that each one plays in the metabolic process.

We have colored each one a distinctive color so that we can keep track of them easier in the sections that follow.

The micronutrients in the soil and their availability to plants are determined by the minerals present in the original rocks and by the climatic weathering processes that have taken place in the soil over the years. In general, the leached highly-weathered soils of warm, moist regions contain smaller amounts of micronutrients than soils in cool, dry regions. However, there are exceptions and the total amount of an element present in soils is often a poor guide to the amount available for plant growth.

Micronutrients	Predominant Chemical Forms in the Soil
Manganese (Mn)	Oxides, silicates
Zinc (Zn)	Oxides, silicates, carbonates
Copper (Cu)	Hydroxy-carbonates, silicates
Iron (Fe)	Oxides, hydrous oxides, silicates
Boron (B)	Borosilicates, borates
Molybdenum (Mo)	Molybdates

Micronutrients	Typical Amounts in the Soil (pounds per Acre)
Iron (Fe)	40,000
Manganese (Mn)	4,000
Zinc (Zn)	200
Boron (B)	100
Copper (Cu)	100
Molybdenum (Mo)	2

Note the relative size of the total amounts of these elements that we might expect to find on an average acre furrow slice. Amounts can range from 40,000 pounds of iron to 2 pounds or less of molybdenum, yet both are considered micronutrients. Just remember that they are "micro" in the sense that only very small amounts are required by the plants.

The Soil as a Source of Plant Nutrients
Essential Element	Percentage in Soil	Amount, Lbs/Acre
Fe	3.5	70,000
K	1.5	30,000
Ca	0.5	10,000
Mg	0.4	8,000
N	0.1	2,000
P	0.06	1,200
S	0.05	1,000
Mn	0.05	1,000
B	0.002	40
Zn	0.001	20
Cu	0.0005	5
Mo	0.0001	2

Here is another representative soil sample. The quantities are a bit different but the idea is the same. There may be as much manganese as there is sulphur or phosphorus in a soil, but only a fraction of a percent as much manganese goes into a plant as does phosphorus.

Be careful, don't let these large amounts fool you! Normally only a very small portion of any of these elements is soluble and available for plant nutrition at any one time. The little bit your NFSA Agronomist has picked loose represents the amount of available nutrient, as compared with the total amount.

We live in the Atomic Age and therefore should apply up-to-date terminology to today's farming. Atoms are the smallest independent units of elements that can exist. When an atom has an electrical charge on it we call the atom an "ion". Some atoms are positively charged and called "cations", some are negatively charged and called "anions". When positive and negative ions get together in an electrically balanced marriage we call the combined result a "molecule," or if the union is electrically unbalanced it may be called a complex ion – either a complex cation or a complex anion, depending upon the nature of the final charge.

An example of a cation is Cu⁺⁺ (copper); an anion is NO₃^- (nitrate). The two together make a copper nitrate molecule: Cu(NO₃)₂.

Chemical Properties of the Micronutrients
Micronutrient Element	Form Used by Plant	Symbol of Form	Reaction of Hydroxide	Relative Solubility* pH 6.0   pH 8.0		Ionic Activity**	Major Cause for Low Availability to Plants
Manganese	Cation	Mn++	Basic	6	6	-1.05	Precipitates as MnO2 at high pH under well-oxidized conditions
Zinc	Cation	Zn++	Weak base	2	3	-0.76	Precipitates and is absorbed by the silicate and oxide fractions of soils
Iron	Cation	Fe++	Very weak base	5	5	-0.44	Precipitates as an insoluble Fe2O3.nH2O at high pH
Copper	Cation	Cu++	Very weak base to acidic	4	4	-0.34	Is absorbed by the silicate, oxide and insoluble organic matter fractions of soils
Boron	Anion	B4O7=	Very weak acid to basic	1	1	+0.10	Under high rainfall soluble boron is leached and residue persists as the insoluble complex silicate tourmaline
Molybdenum	Anion1	MoO4=	Weak acid	3	2	+0.25	Forms insoluble iron and aluminum molybdates in acid soils and more soluble calcium molybdates in neutral and alkaline soils

*Number 1 is most soluble. Number 6 is most insoluble.

** Any metal in a water solution passes atoms into the solution as charged ions, and these in turn tend to return to the metal state. There is a measurable electrical potential between the metal and the ion state. The negative sign on the numerical value indicates that the movement is towards the ionic state. The higher the negative number, the more active the metal is. Note that the anions are no longer pure metals but are oxides of the metal.

This table shows that each of the micronutrients has its own characteristics. When you are considering trace element fertilizers, remember the chemistry of each one individually. We make a serious mistake if we treat a copper deficiency as a manganese deficiency, or iron deficiency as a zinc deficiency. Know each micronutrient and what to expect from it.

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