Role of Secondary and Micronutrients
Farmers try to balance the big three nutrients for plant growth: nitrogen (N) phosphorus (P) and potassium (K) since they are known to increase crop yields. However, for good soil health and good plant health; the secondary nutrients: calcium (Ca) Sulfur (S) Magnesium (Mg) and micronutrients are also important.
Calcium (Ca) is the third most plant abundant nutrient and is a major regulator of plant hormones for plant growth and yield. However, to get Ca into the plant, you need boron (B) a micronutrient. Boron (B) is like a bus driver hauling Ca into the plant. Unfortunately, there needs to be a soil balance because Ca ties up B and vice versa. So too much of either one or too little causes poor growth and low yields. Adequate Ca promotes more roots, bigger shoots, bigger leaves, more flowers, more pollen and ultimately higher yields.
Sulfur (S) is another important nutrient especially for building critical plant amino acids. As an electron donor, S helps plants control light intensity, heal wounds, improves drought resistance, helps plants survive cold weather, and is a great antioxidant by tying up heavy metals. When Acid Rain was prevalent 50 years ago, farmers got their S for free at a rate of 20-30#/Acre. With 1980’s environmental clean-up, now farmers get only 6-10#/A. Sulfur is almost universally low on most soil tests now. Farmers need 1# S for every 10# of N applied. With S levels so low in the soil, adding a little extra may be beneficial because a lot of N & S comes from soil organic matter (SOM) and from microbes and not applied fertilizer. Figure about 2.5# of S for every 1% SOM and roughly 6-10# from the atmosphere. High yielding soybeans need more S.
Magnesium (Mg), as a nutrient, often gets a bum rap. High soil Mg levels make soils tight and compacted. However, Mg is the central element for plant chlorophyll. Most plant available Mg come from SOM, so high levels of SOM buffer the negative effects of Mg and make it plant available. Low SOM and high Mg make soils tight but higher levels of SOM buffer high Mg levels and improve soil aggregation. Sandy soils tend to be low in Mg.
Iron (Fe) is not a micronutrient that farmers worry much about. Most clay soils have a lot of iron but unfortunately, without adequate or high levels of SOM, the iron is either not in the right form for the plants or is tied up by other nutrients. Iron is crucial because it activates the enzyme that makes chlorophyll. Cold wet spring conditions results in yellow corn. This can be a combination of N, S, and Fe deficiency. Iron (Fe) is critical for soil N transformations, so a lack of anyone of these nutrients causes poor growing crops due to low chlorophyll production and poor protein (N & S) production.
Other micronutrients that are important include zinc (Zn), manganese (Mn), and copper (Cu). Zinc (Zn) is a great disease reducing nutrient. Bacteria, fungus, and viruses can not tolerate good to high levels of Zn. Most diseases are associated with a deficiency of at least one or more major, secondary or micro nutrients. Zn activates genes (DNA, RNA), helps plants survive drought and high temperatures, and also helps plants survive water logged soils.
Manganese (Mn) activates 36 enzymes but its key role is splitting the water molecule. Why is important? Water (H20) when split becomes (H+, OH-) which are the building blocks for making plant proteins. Mn increases P and Ca soil availability and helps decrease most bacteria and fungal diseases. Mn is often low in sandy soils and soybeans often respond to foliar Mn applications. However, never apply Mn with glyphosate (Roundup) because they tie each other up. Unfortunately, for best results, these should be separate applications.
Copper (Cu) is the central element in making cellulose and lignin so adequate Cu helps fight thin stalks and lodging. Only about 5% of the Cu in SOM is plant available so it takes several years for it break down. High pH soils tie up Cu while low pH soils make Cu more available.
Finally, three micronutrients we often forget: chlorine (Cl) cobalt (Co) silica (Si). Chlorine is available in most fertilizers especially with potassium (K). Both K, Cl, and nitrates promote the growth hormone called auxin. Cl may be deficient on sandy soils. Cobalt (Co) is important for N fixation especially in soybeans. Silica (Si) strengthens cell walls and the plant’s vascular system creating bigger pipes in the plant for increased nutrient flow. Keeping all nutrients in adequate supply and balanced usually generates the highest and most profitable crop yields.