Sweet 17: Inside the Essentials

Justus von Liebig, a 19th century German chemist, made great contributions to the science of plant nutrition and soil fertility. While Carl Sprengel, a German botanist, formulated the “theory of minimum,” Liebig investigated and popularized the scientific concept we know today as “Liebig’s Law of the Minimum.” This concept demonstrates that plant growth is not controlled by the total amount of available resources but by the scarcest.

Today, we know there are 17 essential plant nutrients. If one of these nutrients is deficient, then plant growth will be reduced — even when all other essential nutrients are adequately supplied. In other words, maximum yield potential can only be achieved when the proper balance of nutrients is in place.

Essential nutrients require three of the following conditions:

  1. The plant is unable to complete its life cycle without this essential nutrient.

  2. The function of that nutrient cannot be duplicated by another nutrient.

  3. It must be directly involved in plant metabolism or plant function.

“It’s important to remember the next step in crop nutrition,” says Curt Woolfolk, senior agronomist at The Mosaic Company. “Balanced crop nutrition is more than just knowing the nutrients required for plant growth — it’s providing a crop the right ratio of nutrients it needs throughout its life cycle.”

Based on crop type, location, soil type and weather conditions, there are varying requirements for success. Multiple factors need to be taken into consideration, including which nutrient(s) is deficient, the rate to apply, and the method and timing of application.

Determining the right ratio for a crop and achieving balanced crop nutrition require a complete soil analysis. “I can’t stress enough the importance of soil testing,” Woolfolk says. “The more complete, the better, so don’t just stop with N, P and K. Measuring the full spectrum of essential nutrients will provide a better picture from year to year.”

With 4R Nutrient Stewardship in mind, after soil testing, utilize a lab, retailers or crop consultants to determine where soil levels need to be in your region. This will help determine a crop nutrition plan that best suits your needs.

Nitrogen (N) is essential for plant growth and is part of every living cell. It plays many roles in plants and is necessary for chlorophyll synthesis. Plants take up most of their N as the ammonium (NH₄⁺) or nitrate (NO₃⁻) ion. Some direct absorption of urea can occur through the leaves, and small amounts of N are obtained from materials such as water-soluble amino acids.

Nitrogen is an important component of many important structural, genetic and metabolic compounds in plant cells. It’s a major element in chlorophyll, the compound by which plants use sunlight energy to produce sugars from water and carbon dioxide, or, in other words, photosynthesis.

Nitrogen deficiency results in chlorosis (a yellowing) of the leaves because of the declining chlorophyll. This yellowing starts first on oldest leaves, then develops on younger ones as the deficiency becomes more severe. Slow growth and stunted plants are also indicators of nitrogen deficiency. Small grains and other grass-type plants tiller less when nitrogen is in short supply.

One of three primary nutrients, phosphorus (P) is essential for plant growth, and a plant must access it to complete its normal production cycle. Plants absorb P from the soil as primary and secondary orthophosphates (H₂PO₄⁻ and HPO₄²⁻).

Phosphorus is a vital component of adenosine triphosphate (ATP), the “energy unit” of plants. ATP forms during photosynthesis, has P in its structure, and processes from the beginning of seedling growth through to the formation of grain and maturity.

Phosphorus deficiency is more difficult to diagnose than a deficiency of N or potassium (K). Crops usually display no obvious symptoms of P deficiency other than a general stunting of the plant during early growth, and by the time a visual deficiency is recognized, it may be too late to correct in annual crops.

Potassium (K) is one of the essential nutrients and is taken up in significant amounts by crops. Potassium is vital to photosynthesis, protein synthesis and many other functions in plants.

While potassium doesn’t constitute any plant structures or compounds, it plays a part in many important regulatory roles in the plant. It’s essential in nearly all processes needed to sustain plant growth and reproduction. Perhaps potassium’s most important function in the plant is that it can activate at least 80 enzymes that regulate the rates of major plant growth reactions. K also influences water-use efficiency.

Plants deficient in potassium don’t grow as robustly and are less resistant to drought, as well as high and low temperatures. They’re also more vulnerable to pests, diseases and nematode attacks. Potassium is also known as the “quality nutrient” because of its important effects on factors such as size, shape, color, taste, shelf life, fiber quality and other qualitative measurements.

Hidden in the heart of each chlorophyll molecule is an atom of magnesium (Mg), making the nutrient actively involved in photosynthesis. Magnesium also aids in phosphate metabolism, plant respiration and the activation of many enzyme systems.

Magnesium nutrition of plants is frequently overlooked, and shortages will adversely impact plant growth. Many essential plant functions require adequate Mg supplies, the most visible being magnesium’s role in root formation, chlorophyll and photosynthesis.

Magnesium’s availability to plants often depends on soil pH. Research has shown that Mg availability to the plant decreases at low pH values. In acidic soils with a pH below about 5.8, excessive hydrogen and aluminum can decrease Mg availability and plant uptake. At high pH values (above 7.4), excessive calcium may greatly increase Mg uptake by plants.

Sulfur (S) is a part of every living cell and is important to the formation of proteins. Unlike the other secondary nutrients like calcium and magnesium (which plants take up as cations), S is absorbed primarily as the SO₄²⁻ anion. It can also enter plant leaves from the air as dioxide (SO₂) gas.

A chain is only as strong as its weakest link. Often overlooked, sulfur can be that weak link in many soil fertility and plant nutrition programs. As of late, there is less S being returned to the soil in rainfall, because of government regulations on the amount of sulfur dioxide (SO₂) that can be released into the atmosphere. So there are several reasons for the increased observance of S deficiencies and increased S needs.

Sulfur, like N, is a constituent of proteins, so deficiency symptoms are similar to those of N. Nitrogen-deficiency symptoms are more severe on older leaves, however, because N is a mobile plant nutrient and moves to new growth. Sulfur, on the other hand, is immobile in the plant, so new growth suffers first when S levels are not adequate to meet the plant’s need. This difference is important in distinguishing between N and S deficiencies, particularly in early stages.

Calcium (Ca) is taken up by the plant as the Ca2+ cation. Ca forms compounds that are part of cell walls to strengthen plant structures. It also helps to balance organic acids within the plant. Boron (B) is a micronutrient that is essential for cell wall formation and rapid growing points within the plant, such as reproductive structures. Interestingly, while higher plants require B, animals, fungi and microorganisms do not need this nutrient.

The need for boron was established in the late 1920’s, but its role and function within the plant continues to be researched and better understood.

Boron deficiencies are widespread across North America. From a global perspective, B is the most widespread micronutrient deficiency after zinc.

Crops vary widely in their need for B, and the line between deficient and toxic amounts is narrower than for any other essential nutrient. B should be used carefully, especially in rotations involving different sensitivities to B. It is important that B fertilizers are applied uniformly in broadcast applications rather than in-furrow situations. Boron placed close to the seed greatly reduces stand counts.

Zinc (Zn) is taken up by plants as the divalent Zn²⁺ cation. It was one of the first micronutrients recognized as essential for plants and the one most commonly limiting yields. Although Zn is required in small amounts, high yields are impossible without it.

Zinc deficiency is growing in the Midwest, and it’s more likely to occur in corn than soybean fields. This is happening in part due to earlier planting of corn in cool and moist soil. Also, more residue resulting from higher grain yields places added stress on seedlings to absorb Zn from soil.

Zinc loss takes place in many ways. Deficiencies are mainly found in sandy soils low in organic matter and in organic soils. They occur more often during cold, wet spring weather, and are related to reduced root growth and activity. Zinc uptake by plants decreases with increased soil pH. High levels of available P and iron in soils also adversely affect the uptake of Zn.

Chloride (Cl) is taken up by plants as the Cl⁻ anion. It is active in energy reactions in the plant. Cl is essential for the activation of many enzyme systems, and is involved in transporting several cations (K, Ca, Mg) within the plant.

Manganese (Mn) is taken up by plants as the divalent cation Mn²⁺. Mn acts as an enzyme activator and is essential for the manufacture of chlorophyll. Manganese accelerates germination and maturity, while increasing the availability of P and Ca.

Iron (Fe) is taken up as the ferrous (Fe²⁺) cation. It is a catalyst to chlorophyll formation and acts as an oxygen carrier in the nodules of legume roots. Because iron is not translocated within the plant, deficiency symptoms first appear on the younger leaves at the top of the plant.

Nickel (Ni) is absorbed by the plant as the divalent cation Ni²⁺. Ni was added to the list of essential plant nutrients late in the 20th century. Ni is important in plant N metabolism. It is required in very small amounts, with the critical level to be about 0.1 ppm.

Copper (Cu) is taken up as both Cu⁺ and Cu²⁺ cations. Cu activates enzymes and catalyzes reactions in several plant-growth processes. Vitamin A production is closely linked to the presence of Cu as well, and Cu helps ensure successful protein synthesis. Organic soils are most likely to demonstrate a Cu deficiency.

Molybdenum (Mo) is taken up by the plants as the MoO₄²⁻ anion. It is required for the synthesis and activity of the enzyme nitrate reductase. Mo is vital for the process of symbiotic N fixation by rhizobia bacteria in legume root nodules. It is also needed to convert inorganic P to organic forms in the plant.

Hydrogen (H), derived almost entirely from water, is one of the 17 essential nutrients necessary for plant growth. Hydrogen, carbon and oxygen are the three primary elements plants use in the largest amounts, and they perform as the building blocks for plant growth.

Oxygen (O) is responsible for cellular respiration in plants. Plants acquire O by breaking down carbon dioxide (CO₂) during photosynthesis, and end up releasing the majority of it as an unnecessary by-product, saving a small portion for future energy.

Carbon (C) is responsible for all life on earth. Carbon dioxide (CO₂) released into the atmosphere is recycled endlessly as part of the carbon cycle. Plants take CO₂ from the air and use the C for energy, helping to build essential biological compounds such as carbohydrates and proteins.

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