Micronutrients for Plant Health

Despite being needed in significantly smaller quantities, micronutrients play just as important a role in crop health as macronutrients like nitrogen, phosphorus and potassium. With insufficient amounts of even one micronutrient, growers could see significant impacts to crop health and yield. To reach their full potential, plants need access to all nutrients throughout their growth cycle, and a strong soil microbiome can help growers provide free nutrition to their crops all season long. 

The list of essential plant micronutrients is long and includes copper, manganese, boron, nickel, iron, molybdenum, zinc and chlorine. These micronutrients, though required in small amounts, are crucial for various physiological processes in plants, such as enzyme activation, photosynthesis, and nutrient metabolism. A deficiency in even one micronutrient can result in stunted growth, poor crop yield, and reduced plant health. Here’s a look at the key micronutrients that support plant growth and how each one contributes to optimal crop yield.

1. Boron (B): While the optimal amount of boron in the soil varies, boron levels greater than 2 parts per million (ppm) are considered excessively high. For boron deficient soils, application rate recommendations are very low; between 0.5 and 2 pounds per acre. Compare this to nitrogen applications, which are often in excess of 100 pounds per acre, depending on a grower’s nutrient use efficiency score and yield potential. Even at these low levels, however, boron plays a very important role in crop health by supporting pollen tube growth and germination, cell wall structure and carbohydrate metabolism. Corn plants deficient in boron may be weak or limp, producing poorly developed ears and reduced yield. With sufficient boron, the crop’s cell walls can reach their full potential, improving standability and boosting yield.
2. Zinc (Zn): Zinc plays a critical role in plant metabolic function, supporting the production of growth hormones, proteins, chlorophyll and carbohydrates. Deficiencies can be spotted in younger leaves first and in corn, will likely appear as bands of pale tissue on either side of the mid rib. Because zinc supports internode length, without sufficient amounts of this nutrient, plants often appear stunted. Sufficient levels of soil zinc using a DPTA zinc soil test are generally between 0.5 – 0.7 pm, but as with all nutrients, other factors like soil texture can impact crop uptake. Phosphorus and zinc have a particularly complex relationship; research has shown that excessive levels of either nutrient can induce deficiency in the other when soil levels are low. Because of how common large phosphorus applications are, however, phosphorus-induced zinc deficiency is much more common. For growers applying substantial amounts of manure or phosphorus fertilizer, it’s important to keep an eye on zinc levels as well to avoid impacting yield.
3. Copper (Cu): plays a vital role in plant growth, supporting photosynthesis, respiration, and the formation of lignin, which strengthens cell walls and improves plant structure. It is also involved in enzymatic processes that drive the synthesis of proteins and carbohydrates necessary for plant development. Soil copper levels are typically measured in parts per million (ppm), with optimal ranges between 0.1 and 0.4 ppm, depending on soil type and crop needs. For copper-deficient soils, application rates are very low compared to macronutrients like nitrogen. Copper deficiencies often result in distorted young leaves, wilting, and poor grain or fruit production. With adequate copper, plants show improved standability and higher yield potential.
4. Manganese (Mn): Manganese is crucial for photosynthesis, activating enzymes involved in chlorophyll production and contributing to the electron transport system. It also plays a key role in nitrogen metabolism and supports disease resistance. Deficiencies in manganese are often identified in younger leaves, where interveinal chlorosis (yellowing between veins) is observed. Manganese levels in soil typically range between 20 and 40 ppm, but uptake can vary based on soil pH and texture. Alkaline soils are especially prone to manganese deficiency, and foliar applications may be required to correct this imbalance.

5. Nickel (Ni): Nickel is required in trace amounts but is essential for the enzyme urease, which helps plants metabolize urea into usable nitrogen and prevents nitrogen toxicity. Nickel also supports seed germination and overall plant vigor. Though deficiencies are rare, they may occur in soils with low organic matter, resulting in yellowing leaves and poor growth. Nickel is particularly important for crops reliant on urea-based fertilizers. While specific soil test levels are not commonly reported for nickel, maintaining balanced soil conditions and organic matter levels is crucial for its availability.

6. Iron (Fe): Iron is critical for chlorophyll production and is directly involved in electron transport during photosynthesis. It also supports several enzyme functions necessary for plant metabolism and energy production. Iron deficiencies are most common in high-pH or poorly aerated soils, where availability is limited. Deficiency symptoms include interveinal chlorosis in young leaves, where veins remain green, but surrounding tissue turns yellow. Soil iron levels typically range between 2.5 and 4.5 ppm, but chelated iron may be required to address deficiencies effectively.

7. Molybdenum (Mo): Molybdenum is essential for nitrogen fixation in legumes and for the conversion of nitrate into ammonium, a form of nitrogen that plants can easily absorb. It also supports enzymes involved in nitrogen metabolism. Deficiencies are rare but occur most often in acidic soils. Plants with low molybdenum show symptoms similar to nitrogen deficiency, such as stunted growth and yellowing leaves. Application rates are extremely low, typically less than 1 ppm, but its impact on nitrogen utilization is significant.

8. Chlorine (Cl): Chlorine is involved in osmosis, water regulation, and photosynthesis by maintaining proper stomatal function. It also helps plants resist diseases and maintain ionic balance in cells. Chlorine deficiencies are rare but can result in wilting, chlorosis, and reduced disease resistance. Toxicity, however, is more common in saline soils or with excessive potassium chloride applications. Optimal soil chlorine levels range from 0.5 to 5 ppm.

Identifying nutrient deficiencies early can help maintain crop health and improve yield. Here are key signs to watch for:

1. Yellowing Leaves (Chlorosis): Nitrogen, sulfur, and iron deficiencies cause yellowing, with nitrogen affecting older leaves and iron younger ones.

2. Stunted Growth: Zinc, phosphorus, and potassium deficiencies can stunt growth, causing plants to appear short and compact.

3. Leaf Spots or Necrosis: Dark spots and edge necrosis often indicate potassium, calcium, or magnesium deficiency.

4. Poor Flowering or Fruit Development: Boron and potassium shortages lead to poor flower set and underdeveloped fruit.

5. Pale or Discolored Leaves: Phosphorus, zinc, and iron deficiencies cause pale or reddish leaves, with zinc leading to a pale green color.

6. Distorted or Rolled Leaves: Molybdenum or magnesium deficiencies cause twisted or curled leaves.

7. Reduced Root Growth: Phosphorus or potassium deficiencies can result in poor root development, hindering nutrient uptake.

By regularly inspecting crops and conducting soil and tissue tests, growers can quickly address deficiencies to support optimal growth.

Micronutrient fertilizers are essential for addressing deficiencies in plants and improving yield. There are several types of micronutrient fertilizers, each with unique applications:

1. Chelated Micronutrient Fertilizers: These fertilizers contain micronutrients that are chemically bonded with organic molecules (chelates), improving their stability and plant availability. Commonly chelated micronutrients include iron (Fe), zinc (Zn), copper (Cu), and manganese (Mn). They are ideal for high-pH soils where nutrient availability is low.

2. Sulphate-Based Micronutrient Fertilizers: Sulphate fertilizers are water-soluble, making them fast-acting and effective in correcting deficiencies. Zinc sulfate and copper sulfate are common examples. They are suitable for soils with low to moderate pH levels.

3. Micronutrient Mixes: These fertilizers combine several essential micronutrients in one product, tailored to meet the needs of specific crops or soil conditions. They are often used for foliar applications or soil treatments.

4. Organic Micronutrient Fertilizers: Organic sources, such as compost or seaweed-based fertilizers, contain trace amounts of micronutrients. These are more gradual but provide additional soil health benefits through their organic matter content.

Choosing the right type of micronutrient fertilizer depends on soil conditions, crop requirements, and the specific deficiencies present. Regular soil testing ensures effective and efficient nutrient application.

No amount of one nutrient can make up for a lack of another; when a crop lacks any nutrient, micro or macro, plant health and yield will suffer. This is Liebig’s Law of the Minimum: growth and productivity of a plant is limited by the scarcest resource. With a deep understanding of their operation and its nutritional limitations, growers can dial in their nutrition program to avoid over-applying nutrients like nitrogen and phosphorus that the crops simply can’t or won’t use, costing growers money. Instead, growers can focus on addressing their fields’ limiting factors and providing the nutrients they need, when and where the plants can use it. 

Soil health plays a key role in ensuring a grower’s crops have access to plant-available forms of micronutrients throughout the season. The organic matter present in healthy soils is an excellent source of many of the micro- and macro-nutrients crops need, in addition to providing other benefits including:

• Increased water-holding capacity
• Improved drainage
• Good soil tilth
• Active soil microbiome

Soil microbes and plants have developed strong, mutually-beneficial relationships. Perhaps the most well known relationships are those between plants and nitrogen-fixing and phosphorus-solubilizing microbes, but a variety of other microbes improve plants’ access to various micronutrients as well.

Products like SOURCE, Sound’s microbiome activator, can also help growers support soil health. SOURCE is a foliar applied chemistry that stimulates and attracts microbes to the root zone. An active soil microbiome supports a bigger root ball, allowing the plant better access to nutrients throughout the season. Tissue tests across the country have consistently shown that plants treated with SOURCE contain more micronutrients. SOURCE works with the microbes already in a grower’s soil to support soil health and provide in-season access to all nutrients.