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Plant Nutrition: Micronutrients

Plant Nutrition: Micronutrients

Understanding plant nutrition can be the difference between vigorous, productive plants and plants that struggle. You may be familiar with the macronutrients like nitrogen or potassium. These are the most commonly discussed nutrients since they are needed in the largest amounts. However, they aren’t the only essential nutrients that your plants need. Your plants will also need micronutrients in order to thrive.

What are Micronutrients?

If we break the word micronutrient down, we can get a better idea of what micronutrients are. ‘Micro-‘ means small. Nutrients are chemicals that plants need in order to survive. Nutrients are essential to proper health and are involved in many of the biological processes. When we put the two parts together, it’s easy to see that micronutrients are essential nutrients that are needed in small quantities.

Don’t get confused and think that micronutrients are small nutrients. All nutrients are chemicals and are very small in size. It’s the amount of the nutrient that the plant needs that is small. Macronutrients are needed in large quantities and micronutrients are needed in small quantities. Although your plants won’t need much of them, micronutrients are still essential and shouldn’t be overlooked.

The Seven Micronutrients

There are seven micronutrients, and they all have specific roles that they play in the plant. Micronutrients can also be referred to as trace minerals. When you’re looking at fertilizers or soil amendments, you’ll want to make sure that you’re getting a well-rounded fertilizer that contains the essential micronutrients as well.

If you were to measure the overall dry weight of your plant, micronutrients would make up less than 1% of the total weight of your plant. Many of the micronutrients are reused in the plant, reducing the amount of the nutrients needed.

Amending your soil with high-quality organic amendments can help ensure that your plants always have access to the micronutrients that they need. Compost and manure will contain many, if not all, of the micronutrients your plants need. You can also use micronutrient blends or foliar sprays to deliver micronutrients to your plants.

The seven micronutrients are iron, manganese, chlorine, boron, zinc, copper and molybdenum.

What Micronutrients Do

The seven micronutrients each have key roles that they play in the plant.

Iron

Iron is a key component in many enzymes that are involved with plant processes. The enzymes that require iron are involved in processes such as nitrogen reduction and fixation, energy transfer, and lignin formation. Lignin is extremely important in plants. It helps to transport liquids and wastes through the plant. Lignin also helps strengthen the plant cell walls, which allows the plant to remain upright.

In legumes, like beans, iron helps to carry oxygen in the root system.

Manganese

Manganese is highly involved in many plant processes, despite the fact that it’s only needed in small amounts. Manganese activates many metabolic reactions, plays a direct role in photosynthesis, speeds up germination and plant maturity, increases the availability of phosphorus and calcium, and helps to produce chlorophyll.

Chlorine

Chlorine plays a role in some major processes in the plant. Chlorine is used to regulate the opening and closing of stomata in the plant. Stomata are pores located on the undersides of leaves that release moisture. Proper stomata function is critical for a plant’s success during dry periods. During photosynthesis, plants break down water molecules. Chlorine is needed to help enzymes break down water molecules during photosynthesis. It’s also used to help the plant adapt to changing water availability and transport nutrients throughout the plant.

Boron

Boron is involved in the proper seed set for plants. When plants are growing in stressful conditions, like drought or when the plant is dealing with disease or pests, boron can help the plant to set seeds. Boron is also a key component in cell walls. Plants don’t have a skeleton to help keep them upright. They rely on rigid cell walls to help hold them up. Boron helps to make the cell walls strong.

Zinc

Of all the micronutrients, zinc is the most limiting. It’s only needed in small amounts but it can have a big impact on plant health. Zinc is required for plants to make proteins and hormones. Both of these can limit how productive and vigorous the plant is. It’s also used in the creation of plant-growth substances and enzymes. Many of the enzymes that require zinc are used in metabolic reactions that take place during early plant growth.

Copper

Copper is involved with several processes in the plant. It’s needed to make chlorophyll. Chlorophyll is the green pigment in leaves that the plant uses to absorb sunlight that is used to make food. Copper also helps to activate enzymes that are involved in many growth processes. Copper is linked to a plant’s ability to produce vitamin A and proteins.

Copper isn’t mobile in the plant, so it’s easy for new growth to develop signs of copper deficiency. Other metals present in the soil, like iron, aluminum, or manganese, can inhibit the plant’s ability to absorb copper properly.

Molybdenum

Molybdenum is a nutrient that is required for the synthesis and use of the enzyme nitrate reductase. Nitrate reductase is responsible for turning nitrate into nitrite. Not all forms of nitrogen that are found in the soil are ones that the plant can use. Molybdenum helps the plant to break unusable nitrogen forms into ones the plant can use.

Signs of a Micronutrient Problem

Iron

Other metals in the soil can interfere with proper iron absorption. Copper, manganese, and molybdenum can compete with iron to be absorbed through roots. When a plant lacks iron, it often shows a distinct type of chlorosis. The leaves will turn pale green. The leaf veins will remain green and the tissue between the veins will turn yellow. This is usually seen on younger leaves and newer growth. If a plant is showing signs of iron deficiency, the best way to treat it is with a foliar spray.

It’s rare for plants to develop an iron toxicity, but it can occur. When plants have too much iron, they will develop bronzed leaves or tiny, brown leaf spots.

Manganese

Plants deficient in manganese will develop yellowing in between leaf veins (called interveinal chlorosis). This can occur on either new growth or older growth, but is most commonly seen first in younger growth. Interveinal chlorosis is followed by brownish-black specks on the leaves. The plant will eventually drop the affected leaves. A deficiency of manganese over time can cause slow growth and failure to mature properly.

A high soil pH can limit the plant’s ability to absorb manganese from the soil. Other nutrients, like calcium, magnesium and iron can compete with manganese uptake and can lead to deficiencies.

Plants with manganese toxicity will develop leaf chlorosis. The overall growth rate will slow down and plants will become less vigorous.

Chlorine

Chlorine deficiency causes leaves to wilt and turn yellow. The affected leaves may develop small, yellow flecks. The leaves will eventually turn bronze. If you were to dig the plant up, you’d see stunted root growth and thickened root tips.

Chlorine toxicity will cause the tips of leaves or leaf margins to appear burned or scorched. The leaves may turn yellow, bronze, and split. Overall growth rate is reduced.

Boron

The line between deficiency and toxicity with boron is narrower than with other nutrients, so it’s important to apply boron properly. Plants are more likely to have boron deficiencies when the plants are in a drought and the roots don’t take in as many nutrients.

Boron deficiency can create symptoms all over the plant. The tips of roots and stems will die. If you dig the roots up, you may notice that the root tips are swollen or discolored. Plants deficient in boron are more susceptible to fungal disease, causing internal rotting. Different plants will show different symptoms on the leaves, including drying, wilting, yellowing, spotting or even purple streaking.

Boron toxicity is usually seen in the leaves. The tip of the leaf will turn yellow and start to die. The yellowing and leaf death will move from the tip towards the stem.

Zinc

Zinc deficiency can lead to smaller leaf size. Younger leaves can have wrinkled leaf margins. You may also notice a shortening between nodes due to reduced growth. As the soil’s pH increases, zinc is harder for plants to absorb.

Zinc in excessive amounts can be extremely toxic and will quickly kill plants. Too much zinc can interfere with iron absorption, which can lead to iron deficiency.

Copper

Younger leaves will often become dark green and twisted when a plant lacks copper. Leaves may die back or start to develop rotting spots. Plants can develop a condition called copper hunger. Copper hunger creates leaves that are wilted and turn a deep blue-green. If the problem isn’t fixed, the leaves will then turn yellow and curl.

In excess, copper is extremely dangerous for plants. When too much copper is present, signs of iron chlorosis, reduced branching, stunted growth, and thick, dark roots are present.

The type of soil you’re growing in can affect your plant’s likelihood to develop a copper deficiency. Organic soils are the most likely to lack copper. Soils that are composed largely of clay are less likely to be deficient in copper.

Molybdenum

Usually, molybdenum deficiency shows up as an overall yellowing and stunted plant. Leaves can have scorched margins or appear cupped or rolled. Thankfully, molybdenum deficiencies are rare. Unlike most other nutrients, as the soil’s pH increases, molybdenum is more easily absorbed.

Written by Shelby from Farminence 

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