With so many products available all promising to make your garden greener, more lush, and bloom brighter, which should you choose? And do you need them or is it all hype? What do micronutrients do?

What are Micronutrients?

We know about the big three macronutrients, Nitrogen (N), Phosphorus (P), and Potassium (K), and how to interpret the numbers on the fertilizer bag. Along with N, P, and K, we commonly list calcium, magnesium, and sulfur as the other macronutrients for plants. 

However, plants also require other essential nutrients besides the big three. Plant scientists have identified 16 or 17 (depending on what camp you subscribe to) essential elements required for healthy plant growth. But don’t panic, you don’t need to provide them all. Plants are pretty good at fending for themselves most of the time.

A micronutrient is a mineral nutrient that a plant uses in very small amounts, but still needs for proper growth, flowering, seed and fruit production, and so on. While a plant might use oxygen, carbon, hydrogen, nitrogen, phosphorus, potassium, and calcium in fairly high amounts, they don’t need much zinc or nickel. Those, along with others, are micronutrients. They’re elements that the plant only needs in trace amounts, but are still important.   

Will a soil test reveal micronutrient deficiencies?

The results provided on a soil test vary depending on which lab you send the sample to. Most test results will provide the pH level, as well as levels of nitrogen, phosphorus, potassium,and percent organic material. For an additional fee, they may test calcium, and magnesium, boron, sulfur, manganese, and zinc. For some micronutrients, soil testing isn’t commonly available, and deficiencies are tested by analyzing plant tissue samples.  

For most of us, the soil test will be more than sufficient to reveal any problems, and most agencies also provide recommendations for any amendments needed. It’s much less expensive to pay for a good soil test than to keep applying expensive amendments year after year hoping to get it right.

First… check your pH

Especially for in-ground plants, a nutrient deficiency is more likely to be caused by a pH issue than a lack of any particular micronutrient. Plants are most successful at taking up nutrients when the soil, and therefore the solution holding the nutrients, is in a specific (but nicely broad) range. 

When pH falls outside this range, the plant struggles to take up nutrients even though they may be present in sufficient amounts in the soil. We refer to this as the nutrients being unavailable. For example, a pH of 7.5 or higher begins to reduce the availability of iron, calcium, and other micronutrients. Gardeners in the western half of the country are more likely to have naturally alkaline, or high pH soils, but if you’ve been adding a lot of lime to your garden, it could be a problem anywhere.  

Six Key Plant Micronutrients

There are several additional micronutrients, but these are the ones most commonly asked about. Of these, only iron is normally found to need adjustment, and that is not often except for potted plants. Usually, a healthy addition of compost yearly will provide all of the below and then some. As mentioned above, check your pH if you suspect any problems.

Iron

Iron plays a crucial role in many plant processes, from the formation of enzymes to cellular respiration. It’s a key component for photosynthesis, which is why an iron deficiency can look like yellowing leaves (chlorosis). Products meant to add iron always promise to “green up” your plants. 

Iron in plants is needed to make the chlorophyll molecule, meaning the plant cannot make enough of the compound to satisfy the need in the leaves. Iron also supports energy transfer and cellular respiration.

An iron deficiency in our garden plants shows up as yellowing leaves, but the veins remain green. It’s not the same as an overall yellow leaf. It’s called interveinal chlorosis, and appears on the new, young leaves first. 

Normally, iron is present in sufficient amounts in soil or can be supplied by adding aged manure and compost. However, if needed, we can correct an iron deficiency with a foliar spray or a liquid fertilizer product when watering. 

Molybdenum (Mo)

Molybdenum is needed by plants for converting nitrate to a usable form, and for nitrogen fixation in legumes (beans and peas). It is needed in extremely low concentrations, but can show deficiencies in sandy, acidic soils, as molybdenum is less available to plants at lower pH. 

Molybdenum deficiency manifests as a low nitrogen symptom due to its effect on nitrogen use by the plant. Eventually, the leaf edges may brown and curl upward. Broccoli and cauliflower leaves may appear wrinkled.

Your soil test is unlikely to report results for this essential element. If you suspect a deficiency, check your soil pH and raise it with lime if it’s below approximately 5.5.

Manganese (Mn)

Manganese deficiency doesn’t typically show up in our gardens, but it exhibits similar symptoms to an iron deficiency, including interveinal yellowing. It’s more likely to affect plants grown in high pH soils, because a high pH reduces manganese availability for plant uptake, not because there aren’t sufficient amounts in the soil.

For home gardeners, lowering the soil pH (after obtaining a soil test) is usually sufficient to correct the problem. No additions are needed. 

Manganese plays a role in photosynthesis, specifically in the breakdown of a water molecule into the hydrogen and oxygen components. It also activates enzymes which help the plant fight off pathogens and adapt to environmental stress.

Zinc (Zn)

Zinc plays a crucial role in nearly every plant process, and a deficiency can significantly hinder plant growth and function, even when all other factors are right. But, it isn’t very commonly deficient in soils, although excessively sandy soils can be lacking. 

Like manganese, a zinc deficiency in our garden plants is more likely due to environmental conditions or root damage. A high pH, or poor plant root health caused by cultivation injury or harmful nematodes, is more likely the culprit.

Zinc deficiencies can produce stunted growth, including smaller than normal leaves and fruit. Adding good compost or aged manure and checking the pH is normally all that is needed. 

Copper (Cu)

Copper makes for beautiful kitchen cookware, and it’s important for plants too. Like many micronutrients, it supports chlorophyll production and enzyme activation for plant health. A copper deficiency can manifest as slow, stunted growth and weak stems. 

Copper in our gardens is a cation which is held present in an available form for plants on cation exchange sites of clay particles and soil organic matter. A very high level of organic material in the soil can hold the copper too tightly for the plants to uptake. Soils with higher pH levels can present a copper deficiency simply because the copper is unavailable to plants. But, problems with a lack of copper are rare in our gardens.

Boron (B)

Boron is used by plants for various functions, including cell wall formation, the movement of sugars within the plant, and even pollination and fruit setting. You likely won’t notice a boron deficiency in your garden, unless you are inspecting pollen grains for vitality. If you grew two plants in a lab, one with adequate boron and one without, you’d notice shorter and less vigorous roots and root tips. It’s not a common issue. I mention it here so you can cross it off your list of worries. 

So, to add micronutrients or not to add?

The common theme is that most garden soils are not actually deficient in the above micronutrients; however, excessive soil pH to either side of the spectrum can cause these elements to be unavailable to plants. 

Before considering any amendments in the garden, start with an accurate soil pH measurement and go from there, correcting any extremes. Most of the plants we like to grow will do well in a pH of 6.0-6.5, although a little to either side is fine. 

Container-grown plants are more likely to suffer from micronutrient deficiency because the potting soil we plant them in usually doesn’t contain any mineral soil. Check the label on the bag next time you are at the store, and you’ll see most don’t contain any actual soil. That’s not a bad thing; they’ve been designed to be light and reduce compaction in the conditions found in pots and planters. But, it means they could be low in those elements naturally occurring in mineral soil.

It’s okay to add a micronutrient product to your potted plants, following the directions on the label. Many liquid products are available which are easy to incorporate into a weekly or biweekly watering schedule. 

For larger applications in a traditional garden bed, follow the recommendations of your soil test. Not only is applying soil amendments over a larger area expensive, but it is also likely unnecessary. If your garden soil is deficient, you’ll be able to target only that micronutrient, and avoid adding excess and possibly causing a toxicity issue.