This group of micronutrients are known as trace elements because only traces are needed by the plant. Again, tiny quantity, still useful in a large way.
Trace elements, partially because of just how little the plants metabolize, are a little tricky to keep track of. Generally, if you’re using compost or manure or rotating your plants, these elements will never be in short supply. That being said, it’s nice to know what’s out there and what these nutrients do for the plant.
Most (if not all) of these nutrients would require some kind of laboratory analysis to check up on.
Boron is necessary for the plant to properly use calcium. Boron is responsible for much of the structural integrity of the cell walls. No boron, no strong, tall plant.
Too much boron can be harmful to the plant. Excess magnesium sulphate can cause a boron imbalance. And, boron deficiency is more common with basic soils (higher than 6.5 pH).
A boron deficiency can show up with poor development of the growing tips of the plant. For crops like broccoli, cabbage, and cauliflower, boron deficiency presents with symptoms such as hollow stems and overall discoloration. (Although, these same symptoms may be similar to and are likely going to be caused by something else).
You can use borax in a pinch to treat a boron deficiency. Unfortunately, borax is also an herbicide. It’s better to avoid overusing magnesium sulphate.
Chlorine is used in the operations of osmosis and ionic balance, and some portions of photosynthesis.
Sodium chloride (table salt) has chlorine in it. Don’t salt your soil, please. The salt is likely already there, and adding any more will be harmful for the plants. Too much chlorine can accumulate in plant leaves, causing a scorched appearance. Alternatively, the leaves may be smaller than usual, turn yellow, and fall off. Chloride toxicity is most common in irrigated, dry regions, seacoast areas, and near roads frequently treated with salt in the wintertime. Also, make sure to flush your potted plants occasionally.
Chlorine gas is a by-product in the manufacture or incineration of glass, plastics, paints, and stains. It is released from refineries or as a result of chemical spills. Reducing air pollution at its source is the best solution to reduce damage to plants and people. Careful watering practices can reduce air pollution damage to plants.UMD Home & Garden Information Center
Copper is required for proper root formation. Copper also activates some enzymes that are responsible for light synthesis (how neat is that term!). It also helps with protein and carbohydrate metabolism, and a few other functions.
Copper also serves to intensify flavor and color in vegetables and color in flowers.PT Horticulture
This is also a pretty rare deficiency, though it’s more common in sandy, peaty, and chalky soils – soils with higher pH levels. Copper is immobile, so the deficiencies will show up in new leaves. Some chlorosis and cupping, maybe some wilting.
Excess copper can be harmful to the plant. It can cause reduced overall growth, reduced root growth, and overall yellowing. It might also compete with iron, zinc, and molybdenum.
A dash of copper sulphate fungicide will take care of your garden forever – though you likely will never need this. Excess potassium or phosphorus, or a high-acidity soil will cause problems with copper regulation instead.
Iron is needed for that all-important chlorophyll molecule, respiration, and photosynthesis (thought all three are really tied together, anyway).
An iron deficiency is very rare (and even harder to test for in the lab). Deficiency will show up with an overall yellowing and lack of liveliness in the plant – chlorosis and stunted growth. Iron deficiency shows up in younger leaves. Soils with high acidity (low pH) will have lower iron availability – keep soil above 5.8 for best results.
Too much iron will bind the potassium and phosphorus in the soil, preventing the plant from accessing any of them. Excess iron will also cause issues with uptake of manganese in the plant.
Manganese is similar to iron in most regards. A deficiency shows up with the same yellowing and lack of vigor, though iron will show in younger leaves and manganese in leaves of all ages. Like iron, it’s also used in photosynthesis, respiration, and nitrogen assimilation.
A deficiency may show up in sandy or peaty soils with high pH (basic) – or at least over 6.5. It’ll present with interveinal chlorosis and tan spotting in the chlorotic areas.
You can add some sulfur to the soil to lower the pH (acidify), taking care of this issue in most cases. Or switch to a fertilizer that isn’t just ‘general purpose’ (which tends to skimp on micronutrients – an issue I’ve had in my own garden).
The last two trace elements are almost never lacking, but can occur in excess if there’s industrial contamination. Proper use of compost will take care of a lack of either (and many of the other nutrients in the meanwhile!)
Molybdenum is required in only truly minute amounts – 50 grams per hectare (over 100,000 square feet!). It’s used in fixing nitrogen (especially for legumes!) and in potassium absorption. A deficiency shows with similar symptoms to nitrogen deficiency. In bad cases, some leaves will present a condition called ‘whiptail’ if molybdenum is too deficient. Still, this typically will not be the first or second problem if your leaves look like they’re dying.
Just pump the pH of the soil to above 5.5 and everything should resolve itself. Molybdenum is more available when soil is alkaline
Zinc is a touchy subject for tomatoes, onions, and beans.
A zinc deficiency will cause problems with chlorophyll production, and presents with a similar appearance to a nitrogen deficiency. In this case, the leaves will yellow while the veins remain green, typically starting with the lower leaves. It’s another form of chlorosis. In the more severe cases, lower leaves will turn purple or die, while the upper leaves finally undergo yellowing.
Viral infections or nitrogen deficiency are more likely culprits if you see a yellowing of the leaves. Still, you can use a kelp spray or other zinc additives to help the plant recover. It’s unlikely you’ll see a zinc overdose.
Balances calcium, is found in borax.
Much like salt, too much is bad for plants.
Found in some fungicides, likely not to be in short supply.
Serious use in chlorophyll, balances with other metals.
Very similar to iron. Don’t use a ‘general purpose’ fertilizer.
Causes similar symptoms to nitrogen deficiency.
Causes similar symptoms as nitrogen deficiency… but with more purple.
Allotment Garden (.org) – Trace Elements
Oxford Academic Journal of Nutrition
LibreTexts – Introductory and General Biology
Alberta Micronutrient Benchmark (pdf)
UMD Extension – Chlorine Toxicity
PT Horticulture – Copper
SF Gate – Effects of Excess Iron
PT Horticulture – Manganese
Gardening KnowHow – Molybdenum