Micronutrients for Plants: 8 Nutrients, Signs and Rates

Plants need 8 micronutrients: boron, chlorine, copper, iron, manganese, molybdenum, nickel and zinc. They are 8 of the 17 essential nutrients, and they make up a very small share of plant dry matter, according to the University of Florida (UF/IFAS). The other nine are the six macronutrients, which include the three main nutrients nitrogen, phosphorus and potassium (N, P, K), plus carbon, hydrogen and oxygen from air and water.

This guide lists all 17, shows how much of each a plant holds, explains the signs of a shortage and gives extension zinc rates for corn. It ends with a calculator for the cost of a zinc application per acre. Always follow the product label and local rules, and get a soil or tissue test before you buy any micronutrient fertilizer.

How many micronutrients do plants need?

Eight. The University of Maryland Extension counts seventeen essential elements: three non-mineral ones from air and water, and fourteen mineral nutrients. Six of the fourteen are macronutrients, needed in large amounts. The other eight are the trace elements, or micronutrients. The University of California and the Government of Saskatchewan give the same count of 17 essential nutrients.

Some older lists name cobalt as a ninth micronutrient. The extension lists above do not include it, so this guide counts eight. The University of Georgia names seven micronutrients for vegetable gardens (boron, chlorine, copper, iron, manganese, molybdenum and zinc) and does not mention nickel.

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The 17 essential nutrients by group

Number of essential elements in each group

All 17 essential nutrients and how much plants hold

UF/IFAS reports that carbon, hydrogen and oxygen make up 94% or more of dry tissue in most plants, and the other 14 elements together make up less than 6%. Macronutrients are measured in percent of dry weight. Micronutrients are measured in parts per million (ppm), so a single percent equals 10,000 ppm. The ranges below are broad ranges across plants, not targets for one crop.

Source for the table: UF/IFAS publication AG462, which reproduces Hochmuth and others (2018). The University of Maryland Extension notes that nitrogen is required in the largest quantity after carbon, oxygen and hydrogen, which is why fertilizer plans start with N, P and K.

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Micronutrient content of plant dry matter

ppm of dry weight, range reported across plants; iron (50-1,000) and chlorine (80-10,000) are left off to keep the scale readable

The 3 main plant nutrients: N, P and K

Nitrogen, phosphorus and potassium are called the primary nutrients because crops take up large amounts and soils often run short of them. The University of California says N, P and K are needed in the greatest quantity and are the ones most commonly deficient. Nitrogen builds protein and chlorophyll. Phosphorus carries energy and drives early root growth. Potassium controls water use and moves sugars.

These are the three numbers on a fertilizer bag. A 10-10-10 bag, for example, holds 10% N, 10% P2O5 and 10% K2O. Check the label to see whether a bag claims any micronutrients. Micronutrients cannot replace N, P and K, and a micronutrient shortage can hold back a crop that has plenty of the big three.

Why micronutrient shortages happen

Soils rarely lack a micronutrient in total. The problem is usually that the plant cannot reach it. Four conditions come up again and again in extension guidance.

• Soil pH. Iowa State University explains that as pH rises, iron, manganese, copper and zinc become less available. Molybdenum is the exception, and it becomes more available as pH rises (Voss, 1998). Oregon State University adds that deficiencies of micronutrients other than boron and zinc are uncommon, and that they rarely occur when soil pH is below 6.5.
• Sandy soil and low organic matter. The Government of Saskatchewan notes that sandy soils are more likely to show micronutrient deficiencies than clay soils, and so are mineral soils very low in organic matter.
• Eroded or levelled ground. Colorado State University Extension links zinc deficiency to areas where topsoil was removed by leveling for irrigation, erosion or terrace construction, and to high soil phosphorus.
• Cold, wet springs. Saskatchewan reports that symptoms seen in cool, wet soil can disappear when the soil warms. Colorado State says zinc deficiency is more severe in years with cold, wet springs.

Colorado State states that the response to zinc is much greater under high yield conditions, so put the money where a test shows a likely response.

The eight micronutrients: first signs and where they run short

Where a symptom first appears depends on whether the nutrient moves inside the plant. Iron does not move, so Colorado State notes that deficiency shows on new growth first. Saskatchewan makes the same point for boron. The table below pulls together what the extension services report for each element.

Sources for the table: UF/IFAS AG462 for symptoms of Zn, Fe, Mn, B, Cu, Mo, Cl and Ni; Colorado State University Extension for corn zinc and for iron; Virginia Tech Soil Test Note 4 for manganese on soybeans and peanuts; Government of Saskatchewan for boron, copper and chloride.

Is it a micronutrient problem? How to check

Yellow, pale or stunted plants have many possible causes. The Government of Saskatchewan warns that diagnosing deficiencies from symptoms is difficult even for trained agronomists, and that drought, salinity, disease, insects, herbicide injury and a macronutrient shortage can look alike. It advises looking for “multiple evidence”: crop symptoms, tissue tests, soil tests, test strips and cropping history.

1. Map where it happens. Micronutrient problems tend to show in patches, such as eroded knolls, sandy rises or leveled areas, rather than across the whole field.
2. Sample good and bad areas separately. Saskatchewan recommends a soil and tissue test from affected and unaffected areas in the same field. Take tissue from the same plant part and growth stage that the lab asks for, and do not wash the leaves.
3. Read the lab report with the lab’s own ranges. UConn Extension notes that a tissue report lists sufficiency ranges for each nutrient, but gives no recommendations, so talk to a local extension professional about an action plan.
4. Check soil pH. A high pH (above 7 for zinc) points to zinc and iron, and manganese shortage is found on slightly acid to alkaline soils. A pH below 5.5 points to molybdenum in legumes.
5. Run a test strip. Apply the nutrient on one strip and leave a strip untreated, then compare yield. Saskatchewan advises this when a soil test is marginal.

Satellite maps can help with step 1. The Farmonaut Satellite + AI Farm Monitoring System shows vegetation index maps (NDVI) of your fields through the season at sat.farmonaut.com, and see Farmonaut’s satellite data (API) for developers. A map shows where plants are greener or paler. It cannot tell you which nutrient, or whether the cause is a nutrient at all, so use it to decide where to walk and sample.

▶ How satellite NDVI maps show crop stress across a large farm before it is easy to see from the ground. · 4:09

Zinc rates from extension services

Zinc is a good worked example because several states publish corn rates for it. Rates depend on the soil test, the method and the region, and they differ between states.

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Zinc to apply on corn, by soil test (Colorado)

lb of zinc per acre; AB-DTPA test: low 0-0.9 ppm, marginal 1.0-1.5 ppm, adequate above 1.5 ppm

Colorado State University Extension ties the rate to the AB-DTPA soil test. For corn, sorghum, beans and potatoes on irrigated land, a low test (0 to 0.9 ppm Zn) calls for 10 lb of zinc per acre, a marginal test (1.0 to 1.5 ppm) calls for 5 lb, and an adequate test (above 1.5 ppm) calls for none. On dryland the rates are 5, 0 and 0 lb.

Virginia Tech gives rates by method, using lb of elemental zinc from zinc sulfate or oxide: 10 to 12 lb broadcast on Coastal Plain soils, 20 to 25 lb on Piedmont and Appalachian soils (corrects the deficiency for 3 to 5 years), 6 to 8 lb in a sideband with starter fertilizer (repeat every year), 1 lb in seed contact and 1 lb per foliar spray. North Dakota State University uses a different test, with a critical level of 1 ppm DTPA, and suggests a broadcast of at least 30 lb per acre of 36% zinc sulfate, which is about 11 lb of zinc, to avoid a potential deficiency. Oregon State says a DTPA test above 1.5 ppm is sufficient for most crops and quotes zinc sulfate rates of 5 to 15 lb of zinc per acre.

Do not copy a rate from another state. Use your own state’s soil test, crop and label. Seed contact zinc can injure germination, so follow the label.

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Zinc by application method (Virginia)

lb of elemental zinc per acre from zinc sulfate (seed contact) or sulfate or oxide (other methods); foliar is per spray

Fertilizer forms: sulfates, oxides and chelates

The Government of Saskatchewan describes the common forms. Sulfates of copper, zinc, iron and manganese are water soluble and are the forms used most often for field crops. Borate is the matching form for boron. Chelated products are many times dearer per pound of nutrient than sulfates or oxides, but the rate is lower, and at label rates they have limited residual value. Manure can also supply copper and zinc. Fertilizer labels list the percent of the nutrient in the product, and that percent is what you need for the calculator below.

For iron, Colorado State says soil application has not been economic on a field scale. It describes foliar sprays of ferrous sulfate or iron chelates, repeated if yellowing continues, and notes that sprays may restore color without restoring top yield. Follow the product label for the rate and timing.

▶ A 20-minute walkthrough of reading NDVI maps and pulling a report for your own field. · 19:36

What a zinc application costs per acre

The calculator turns a rate in pounds of zinc per acre into pounds of product, a cost per acre and a cost per year, then shows the yield gain that would pay for it. All values are examples; replace them with your own soil test advice and today’s prices.

Try the numbers for your own farm below.

Too much can hurt: boron and other limits

More is not better. The Government of Saskatchewan states that over-applying micronutrients can cause toxicity and yield loss, with boron the main concern because the gap between deficiency and toxicity is narrow. Where a boron deficiency has been proven it suggests broadcast rates of no more than 0.5 lb of boron per acre for cereals and 1.5 lb for canola, and a foliar maximum of 0.3 lb. Boron is toxic in contact with seed. Saskatchewan also warns that applying micronutrients when they are not needed may reduce yields or economic returns.

Other limits: Virginia Tech says to apply only the trace elements the soil test report recommends, at the recommended rates. UF/IFAS notes that excess copper may induce iron deficiency, and so may a high manganese level. These are the reasons to test before you spray.

▶ Seven simple ways to protect soil and water, explained with memes. · 4:34

Gardeners and small plots

For home vegetable beds the same logic holds. The University of Georgia suggests buying a premium fertilizer that also contains micronutrients when fertilizing at fruit set, and the University of Maryland notes that boron, zinc and manganese can sometimes be deficient in Maryland, especially in Coastal Plain soils. Manure can add copper and zinc, according to Saskatchewan, so test before adding more.

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FAQ

How many micronutrients are there for plants?

Eight: boron, chlorine, copper, iron, manganese, molybdenum, nickel and zinc. They are 8 of the 17 essential nutrients listed by UF/IFAS, the University of Maryland and the Government of Saskatchewan. The other nine are six macronutrients and carbon, hydrogen and oxygen.

What are the 17 essential nutrients for plants?

Carbon, hydrogen, oxygen, nitrogen, phosphorus, potassium, calcium, magnesium, sulfur, boron, chlorine, copper, iron, manganese, molybdenum, nickel and zinc. The first three come from air and water. The other 14 come from the soil or from fertilizer.

What are the 3 main plant nutrients?

Nitrogen, phosphorus and potassium. They are the primary macronutrients, taken up in the largest amounts, and they are the three numbers on a fertilizer bag.

Do plants need iron?

Yes. Iron is one of the eight micronutrients and is needed to make chlorophyll. Shortages show as yellow leaves with green veins on new growth, and they are linked to high pH soils with free lime and low organic matter (Colorado State University Extension). Soil tests and tissue tests confirm it.

Is cobalt a plant micronutrient?

The extension lists used for this guide (UF/IFAS, University of Maryland, Government of Saskatchewan) do not count cobalt among the 17 essential nutrients, so it is not part of the standard list of eight.

Can plants get too many micronutrients?

Yes. Boron has a narrow gap between deficiency and toxicity, and too much of any micronutrient can cut yield. Use soil or tissue test results, keep to label rates and ask your extension service or a certified crop adviser before applying.

How we checked the numbers

Element counts, plant content ranges and symptoms come from UF/IFAS AG462, the University of Maryland Extension, the Government of Saskatchewan and the University of California. Soil factors come from Iowa State University (1998), Oregon State University and Colorado State University. Zinc rates come from Colorado State, Virginia Tech, North Dakota State and Oregon State. The calculator uses your inputs only. This page is general information, not a fertilizer recommendation for your field.