Micronutrients in Soil
Micronutrients are nutrients required by plants in very small amounts. Micronutrient deficiencies are uncommon in the Northeast if soil pH is between 6 and 6.8 on mineral soils and between 5.4 and 6 on high organic matter muck or peat soils. Likewise, sulfur, a secondary nutrient, is unlikely to be deficient due to additions from acid rain and/or manure. That said, boron and zinc are the two most common deficiencies seen in the Northeast. It is also important to note that crops differ in their micronutrient requirements. The optimum level for one crop may be toxic for another crop. This is especially true for boron. At the opposite extreme, phytotoxic levels of copper and zinc are a growing concern on some dairy farms due to the increased use of copper sulfate and zinc sulfate hoof baths.

Soil testing and tissue analysis is the best way to confirm if corrective measures are needed. Micronutrient recommendations should be tempered by first hand knowledge of field history including any soil amendment, soil type, soil pH, soil drainage, soil organic matter, crop requirement and yield potential.

Below is a general discussion of the function of micronutrients in plants and some of the conditions favoring deficiency or excessive levels in the soil.

Sulfur (S)
Sulfur (S) is a component of several amino acids. Sulfur is generally sufficient in the Northeast due to additions from acid rain and manure. It is taken up by the plant as sulfate (SO4) or sulfur dioxide (SO2). Like nitrogen, it is mobile in the plant and in the soil. Deficiency symptoms are quite similar to nitrogen deficiency and appear as a pale green to yellow color beginning in older leaves first. Use tissue analysis in conjunction with soil testing to confirm suspected problems.

Boron (B)
Boron (B) is needed for the growth and development of new cells. It is absorbed mainly as boric acid (H3BO3). Boron deficiency is more common on coarse, well drained sandy soils since it is water soluble and is quite mobile in the soil. Availability and uptake are also reduced by increasing soil pH.

Boron is a common deficiency for high demand crops like alfalfa, root crops (beets), or cole crops (cabbage, cauliflower, etc.) especially on well drained, coarse textured soils. Deficiency symptoms vary by crop but generally appear as stunted growth, yellow to white color of new leaves, brown discolorations on flowers and foliage of cole crops, brown spots on beet roots, etc.. Symptoms first show up on new growth since it is immobile in the plant.

One - two pounds of B per acre incorporated before planting is recommended for high response crops. Established stands of alfalfa should be topdressed with an additional 1-2 lbs./acre B each year. Optimum levels for high demand crops may be toxic for sensitive crops like peas or small grains. An application of ½ - 1 pound B per acre is generally sufficient for low response crops if soil test levels are low.

Zinc (Zn)
Zinc is needed for protein synthesis and is involved in the activation of several enzyme systems in plants. Availability is influenced by several factors including soil pH, soil organic matter content and soil phosphorus levels. Deficiency is sometimes seen on high demand crops like corn or snap beans when soil pH is > 7 and soil test P is high. Zinc is taken up as Zn++ and is immobile in the plant, so deficiency symptoms will first show up on new growth. Deficiency in corn will show up as a light striping to overall whitening of leaf tissue and plants will appear stunted. Bean leaves will be small with yellow to bronze color between veins.

Zinc can accumulate to toxic levels in the soil. Consider corrective measures if soil test and plant tissue levels are high and additional zinc from zinc sulfate hoof baths or other sources is likely.

Copper (Cu)
Copper acts as an activator for important enzyme systems in plants. Copper deficiency is very rare on mineral soils but can be a concern on high value vegetable crops grown on low pH organic (muck or peat) soils. Copper is taken up as Cu++ and like zinc, it is immobile in the plant so deficiency shows up in new growth first. Deficiency on onions shows up as thin, pale yellow scales, chlorotic leaves and bulbs that lack adequate firmness. Potatoes will be stunted and may have a bluish green color.

Copper can accumulate to toxic levels in the soil. Excess copper will inhibit iron uptake so plants will exhibit "iron chlorosis". Consider corrective measures if soil test values exceed 30 ppm, plant tissue levels are high, and annual copper addition from copper sulfate hoof baths or other sources exceeds 5 pounds copper per acre.

Manganese (Mn)
Manganese is used to activate several enzyme systems in plants and is required for photosynthesis. Manganese is taken up as Mn++. It is immobile in the plant so deficiency shows up in new growth first. Deficiency symptoms include stunting and a general yellowing of plants. Leaves may also show dark brown, dead spots. That said, deficiency is quite rare if soil pH is 7 or lower. Banding an acid forming starter fertilizer or adding 1 pound per acre Mn in the fertilizer band may be sufficient to correct a deficiency if soil pH is > 7 for crops with a high Mn requirement (soybeans, small grains, lettuce, onions, peas, radish, beets, spinach, potatoes, beans). Manganese can reach toxic levels in low pH (<5.2) and/or waterlogged soils. Proper drainage and pH is recommended for best crop performance.

Molybdenum (Mo)
Molybdenum levels are generally low but deficiency is extremely rare. Deficiency is most likely on acid sandy soils. Liming to the optimum pH will prevent most problems. Mo is required for N fixation in legumes. Consider using an inoculant seed treatment containing ½ oz sodium molybdate per bushel of legume seed if soil pH is below the optimum for that crop. Excessive Mo is rare but should be confirmed by tissue analysis if suspected since excessive levels may be harmful to livestock.

Iron (Fe)
Iron is used in chlorophyll formation and is essential for nitrogen fixation in legumes. Deficiency is most common on calcareous soils and at high soil P levels. It appears as yellow leaves with green veins. Iron chlorosis is fairly common in the Midwest on soybeans grown on high pH soils. Planting varieties with a good "Iron Chlorosis Rating" is usually sufficient to correct the problem. Deficiency is very uncommon in agronomic crops in the northeast, but is sometimes seen on high value ornamentals and fruit trees. Suspected deficiencies should be confirmed via tissue analysis to determine if corrective measures are needed.

Element	Symbol	Form of Uptake	Plant Tissue Sufficiency Range (% on a DMB)	Comments
Structural Elements, seldom limiting in the field
Carbon	C	CO2, H2CO3, H(CO3)2
Hydrogen	H	H2O
Oxygen	O	O2
Major Fertilizer Nutrients (Macronutrients)
Nitrogen	N	NO3-, NO2-, NH4+, Organic & amine forms	1 - 5%	Mobile in plant Component of plant proteins.
Phosphorus	P	H2PO4-, HPO4--	0.1 - 0.5%	Somewhat mobile in plant. Needed for cell growth & energy transfer, component of enzymes and proteins
Potassium	K	K+	1 - 4% in vegetative tissue	Very mobile in plant Needed for water uptake & retention, cell growth & stalk strength.
Sulfur	S	SO4---, SO3--	0.1 - 0.5%	Mobile in plant Component of several amino acids
Lime Elements (Macronutrients)
Calcium	Ca	Ca++	0.1 - 1%	Immobile in plant Cell wall component
Magnesium	Mg	Mg++	0.1 - 0.5%	Somewhat mobile in plant Primary component of chlorophyll
Micronutrients (required in very small quantities)
Iron	Fe	Fe++	50-250 ppm	Immobile in plant Essential for chlorophyll formation. Involved in N fixation in legumes.
Boron	B	H3BO3	5 - 60 ppm	Immobile in plant Needed for growth & development of new cells
Manganese	Mn	Mn++	20 - 500 ppm	Immobile in plant Used for activation of several enzyme systems & essential for photosynthesis
Copper	Cu	Cu++	4 - 20 ppm	Immobile in plant Used for enzyme activation
Zinc	Zn	Zn++	25 - 150 ppm	Immobile in plant Involved in protein formation & enzyme activation
Molybdenum	Mo	MoO4-	< 1ppm	Immobile in plant? Required for N fixation in legumes
Chlorine	Cl	Cl-	0.2 - 2%	Mobile in plant Involved in photosynthesis
Other elements accumulated in plants
Silicon	Si	SiO3		Response in some grasses
Aluminum	Al	Al+++		Excess toxic, acid soil injury
Selenium	Se	SeO3		Deficient for animal needs in Northeast. Excess toxic to animals.
Sodium	Na	Na+		Response in some crops, especially beets
Iodine	I	I-		Deficiency in animals & humans
Cobalt	Co	Co++		Deficiency in animals

Adapted from a handout developed by W.S. Reid, Cornell University.