5.2: Nutrients in the Plant Body

What Nutrients are in the Plant’s Body?

Carbon, hydrogen, oxygen. Although hydrogen (H), oxygen (O), and carbon (C) are elements, they are generally not called “nutrients” because they are usually abundant in water and air. Remember also that the plant body is made up mostly of carbon, hydrogen, and oxygen. These elements are synthesized, or knitted together using energy from the sun to form carbohydrate compounds, often referenced as CH₂0. Recall that this is the plant process we call photo (sun) synthesis (knitted).

The table below summarizes the 13-15 elements essential plant for plant growth that are considered nutrients. The table lists the elements in order according to an average concentration in plant tissues. The table below also shows the amount of the element found in a typical soil. It also gives a brief description of the nutrient’s role in the plant.

Table \(\PageIndex{1}\): Elements required by plants listed in order of how much of it (%) is in a typical plant’s body.

			Macronutrients	Its available form in the soil ^A			% in typical soil	% in typical plant (%DM)^B	Where is it found in the plant? What role does it play?
N Anions have (–) charges			nitrogen	NO3^- nitrate or ammonium NH4^+			0.1	1-3%	Amino acids, proteins, DNA, chlorophyll, enzymes
K Cations have (+) charges			potassium	K^+			1.5	0.3 - 6%	Enzymes, amino acids. Makes proteins. Activates enzymes, opens/closes stomata
Ca Cations have (+) charges			calcium	Ca^+2			0.5	0.1-3.5%	Cell walls, enzyme cofactor, cell permeability
P Anions have (–) charges			phosphorus	phosphate H2PO4 or HPO4^-2			0.06	0.05-1.0%	Energy compounds i.e. ATP, co-enzymes, Nucleic Acids i.e. DNA. Phospholipids
S Anions have (–) charges			sulfur	sulfate SO4^-2			0.05	0.05-1.5%	Part of some amino acids and proteins and the very important Co-enzyme A molecule
Mg Cations have (+) charges			magnesium	Mg^+2			0.4	0.05 - .07%	A part of the chlorophyll molecule. Activates many enzymes.

		Micronutrients	Its available form in the soil ^A	% in typical soil	Parts per million (ppm)	Where is it found in the plant? What role does it play?
Fe		Iron	Fe^+2 or Fe^+3	3.5	10-1500	Synthesis of chlorophyll, cytochrome, and ferredoxin
Mn		Manganese	Mn^+2	0.05	5-1500	Activates some enzymes
B		Boron	BO^-3 or B4O7^-2	0.002	2-75	Influences Ca^+2 utilization, other unknown functions
Zn		Zinc	Zn^+2	0.001	3-150	Activates some enzymes
Cu		copper	Cu^+2	0.0005	2-75	Activates some enzymes
Mo		molybdenum	MoO4^-2	0.0001	0.1-5.0	Nitrogen metabolism
Cl		Chlorine	Cl^-	-	100-10,000	Cell osmosis, O2 production in photosynthesis

	Essential in some plants		Its available form in the soil ^A		% in typical soil		Parts per million (ppm)		Where is it found in the plant? What role does it play?
Co	Cobalt		Co^+2		-		trace		Required by nitrogen-fixing microbes associated with legumes
Na	Sodium		Na^+2				trace		Osmotic balance in some desert plants and C4 plants

Available form. Nitrogen, phosphorus, and sulfur are anions and the form in which the plant takes them up from the soil has always been thought to be in conjunction with oxygen, as shown in the 3^rd column. Cationic nutrient elements exist in the soil solution in ionic forms and are taken up by the plant as cations.

Macronutrients. A macronutrient is needed in relatively large amounts by the plant as measured in the “percent of dry matter” of the plant’s body. In other words, if you took 100 grams of the plant, dried it, and got rid of all the water, the ashy material that is left may contain about 2.5 grams (g) nitrogen, 2g potassium, 2 g calcium, 0.7g phosphorus, 0.06g sulfur, and 0.06g magnesium – depending on the plant.

Primary macronutrients are nitrogen, phosphorus, and potassium.

Secondary macronutrients are calcium magnesium, and sulfur.

Micronutrients are measured in parts per million rather than a percent of dry matter. This tells us that micronutrients, while essential to plant metabolism are needed only in very tiny amounts. The micro-nutrients are not “bodybuilding” components, but rather they are keys or tiny locks in enzymes that allow the plant to function.

For an analogy, envision a car. The bulk of the car is composed of metal in the frame and the body – that is like the CH₂O carbohydrates in a plant body. The primary macronutrients might be rubber, glass, and fiberglass/plastic–much like our N, P, and K. The secondary macronutrients (Ca, S, Mg) may be gas and oil. The micronutrients are the very small things, by volume that are still very critical for function–the sparkplugs, brakes, keys. It’s not a perfect analogy but it gives the general idea that a plant’s body, like a car, has some components in huge amounts, some in medium, and some in tiny amounts.

Table \(\PageIndex{2}\): Difference between what the soil can provide and what the plant needs

Macronutrients	% in typical soil	% in typical plant (%DM)^B	Difference	Exchangeable - available
N Nitrogen (anion)	0.1	1-3%	3%	lowest
K Potassium +1 (cation)	1.5	0.3 - 6%	1%	low
Ca Calcium +2 (cation)	0.5	0.1-3.5%	1%	medium
P Phosphorus (anion)	0.06	0.05-1.0%	1%	low
S Sulfur (anion)	0.05	0.05-1.5%	1%	medium
Mg Magnesium +2 (cation)	0.4	0.05 - .07%	abundant	high

Nitrogen: (N). The soil never seems to be able to keep up with the nitrogen demands of plants. Plants need a lot compared to what’s in a typical soil. Nitrogen takes many forms when it combines with Oxygen and Hydrogen but it must always be in ionic form when taken in by the plant roots, usually as NH4+ (ammonia). We’ll examine the N cycle in other places in the book. We will also go into more detail on nitrogen fertilizers, both organic and synthetic in another chapter.

Farmers & gardeners understandably focus on nitrogen and phosphorus, because additions of these nutrients are commonly needed in order to maintain crop productivity; large quantities are normally used. But both N and P have the potential for environmental problems which we investigate later. While K deficiency is also fairly common, most other nutrients are not normally deficient. Overuse of fertilizers and amendments other than N and P seldom causes problems for the environment, but it may waste money and reduce yields. There are also animal health considerations with excess amounts. For example, excess potassium in feeds for dry cows (cows that are between lactations) results in metabolic problems, and low magnesium availability to dairy or beef cows in early lactation can cause grass tetany. As with most other issues we have discussed, focusing on the management practices that build up and maintain soil organic matter will help eliminate many problems, or at least make them easier to manage.

Phosphorus: (P) is needed for many plant functions, just like nitrogen. It tends to be in slightly short supply in most soils unless the farm is a dairy farm. On those farms, phosphorus has built up in the soil over time, more than the plant can use, and then the excess becomes a pollutant. We will discuss phosphorus fertilizers and the phosphorus cycle more in another chapter because they deserve special attention.

Potassium: (K) is one of the N-P-K “big three” primary nutrients needed in large amounts, and in humid regions, it is frequently not present in sufficient quantities for optimum crop yields. Deficiencies occur most often when an entire crop is harvested and removed versus the grain only. Unlike N and P, Potassium is more concentrated in stalks and stems that remain in the field as stover/straw if only the grain is harvested, thereby recycling most of the K for the next crop. K is generally available to plants as a cation, and the soil’s cation exchange capacity (CEC) is the main storehouse for this element for a given year’s crop. Potassium availability to plants is sometimes decreased when a soil is limed to increase its pH by one or two units. The extra calcium, as well as the “pull” on K exerted by the new cation exchange sites (see the next section, “Cation Exchange Capacity Management”), contributes to lower K availability. Problems with low K levels are usually easily dealt with by applying muriate of potash (potassium chloride), potassium sulfate, or K-mag (potassium magnesium sulfate, also sold as Sul-Po-Mag or Trio). Manures also usually contain large quantities of K. Some soils have low amounts of CEC, such as sandy and sandy loams low in both organic matter and clay. However, if the type of clay has low CEC, such as kaolinitic clays found in the U.S. Southeast, low CEC may make it impossible to store large amounts of readily available K for plants to use. If a lot of fertilizer K is added at one time—an amount that may be reasonable for another soil—a significant portion may be leached below the root zone before plants can use it. In these situations, split applications of K may be needed. Since most complete organic fertilizers are low in K, organic growers with low CEC soils need to pay special attention to maintaining the K status of their soils.

Secondary Macronutrients

Calcium: Why is Ca considered a secondary macronutrient while it looks like plants have more of that in their body than phosphorus? The answer has to do with the amount of each of those nutrients the soil can provide compared to how much is in the plant body. Review the table’s last column “exchangeable–available” to see why calcium and sulfur are considered secondary macronutrients while potassium and phosphorus are primaries. It turns out that there is not much “exchangeable-available” K or P for plants in the soil because of soil chemistry. Clay and other soil particles “grip” the +2 Ca and +2 Mg more than the +1 K. The K slips away via leaching and therefore needs to be replenished by fertilizers. The Ca & Mg are held and “time-released” when they become depleted in the soil solution. On the other hand, the anion -2 Phosphorus can be gripped too tightly by adsorption onto soil particles. The phosphorus needed for plant growth may be limited unless fertilizers are added. Sulfur seems to act more like Ca and Mg and is more available than P. That’s why it is viewed as a secondary nutrient as well.

Calcium deficiencies are generally associated with low pH soils and soils with a low CEC. The best remedy is usually to lime and build up the soil’s organic matter. However, some important crops, such as peanuts, potatoes, and apples, commonly need added calcium. Calcium additions also may be needed to help alleviate soil structure and nutrition problems of sodic soils or soils that have been flooded by seawater (see “Remediation of Sodic [Alkali] and Saline Soils”). In general, there will be no advantage to adding a calcium source, such as gypsum, if the soil does not have too much sodium, is properly limed, and has a reasonable amount of organic matter. However, soils with very low aggregate stability may sometimes benefit from the extra salt concentration and calcium associated with surface gypsum applications. This is not a calcium nutrition effect but is a stabilizing effect of the dissolving gypsum salt. Higher soil organic matter and surface residues should do as well as gypsum to alleviate this problem.

Magnesium: At the bottom of Table \(\PageIndex{2}\) is Mg; it is more abundant in soil than what a plant needs. There are a few exceptions, but most farmers do not supply Mg because there is enough in the soil. Deficiency is easily corrected, if the soil is acidic, by using a magnesium (dolomitic) lime to raise the soil pH (see “Soil Acidity”). If K is also low and the soil does not need liming, potassium magnesium sulfate is one of the best choices for correcting a magnesium deficiency. For soil that has sufficient K and is at a satisfactory pH, a straight magnesium source such as magnesium sulfate (Epsom salts) would be a good choice.

Sulfur deficiency is common on coarse-texture soils with low organic matter, in part because it is subject to leaching in the oxidized sulfate form (similar to nitrate). Some soil testing labs around the country offer a sulfur soil test. (Those of you who grow garlic should know that a good supply of sulfur is important for the full development of garlic’s pungent flavor.) Much of the sulfur in soils occurs as organic matter, so building up and maintaining organic matter should result in sufficient sulfur nutrition for plants. Sulfur deficiency is becoming more common in certain regions now that there is less sulfur air pollution, which previously originated from the combustion of high-sulfur forms of coal. (Now it is captured in power plant exhaust scrubbers, and the residue is sold as gypsum.) Organic farmers cannot use gypsum from power plants because of the potential contaminants in that product. In the Great Plains, on the other hand, irrigation water may contain sufficient quantities of sulfur to supply crop needs even though the soils are deficient in sulfur. Some fertilizers used for other purposes, such as potassium sulfate, potassium magnesium sulfate, and ammonium sulfate, contain sulfur. Calcium sulfate (gypsum) also can be applied to remedy low soil sulfur. The amount used on sulfur-deficient soils is typically 15–25 pounds of sulfur per acre.

The risk for sulfur deficiency varies with the soil type, the crops grown on the soil, the manure history and the level of organic matter in the soil. A deficiency is more likely to occur on acidic, sandy soils; soils with low organic matter levels and high nitrogen inputs; and soils that are cold and dry in the spring, which decreases sulfur mineralization from soil organic matter. Manure is a significant supplier of sulfur, and manured fields are not likely to be S deficient; however, sulfur content in manure can vary.

—S. Place et al. (2007)

Micronutrients

Micronutrient fertilizers generally are required in cases where the micronutrients are naturally unavailable in the soil, or when many years of intensive crop production has reduced much of the natural soil supply. We focus here mostly on the mineral nutrients that are critical for healthy plants, but some trace elements are also important for animal and human health, including zinc, iron, iodine, calcium, magnesium, selenium, and fluorine, which need to be supplied through the food chain (soil-plant-animal/human) or added as nutritional supplements.

As of the writing of this edition, there are discussions about how glyphosate-based herbicides affect micronutrient availability. Glyphosate is the most frequently applied herbicide worldwide and, like soil organic matter, has chelating abilities. Remember, chelation in this context means 'grabbing' and hanging onto a nutrient so it becomes unavailable for the plant. It is still an open debate whether this has a significant impact on plant micronutrient availability or affects soil, plant health, or human health. However, there is no conclusive evidence that it is overall more harmful than the chemicals it replaces.

Zinc deficiencies occur with certain crops on soils low in organic matter and in sandy soils or soils with a pH at or above neutral. Zinc problems are sometimes noted on silage corn when manure hasn’t been applied for a while. Zinc also can be deficient following topsoil removal from parts of fields as land is leveled for furrow irrigation. Cool and wet conditions may cause zinc to be deficient early in the season. Sometimes crops outgrow the problem as the soil warms up and organic sources become more available to plants. Zinc deficiencies are also common in other regions of the world, especially Sub-Saharan Africa, South and East Asia, and parts of Latin America. Applying about 10 pounds of zinc sulfate per acre (which contains about 3 pounds of zinc) to soils is one method used to correct zinc deficiencies. If the deficiency is due to high pH, or if an orchard crop is zinc deficient, a foliar application is commonly used. If a soil test before planting an orchard reveals low zinc levels, zinc sulfate should be applied.

Boron deficiencies occur most frequently on sandy soils with low organic matter and on alkaline/calcareous soils. It shows up in alfalfa when it grows on eroded knolls where the topsoil and organic matter have been lost. Deficiencies are common in certain regions with naturally low boron, such as in the Northwest maritime area, and in many regions in other parts of the world. Root crops seem to need higher soil boron levels than do many other crops. Cole crops, apples, celery, and spinach are also sensitive to low boron levels. The most common fertilizer used to correct a boron deficiency is sodium tetraborate (about 15% boron). Borax (about 11% boron), a compound containing sodium borate, also can be used to correct boron deficiencies. On sandy soils low in organic matter, boron may be needed on a routine basis. Applications for boron deficiency are usually around 1–2 pounds of boron per acre. No more than 3 pounds of actual boron (about 27 pounds of borax) per acre should be applied at any one time; it can be toxic to some plants at higher rates.

Manganese deficiency, usually associated with soybeans and cereals grown on high-pH soils and on vegetables grown on muck soils, is corrected with the use of manganese sulfate (about 27% manganese). About 10 pounds of water-soluble manganese per acre should satisfy plant needs for a number of years. Up to 25 pounds per acre of manganese is recommended if the fertilizer is broadcast on very deficient soil. Natural, as well as synthetic, chelates (at about 5% to 10% manganese) usually are applied as a foliar spray.

Iron deficiency occurs in blueberries when they are grown on moderate- to high-pH soils, especially with a pH of over 6.5. Iron deficiency also sometimes occurs in soybeans, wheat, sorghum, and peanuts growing on soil with a pH greater than 7.5. Iron (ferrous) sulfate or chelated iron is used to correct iron deficiency. Reducing plant stressors such as compaction and selecting more tolerant crop varieties are also ways of reducing iron deficiency damage to crops. In addition, research in Minnesota indicates that companion planting a small amount of oats (whose roots are able to mobilize iron) with soybeans reduces iron deficiency symptoms. Manganese and iron deficiencies are frequently corrected by adding inorganic salts in a foliar application.

Copper is another nutrient that is sometimes deficient in high-pH soils. It can also be deficient in organic soils (soils with 10–20% or more organic matter). Some crops—for example, tomatoes, lettuce, beets, onions, and spinach—have a relatively high copper need. A number of copper sources, such as copper sulfate and copper chelates, can be used to correct a copper deficiency.

High-end fertilizer materials have been developed that combine many macro and micronutrients into a single product that can be applied as seed coatings, leaf sprays (foliar), directly to the soi,l or through fertigation systems, and they are especially of interest for high-value crops.

Nutrient Deficiencies

Nutrient deficiency symptoms can appear on plant leaves when soils are low in a particular nutrient (Table \(\PageIndex{3}\)). (Note that crop nutrient deficiencies can sometimes result from compaction and poor aeration, even though enough nutrients are present in the soil). Many nutrient deficiency symptoms look similar, and they may also vary from crop to crop. In addition, typical symptoms may not occur if the plant is suffering from other stresses, including more than one nutrient deficiency. However, some symptoms on some crops are easy to pick out.

For example, N-deficient plants are frequently a lighter shade of green than plants with sufficient N. Nitrogen deficiency on corn and other grasses appears on the lower leaves first as a yellowing around the central rib of the leaf. Later, the entire leaf yellows, and leaves farther up the stem may become yellow. However, yellowing of the lower leaves near maturity is common with some plants. If the lower leaves of your corn plant were all nice and green at the end of the season, there was more N than the plant needed. Potassium deficiencies in corn also show as yellowing on lower leaves, but in this case around the edges. Phosphorus deficiency is normally noted in young plants as stunted growth and reddish coloration. In corn, this may appear early in the season due to wet and cold weather. When the soil warms up, there may be plenty of phosphorus for the plants. For pictures of nutrient deficiencies on field crops, see Iowa State University’s publication Nutrient Deficiencies and Application Injuries in Field Crops (IPM 42).

Table \(\PageIndex{3}\): Examples of Nutrient Deficiency Symptoms

Nutrient	Deficiency symptoms
Calcium (Ca)	New leaves (at the top of the plant) are distorted or irregularly shaped. Causes blossom-end rot.
Nitrogen (N)	General yellowing of older leaves (at the bottom of the plant). The rest of the plant is often light green.
Magnesium (Mg)	Older leaves turn yellow at the edge, leaving a green arrowhead shape in the center of the leaf.
Phosphorus (P)	Leaf tips look burnt, followed by older leaves turning a dark green or reddish purple.
Potassium (K)	Older leaves may wilt and look scorched. Loss of chlorophyll between veins begins at the base, scorching inward from leaf margins.
Sulfur (S)	Younger leaves turn yellow first, sometimes followed by older leaves.
Boron (B)	Terminal buds die; the plant is stunted.
Copper (Cu)	Leaves are dark green; the plant is stunted.
Iron (Fe)	Yellowing occurs between the veins of young leaves. The area between veins may also appear white.
Manganese (Mn)	Yellowing occurs between the veins of young leaves. These areas sometimes appear “puffy.” The pattern is not as distinct as with iron deficiency. Reduction in size of plant parts (leaves, shoots, fruit) generally. Dead spots or patches.
Molybdenum (Mo)	General yellowing of older leaves (at the bottom of the plant). The rest of the plant is often light green.
Zinc (Zn)	Terminal leaves may be rosetted, and yellowing occurs between the veins of the new leaves. The area between veins on corn leaves may appear white.

Source: Modified from Hosier and Bradley (1999)