Essential Elements in Corn

A.M. JOHNSTON¹  and R. DOWBENKO²

¹ Potash & Phosphate Institute of Canada, Saskatoon, Saskatchewan; ² Agrium Inc., Calgary, Alberta.

Plants require 16 nutrient elements for their growth. Three of the nutrient elements (carbon, hydrogen and oxygen) are derived from air and water. The other 13 are normally obtained by the plant from the soil or applied as amendments (fertilizer, manure, etc.) if they are inadequate or unavailable in the soil. Nitrogen (N), phosphorus (P) and potassium (K) are required by plants in relatively large quantities and are most frequently required as soil amendments for maximum crop growth. In fact, the first three numbers on fertilizer products (e.g., 18-18-18) are the percentages of N, P (expressed in oxide form as P2O5) and K (also expressed as an oxide as K2O). For this discussion, these three nutrients are grouped as “primary” nutrients. Sulphur (S), calcium (Ca) and magnesium (Mg) are required by the plant in moderate quantities and are grouped as “secondary” nutrients. The remaining seven nutrients are grouped as “micronutrients” as they are required in small quantities or applied to crops less frequently. Comparative amounts of these nutrients are shown in Table 1 for a corn crop yielding 18.7 t/ha dry matter. This constitutes the amounts of these nutrients that would be removed from the field if the corn is used as silage.

Table 1.  Comparison of Quantities of 13 Essential Nutrients typically found in a Corn Crop Yielding 18.7 t/ha dry matter (9 T/ac; for lb/ac multiply by 0.9)

Nutrient

kg/ha

Nutrient

kg/ha

Nitrogen (N)

240

Chlorine (Cl)

110

Phosphorus (P)

44

Iron (Fe)

3

Potassium (K)

200

Manganese (Mn)

0.6

 

 

Zinc (Zn)

0.6

Sulphur (S)

34

Copper (Cu)

0.2

Calcium (Ca)

45

Boron (B)

0.1

Magnesium (Mg)

56

Molybdenum (Mo)

> 0.1

Primary Nutrients

Nitrogen (N)
Nitrogen is necessary for making chlorophyll and is directly involved in photosynthesis.  Each molecule of chlorophyll contains four N atoms surrounding a magnesium (Mg) atom at the core.  All proteins and enzymes contain N in the form of amino acids. Nitrogen is also a component of vitamins.  Increasing N supply enhances plant production of carotene (a precursor to vitamin A), the B vitamins (riboflavin, thiamin, and nicotinic acid), and cytokinin (a plant growth hormone).

Nitrogen deficiency in young corn reduces chlorophyll production and causes the entire plant to be pale and yellowish green, with spindly stalks. Later, V-shaped yellowing may appear on the tips of leaves. Since N is mobile within the plant, yellowing begins on the older lower leaves and progresses up the plant as the N is moved up to the newer growing tissues.  When N is deficient, the production of chlorophyll and enzymes for photosynthesis is limited, slowing the supply of energy and reducing plant vigour.  Also, the root system becomes less prolific, slowing uptake of other nutrients (Table 2).  Note the small increase in K concentration.  Nitrogen tends to increase K uptake, but only when soil K levels are high.  When soil K is low, added N stimulates growth dilution of the K in the plant.

Corn takes up N as inorganic nitrate (NO3 -) and ammonium (NH4+) ions.  Ammonium is rapidly converted to nitrate in most soils, therefore, most N uptake is in the nitrate form.  However, certain corn hybrids prefer ammonium. Taking up N as ammonium saves the plant a great deal of energy which is required to convert nitrate to ammonium before it is used for protein synthesis. Too much ammonium can be toxic to plants.

 

Table 2.  Example of how nitrogen application can increase the
concentrations of other nutrients in ear leaf at silking or anthesis
(adapted from Illinios and Ontario university data)

Nutrient

Zero N

With N

Increase

Nitrogen (%)

2.45

3.20

31%

Phosphorus (%)

0.22

0.29

30%

Potassium (%)

1.93

1.96

2%

Calcium (%)

0.88

0.90

2%

Magnesium (%)

0.43

0.48

12%

Zinc (ppm)

20

29

45%

Boron (ppm)

9

12

34%

Copper (ppm)

8

12

44%

Grain Yield, kg/ha*

5707

7212

1505 (26%)

Phosphorus (P)
Although P is not present in the plant in large quantities, it is involved in many critical metabolic functions that occur in the plant’s cells. It influences the activity of many enzymes, is a carrier of energy within the cell and can also store energy as in phytin. Because of the ability of inorganic P compounds to dissociate into various forms, it is an important component to buffer the pH within the cell. It is a component of a variety of organic molecules, such as phospholipids, which are involved in a range of activities and functions. Phosphorus is important for photosynthesis, maintenance and transfer of genetic code, development and growth of new plant cells, and germination and formation of seed.

Phosphorus deficiency usually appears when plants are young. Young plants develop shoots faster than roots, particularly when the air is much warmer than the soil, causing a high P demand per unit of root length (See Applying Starter Fertilizer section). Often soil solution P concentrations are inadequate to meet these high requirements, leading to buildup of carbohydrates and sugars, which causes a dark green or reddish-purple leaf color. Extreme symptoms include spindly stalks which are either barren or have twisted ears with incomplete grain fill. Inadequate P, even in seedlings, frequently delays maturity. Phosphorus enters the corn plant through root hairs, root tips, and the outermost layers of root cells. Beneficial fungi, called mycorrhizae, also enhance P uptake (see Early Phosphorus Nutrition in Corn and the Role of Mycorrhizae section).

Phosphate fertilizers typically contain soluble forms of P that are immediately available to plants. However, soluble P in soil reacts with iron (Fe), aluminum (Al), calcium (Ca), and magnesium (Mg) forming new compounds that are less plantavailable, a process called “fixation.” Maintaining the soil pH between 6 and 7 usually maximizes P availability. The fixed P may eventually become plant-available but this can take months or years. Phosphorus is also contained in soil organic matter and manure, and is gradually released as plant-available forms in a process called mineralization. The P used by a corn crop therefore comes from fertilizer and manure applied for that crop and from such materials applied in previous years.

Potassium (K)
Potassium is involved in photosynthesis, conversion of sugars into energy storage compounds such as starch, conversion of amino acids into proteins, the activation of over 60 enzyme systems, and regulation of leaf pore function. In addition to these factors increasing yield and quality of silage corn, K improves disease resistance, N use efficiency and water utilization by preventing excessive respiration. Potassium in cell water helps protein molecules keep their conformation or shape and maintain their activity.

Potassium deficiency is not always evident visually and can be masked by other crop stress symptoms. Look for leaf discoloration, with older leaf edges turning yellow then brown while the midrib stays green. Conditions that may indicate K deficiency include: thin stands due to poor seedling vigor and disease, slow growth rate and poor N response due to reduced enzyme activity, injury due to stress and leaf diseases, delay of silk emergence by up to one week, stalk lodging due to breakdown of internal stalk tissue and invasion by stalk rot, and chaffy, loose-grain ears and poor grain fill near the ear tip.

Potassium is provided both from the soil’s nutrient reservoir and from fertilizer. The amount of K needed by a corn crop is very site-specific, varying between fields and within fields. The fertilization program should consider several factors that affect availability and uptake of K: soil analysis, nutrient requirements for the target yield, previous crop management practices, cultural practices (tillage) and climatic conditions which interact with K availability and uptake by corn. Soil testing provides a measure of the soil’s K nutrient reserves and an estimate of likely crop response to applied fertilizer. Because K is relatively immobile in most soils, no more than 50 to 60% of fertilizer K can be absorbed by corn during the season of application, even when soil supplies are low and growing conditions are favorable. In the long term, K rates should be such that soil test K is built to and maintained at levels that ensure maximum economic yields.

Timing of Potassium Uptake The relative amounts of nutrients taken up at each stage of growth will differ and is best shown by uptake of major nutrients and dry matter production during 25-day periods which represent 5 different stages of growth (Table 3). Nearly 75% of the nitrogen, 65% of the P and 85% of the K used by the crop are taken up by the time the ears are tasseling, which is usually about the mid-point of the growing season.

Corn takes up very little N in the first month after planting, but once the crop reaches a height of about one foot uptake becomes very rapid (Table 3). Nitrogen supply before silking can affect the number of kernels set in the ear. Most corn hybrids take up less than 30% of their total N after silking. However, newer hybrids with “stay-green” and multileaf traits take up 40% or more of their N after silking. Removing nitrate from the soil late in the season reduces the amount left in the soil that may be lost over winter (see Post Harvest Nitrate Test section).

When does corn need K? Every day. The amount increases by growth stage until seedlings become fully mature plants. Potassium must be available early for optimum corn development because over 70% of the total K requirement must be in the plant by silking stage, or about 65 days after planting (Table 3). The peak rate of absorption per day occurs just prior to silking. Uptake is slower but not less important during the critical period of grain formation. Uptake per unit of root length is also an important expression of K requirement. Corn grows shoots faster than roots during early growth stages. Therefore, soils sometimes fail to meet the high requirement of each root segment and K deficiency results during the vegetative growth stage. For these situations, K supplemented by a side dress application prior to silking can improve crop productivity.

Balancing N and K The right balance of N with K improves grain and silage yields, boosts forage quality, improves input use efficiency and provides better protection of groundwater. For example, application of 160 kg/ha (145 lb/ac) of K improved the quality of N-fertilized corn silage by reducing fermentation losses by 5 percent, increasing the carotene content six-fold and nearly tripling total protein production. Application of K also improved N-use-efficiency, helping the corn plants capture nearly 25 percent more N (Table 4) thus lowering the risk of unused N moving down to ground water.

Table 3. Example of the Pattern of Uptake of Primary Nutrients by a Corn Crop

Growth Stage

Seedling

Rapid Vegetative

Silking

Grain Fill

Maturity

Total

(days)

(1-25)

(26-50)

(51-75)

(76-100)

(101-125)

(1-125)

Amount of growth and nutrient uptake during each growth stage (kg/ha)¹

Dry Matter

524

3,597

6,369

6,745

1,499

18,734

Nitrogen (N)

19

84

75

48

14

240

Phosphorus (P)

2

12

16

11

3

45

Potassium (K)

18

88

62

28

4

200

Proportion of uptake during each growth stage (%)

Nitrogen (N)

8

35

31

20

6

100

Phosphorus (P)

5

27

36

25

7

100

Potassium (K)

9

44

31

14

2

100

 

Table 4.  Example of the Influence of K on Corn Grain Yield and N Efficiency

K applied

Grain

N-Efficiency

N Uptake

(kg/ha)¹

(kg/ha)¹

(kg grain/kg N)

(kg/ha)¹

0

8,340

49

194

50

8,780

52

204

100

10,347

62

240

¹ For lb/ac multiply by 0.9

Secondary Nutrients

Sulphur (S)
Sulphur is required by plants for the synthesis of certain amino acids (cysteine and methionine), protein formation, and photosynthesis. Symptoms of S and N deficiency are often confused. Sulphur is immobile within the plant and does not readily move from old to new growth. Hence, with S deficiency, yellowing symptoms first appear in younger leaves, unlike N deficiency, where yellowing appears on the older leaves first. Both N and S deficiencies may appear as stunted plants, with a general yellowing of leaves.

Sulphur has been overlooked in many soil fertility programs. But recently, increased crop yields, reduced deposition of atmospheric S, increased use of high analysis fertilizers, and greater awareness are contributing to increased requirements for S fertilizer applications. Most S in the soil is bound in the organic matter and cannot be used by plants until it is converted to the soluble sulphate (SO4 2-) form by soil bacteria (mineralization). Corn growing on deep sandy soils with low organic matter and little clay in the subsoil will likely respond to applications of 10 to 20 kg/ha (9 to 18 lb/ac) SO4-S with pre-plant fertilizers, or with the first N side dressing. If elemental S is used, it may be necessary to apply more than 50 kg/ha (45 lb/ac) in autumn so that at least 10 to 20 kg/ha (9 to 18 lb/ac) of SO4-S is available by early spring.

Calcium (Ca)
Calcium activates growth-regulating enzyme systems, and is needed for cell wall formation and cell division. It improves the root absorption and translocation of other nutrients, and contributes to improved disease resistance. Calcium is taken up by plants as the divalent cation, Ca2+. Along with Mg and K, Ca helps to balance organic acids, which form during cell metabolism.

Symptoms of Ca deficiency are seen in the new growth because Ca is not readily translocated. The symptoms include slowed root development and slowed new leaf growth, with leaf tips sticking together. Calcium deficiency is not likely to occur when the soil is properly limed (See Liming for Optimum Corn Production section). In acidic soils, crop growth is restricted more by toxic concentrations of aluminum and manganese rather than a Ca shortage. Deficiencies are most commonly observed in acid, sandy soils where Ca has been leached by rain or irrigation water, or in strongly acid peat and muck soils with very low soil Ca content.

Magnesium (Mg)
Magnesium, as the central atom in the chlorophyll molecule, is needed for photosynthesis. It is also required for cell division, protein formation, P metabolism, plant respiration, and the activation of several enzyme systems. Magnesium is taken up by the plant as the divalent cation, Mg2+. It is mobile and easily translocated from older to younger tissues.

When deficiencies occur, the older leaves are affected first with a loss of color between the leaf veins, beginning at the leaf margins or tips and progressing inward. Leaves appear striped, with yellowing and browning of leaf tips and edges as symptoms progress (which may be confused with K deficiency), resulting in less photosynthesis and overall crop stunting. Small amounts of Mg can be applied to growing crops through foliar fertilization to correct or prevent developing deficiencies, but the preferred approach is to soil-apply the required amounts before planting.

Plant S, Ca and Mg diagnosis
The best way to diagnose S, Ca and Mg deficiencies is with plant tissue analysis, using specific tissues at different growth stages (Table 5).  In the case of S, ratios of total N to total S range from 7:1 to 15:1. Wider ratios may point to possible S deficiency, but should be considered along with actual S and N concentrations in making diagnostic interpretations.

Table 5. Sufficient concentrations of S, Ca and Mg in corn tissues
at different growth

Tissue Selected

S (%)

Ca (%)

Mg (%)

Whole plants less than 12 inches tall

0.15-0.50

0.30-0.70

0.15-0.45

Leaf below the whorl prior to tasseling

0.15-0.50

0.25-0.50

0.13-0.30

Ear leaf at initial silking

0.21-0.50

0.21-1.00

0.20-1.00

Micronutrients

The function of any nutrient is the origin of the symptom of its deficiency. A listing of the specific functions of each micronutrient (Table 6) helps to illustrate why the detection of a deficiency in corn is often difficult. For example, Fe, Mn and Cu are each involved with chlorophyll formation and a shortage will likely trigger a visible yellowing of plant tissue. Zinc, B, and Mo are each involved with protein formation, which is less likely to trigger a visible symptom, although leaves of zinc deficient plants tend to have yellowish interveinal striping.

Table 6.  The functions of micronutrients in plant development

Plant Growth Function

Cl

Fe

Mn

Zn

Cu

B

Mo

enzyme systems

 

X

X

X

X

 

X

protein formation

 

X

 

X

X

X

X

hormones and cell division

 

 

 

X

 

X

 

chlorophyll formation

 

X

X

 

X

 

 

disease resistance

X

 

 

 

 

 

 

photosynthesis

X

X

X

 

 

 

 

N, Fe and/or P metabolism

 

X

X

X

X

X

X

crop maturity

X

 

 

 

 

 

 

seed formation

 

 

 

X

X

X

 

sugar/starch translocation

X

 

 

X

 

X

Midwest U.S. researchers report that the sensitivity of corn to micronutrient deficiency is low for B and Mo, medium for Cu, Fe and Mn and high for Zn. Liming a strongly acidic soil to a pH level of about 6.0 to 6.5 impacts the availability of all micronutrients except Cl; Fe, Zn, Cu, B and Mn become less available to corn while Mo availability actually increases. Soils high in organic matter are often in need of Cu and B. Alkaline soils, that are also high in P, tend to be responsive to applied Zn. Sandy soils are more likely to be in need of micronutrients than soils high in clay content. Cold, wet soils often trigger Zn deficiency in young corn plants. Land leveling or removal of higher organic matter surface soils also triggers a shortage of Zn. Dry soils late in the season can lead to inadequate B absorption by corn roots.

The total concentration of a micronutrient in the soil is usually a poor indicator of its availability to the corn plant. For example, considerable Fe and Mn might exist in a soil, yet be limiting to plant growth because the nutrients are in a form unsuitable for absorption by roots. The content of B, Zn, Cu or Cl in soils might range from a few to several hundred kg/ha (lb/ac) and adversely affect plant growth as either a deficiency or toxicity. Thus, micronutrient management for high yield corn production should include consideration of the conditions regulating their availability—soil acidity, soil temperature and moisture, genetics, and interactions with other inputs.

Excess concentrations of micronutrients in plants can also be of concern for corn growers. The boundaries for deficiency and excess are close for B, Cu and Zn. Excess levels of Fe and Mn are alleviated by liming acid soils. Excess levels of B, Zn or Cu are seldom a problem in corn production. Occasionally, an excess of Cu or Mn will inhibit Fe metabolism and vice versa.