Essential Plant Nutrients and Their Roles in Turf Health

By Achille Correggia, B.Sc. (Agri.) Agronomy, CCA-ON

Macronutrients

Nitrogen (N)

Nitrogen is considered one of the most essential fertilizer elements for healthy turfgrass growth; it is the most abundant fertilizer element in plant tissue. Nitrogen is a component of many important molecules within the plant, including protein, amino acids, chlorophyll, hormones, (e.g. Cytokinin) and many other plant processes and structures.

Plants absorb nitrogen from the soil in the form of nitrate ions (NO₃-) and in few situations plants will absorb ammonium (NH₄+). Once in the plant, certain nitrogen compounds (soluble amines and amides) are highly mobile allowing them to move or reallocate from older leaves to younger, more rapidly developing leaves. Consequently older leaves are the first to show deficiency symptoms and die prematurely when nitrogen deficiencies occur within the plant.

Nitrogen deficiencies are relatively common due to nitrogen's ability to move within the soil. The most common causes for the deficiency are under fertilization; soil leaching, and denitrification caused by waterlogged soils. Typical symptoms of nitrogen deficiencies include: stunted plant growth, general yellowing of leaves starting with older leaves, and the possible loss of leaves under severe deficiency.

Nitrogen sources include urea, sulphur coated urea (S.C.U ®), Nutralene ®, Nitroform ®, ammonium sulphate, and isobutylidene diurea (IBDU ®).

Phosphorous (P)

Phosphorous, like nitrogen, is a macronutrient required for healthy turf development and is contained in every cell of living turf. The application of phosphorous to turf has a proactive effect on topical growth, rooting and root branching and is critical during the early stages of grass seed establishment and development.

Without the application of trace amounts of phosphorous through fertilizers, deficiencies may begin to appear in turf. Signs of phosphorous deficiency include reductions in both turf density and turf root growth, reddening or yellowing of leaf margins and death of mature leaves.

It must be noted that all soils have some phosphorous in reserve. Although these reserves may be adequate, turf may still suffer from a phosphorous deficiency since less than 5% of a soil's total phosphorous is available or slowly available to the plant at any time. The remainder of soil phosphorous may be held in organic matter, or a number of mineral complexes. The release rate of phosphorous from organic matter is dependent on the decomposition rate of the organic matter. Factors that contribute to the availability of phosphorous to turf include pH, soil moisture, soil temperature, fertilizer application, soil clay content and crop residue. A change in any one of these factors can affect the availability of phosphorous to the plant.

Sources of phosphorous include mono-ammonium phosphate (MAP), di-ammonium phosphate (DAP) and triple super phosphate (TSP).

Potassium (K)

Potassium is involved in many plant processes; it serves as an activator of a number of enzymes involved in respiration, is vital for chlorophyll production, and regulates osmotic pressure in cellular tissue. Protein and starch production as well as plant movement (stomata opening & closing) are also dependent on the availability of potassium. Potassium deficiencies will begin in the older leaves appearing as chlorosis, followed by necrotic lesions (spots of dead tissue) at the leaf margins. The most common causes of potassium deficiencies are under fertilization, restricted root growth from soil compaction and dry weather conditions on sand soils.

Most soils have an abundant amount of potassium, unfortunately only a small amount is available to plants. Potassium is available in many forms within the soil and can be categorized into three forms of availability: relatively unavailable, slowly available and readily available.

Ninety to ninety-eight percent of all potassium in the soil is relatively unavailable. This form of potassium is locked in insoluble primary material such as feldspars and micas, which are quite resistant to weathering and supply a relatively small amount of potassium. One to ten percent of the potassium in the soil is available in a slowly available form. This potassium form is trapped between layers of silica and alumina clays. These soil materials have a tendency to shrink and swell during dry and wet conditions resulting in the release of potassium during wet conditions and the trapping of potassium during dry conditions. The final 0.2-2% of potassium in the soil is in the readily available form, this form of potassium is held in soil solution or in an exchangeable form where it can be absorbed by the plant roots.

Potassium sources include sulphate of potash (SOP) and potassium chloride (KCl).

Secondary Nutrients

Calcium (Ca)

Calcium within the plant is required for cell wall formation, movement of sugars throughout the plant, root hair formation, neutralizing any poisons produced in the plant, improving general plant vigor and quality of plant tissue. Calcium is relatively immobile within the plant; therefore any signs of calcium deficiencies in the plant will first appear in the youngest tissue. General signs of Calcium deficiency are die back of plant borders, poor root hair development, deformed and necrotic young leaves and the death of the meristem in severe deficiency situations.

Calcium is taken up by the plant in the form of Ca²⁺ from the soil solution. With calcium being a cation within the soil, equilibrium exists between the soil solution phase and the soil exchange sites. As calcium is lost from the soil solution through plant absorption or leaching, calcium ions located on the exchange sites will be released to re-establish the equilibrium. This process also works in the opposite direction; if the Ca²⁺ ion concentration increases in the soil solution, more Ca²⁺ ions will attach to the exchange sites. Calcium should occupy 65% to 75% of the exchange sites, with proper amounts of calcium in the soil; texture improves, phosphorous availability increases and enhances root system formation.

The availability of calcium to the plant is affected by several different factors, which include:

Total calcium supplied
Soil pH - soils with lower pH have less Calcium available
C.E.C. - soils with higher C.E.C have more exchangeable sites for Ca²⁺ attachment
Soil type – higher amounts of Calcium will leach from coarse textured soils
Other plant nutrient concentrations – excessive amounts of potassium, magnesium, manganese and aluminum will depress calcium uptake

Sources of calcium include gypsum (Calcium Sulphate) and dolomite (Calcium magnesium carbonate).

Magnesium (Mg)

Plants absorb magnesium in the form of Mg²⁺. Magnesium, is used to help regulate the uptake of other plant nutrients, is a carrier of phosphorus within the plant, assists protein production, aids in phosphate metabolism and is essential for chlorophyll production. Figure 1. Shows the chemical makeup of chlorophyll and displays the importance of magnesium, which is the cornerstone of the chlorophyll molecule. Without this essential nutrient the formation of the chlorophyll molecule can not occur.

Magnesium concentration within the soil can vary from 0.05% to 1.34%. This variation can be attributed to the type of parent material in the growing medium. As these parent materials decompose, magnesium is slowly released and is absorbed by both clay particles and organic material. Magnesium, like calcium, is a cation that is strongly attracted to the exchange sites of a soil particle. As the concentration of magnesium decreases in the soil solution, magnesium that is attached to the soil particle exchange sites is released to re-establish this equilibrium between the soil solution and the soil exchange site. This process also works in the opposite direction; if the magnesium concentration increases in the soil solution, more magnesium will attach to the exchange sites.

The availability of magnesium to the plant is directly influenced by other plant nutrients. High levels of soil potassium can interfere with the uptake of magnesium. Elevated levels of ammonium can also affect the uptake of magnesium, and typically occurs with high application rates of ammonium fertilizer to soils already low in magnesium.

Signs of magnesium deficiencies typically begin in the lower leaves, due to the nutrient being mobile within the plant. Plants with magnesium deficiency will have a general

loss in green colour starting with the older leaves, and chlorosis between the veins due to chlorophyll breakdown. Magnesium, like calcium can be deficient in soils with pH problems and coarse texture.

Fertilizers that contain magnesium include dolomite (Calcium magnesium carbonate).

Figure1. The above illustration shows the Mg atom at the center of the chlorophyll molecule.

Sulphur (S)

Sulphur is necessary for protein production, maintaining dark green growth, and also encourages more vigorous plant growth. Signs of sulphur deficiencies typically appear in the new growth first. Young leaves will turn bright yellow with veins being an even lighter yellow. Deficiency signs of both sulphur and nitrogen are very similar, however the one distinguishing difference between the two nutrients is that signs of nitrogen deficiencies begin in the older, not the younger leaves.

Sulphur occurs in the soil in many different forms, both organic and inorganic. The largest reserve of sulphur in the soil is contained in the soil organic matter. Sulphur becomes available to the plant through organic matter degradation. Once degraded, plants absorb sulphur in the SO₄^2- form from the soil solution.

It has been suggested that the best method of building sulphur reserves in the soil is by adding available organic materials and maintaining an adequate organic matter content.

Micronutrients

Iron (Fe)

The role of iron within the plant includes chlorophyll synthesis, plant metabolism

and the formation of certain plant proteins. Plants that are deficient in iron will show signs of being pale green, then become yellow and finally show white spots between the veins. New leaves will have a light green band along the leaf edge.

Of all the micronutrients, iron is required in the largest amount by the plant. For this reason some consider it to be a macronutrient. Iron is available to the plant in two different forms, Fe³⁺ and Fe²⁺, with the latter being more available due to its greater solubility. Iron availability is dependent on a few factors, including:

Soil pH – availability decreases as pH increases
Organic matter percentage – soils with low organic matter can have low availability of iron
Other plant nutrient concentrations – high levels of phosphorous will depress iron absorption and an improper balance of molybdenum, copper, and manganese can also affect iron availability

Manganese (Mn)

Manganese is absorbed by the plant from the soil solution in the Mn²⁺ form. The availability of manganese to the plant is affected by several different factors, which include:

Soil pH – as soil pH increases available manganese decreases, optimum pH range is 5.0 to 6.5
Organic Matter – high organic matter in the soil will decrease the availability of manganese
Other plant nutrient concentrations - nitrogen fertilizers that have an acidifying affect can enhance the uptake of manganese

Once in the plant, manganese aids in the production of chlorophyll and is involved in the photosynthetic process.

Manganese deficiencies typically occur when soil pH is outside the optimum range and/or poor soil drainage occurs. Signs of manganese deficiency will typically begin in new growth first. Leaf tissue between the veins will fade to a medium yellow. Note that the size and texture of the leaf is not affected by manganese deficiency, only the colour.

Boron (B)

Boron within the plant is required for sugar transport, cell wall formation, root growth, nitrogen assimilation and respiration. Plants that experience boron deficiencies will have poor root growth and appear to encourage higher insect and disease pressure. The higher attraction of insects is the result of phenol (an organic scent) production with the plant. With the presence of adequate boron simple sugars are converted into complex sugars. When the plant is lacking boron these simple sugars are converted into phenol which attracts insects to the plant.

Boron is one of the most commonly deficient of all micronutrients. The availability of boron to the plant is affected by several different factors, which include:

soil pH – the availability of boron is related to soil pH, optimum range is 5.0 to 7.0
soil moisture – soils experiencing drought conditions will have limited amounts of boron available
organic matter percentage – organic matter serves as a major source of boron for many soils
leaching – boron is readily leached from acid, sandy soils

Zinc (Zn)

Zinc is absorbed by the plant in the form of Zn²⁺. It is involved in growth hormone production, control of other elements in the plant, enzyme formation and starch production. Signs of zinc deficiencies in the plant will appear as white areas between the veins on the leaves.

Zinc in the soil is relatively immobile; therefore leaching does not pose a problem. The availability of zinc in the soil to the plant is reduced when the pH is increased above 9.0 and may be reduced in soils where the phosphorous levels are too high.

Zinc is also vital for soil microorganisms.

Copper (Cu)

Copper, once in the plant is involved in chlorophyll formation, plant metabolism,

root development, amino acids and helps to prevent chlorosis. It has also been suggested that copper may have some disease suppression characteristics.

In well aerated soils, copper is available to the plant in the form of Cu²⁺. Copper availability is dependent on a few factors, including:

soil pH – copper mobility decrease as pH increases
soil texture – copper is less available in sand soils
organic matter percentage – soils with low organic matter can have lower available zinc
Other plant nutrient concentrations – high levels of zinc, aluminum, phosphorous and iron will depress copper absorption

Molybdenum (Mo)

Molybdenum is involved in metabolizing nitrogen. Plants absorb molybdenum from the soil in the form of MoO₄^2-. Unlike all the other micronutrients, the availability of molybdenum increases as the soil pH increases to 7.0 or above. The availability of the element is not only dependent on pH, but also soil moisture. Soil's that experience drought conditions will have low amounts of molybdenum available for plant absorption. With the application of phosphorous, the uptake of this essential element by plants is enhanced.

Chlorine (Cl)

Chloride plays an important role in stomatal regulation, water flow, and is also involved in photosynthesis.

Chlorine is only found in nature as chloride (Cl^-) and is widely present. This ion is highly soluble, highly mobile and readily taken up by the plant. The availability of chloride to plants is not affected by pH. The accumulation of chloride in the plant is affected by nitrate and sulphate levels. Higher nitrate and sulphate levels within the plant will reduce the plant chloride concentration. Plants deficient in chloride will show signs of reduced growth, wilting of leaf tips, and general chlorosis.

Sources of chlorine include potassium chloride.

Reference:

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S.C.U. is a registered trademark of Zeneca Corp. Nutralene and Nitroform are registered trademarks of Nu-Gro America Corp.

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