brasmith
Well-Known Member
This is an excellent resource for anyone interested in what our girls need to thrive and bloom. It describes in simple terms why and how the plants need and use the elements we feed our plants. The information I found most interesting is the reading on calcium and magnesium.
Hope you find this article as beneficial as I do.
What is Plant Nutrition?
by David Neal
2008-10-01
Plants use inorganic minerals for nutrition, whether grown in the field or in a container. Complex interactions involving weathering of rocks, decaying organic matter, animals and microbes take place to form inorganic minerals in soil. Roots absorb mineral nutrients as ions, dissolved in soil water. Many factors influence nutrient uptake for plants. Ions can be readily available to roots or could be tied up by other elements or the soil itself. Soil that is too high in pH (alkaline) or too low in pH (acidic) makes many minerals unavailable to plants.
Fertility or Nutrition
The term fertility refers to the inherent capacity of a soil to supply nutrients to plants in adequate amounts and in suitable proportions. The term nutrition refers to the interrelated steps by which a living organism assimilates food and uses it for growth and replacement of tissue. Previously, plant growth was thought of in terms of soil fertility or how much fertilizer should be added to the soil to increase the levels of mineral elements. Most fertilizers were formulated to account for deficiencies of mineral elements in the soil. The use of soilless mixes and increased research in nutrient cultures and hydroponics as well as advances in plant tissue analysis has led to a broader understanding of plant nutrition. Plant nutrition is a term that takes into account the interrelationships of mineral elements in the soil or growing medium as well as their role in plant growth. These interrelationships involve a complex balance of mineral elements, which are essential or beneficial for optimum plant growth.
Nitrogen is a major component of proteins, hormones, chlorophyll, vitamins and nucleic acids.
Essential versus Beneficial
Arnon and Stout proposed a definition of an essential mineral element (or mineral nutrient) in 1939. They concluded that three criteria must be met for an element to be considered essential. These criteria are:
1. A plant must be unable to complete its life cycle in the absence of the mineral element.
2. The function of the element must not be replaceable by another mineral element.
3. The element must be directly involved in plant metabolism.
These criteria are important guidelines for plant nutrition but ignore beneficial mineral elements. Beneficial elements are those that can compensate for toxic effects of other elements and replace mineral nutrients in some other less specific function such as the maintenance of osmotic pressure or provide other nonessential benefits. The omission of beneficial nutrients in commercial production could mean that plants are not being grown to their optimum genetic potential but are merely produced at a subsistence level.
In other words, Arnon and Stouts proposed definition of an essential element is a narrow one. A plant cannot survive without these elements. A better definition of an essential mineral element would include all elements essential for optimum growth and health of any plant. This discussion of plant nutrition includes both the essential and beneficial mineral elements.
What Mineral Elements Do Plants Need?
There are 20 mineral elements essential for plant growth. Carbon (C), hydrogen (H) and oxygen (O) are supplied by air and water. The six macronutrients: nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg) and sulfur (S) are required by plants in large amounts. The rest of the elements are micronutrients and are only required in trace amounts. Essential trace elements include boron (B), chlorine (Cl), copper (Cu), iron (Fe), manganese (Mn) sodium (Na), zinc (Zn), molybdenum (Mo), nickel (Ni), silicon (Si) and cobalt (Co). Note that silicon and cobalt are not considered essential by the Arnon and Stout definition to all plants but are essential to some. Studies have also shown that other mineral elements are beneficial to the growth of some plants. The distinction between beneficial and essential is often difficult in the case of some trace elements. Cobalt for instance is essential for nitrogen fixation in legumes. It may also inhibit ethylene formation and extend the life of cut roses. Silicon, deposited in cell walls, has been found to improve heat and drought tolerance and increase resistance to insects and fungal infections. Silicon, acting as a beneficial element, can help compensate for toxic levels of manganese, iron, phosphorus and aluminum as well as zinc deficiency. A more holistic approach to plant nutrition would not be limited to nutrients essential to survival but would include mineral elements at levels beneficial for optimum growth. With developments in analytical chemistry and the ability to eliminate contaminants in nutrient cultures, the list of essential elements may well increase in the future. For example, nickel, the most recent addition to the list of essential elements, was added in the early 1990s as a result of several years of research.
The Mineral Elements in Plant Production
The use of soil for greenhouse production before the 1960s was common. Today few growers still use soil in their mixes. The bulk of production is in soilless mixes. Soilless mixes must provide support, aeration, nutrient and moisture retention just as soils do, but the addition of fertilizers or nutrients are different. Many soilless mixes have calcium, magnesium, phosphorus, sulfur, nitrogen, potassium and some micronutrients incorporated as a pre-plant fertilizer. Nitrogen and potassium still must be applied to the crop during production. Difficulty in blending a homogeneous mix using pre-plant fertilizers may often result in uneven crops and possible toxic or deficient levels of nutrients. Soilless mixes that require the addition of micro and macronutrients applied as liquid throughout the growth of the crop actually give the grower more control of his crop. To achieve optimum production, the grower can adjust nutrient levels to compensate for other environmental factors during the growing season. The uptake of mineral ions is dependent on a number of factors in addition to weather conditions. These include the cation exchange capacity or CEC and the pH of the growing medium and water supply, as well as the total alkalinity of the irrigation water.
CEC or Cation Exchange Capacity
CEC refers to the ability of the growing medium to hold exchangeable mineral elements within its structure. These cations include ammonium (nitrogen), potassium, calcium, magnesium, iron, manganese, zinc and copper. Peat moss, coir and mixes containing bark, sawdust and other organic materials all have some level of cation exchange capacity.
pH: What Does it Mean?
The term pH refers to the alkalinity or acidity of a growing medium water solution. This solution consists of mineral elements dissolved in ionic form in water. The reaction of this solution whether it is acid, neutral or alkaline will have a marked effect on the availability of mineral elements to plant roots. When there is a greater amount of hydrogen (H+) ions the solution will be acidic (pH<7.0). If there are more hydroxyl (-OH) ions, the solution will be alkaline (pH>7.0). A balance of hydrogen and hydroxyl ions results in a pH neutral soil (pH=7.0). The optimum pH range for most crops is 5.5 to 6.2 or slightly acidic. This creates the greatest average availability of all essential plant nutrients. Different mineral elements are available at different pH levels. Extreme variations in pH can cause mineral deficiencies or toxicity by binding or releasing large amounts of various elements.
Enzymes: The Workhorses of Life
Enzymes are proteins, which are involved in increasing the rate and efficiency of biochemical reactions. Most enzymes require metal ions for activation and function. Without proper enzyme function, growth would cease in an organism. Most of the essential mineral elements affect enzymes in multiple ways. Many of the roles of mineral elements in enzymes are discussed in the following paragraphs.
The Elements of Complete Plant Nutrition
The following is a brief summary of the role of essential and beneficial mineral nutrients crucial to plant growth. If any one of the essential elements is eliminated from a plants nutrition, it will display abnormalities of growth, symptoms of deficiency and may not reproduce normally.
Essential Macronutrients
Nitrogen is a major component of proteins, hormones, chlorophyll, vitamins and nucleic acids. Nitrogen metabolism is a major factor in stem and leaf growth (vegetative growth). Excessive nitrogen can be detrimental to growth by delaying flowering and fruiting. Deficiencies can reduce yields, cause yellowing of the leaves and stunt growth.
Phosphorus is necessary for seed germination, photosynthesis, protein synthesis and almost all aspects of growth and metabolism in plants. Phosphorus is a component of RNA and DNA, the genetic makeup of life. It is essential for flower and fruit formation. Low pH (<4) results in phosphate being chemically locked up in organic soils. Deficiency symptoms include stunted growth, reduced yields of fruit and flowers and premature drop of fruit and flowers. Purple coloring may also appear due to anthocyanin and accumulation. Large applications of phosphorus without adequate levels of zinc can cause a zinc deficiency.
Potassium is an activator of many enzymes that are required in photosynthesis and respiration. It is involved in osmotic potential in cells. Potassium is also required for sugars, carbohydrates, cell division, protein synthesis and phloem transport. It helps adjust water balance, improves stem rigidity and cold hardiness, enhances flavor and color in fruit and vegetable crops, increases oil content and is important in leafy crops. Deficiencies result in low yields, mottled, spotted or curled leaves and a scorched or burned look to leaves.
Sulfur is a structural component of amino acids, enzymes, proteins and vitamins. Sulfur is essential in respiration and lipid metabolism. It imparts flavor to many vegetables. Deficiency symptoms appear as chlorosis throughout the leaves. Sulfur is readily lost by leaching from soils and should be applied with a nutrient formula. Many water supplies contain sulfur.
Magnesium is a critical structural component of the chlorophyll molecule and is necessary for functioning and/or activation of plant enzymes to produce carbohydrates, sugars, proteins and fats. It is used for fruit and nut formation and is essential for germination of seeds. In essence, magnesium is essential to every metabolic pathway in plants. Deficient plants appear chlorotic meaning they show yellowing between the veins of older leaves and the leaves may droop. Magnesium is leached by watering and must be supplied when feeding. It can be applied as a foliar spray to correct deficiencies.
Calcium activates enzymes, is a structural component of cell walls, influences water movement in cells and is necessary for cell growth and division. Calcium is required for membrane function in all cells. Some plants must have calcium to take up nitrogen and other minerals. Calcium is easily leached. Calcium, once deposited in plant tissue, is immobile (non-translocatable). Accordingly, a constant supply of calcium is essential for growth. Deficiency causes stunting of new growth in stems, flowers and roots. Symptoms range from distorted new growth to black spots on leaves and fruit.
Essential Micronutrients
Iron is a component of many structural and enzymatic proteins. It is essential for electron transport and chlorophyll biosynthesis. It is, therefore, required for photosynthesis and respiration. Iron is also essential for lipid metabolism. A well-known symptom of iron deficiency is interveinal chlorosis. High soil pH can cause iron deficiency. Toxic levels of iron are associated with waterlogged soils as iron is immobile.
Manganese activates many enzymes, but to date, only two are considered manganese-containing enzymes. One of these enzymes is directly involved with the photosynthetic evolution of oxygen. Manganese is required for respiration and carbohydrate and lipid metabolism. Deficiency symptoms in dicots appear as chlorosis between the veins (interveinal) of young leaves, as manganese is not mobile in plants. In grasses, greenish gray spots on the more basal leaves are a sign of manganese deficiency. In neutral or alkaline soils, plants often show deficiency symptoms. In highly acidic soils, manganese may be available in toxic levels.
Zinc is a structural component of many enzymes and also acts as a cofactor in others. Zinc is essential to DNA replication, gene expression, protein synthesis, IAA synthesis, membrane integrity and carbohydrate metabolism. Deficiency symptoms in dictos include shortened internodes and a reduction in leaf size. Chlorosis often accompanies these symptoms. At low soil pH, zinc may accumulate to toxic levels. Raising the pH is the most effective method of reducing zinc availability in soils.
Copper is an integral component of several enzymes and other critical biological proteins. It is required in photosynthesis, respiration and lignin biosynthesis and in carbohydrate, nitrogen and lipid metabolism. Copper is also required for pollen grain formation. A copper deficiency can cause die back of the shoot tips, stunted growth and terminal leaves may develop black necrotic spots. Copper deficiency affects fruit and seed formation much more drastically than vegetative growth. Copper is bound tightly to organic matter and deficiencies may result in highly organic soils even though copper is present. Copper becomes toxic to plants at high levels.
Molybdenum is a structural component of the enzyme nitrate reductase that reduces nitrates to ammonia. This enzyme is found in all higher plants. Many plants (i.e. legumes) reduce atmospheric nitrogen to ammonia via bacteria located in root nodules. These bacteria use the enzyme nitrogenase, which also contains molybdenum. Without adequate levels of molybdenum, the synthesis of proteins is blocked, plant growth ceases and seeds may not form completely. Not surprisingly, other biologically important enzymes beside the two mentioned contain molybdenum. One of the most common signs of molybdenum deficiency is interveinal chlorosis of young leaves. Other symptoms include stunted seedling growth and those symptoms associated with nitrogen deficiency, including rolled or cupped leaf margins.
Chlorine is involved in osmoregulation, the regulation of movement of water and other solutes into and out of cells. Chlorine is essential for cell division in leaves and in the regulation of opening and closing of stomata. Chlorine is also involved in the photosynthetic evolution of oxygen and nitrogen metabolism. Deficiency symptoms include wilting of leaves, chlorosis and stunted root growth. High levels of chlorine can be severely detrimental to plant growth.
Nickel has recently been determined to be an essential trace element for plants by a group of scientists at the USDA Agricultural Research Service (ARS) in Ithaca, New York. It is required for the enzyme urease, which most plants use to break down urea into usable forms of nitrogen. Nickel is also a necessary component for the function of other enzymes. Nickel is essential for iron absorption. Seeds require nickel in order to germinate. Plants grown without an adequate supply of nickel will gradually reach a deficient level at about the time they mature and begin reproductive growth.
Boron plays an essential role in membrane integrity, calcium uptake, root elongation, nucleic acid metabolism, cell wall synthesis and pollen tube formation. Boron affects at least 16 functions in plants. These functions include flowering, pollen germination, fruiting, cell division, water relationships and the movement of hormones. Boron is non-translocatable and, therefore, must be available throughout the life of the plant. Its uptake is closely related to the soil pH. It becomes more readily available as pH increases. Deficiency symptoms include discoloration or death of young leaves and terminal bud leaving a rosette effect on the plant. Plants will also fail to set seed and fruit. Leaves are thick, curled and brittle. Fruits, tubers and roots are discolored, cracked and flecked with brown spots.
Beneficial Micronutrients
Sodium is involved in osmotic (water movement) and ionic balance and is required for some plants. Sodium is essential for many but not all C4 plants.
Cobalt is required for nitrogen fixation in legumes and in root nodules of non-legumes because it is a component of enzymes essential for nitrogen fixation. Deficient levels could result in nitrogen deficiency symptoms.
Silicon is found as a component of cell walls. Plants with supplies of soluble silicon produce stronger, tougher cell walls creating a mechanical barrier to the mouthparts of piercing and sucking insects. Silicon significantly enhances plant heat, drought and cold tolerance. Silicon stimulates the production of polyphenols, part of a plants natural defenses against fungal and insect attack. Foliar sprays of silicon have also shown benefits reducing populations of aphids on field crops. Tests have also found that silicon can be deposited by the plants at the site of a fungal infection to combat the penetration of cell walls by the attacking fungus. Improved leaf erectness, stem strength and prevention or depression of iron and manganese toxicity has all been noted as effects from feeding soluble silicon. Silicon, known to be essential to members of the poaceae family (grasses), has demonstrated benefits to a wide variety of plants. Silicon is the second most common element in the earths crust. However, it is largely tied up in the form of insoluble rock. Hence, it is only available in very low levels in nature, though it is ubiquitous in soil solutes and all water supplies, even rainwater. Even double distilled water contains not less than five ppb of silicon. Accordingly, it is not possible to deprive any plant of all silicon, a requirement for the Arnon and Stout definition of essentiality
Hope you find this article as beneficial as I do.
What is Plant Nutrition?
by David Neal
2008-10-01
Plants use inorganic minerals for nutrition, whether grown in the field or in a container. Complex interactions involving weathering of rocks, decaying organic matter, animals and microbes take place to form inorganic minerals in soil. Roots absorb mineral nutrients as ions, dissolved in soil water. Many factors influence nutrient uptake for plants. Ions can be readily available to roots or could be tied up by other elements or the soil itself. Soil that is too high in pH (alkaline) or too low in pH (acidic) makes many minerals unavailable to plants.
Fertility or Nutrition
The term fertility refers to the inherent capacity of a soil to supply nutrients to plants in adequate amounts and in suitable proportions. The term nutrition refers to the interrelated steps by which a living organism assimilates food and uses it for growth and replacement of tissue. Previously, plant growth was thought of in terms of soil fertility or how much fertilizer should be added to the soil to increase the levels of mineral elements. Most fertilizers were formulated to account for deficiencies of mineral elements in the soil. The use of soilless mixes and increased research in nutrient cultures and hydroponics as well as advances in plant tissue analysis has led to a broader understanding of plant nutrition. Plant nutrition is a term that takes into account the interrelationships of mineral elements in the soil or growing medium as well as their role in plant growth. These interrelationships involve a complex balance of mineral elements, which are essential or beneficial for optimum plant growth.
Essential versus Beneficial
Arnon and Stout proposed a definition of an essential mineral element (or mineral nutrient) in 1939. They concluded that three criteria must be met for an element to be considered essential. These criteria are:
1. A plant must be unable to complete its life cycle in the absence of the mineral element.
2. The function of the element must not be replaceable by another mineral element.
3. The element must be directly involved in plant metabolism.
These criteria are important guidelines for plant nutrition but ignore beneficial mineral elements. Beneficial elements are those that can compensate for toxic effects of other elements and replace mineral nutrients in some other less specific function such as the maintenance of osmotic pressure or provide other nonessential benefits. The omission of beneficial nutrients in commercial production could mean that plants are not being grown to their optimum genetic potential but are merely produced at a subsistence level.
In other words, Arnon and Stouts proposed definition of an essential element is a narrow one. A plant cannot survive without these elements. A better definition of an essential mineral element would include all elements essential for optimum growth and health of any plant. This discussion of plant nutrition includes both the essential and beneficial mineral elements.
What Mineral Elements Do Plants Need?
There are 20 mineral elements essential for plant growth. Carbon (C), hydrogen (H) and oxygen (O) are supplied by air and water. The six macronutrients: nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg) and sulfur (S) are required by plants in large amounts. The rest of the elements are micronutrients and are only required in trace amounts. Essential trace elements include boron (B), chlorine (Cl), copper (Cu), iron (Fe), manganese (Mn) sodium (Na), zinc (Zn), molybdenum (Mo), nickel (Ni), silicon (Si) and cobalt (Co). Note that silicon and cobalt are not considered essential by the Arnon and Stout definition to all plants but are essential to some. Studies have also shown that other mineral elements are beneficial to the growth of some plants. The distinction between beneficial and essential is often difficult in the case of some trace elements. Cobalt for instance is essential for nitrogen fixation in legumes. It may also inhibit ethylene formation and extend the life of cut roses. Silicon, deposited in cell walls, has been found to improve heat and drought tolerance and increase resistance to insects and fungal infections. Silicon, acting as a beneficial element, can help compensate for toxic levels of manganese, iron, phosphorus and aluminum as well as zinc deficiency. A more holistic approach to plant nutrition would not be limited to nutrients essential to survival but would include mineral elements at levels beneficial for optimum growth. With developments in analytical chemistry and the ability to eliminate contaminants in nutrient cultures, the list of essential elements may well increase in the future. For example, nickel, the most recent addition to the list of essential elements, was added in the early 1990s as a result of several years of research.
The Mineral Elements in Plant Production
The use of soil for greenhouse production before the 1960s was common. Today few growers still use soil in their mixes. The bulk of production is in soilless mixes. Soilless mixes must provide support, aeration, nutrient and moisture retention just as soils do, but the addition of fertilizers or nutrients are different. Many soilless mixes have calcium, magnesium, phosphorus, sulfur, nitrogen, potassium and some micronutrients incorporated as a pre-plant fertilizer. Nitrogen and potassium still must be applied to the crop during production. Difficulty in blending a homogeneous mix using pre-plant fertilizers may often result in uneven crops and possible toxic or deficient levels of nutrients. Soilless mixes that require the addition of micro and macronutrients applied as liquid throughout the growth of the crop actually give the grower more control of his crop. To achieve optimum production, the grower can adjust nutrient levels to compensate for other environmental factors during the growing season. The uptake of mineral ions is dependent on a number of factors in addition to weather conditions. These include the cation exchange capacity or CEC and the pH of the growing medium and water supply, as well as the total alkalinity of the irrigation water.
CEC or Cation Exchange Capacity
CEC refers to the ability of the growing medium to hold exchangeable mineral elements within its structure. These cations include ammonium (nitrogen), potassium, calcium, magnesium, iron, manganese, zinc and copper. Peat moss, coir and mixes containing bark, sawdust and other organic materials all have some level of cation exchange capacity.
pH: What Does it Mean?
The term pH refers to the alkalinity or acidity of a growing medium water solution. This solution consists of mineral elements dissolved in ionic form in water. The reaction of this solution whether it is acid, neutral or alkaline will have a marked effect on the availability of mineral elements to plant roots. When there is a greater amount of hydrogen (H+) ions the solution will be acidic (pH<7.0). If there are more hydroxyl (-OH) ions, the solution will be alkaline (pH>7.0). A balance of hydrogen and hydroxyl ions results in a pH neutral soil (pH=7.0). The optimum pH range for most crops is 5.5 to 6.2 or slightly acidic. This creates the greatest average availability of all essential plant nutrients. Different mineral elements are available at different pH levels. Extreme variations in pH can cause mineral deficiencies or toxicity by binding or releasing large amounts of various elements.
Enzymes: The Workhorses of Life
Enzymes are proteins, which are involved in increasing the rate and efficiency of biochemical reactions. Most enzymes require metal ions for activation and function. Without proper enzyme function, growth would cease in an organism. Most of the essential mineral elements affect enzymes in multiple ways. Many of the roles of mineral elements in enzymes are discussed in the following paragraphs.
The Elements of Complete Plant Nutrition
The following is a brief summary of the role of essential and beneficial mineral nutrients crucial to plant growth. If any one of the essential elements is eliminated from a plants nutrition, it will display abnormalities of growth, symptoms of deficiency and may not reproduce normally.
Essential Macronutrients
Nitrogen is a major component of proteins, hormones, chlorophyll, vitamins and nucleic acids. Nitrogen metabolism is a major factor in stem and leaf growth (vegetative growth). Excessive nitrogen can be detrimental to growth by delaying flowering and fruiting. Deficiencies can reduce yields, cause yellowing of the leaves and stunt growth.
Phosphorus is necessary for seed germination, photosynthesis, protein synthesis and almost all aspects of growth and metabolism in plants. Phosphorus is a component of RNA and DNA, the genetic makeup of life. It is essential for flower and fruit formation. Low pH (<4) results in phosphate being chemically locked up in organic soils. Deficiency symptoms include stunted growth, reduced yields of fruit and flowers and premature drop of fruit and flowers. Purple coloring may also appear due to anthocyanin and accumulation. Large applications of phosphorus without adequate levels of zinc can cause a zinc deficiency.
Potassium is an activator of many enzymes that are required in photosynthesis and respiration. It is involved in osmotic potential in cells. Potassium is also required for sugars, carbohydrates, cell division, protein synthesis and phloem transport. It helps adjust water balance, improves stem rigidity and cold hardiness, enhances flavor and color in fruit and vegetable crops, increases oil content and is important in leafy crops. Deficiencies result in low yields, mottled, spotted or curled leaves and a scorched or burned look to leaves.
Sulfur is a structural component of amino acids, enzymes, proteins and vitamins. Sulfur is essential in respiration and lipid metabolism. It imparts flavor to many vegetables. Deficiency symptoms appear as chlorosis throughout the leaves. Sulfur is readily lost by leaching from soils and should be applied with a nutrient formula. Many water supplies contain sulfur.
Magnesium is a critical structural component of the chlorophyll molecule and is necessary for functioning and/or activation of plant enzymes to produce carbohydrates, sugars, proteins and fats. It is used for fruit and nut formation and is essential for germination of seeds. In essence, magnesium is essential to every metabolic pathway in plants. Deficient plants appear chlorotic meaning they show yellowing between the veins of older leaves and the leaves may droop. Magnesium is leached by watering and must be supplied when feeding. It can be applied as a foliar spray to correct deficiencies.
Calcium activates enzymes, is a structural component of cell walls, influences water movement in cells and is necessary for cell growth and division. Calcium is required for membrane function in all cells. Some plants must have calcium to take up nitrogen and other minerals. Calcium is easily leached. Calcium, once deposited in plant tissue, is immobile (non-translocatable). Accordingly, a constant supply of calcium is essential for growth. Deficiency causes stunting of new growth in stems, flowers and roots. Symptoms range from distorted new growth to black spots on leaves and fruit.
Essential Micronutrients
Iron is a component of many structural and enzymatic proteins. It is essential for electron transport and chlorophyll biosynthesis. It is, therefore, required for photosynthesis and respiration. Iron is also essential for lipid metabolism. A well-known symptom of iron deficiency is interveinal chlorosis. High soil pH can cause iron deficiency. Toxic levels of iron are associated with waterlogged soils as iron is immobile.
Manganese activates many enzymes, but to date, only two are considered manganese-containing enzymes. One of these enzymes is directly involved with the photosynthetic evolution of oxygen. Manganese is required for respiration and carbohydrate and lipid metabolism. Deficiency symptoms in dicots appear as chlorosis between the veins (interveinal) of young leaves, as manganese is not mobile in plants. In grasses, greenish gray spots on the more basal leaves are a sign of manganese deficiency. In neutral or alkaline soils, plants often show deficiency symptoms. In highly acidic soils, manganese may be available in toxic levels.
Zinc is a structural component of many enzymes and also acts as a cofactor in others. Zinc is essential to DNA replication, gene expression, protein synthesis, IAA synthesis, membrane integrity and carbohydrate metabolism. Deficiency symptoms in dictos include shortened internodes and a reduction in leaf size. Chlorosis often accompanies these symptoms. At low soil pH, zinc may accumulate to toxic levels. Raising the pH is the most effective method of reducing zinc availability in soils.
Copper is an integral component of several enzymes and other critical biological proteins. It is required in photosynthesis, respiration and lignin biosynthesis and in carbohydrate, nitrogen and lipid metabolism. Copper is also required for pollen grain formation. A copper deficiency can cause die back of the shoot tips, stunted growth and terminal leaves may develop black necrotic spots. Copper deficiency affects fruit and seed formation much more drastically than vegetative growth. Copper is bound tightly to organic matter and deficiencies may result in highly organic soils even though copper is present. Copper becomes toxic to plants at high levels.
Molybdenum is a structural component of the enzyme nitrate reductase that reduces nitrates to ammonia. This enzyme is found in all higher plants. Many plants (i.e. legumes) reduce atmospheric nitrogen to ammonia via bacteria located in root nodules. These bacteria use the enzyme nitrogenase, which also contains molybdenum. Without adequate levels of molybdenum, the synthesis of proteins is blocked, plant growth ceases and seeds may not form completely. Not surprisingly, other biologically important enzymes beside the two mentioned contain molybdenum. One of the most common signs of molybdenum deficiency is interveinal chlorosis of young leaves. Other symptoms include stunted seedling growth and those symptoms associated with nitrogen deficiency, including rolled or cupped leaf margins.
Chlorine is involved in osmoregulation, the regulation of movement of water and other solutes into and out of cells. Chlorine is essential for cell division in leaves and in the regulation of opening and closing of stomata. Chlorine is also involved in the photosynthetic evolution of oxygen and nitrogen metabolism. Deficiency symptoms include wilting of leaves, chlorosis and stunted root growth. High levels of chlorine can be severely detrimental to plant growth.
Nickel has recently been determined to be an essential trace element for plants by a group of scientists at the USDA Agricultural Research Service (ARS) in Ithaca, New York. It is required for the enzyme urease, which most plants use to break down urea into usable forms of nitrogen. Nickel is also a necessary component for the function of other enzymes. Nickel is essential for iron absorption. Seeds require nickel in order to germinate. Plants grown without an adequate supply of nickel will gradually reach a deficient level at about the time they mature and begin reproductive growth.
Boron plays an essential role in membrane integrity, calcium uptake, root elongation, nucleic acid metabolism, cell wall synthesis and pollen tube formation. Boron affects at least 16 functions in plants. These functions include flowering, pollen germination, fruiting, cell division, water relationships and the movement of hormones. Boron is non-translocatable and, therefore, must be available throughout the life of the plant. Its uptake is closely related to the soil pH. It becomes more readily available as pH increases. Deficiency symptoms include discoloration or death of young leaves and terminal bud leaving a rosette effect on the plant. Plants will also fail to set seed and fruit. Leaves are thick, curled and brittle. Fruits, tubers and roots are discolored, cracked and flecked with brown spots.
Beneficial Micronutrients
Sodium is involved in osmotic (water movement) and ionic balance and is required for some plants. Sodium is essential for many but not all C4 plants.
Cobalt is required for nitrogen fixation in legumes and in root nodules of non-legumes because it is a component of enzymes essential for nitrogen fixation. Deficient levels could result in nitrogen deficiency symptoms.
Silicon is found as a component of cell walls. Plants with supplies of soluble silicon produce stronger, tougher cell walls creating a mechanical barrier to the mouthparts of piercing and sucking insects. Silicon significantly enhances plant heat, drought and cold tolerance. Silicon stimulates the production of polyphenols, part of a plants natural defenses against fungal and insect attack. Foliar sprays of silicon have also shown benefits reducing populations of aphids on field crops. Tests have also found that silicon can be deposited by the plants at the site of a fungal infection to combat the penetration of cell walls by the attacking fungus. Improved leaf erectness, stem strength and prevention or depression of iron and manganese toxicity has all been noted as effects from feeding soluble silicon. Silicon, known to be essential to members of the poaceae family (grasses), has demonstrated benefits to a wide variety of plants. Silicon is the second most common element in the earths crust. However, it is largely tied up in the form of insoluble rock. Hence, it is only available in very low levels in nature, though it is ubiquitous in soil solutes and all water supplies, even rainwater. Even double distilled water contains not less than five ppb of silicon. Accordingly, it is not possible to deprive any plant of all silicon, a requirement for the Arnon and Stout definition of essentiality
