Classwork Exercise and Series (Biology-SS1): Mineral Requirements Of Plants

Introduction

Plants require mineral nutrients or elements obtained from the soil in form of solution for good growth and healthy development. The soil is the main source of mineral salts while gaseous elements such as oxygen, hydrogen and carbon are mainly derived from the atmosphere. These elements or plant nutrients are grouped into two classes, depending on the quantity that is required by plants. They are as follows:

Macro-Nutrients or Major elements: These are mineral elements or nutrients required in large quantities for healthy growth of plants. Examples of macro-nutrients are nitrogen, phosphorus, potassium, magnesium, calcium, oxygen, hydrogen, carbon, sulphur and iron. These macro-nutrients are sometimes called essential elements.

Micro-Nutrients or Trace elements: These are mineral elements or nutrients required in small quantities for healthy growth of plants. Examples of micro-nutrients are zinc, copper, boron, molybdenum, cobalt, chlorine and manganese. 

The Elements of Complete Plant Nutrition

The following is a brief guideline of the role of essential and beneficial mineral nutrients that are crucial for growth. Eliminate any one of these elements, and plants will display abnormalities of growth, deficiency symptoms, or may not reproduce normally.

Macronutrients

Nitrogen is a major component of proteins, hormones, chlorophyll, vitamins and enzymes essential for plant life. Nitrogen metabolism is a major factor in stem and leaf growth (vegetative growth). Too much can delay flowering and fruiting. Deficiencies can reduce yields; can cause yellowing of the leaves and stunt growth.

Phosphorus is necessary for seed germination, photosynthesis, protein formation and almost all aspects of growth and metabolism in plants. It is essential for flower and fruit formation. Low pH (<4) results in phosphate being chemically locked up in organic soils. Deficiency symptoms are purple stems and leaves; maturity and growth are retarded. Yields of fruit and flowers are poor. Premature drop of fruits and flowers may often occur. Phosphorus must be applied close to the plant’s roots in order for the plant to utilize it. Large applications of phosphorus without adequate levels of zinc can cause a zinc deficiency.

Potassium is necessary for formation of sugars, starches, carbohydrates, protein synthesis and cell division in roots and other parts of the plant. It helps to adjust water balance, improves stem rigidity and cold hardiness, enhances flavour and colour on fruit and vegetable crops, increases the oil content of fruits and is important for leafy crops. Deficiencies result in low yields, mottled, spotted or curled leaves, scorched or burned look to leaves.

Sulphur is a structural component of amino acids, proteins, vitamins and enzymes and is essential to produce chlorophyll. It imparts flavour to many vegetables. Deficiencies show as light green leaves. Sulfur is readily lost by leaching from soils and should be applied with a nutrient formula. Some water supplies may contain Sulphur.

Magnesium is a critical structural component of the chlorophyll molecule and is necessary for functioning of plant enzymes to produce carbohydrates, sugars and fats. It is used for fruit and nut formation and essential for germination of seeds. Deficient plants appear chlorotic, show yellowing between veins of older leaves; 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. 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) so there must be a constant supply 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. Yellow leaf margins may also appear.

Micronutrients

Iron is necessary for many enzyme functions and as a catalyst for the synthesis of chlorophyll. It is essential for the young growing parts of plants. Deficiencies are pale leaf colour of young leaves followed by yellowing of leaves and large veins. Iron is lost by leaching and is held in the lower portions of the soil structure. Under conditions of high pH (alkaline) iron is rendered unavailable to plants. When soils are alkaline, iron may be abundant but unavailable. Applications of an acid nutrient formula containing iron chelates, held in soluble form, should correct the problem.

Manganese is involved in enzyme activity for photosynthesis, respiration, and nitrogen metabolism. Deficiency in young leaves may show a network of green veins on a light green background similar to an iron deficiency. In the advanced stages the light green parts become white, and leaves are shed. Brownish, black, or grayish spots may appear next to the veins. In neutral or alkaline soils plants often show deficiency symptoms. In highly acid soils, manganese may be available to the extent that it results in toxicity.

Boron is necessary for cell wall formation, membrane integrity, calcium uptake and may aid in the translocation of sugars. 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 must be available throughout the life of the plant. It is not translocated and is easily leached from soils. Deficiencies kill terminal buds leaving a rosette effect on the plant. Leaves are thick, curled and brittle. Fruits, tubers and roots are discolored, cracked and flecked with brown spots.

Zinc is a component of enzymes or a functional cofactor of a large number of enzymes including auxins (plant growth hormones). It is essential to carbohydrate metabolism, protein synthesis and internodal elongation (stem growth). Deficient plants have mottled leaves with irregular chlorotic areas. Zinc deficiency leads to iron deficiency causing similar symptoms. Deficiency occurs on eroded soils and is least available at a pH range of 5.5 – 7.0. Lowering the pH can render zinc more available to the point of toxicity.

Copper is concentrated in roots of plants and plays a part in nitrogen metabolism. It is a component of several enzymes and may be part of the enzyme systems that use carbohydrates and proteins. Deficiencies cause die back of the shoot tips, and terminal leaves develop brown spots. Copper is bound tightly in organic matter and may be deficient in highly organic soils. It is not readily lost from soil but may often be unavailable. Too much copper can cause toxicity.

Molybdenum is a structural component of the enzyme that reduces nitrates to ammonia. Without it, the synthesis of proteins is blocked and plant growth ceases. Root nodule (nitrogen fixing) bacteria also require it. Seeds may not form completely, and nitrogen deficiency may occur if plants are lacking molybdenum. Deficiency signs are pale green leaves with rolled or cupped margins.

Sodium is involved in osmotic (water movement) and ionic balance in plants.

Cobalt is required for nitrogen fixation in legumes and in root nodules of non legumes. The demand for cobalt is much higher for nitrogen fixation than for ammonium nutrition. 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 making them a mechanical barrier to piercing and sucking insects. This significantly enhances plant heat and drought tolerance. 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 infection by fungus to combat the penetration of the 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 silicon. Silicon has not been determined essential for all plants but may be beneficial for many.

Nutrient Cycles

Inorganic nutrients occur in limited quantities and their loss to an ecosystem or retention and re-use is of great importance. The cycles of chemical elements in an ecosystem are known as nutrient cycles. If there is no loss to the ecosystem the cycle is said to be a ‘perfect cycle’ and if loss does occur the cycle is said to be ‘imperfect’. The decomposers play an important role in these cycles because they break down dead organisms and make the nutrient components available once more to other organisms.

The carbon and nitrogen cycle are two such cycles.

The Carbon Cycle

All organic compounds contain carbon and the most important sources of all inorganic carbon are carbon dioxide in the atmosphere.

o    Carbon dioxide is taken up by autotrophic organisms during photosynthesis and the carbon is incorporated into carbohydrates and other compounds , such as proteins and fats;

o    Consumers (heterotrophic organisms) feed on plants, and their bodies assimilate carbon compounds derived from the plants;

o    All organisms, including plants, release carbon dioxide during respiration as a by-product. (Fermentation releases of carbon dioxide);

o    When autotrophic and heterotrophic organisms die or lose body parts such as leaves, carbon dioxide is released as a result of decomposition;

o    Combustion of dead animal and plant material also releases carbon dioxide;

o Under high pressures, dead plants and animals are carbonized, forming fossil-fuels, such as coal and crude-oil. These release carbon dioxide during combustion.

A Diagrammatic representation of the Carbon Cycle

The Nitrogen Cycle

Nitrogen is an element essential in all organisms, occurring in proteins and other nitrogenous compounds, e.g. nucleic acids. Although organisms live in nitrogen-rich environments (78% of the atmosphere is nitrogen) the gaseous forms of nitrogen can only be used by certain organisms. Free nitrogen must first be fixed into a useable form.

o    Free nitrogen in the atmosphere is mainly fixed by two groups of bacteria, Azotobacter and Clostridium. The nitrogen is then used to manufacture proteins in their bodies, when they die, their proteins are broken down by decomposers (mainly bacteria and other micro-organisms), and converted into ammonia (blue-green algae, cyanobacteria, can also be use free nitrogen from the atmosphere);

o    During electrical changes in the atmosphere(e.g. lightning), free nitrogen is fixed (combined) finally forming nitrate;

o    Nitrates are taken up by plants which use them to manufacture proteins;

o    Animals (herbivores) eat plants and convert plant proteins to animal proteins, while carnivores obtain their plant proteins by indirect means (by eating herbivores);

o    When plants and animals die, the proteins in their bodies are broken down into ammonia by decomposers. The process is known as ammonification;

o    Ammonia is converted to nitrites by nitrite bacteria (Nitrosomonas and Nitrosococcus). Nitrites are again converted to nitrates by nitrate bacteria (Nitrobacter )This process is known as nitrification;

o Different types of bacteria are also able to break down nitrates, nitrites and ammonia which results in the release of nitrogen. This process is known as denitrification.