Role of Macro- and Micro-nutrients

Two Groups by Amount Needed

Once the essentiality criteria are applied, plants turn out to need a fairly short list of mineral elements, and these are split into two groups purely by how much of each the plant requires.

• Macronutrients are needed in large amounts — more than 10 millimoles per kilogram of dry matter. They are carbon, hydrogen, oxygen, nitrogen, phosphorus, sulphur, potassium, calcium and magnesium. Of these nine, carbon, hydrogen and oxygen are not really absorbed from the soil at all — the plant gets them mainly from carbon dioxide in the air and from water.
• Micronutrients (also called trace elements) are needed in much smaller amounts — less than 10 millimoles per kilogram of dry matter. They are iron, manganese, copper, molybdenum, zinc, boron, chlorine and nickel.

Beyond these seventeen essential elements, some plants also benefit from a few extra, 'beneficial' elements — sodium, silicon, cobalt and selenium — which help growth in particular species without being essential across all plants.

Four Kinds of Jobs Nutrients Do

Mineral nutrients earn their keep in four broad ways:

• Structural components of biomolecules — carbon, hydrogen, oxygen and nitrogen build the plant's organic molecules.
• Components of energy-related compounds — magnesium sits at the centre of every chlorophyll molecule, and phosphorus is a building block of ATP.
• Activators or inhibitors of enzymes — magnesium ions switch on RuBisCO and PEP carboxylase, the two key enzymes of photosynthetic carbon fixation; zinc ions activate alcohol dehydrogenase; and molybdenum activates nitrogenase, the enzyme central to nitrogen metabolism.
• Regulators of osmotic potential — potassium, for example, controls the opening and closing of stomata by changing the water potential of the guard cells.

What Each Nutrient Actually Does

• Nitrogen is needed in the largest amount of any nutrient. It is absorbed mainly as nitrate, with some taken up as nitrite or ammonium, and is used everywhere in the plant, especially in actively growing meristem tissue — it is a core building block of proteins, nucleic acids, vitamins and hormones.
• Phosphorus is absorbed as phosphate ions and becomes part of cell membranes, several proteins, all nucleic acids and nucleotides, and every reaction that adds a phosphate group to a molecule.
• Potassium is absorbed as the K+ ion, concentrates in meristems, buds, leaves and root tips, keeps the balance between positive and negative ions in the cell, and takes part in protein synthesis, stomatal movement, enzyme activation and maintaining cell turgor.
• Calcium is absorbed as Ca2+, is needed by dividing and differentiating tissue, is used to build the cell wall (as calcium pectate in the middle lamella) and to form the spindle during cell division, accumulates in older leaves, keeps membranes functioning properly, and activates certain enzymes.
• Magnesium is absorbed as Mg2+, activates enzymes of respiration and photosynthesis, is needed for making DNA and RNA, forms part of the chlorophyll ring itself, and helps hold ribosomes together.
• Sulphur is absorbed as sulphate, is found in the amino acids cysteine and methionine, and is part of several co-enzymes and vitamins, including thiamine, biotin and coenzyme A, as well as ferredoxin.
• Iron is absorbed as the ferric ion and is needed in a larger amount than the other micronutrients. It is a key part of electron-carrying proteins such as ferredoxin and the cytochromes, cycling between its two oxidation states as it passes electrons along; it also activates the enzyme catalase and is essential for making chlorophyll.
• Manganese is absorbed as the manganous ion and activates enzymes of photosynthesis, respiration and nitrogen metabolism; its best-known role is splitting water to release oxygen during photosynthesis. …