An Introduction to Plant Nutrition

Noel Grundon

Plant Nutrients

Plants need 16 elements to be able to grow and produce maximum yields and quality. These elements are divided into major and minor elements (macronutrients and micronutrients respectively).

The major elements are carbon, oxygen, nitrogen, phosphorus, sulphur, potassium, calcium and magnesium. The minor elements are iron, manganese, copper, zinc, boron, molybdenum and chlorine. The elements carbon, hydrogen and oxygen are obtained by the plants from air and water, but the other necessary mineral elements must be taken up by the roots from soil or nutrient solution, in the case of hydroponics.

NITROGEN is a constituent of amino acids, the building units of proteins. It is also part of chlorophyll and enzymes. As such, nitrogen has an over-riding effect on growth and a dominant effect on other nutrients in the plant. It is especially high in the young tissues.

PHOSPHORUS functions in the movement and storage of energy within the plant. Consequently, it has a controlling role in photosynthesis (the conversion of light energy into chemical energy, sugars and starch) and the use of that energy through respiration. It also plays an important role in cell division and protein formation, promotion of root development and regulation of maturity. A large proportion of plant phosphorus is found in seeds and the growing points.

POTASSIUM - Although potassium is needed in large amounts, the details of its functions are a little vague. From studying the effects of potassium deficiency, it is obvious that it performs many vital functions in cell organisation, permeability, water relations, in both carbohydrate and nitrogen metabolism, and in photosynthesis. It activates several enzymes, controls pH within the cell and is needed to open stomata.

CALCIUM - Much of the calcium in the plant is fixed in cell walls which it appears to stiffen. Calcium is important for the growth and functioning of roots, in cell enlargement, an activator of some enzymes and in chromosome structure. It helps to balance the effects of an excess of other elements, particularly trace elements.

MAGNESIUM is the central atom of every chlorophyll molecule. It is also an important activator of several enzymes and concentrates in seeds.

SULPHUR forms a part of several essential amino acids and all plant proteins contain sulphur. It is also a part of several enzymes in carbohydrate, protein and lipid formation. Sulphur compounds also give certain plants characteristic flavours and odours.

IRON is necessary for the synthesis of chlorophyll and is part of several important enzymes. It functions in photosynthesis, nitrate reduction and nitrogen fixation by legumes. MANGANESE is required for the activity of several enzymes including the reduction of nitrate prior to protein formation, in photosynthesis carbohydrate metabolism and oxidation systems within the plant.

BORON is important in the movement of sugars, in cell wall structure and is closely related to some functions of calcium.

ZINC is essential for the development and activity of the growth controlling hormones, auxin and indole acetic acid. It is also needed for important enzyme systems in nitrogen metabolism and respiration.

COPPER is an activator of a number of enzymes and is present in high concentration in the chloroplasts of leaves.

MOLYBDENUM is required for the reduction of nitrate prior to protein formation.

Balanced Nutrition

Plants require a balance in their diet of minerals. Just as they require a minimum amount of all essential nutrients to grow normally, plants can also be adversely affected if the supply of any one element or group of elements is too high. Since many native plants grow very slowly compared with the growth of crop species, they may require less nutrients for normal growth than do crop species.

Furthermore, native plants often grow naturally in either very acid or very alkaline soils and may prefer an acid or alkaline pH. So, to provide any plant with a balanced diet, remember three things:

• Make sure all nutrients are supplied.
• Supply the nutrients only in the amounts required by the plant. That is, less nutrients for slow growing species and supply the nutrients when the plant is growing actively.
• Make sure the plant is growing in a pH range it prefers.

Organic or Non-organic Nutrition

Do not get confused between the benefits of applying organic mulches and the benefits of nutrients added in organic compounds.

Organic mulches provide benefits of improved soil structure, water penetration and water retention by supplying organic matter to the soil. Although organic matter can supply nutrients for plant use as it breaks down, the amounts of nutrients supplied are small and can take a long time to become available. For some plants, especially some native species, organic mulches can supply the correct balanced diet at the correct rate of supply.

Organic fertilizers are no different to non-organic fertilizers from a plant's point of view. A plant cannot absorb the organic compounds through its roots. These compounds must firstly be converted to the non-organic forms that plants absorb before a plant can make use of the nutrients in organic fertilizers. So the benefits of applying organic fertilizers are the same as applying that nutrient at the same rate in its non-organic form.

From the newsletter of the Pine Rivers Branch of the Society for growing Australian Plants, August 2007.

Australian Plants online - 2007
Association of Societies for Growing Australian Plants