Fertilizers

Nitrogen deficiency

J Slow, stunt I'd ^;mwtl\; pale green leaves

Nitrogen deficiency

J Slow, stunt I'd ^;mwtl\; pale green leaves

There is much misunderstanding of the role that plant nutrients play in the growth of the plant and hence they are often misused in the propagation and establishment phases of plant production.

The three so-called major elements that are required for successful plant growth are nitrogen, potassium and phosphorus. These materials are needed in addition to carbon, hydrogen and oxygen as the basic building blocks of plant material. At a secondary level elements such as calcium, sulphur and magnesium are needed in fair quantities, while trace elements, which include iron, manganese, boron, molybdenum and cobalt, are used by the plant in small to minute quantities.

Nitrogen is required wherever and whenever plant growth is anticipated. A shortage of available nitrogen is typified by the slow, stunted development of the plant and pale green leaves. It is usually taken up by the plant in the form of either nitrate or ammonium salts. Nitrogen's role in the plant is predominantly as a basic ingredient of protein and it is thus a necessary feature of developing new plant tissue. Often it is referred to as being "important for leafy growth", which is essentially true because leaves constitute a major part of plant tissue; however it ifc equally a necessary component of stem, root, flower and seed production. As a protein component it is also a vital feature of chromosome development.

The role of potassium in the plant is rather less easily explained. Potash, as it is commonly called, is needed as a catalyst wherever chemical reactions occur. It is especially associated with the food-making process of photosynthesis and with supplying nourishment around the plant. This again explains its general association with "leafy growth". However potash is equally important in virtually all parts of the plant where chemical reactions are occurring. Potassium deficiency in plants is usually manifest when the edges of the leaves turn pale yellow, and as this discoloration moves inwards the outer edges turn brown and appear scorched.

Phosphorus, which is normally used in the form of phosphate, has two major roles to fulfil in plant growth. Firstly it is an essential component of those very specialized proteins that constitute the chromosomes. Secondly it is the basis on which the energy needed for plant growth and development is collected, transported and released within the various chemical reactions of the plant. A phosphate shortage is much more difficult and uncertain to describe especially when it is only a marginal amount, but generally stunted growth associated with a purple or red leaf discoloration is a typical symptom; however a similar situation often arises with root damage caused by pests or rots.

Most other plant nutrients occur in sufficient quantities as minor components or impurities in the main fertilizers, and they do not specifically need to be applied as individual fertilizers.

The only two nutrients that may cause problems are magnesium and iron. The prime role of magnesium is in the formation of chlorophyll, the green colouring in the plant. A lack of it is typified by a yellowing of the older leaves as the plant transfers magnesium from its older parts to its newly created parts so causing the "chlorosis" in the old leaves.

Iron has a similar function to magnesium but it is not reusable in the plant. Its deficiency causes the young leaves to turn yellow although the veins remain green, and by this characteristic it is possible to distinguish a shortage of this element from a magnesium deficiency.

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