Nitrogen

Abstract

From the viewpoint of the plant physiologist, all of the essential elements can be regarded as being of equal importance in the nutrition of the plant, but in terms of the practical management of lightweight mixes nitrogen is probably the most important single element. It forms between 2 and 4% of the dry weight of the average pot plant and occurs in all plant proteins. It is also an essential component of chlorophyll, the green pigment present in leaves which enables plants to utilise the sun’s energy to combine carbon dioxide from the atmosphere with water vapour to form carbohydrates. This process is known as photosynthesis and provides man with his primary source of food. When nitrogen is deficient there is a marked reduction in the growth rate. Plants are tall and spindly, the upper leaves being narrow and erect while the lower leaves turn first pale green and then yellow as the nitrogen is withdrawn and transported to the actively growing regions. Often the older leaves produce bright colours before they die. When the nitrogen level in the medium is high in relation to the amounts of phosphorus and potassium, plants tend to make soft, vegetative growth and be less reproductive with delayed flowering. This situation is intensified when the plants are grown under conditions of low light intensity, high humidity and low water stress. Soft vegetative plants are more susceptible to fungal diseases such as botrytis. If the nitrogen level in the medium is further increased there will be a corresponding increase in the soluble salt level, and eventually this will have the effect of producing hard, stunted plants.