2 - Improper fertilisation influences the nutrient availability in the rhizosphere and is another very important factor for weakening the root system, especially in the case of bonsai trees which grow in a restricted space. The three major causes are :

A -Excess fertilisation can injure or kill the roots by causing an excessive concentration of salts in the rhizosphere which results in reverse osmosis. The diffusion mechanism of osmosis is an important phenomenon by which ions in the soil medium move from a point of higher concentration to a point of lower concentration. Whenever the concentration of salts of the soil solution is near to or greater than that of the cell’s contents, the root cells are unable to take up sufficient water for growth or may even dry out and die. Fortunately, for potted trees, it is relatively easy to remove excess nutrients by a proper drench of the soil mixture.

B -Under fertilisation or the use of an inappropriate fertiliser formula applied to the soil weakens the roots to the point that it offers less resistance to invasion by soil parasites.

C -The wrong timing of fertiliser application can also greatly influence the health of roots. For example, deciduous trees and conifers should not be fertilised when root growth is at a minimum during its root growth seasonal cycles, otherwise salts will accumulate in the rhizosphere and weaken the existing roots thereby making them more susceptible to the root rot organisms.

3 – Excessive alcality or acidity in the rhizosphere weakens the root system because pH has a major influences for the availability of plant nutrients. The nutrient pH is responsible for the solubility of the various elements that are absorbed by the roots. At a given pH some of the nutrients may exist as insoluble forms but become soluble as soon as it changes to a more favorable range. The opposite is also true. For example, as pH increases, iron and manganese elements form molecules that precipitate in water, rather than ionize, and are not absorbed . If they are not absorbed, several of the elements essential for photosynthesis and chlorophyll formation will not be available thereby resulting in leaf chlorosis.

Scientific studies have shown an optimal pH range of 6.0 to 6.5 for most trees because this range allows adequate uptake conditions for the primary, secondary and micronutrients elements.

Among other undesirable environmental factors which could make trees susceptible to root rot are: salt pollution, especially sodium, when they accumulate in excess in the soil mixture; excess competition between roots when they are in a pot-bound condition; and finally, any environmental condition where light, humidity or temperature is too low for maintaining healthy growth of trees, especially tropicals and sub-tropicals, that are kept indoors during winter.