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  Section: Principles of Horticulture » Soil pH
 
 
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Plant selection

 
     
 
Content
Soil pH
  The importance of soil pH
  Measuring pH
  Changes in soil pH
  Effects of soil pH
  Plant selection
  Changing soil pH

Plant tolerance to soil pH and calcium levels varies considerably, but all plants are adversely affected when the soil becomes too acid. At very low pH some elements, such as aluminium, become soluble at levels that are toxic to plants. Table 20.1 shows the point below which the growth of common horticultural plants is significantly reduced. Although aluminium is not an essential nutrient for plants it is required to produce a blue flower rather than a pink one in hydrangeas (see Figure 20.1). Commercially, growers do not just use a compost with a low pH, but also add aluminium sulphate to get the blue colouring.
Table 20.1 Soil acidity and plant tolerance
Table 20.1 Soil acidity and plant tolerance
Figure 20.4 Effect of soil pH on nutrient availability. The availability of a given amount of nutrient is indicated by the width of the band. The growing media should be kept at a pH at which all essential nutrients are available. For most plants the optimum pH is 6.5 in mineral soils and 5.8 in peats
Figure 20.4 Effect of soil pH on nutrient availability. The availability of a given amount of
nutrient is indicated by the width of the band. The growing media should be kept at a pH at
which all essential nutrients are available. For most plants the optimum pH is 6.5 in mineral
soils and 5.8 in peats
In the case of calcifuges the highest point before growth is affected by the presence of calcium should be noted, e.g. for Rhododendrons and some Ericas this is at pH 5.5 in mineral soils. Such plants are unable to metabolize many of the nutrients when there is more calcium present; typically they show signs of lime induced chlorosis.

In contrast, calcicoles are well adapted to utilizing soil nutrients in the presence of calcium, but are unable to survive in acid conditions where they shown signs of aluminium toxicity (dead tissues) and phosphate deficiency (stunting and blue or reddish stem and leaves).

 
     
 
 
     



     
 
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