Other effects on soil

Beneficial soil organisms (see p320) are affected by soil acidity and liming. A few soil-borne disease-causing organisms tend to occur more frequently on lime deficient soils (see clubroot), whereas others are more prevalent in well-limed soils. Calcium sometimes improves soil structure and soil stability. It is probable that this is mainly because it encourages root activity and creates conditions favourable for decomposition of organic matter, yielding humus (see p326). Free lime in clay soils sometimes, but not always, leads to better crumb formation on drying and shrinking.

Plant selection

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

Mineral soils

Peats

Mineral soils

Neutral

Optimum For Most Mineral Soils
4.0 5.0 6.0 I 7.0 8.0 Optimum 'pH' for most mineral soils

Neutral

Peats

Neutral

Availability Growing Media
5.0 6.0 7.0 Optimum 'pH' for most 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

Table 20.1 Soil acidity and plant tolerance pH below which plant growth may be restricted on mineral soils:

Table 20.1 Soil acidity and plant tolerance pH below which plant growth may be restricted on mineral soils:

Celery

B.3

Rose

S.B

Daffodil

B.1

Raspberry

S.S

Bean

B.G

Cabbage

S.4

Lettuce

B.1

Strawberry

S.1

Carnation

B.G

Tomato

S.1

Chrysanthemum

S.7

Apple

S.G

Carrots

S.7

Potato

4.9

Hydrangea (pink)

S.9

Hydrangea (blue)

4.1

Calcifuge or 'lime-hating' plants do not tolerate the level of calcium in soils normally found at pH 5.5. Consequently, they must be grown in more acid conditions.

Calcicoles, or 'lime-loving' plants, have evolved a different metabolism and are tolerant of high soil pH.

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.

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 (see p372).

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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