Geoscience Reference
In-Depth Information
•
Which crops or pastures will grow best in a soil of a particular level of acidity
or alkalinity?
•
What is the availability or non availability of soil nutrients due to the soil pH level?
•
What are the trends in soil pH levels over time?
Plant responses
Most plants prefer a pH(w) range between 6.0 and 7.5, but will grow outside this
range (although yield may be affected). How far outside this range plants will grow
varies between the species (Figure 4).
Soil biological activity is also affected by soil pH. This becomes important
approaching the extremes of acidity or alkalinity, when, for example, various
species of earthworms and nitrifying bacteria disappear. Rhizobia strains vary in
their sensitivity to soil pH, and have preferred ranges in which they are effective
(Figure 5). Most soil organisms function best between pH 6.0-7.0.
Availability
Soil pH will inf luence both the availability of soil nutrients to plants and how the
nutrients react with each other. At a low pH, many elements become less available
to plants (see Figure 4), while others, such as iron, aluminium and manganese
become toxic to plants and, in addition, aluminium, iron and phosphorus combine
to form insoluble compounds. In contrast, at high pH levels calcium ties up
phosphorus, making it unavailable to plants, and molybdenum becomes toxic in
some soils. Boron may also be toxic at high pH levels in some soils and deficient in
acid soils (Figure 6).
Acidity
Alkalinity
Mild
High
Moderate Mild Neutral
Moderate High
Crops
3
No agricultural
plants thrive
4
Acid tolerant
plants survive
5
Acid tolerant
plants thrive
6
7
Alkaline tolerant
plants thrive
8
Boron and zinc
deficiency increases
9
Pasture
Optimum
range
Barley
Red clover
Canola
Subclover
Linseed
White clover
Lupins
Cocksfoot
Maize
Cowpeas
Millet
Medics
Oats
Phalaris
Peas
Ryegrass
Rice
Serradella
Ryecorn
Sorghum
Soybeans
Sunflowers
Triticale
Wheat
Figure 4
Preferred pH(w) ranges for plants. (
Soil Sense
C-03)
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