Soil pH and nutrient availability
Soil pH is a measure of a soil’s acidity or alkalinity on a scale of 0
to 14.
Influence of pH on nutrient availability
A pH of 7 is neutral, with acidity increasing with every number
down the scale and alkalinity increasing with every number up
the scale.
The pH scale is logarithmic, which means a pH of 6 is 10 times
more acidic than 7 and a pH of 5 is 100 times more acidic than 7.
The best way to understand your soil pH is to conduct a soil test.
It is important to know the pH of soils because it can affect the
availability of nutrients to plants. See figure on right.
In very acidic soils, the availability of some nutrients such as
phosphorus, calcium, magnesium and molybdenum is very low.
The rhizobium bacteria in the nodules of legumes that fix
atmospheric nitrogen are less effective in strongly acidic
conditions. Molybdenum, required by these rhizobia, is less
available at pH (CaCl2) levels below 5.0.
In strongly acidic soils, aluminium and manganese, which are
usually insoluble in the soil, become more soluble to the point
where they can become toxic. Plants which prefer more alkaline
conditions, such as lucerne, are very susceptible to aluminium
toxicity.
Soil acidity is corrected by applying liming products.
Lime, or calcium carbonate, is made from pulverising limestone,
marble, shells or coral to create a product that can be spread. If
lime contains more than 8% magnesium, it is called dolomite.
Lime works by neutralising hydrogen ions in the soil, which leads
to the soil’s pH increasing.
By applying a liming product, the availability of phosphorus,
calcium, magnesium and molybdenum can increase, while
reducing the availability of aluminium, manganese and iron.
Source: Australian Soil Fertility Manual, J.S. Glendinning,
Fertilizer Industry Federation of Australia and CSIRO, 1999.
How much lime?
Soil test
CEC (meq
/100g)
Lime required (t/ha) to lift the pH of the top 10 cm
From 4.0
to 5.2
From 4.3
to 5.2
From 4.7
to 5.2
From 5.2
to 5.5
Gypsum
1
1.6
0.8*
0.3*
0.2*
2
2.4
1.2
0.5*
0.4*
Gypsum does not alter soil pH, but can be used to address the
breakdown of soil structure, in particular, sodic soils. Excess
sodium disperses the soil particles, limiting movement of air and
water, so water tends to pond on the soil surface.
3
3.5
1.7
0.7
0.5*
4
3.9
2.1
0.9
0.6
To better understand your soil’s lime or gypsum requirements,
conduct a soil test and consult your agronomist.
5
4.7
2.5
1.1
0.7
6
5.5
3.0
1.2
0.8
7
6.3
3.3
1.4
1.0
8
7.1
3.8
1.6
1.1
9
7.9
4.2
1.8
1.2
10
8.7
4.6
1.9
1.3
15
12.5
6.7
2.8
1.9
This table shows the limestone required (fine and NV>95) to lift the pH of the top
10 cm of soil. *Low rates of lime are impractical to apply, but over-liming can cause
nutrient imbalances, particularly in light soils. Do not apply more than 4 t/ha in
a single application. Source: AgFacts ‘Soil acidity and liming’, NSW DPI, B.Upjohn,
G.Fenton, M. Conyers, 1995.
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