Page 152 - Soil Degradation, Conservation and Remediation

5.3 Acidification 141

difficult and time-consuming to achieve the desired distribution. Deep placement
is only recommended for soils in which subsurface acidity is constraining
production, and before it is attempted, detailed consideration should be given to
whether it is likely to be profitable.
Surface application: The main aim of applying lime to the surface should be an even
coverage of the ground. The spreading width should be approximately 6–8 m,
depending on wind conditions to get good coverage of fine particles less than
0.5 mm. Spreading too wide results in uneven treatment of soil acidity. Larger
particles will spread up to 15 m, but the effective distribution is poor and will
result in variable change in pH.
Surface applied then deep-rip: Applying lime by spreading on surface prior to any
soil disturbance and deep-ripping by plowing is a good practice. This enables
better distribution of the lime particles and greater contact with the acidic soil.
Liming before deep-ripping for compaction adds value to both treatments.
Shallow incorporation: Some farmers use shallow incorporation of 200–300 kg ha
−1
of lime at seeding. Little work has been done to test whether this makes a difference
to the rate or amount of amelioration that can be achieved. Research shows
that low rates of lime that do not increase the surface pH to 5.5 or above are
insufficient to prevent ongoing subsurface acidification.
Direct injection: Deep and direct injection involves specific machinery. Research
has shown that successful direct injection is possible and subsurface acidity can
be quickly removed as a production constraint. When the distribution of lime is
correct, responses of 20–30 % in wheat are common. However, increased costs
due to machinery modification and slow operation need to be considered. Poor
distribution can result in the lime being placed below an untreated acidic layer,
and root growth will still be restricted. Only where compaction is also a constraint
may direct injection be worth considering.

5.3.4 Acid Sulfate Soils

Most acid sulfate soils occur in the tropics, in low-lying coastal lands formerly
occupied by mangrove swamps. Their most important characteristics are a field pH
of below 4, owing to the oxidation of pyrite to sulfuric acid, and generally high clay
content. If samples of the pyrite layers are air dried in the laboratory, the pH may
drop by a further 2 units. Potential acid sulfate soils have a near-neutral pH under
field conditions but become strongly acid upon drainage and oxidation. The total
area of actual and potential acid sulfate soils is rather small: about 10 M ha are
known to occur in the tropics, and the world total probably does not exceed 14 M ha.
In addition, some 20 M ha of coastal peats, mainly in Indonesia, are underlain by
potential acid sulfate soil. Clearance of the native vegetation, deforestation, shrimp
farming, etc. have converted many potential acid sulfate soils into extremely acidic
soils. Potential acid sulfate soils can be used for satisfactory rice production, but