The fact that to the present day most acid upland soils are not used for agriculture is an
indication that it is very difficult to practice sustainable agriculture on them. Before the
introduction of chemical fertilisers and high yielding crop varieties. many acid soils were
considered unsuitable for arable crops. However. tree crops have been grown on them
successfully for many decades because trees represent a rather natural succession to the rain
forest, the natural cover of those soils.
Acid upland soils (Ultisols and Oxisols) of tropical Asia are, in general, inherently infertile
and AI or Mn toxic. Their shallow topsoils, the Ultisols particularly. are highly susceptible to
erosion. The organic matter can be easily degraded and lost by conventional land-clearing
practices. including burning and direct exposure to sun and rain. If not managed properly after
clearing. these soils can rapidly lose much of their original fertility and beneficial physical
properties.
To sustain production on these soils requires inputs far exceeding the means of poor farmers.
Unfortunately, there is no such thing as a 'low input' technology for the poor acid soils of the
tropics. If poorly managed with no or low input to replace nutrients removed by cropping or
erosion, these soils will within a few years be incapable of producing a decent crop and will be
invaded by lmperata cylindrica. Eventually they may become barren if the degradation is not
stopped.
At present most acid soils are still covered by primary or secondary forest. Before any
forested area is to be opened up for agricultural use, short as well as long term costs and
benefits and possible damage to the environment should be considered. In the long run, we may
need the acid soils more as a source of timber and a sink for carbon dioxide than for food
production.
Introduction
Once soils have lost their fertile surface layer,
they cannot, even with the best management, be
returned to their former productivity in less than
10 years, if ever. Furthermore, the present-day
management of agricultural soils must protect
them against erosion and leaching losses, if their
productivity, even at high input, is to be
sustained. Much technology to make acid
infertile upland soils productive and profitable
on a long-term sustainable basis is already
,.. Potash and Phosphate Institute and International
Potash Institute, 126 Watten Estate Road, Singapore,
1128.
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